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Motor Vibration and Low-end Torque

Motor Vibration and Low-end Torque

The following is my assessment of the VTX regarding vibration & lugging, based on my expertise as a mechanic, as an engine designer, and as a physicist for the past 20 years, and also relies on numerous book references and professional experience in automotive engine research and development. I have ridden my VTX just under 20,000 miles, under all types of conditions, sometimes covering 700 miles in a day. Take it or leave it: The cylinders of the VTX are splayed 52 degrees. The pins on the crankshaft are separated by 76 degrees. Do you (not you, D-Hav, but "you" the collective vibration/lugging-question-group) know why the cylinder angle of separation doesn't match the crank-pin angle of separation? Standard engineering practices would dictate that these two values would ALWAYS match if the usual goals of engine power and engine efficiency are the design objective. The reason they don’t match on the VTX is to CAUSE vibration. Read again: To GENERATE vibration in the motorcycle for the rider to feel. Honda's have long been criticized for being too refined, for having no "character". When Honda entered the real cruiser market (ie not the Magna/hot-rod series of bikes) to take on Harley, Honda’s top brass thought, “We'll make our bikes better than a Harley and people will buy them”. Guess what. In doing so, Honda made V-Twins so good, so smooth and versatile, that they lost the very mystique that was leading people to buy Harley; Honda’s engineers eliminated the little peculiarities and quirks that make Harleys fun to ride. The biggest one of these charismatic peculiarities is the vibration, the so-called "cadence" of the motor. Ride a Shadow 1100 sometime. Fantastic motorcycle in many regards (mine has 70K+ miles and has required no adjustments and no repairs). But Shadows are boring in that they are too refined. The engine feels...well....perfect…perfectly balanced (even though it is hard mounted to the frame, there is NO vibration to speak of; the mirrors are usable at 100mph and all speeds below). After experimenting around in the cruiser market for a number of years, Honda finally realized that a big reason people enjoy Harleys, and a big reason Harleys are so popular is because they feel (and sound) nostalgic, like riding something very old, and a big part of that feel is the slow, physical rhythm of the engine; the vibration. When Honda designed the VTX, they specifically, intentionally, made the engine shake. Yes, the engine has balance shafts to remove the high-frequency, annoying vibration, but the low-frequency, “lumpy” shake is there, ON PURPOSE, to remind you that you have locomotive-like 4 inch pistons between you’re legs, rising and falling like some huge antiquated factory machine, or a 1930’s farm implement. The people who purchased the VTX to “hop” up and “max it out” on a dyno, to make it fast(er), simply missed the point. They could sell their VTX, buy a ZRX-1200, deposit 3,000 dollars into their bank account, and have an infinitely faster, better performing, and more capable motorcycle than the VTX will EVER be. But in doing so, they wouldn’t have the feel of those BIG lumpy pistons, making the gobs of low-end torque, enough low-rpm torque to make a farm tractor envious. This engine IS ABOUT low RPM torque. As for lugging, there are two things happening. The first type of lugging occurs when an engine is run so slowly, that the hydrodynamic oil pressure decreases to a pressure that is less than the load on the bearing surface. This is very hard to explain easily, but think about a water skier behind a powerboat. If the boat slows down, at some slow speed the skier will finally sink into the water, below the surface; the water can no longer support his weight. Speed the boat up a little, and the skier comes back to the surface and is supported by the water. This is EXACTLY how oil works in your engine, and for a given load, if you slow your engine down enough, the parts “sink” through the surface of the oil and start touching each other. This occurs when the hydrodynamic pressure is less than the load pressure. This type of lugging will not happen on the VTX, or any other type of modern automotive 4-stroke engine (including bikes), except, possibly, during starting. Commercial/vehicle engines today are designed with large enough bearing surfaces, tight enough tolerances, and enough oil pressure that you do not have to worry about this form of lugging. (This could change, however, if you were to use oil that is too thin (grossly wrong viscosity), or if the oil is too hot (very overheated engine).) I am excluding aircraft engines because my sources and experience are not specifically applicable to them, although, with one exception I would think everything here still would apply. The second type of lugging occurs when the engine vibration frequency is roughly equal to the resonant frequency of the entire motorcycle structure, including the rider. This gets a little tricky because there are engine vibration components in the three dimensional Cartesian coordinate system, and there are also torsional components within the drive train, and both sets are significant, even crucial. As complex as they are, if you excite either of these modes of vibration (Cartesian or torsional), it becomes very obvious very quickly. It is a bit subjective, but I think you will realize it when it happens: very sharp, sever vibrations, accompanied by jerky movements of the bike and a hammering noise that is the lash in the drive train slapping forward and backward. There might also be ignition knock but this can be controlled with higher-octane fuel (and has never been a problem on my bike). Lugging IS NOT the lopey, low frequency vibration you feel at 55 mph in 5th gear. That is the “character” designed in, and you paid for that! (See explanation, above.) Honda went to great lengths to ensure that this engine is difficult to lug : count them…there are 5 (five) torsional dampers in the drive train, and unusually large rubber engine mounts to the frame, to isolate the engine vibration (both types) and keep the engine from creating an excitation frequency in the frame. Simply put, all of this keeps the rpm-lugging point at the lowest point possible. It worked. On my bike, totally stock except for the pipe, the engine is completely happy at 50mph in 5th gear, even up average sized hills. All you feel are the power pulses, not lugging. In 4th, I can go down to 15-20 mph., even under load. There is no reason to spin this motor up, other than to generate more power (passing, drag race, etc.), or unless you simply want to eliminate the power pulses. (The power pulses DO NOT indicate lugging.) I will stop just short of saying you can’t lug it. But it is hard to. This engine simply works too well at low rpm to worry about. Look for the telltale signals I described above (particularly the jerky motion), and if they are not there, dig into that low-end torque as much as you want and enjoy it. I threw this together very quickly, and I apologize for any mistakes/typos, in advance.
Last update: 2004-06-25 09:28
Author: T-Rex

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Discussion Starter · #4 ·
How Motor works article

Big, How Motor works article - info on function, fuel injection, vacuum pressure, aftermarket fuel computers, backfiring, and more. By Tapper
This is long, so be prepared. Hopefully, its worth the effort. This is part of something else I'm doing, but I thought you might enjoy it.

Having read hundreds of posts on these topics, I decided to take a stab at writing something to clarify the many misconceptions on this subject. Whether it’s interminable questions about backfiring/popping, or arguments about “load”, or discussion about whether aftermarket fuel computers are necessary and which is best, we see a lot of bad information and mistaken ideas being tossed around as fact. So lets take a look at the basics, and then talk about a few specifics.

Pressure is the thing

The single most important thing to understand about an internal combustion engine is that at its very root, it’s nothing but an air pump. If you can get your mind around that one simple concept, understanding everything else is a lot simpler and more straightforward.

Ever consider your own lungs? As it happens, your body is also a pretty efficient air pump. When you take a breath, your diaphragm muscle contracts and pulls your lungs downwards in your chest, and this increases their size by stretching them. This increase in volume causes a decrease in pressure (applies vacuum) to your windpipe, and this causes air to rush into your chest. Air always flows from an area of high pressure to an area of low pressure. How fast it flows, depends on the difference in pressure. The bigger the difference, the faster air moves. The bigger the volume the more air needs to move to equalize the pressure. When you exhale, the reverse happens – your diaphragm relaxes, causing the volume in your lungs to decrease, which increases the pressure inside your lungs, which makes air flow out of your windpipe – to the outside air, which is now at a lower pressure than your lungs.

The pressure is the thing. Differences in pressure make the air move. All around you, at this very minute, is a bunch of air that stretches up above you for several miles. The weight of all that air, presses down on the air below it, and causes the air that surrounds you to be under pressure. This is called “atmospheric pressure”. It is the baseline around which we talk about the pumping of air. You should understand something else here, the concept of “vacuum”. Vacuum, at least with respect to motors, just means “lower pressure than atmospheric”. If you create a vacuum, what you are really doing, is creating an area where the air pressure is lower that the surrounding atmospheric pressure. If you give it a way to do so, air is going to try real hard to kill that vacuum, by sending air into it. Nature abhors a vacuum. Air always moves from an area of high pressure to an area of low pressure, and the amount of force it does so with, is proportional to the difference between the two pressures. Sounds simple, no? It really is, but it also confuses folks.

Remember I said a motor was just a big air pump? It is, and it works exactly the same way your lungs do. On the intake stroke, as a piston travels down its cylinder, the area in the cylinder increases rapidly – and causes a vacuum. When the intake valve opens, air rushes into the cylinder, and causes a drop in pressure in the intake manifold that is proportional to the amount air moving into the cylinder. On the exhaust stroke, the burning fuel and air has created very high pressure in the cylinder, and when the exhaust valve opens, the pressure in the exhaust header is much lower than the cylinder, so the gas rushes out into the exhaust pipe.

If this sounds simple or obvious to you, then great. But make sure you really do understand these concepts, because understanding the way a motor works means understanding the sequence of pressure changes. If you don’t understand, then none of the rest of this is going to gel for you.

Different Strokes Move the World, Willis.

Ok, now its time to see what the air pump really does, and what drives it, and that means understanding strokes. For the purposes of this article, we’ll focus on a 4-stroke motor, since our bikes use them, and since the vast majority of motors are 4-strokes.

Lets simplify, and look at each stroke.

Intake stroke – This is the first stroke in our series of four. In this one, the piston is moving downward in the cylinder with the intake valve open, causing a vacuum to be created in the cylinder and more importantly, in the intake manifold where the carburetor or fuel injectors live. The vacuum (area of low pressure) created in the intake manifold is the power that drives the fuel system, and gets the fuel into the cylinder where we want it. The larger the vacuum (lower the pressure), the more fuel is pumped into the cylinder. The faster the piston moves downward in the cylinder, the bigger and faster the vacuum is created, and the more fuel gets pumped. Therefore, it also follows, that the higher the engine’s RPM, and the harder an engine is working, the higher the vacuum (or lower the pressure) exists in the intake manifold.

At this moment, I hope your brain is saying to you “whoa – that vacuum measurement is sure an important number to know if I’m going to know what my motor is doing”. Damn skippy it is – so lets define something important right now:

The amount of vacuum produced by a motor is a direct measurement of the load placed on the motor. Therefore, when discussing motors, Load means the amount of air being demanded by a motor, which is equivalent to the amount of vacuum developed in the intake manifold, which is the same as the pressure difference inside the manifold and the outside air.

Woo! Now we understand the mysterious load. It’s a great number to know, and is used by fuel injection computers to determine how much fuel to squirt into the intake manifold. It’s also the energy that makes a carb shoot a given amount of fuel out of its jets. We’ll talk about carbs and fuel injection a little later, so lets move on with the strokes.

Compression Stroke – Ok, the piston has moved as far down in the cylinder as it possibly can on the intake stroke (called bottom dead center or BDC), and starts its way back upward. The intake valve closes and so we have a volume of air and fuel trapped in the cylinder, and suddenly the space it occupies is gonna get squished real hard (compressed), as the piston begins moving upward in the cylinder. This causes the pressure in the cylinder to increase very rapidly, since the charge has nowhere to go. One thing to remember here – fuel burns much faster under pressure.

Power Stroke – Ok, all the valves are closed, we’ve got our air/fuel charge compressed really tight, and put it under a lot of pressure as the piston gets to the very tippity top of it’s path through the cylinder (called Top Dead Center). At this point we set the charge on fire, and as it burns it produces a whole bunch of gas, which rapidly increases the pressure in the small space, and pushed the cylinder downward very hard. This is where all the power in your motor comes from, this rapidly expanding gas pressure. A couple of things to note here.

First, we ignite the mix using the spark plug. If the plug were to fire at exactly top dead center (TDC), we could arbitrarily call that the zero position of the crankshaft, and indeed, that’s exactly what we do. Remember, the crankshaft is rotating in a circle throughout this process. One rotation of the crankshaft (360 degrees) means one complete up and down motion of the piston. When we talk about “timing” in a motor, what we’re really talking about is the relationship of the spark to the moment of TDC or the zero position of the crankshaft. If we fire the spark plug before TDC (BDTC, or during the end of the compression stroke), we call that advancing the spark. If we fire the plug after (ATDC, or during the power stroke), we call that retarding the spark.

Exhaust Stroke – Ok, the piston is now hauling ass downwards, and the cylinder is full of hot and probably still burning gas and residual fuel/air. As the piston crosses bottom dead center (BDC) again and starts upward, we enter the exhaust stroke. During the stroke, the exhaust valve opens, and since the cylinder is now at a much higher pressure than the exhaust pipe, all that hot gas and stuff goes flying into the exhaust header, and out the pipe. That’s the end of the 4 strokes, and when the piston crosses TDC again, the whole process starts anew, and away we go.

A couple of things to note here. You’ll notice, that during all four strokes, the crankshaft rotates two full rotations, and travels through 720 full degrees. The camshafts, which actuate the valves, have traveled through one full rotation, or 360 degrees. You often see guys tossing around degree measurements when discussing things like spark timing, cam timing, cam profiles, and so forth. So it’s important to be able to relate the concept of degrees, to the actual rotation of the parts. If you were really good at math in school, you’ll immediately notice that any given instant in the cycling of a motor can be described using simple trigonometry, which should give you some pretty important insights into the ways fuel injection computers and electronics work on your motors. But that’s a bit lofty a subject for this article, so lets move along, shall we?

It Gives me Gas

Well, now that we’ve got the strokes, let talk about gas, and the fuel system in general. I’m going to focus on describing a fuel injection system, because once you understand that, understanding a carbureted system is vastly easier.

So. We know that load (or vacuum) is the way fuel is delivered to the cylinder, but how does the motor know how much fuel? And how much fuel is actually needed?

Lets start, by talking about the air/fuel ratio.

A ratio, just means “amount of one thing compared to amount of another”. In our case, it means amount of air compared to the amount of fuel. When you see someone talk about air/fuel ratios, they usually put the air part first, followed (or divided) by the fuel part. So an A/F ratio of 13 to 1, or 13-1, or 13/1 just means “thirteen parts air to one part fuel”.

Now, lets talk a little simple chemistry. The burning of air and fuel is just a chemical reaction called combustion. Chemical reactions are described by chemists in chemical equations, and the science of balancing these equations, or determining how much of each chemical reacts, is called “stoichiometry”. Lets look at a simple one:

1O2 + 2H2 --> 2H2O

It’s a little hard to read that, but it just means “One molecule (consisting of two atoms) of Oxygen plus two molecules (consisting of two atoms each) of Hydrogen will react and form 2 molecules of water”. You’ll notice, that in this reaction, everything gets used up, and no atoms are left over, and so the reaction is said to be “stoichiometric” (i.e. nothing is left over), and now you know where that big word comes from that gets tossed around when people talk about A/F ratios. It just means “completely burnt with nothing left over”. Glad we got that out of the way.

Now that we understand stoichiometric A/F ratios, lets define a couple of things.

An A/F ratio is said to be “Lean”, when there is air left over after burning.

An A/F ratio is said to be “Rich”, when there is gasoline left over after burning.

Now, the perfect stoichiometric A/F ratio varies based on a lot of factors, like the formulation of the gasoline, or the density of the air, but in general, we can say it’s in the neighborhood of 13 parts of air to one part of gas, or 13/1.

So now we understand ratios, and what ratio we are aiming at, but how does the motor get the right amount of gas in to make that ratio? Vacuum, that’s how.

A carbureted motor does it directly, by varying the size of its jets. The carb sits in-line between the outside air pressure (air filter) and the manifold pressure (load or vacuum). The action of the air flowing through it into the manifold causes suction in the jets, which pull gas out of the bowl, and spit it into the intake manifold. The bigger the jet, the more gas gets spit out for a given vacuum. But it’s a little different in a fuel-injected motor, since the fuel injectors are turned off and on by the computer (ECU). So how does the computer measure the load to know how long to turn the injectors on? By using sensors.

There are several involved in this process, with the most important being the “MAP” (manifold absolute pressure) sensor, which directly measures the pressure in the intake manifold. The computer then compares this pressure to the outside air pressure, or barometric pressure (obtained by reading the BAR sensor), and now knows just how much vacuum the motor is pulling. In order to get load though, the computer needs to know how much air is being pulled into the cylinders, so it needs to know the air density as well. It does this by looking at the intake air temperature measure by the “IAT” sensor. The ECU has a table of density values in it that compares density at various temperatures and pressures. It just takes the values it read from its IAT and (adjusted) MAP sensor, looks up the density value for these readings in its table, and “hey presto”, your ECU now has a direct measurement of the load on your motor. Now that the ECU knows the load, it just has to figure out how long to turn the injectors on to get the right A/F ratio (which it does by looking it up in a fuel table by comparing load to RPM), and boom – we got gas. The same table is also used to look up the right time to fire the spark plugs (timing). Honda calls this “three dimensional fuel programming”, and I guess that’s sort of accurate.

There are other sensors that play less important roles in the process, like O2 sensors on California bikes, WTS (water temperature sensors), and so forth. But the MAP senor is the big one.

After Market Fuel Computers

This is a good time to talk about aftermarket fuel controllers, so lets hit on what they do real quick. The 1300 guys can manipulate their A/F ratios by changing jets, but it’s a bit more complex (and expensive) for the 1800 riders. For the VTX, there are basically three types of controller available. All three do exactly the same thing – they manipulate the amount of time the injectors are turned on during the intake stroke, by intercepting the voltage the ECU sends to the injectors. How the decide how much longer or shorter to turn the injector on, varies a bit though, and that variety is important thing to know when selecting which controller to use on your bike (if in fact you choose to use on at all – none of them are ever really necessary, regardless of what pipes or airboxes you add on to the bike.)

It’s also important to understand this: None of these controllers is ever a necessity, regardless of whether you change pipes or airboxes on your bike. The stock ECU will, in almost every case, adjust to the changes in airflow you’ve caused, and give you a reasonably good A/F ratio. But you should understand, that the ECU is not programmed to give you an A/F ratio that is optimized for horsepower from the factory – instead, Honda worries about things like pollution, engine temperature, and rider perception, and so the ECU can be said to be “de-tuned” in order to address these other concerns. So the real function of these add-on controllers is to correct the error (or eliminate the de-tuning) that Honda induced in your fuel curve on purpose, in order to reclaim the lost horsepower and improve engine efficiency (possibly at the cost of making more pollution, hearing more deceleration backfiring, etc). Adding on aftermarket pipes or airboxes can sometimes exaggerate this de-tuning as well, so we need to be able to modify our fuel curves to match the configuration of our bikes. Got that? These boxes aren’t necessary, but if you’re hunting more horses, they can sure find them.

So lets talk about the specific boxes:

TFI – This unit is marketed under several brand names by Techlusion, Cobra, and others. It does one thing, and one thing only – it extends the fuel pulses produced by the ECU, and therefore, it can only richen the fuel mix – and never ever lean it. Typically, these units are designed to act like variable fuel jets – by adjusting two or three settings, you can add more or less fuel to the mix for a certain RPM range – usually low, middle, and high. It determines RPM by looking at the signal frequency produced by the ECU. Because these units are extremely simplistic, they won’t really optimize your fuel curve, although that can reduce or eliminate decel popping, or sometimes correct a really lean condition, But in all cases, your fuel curve will vary from lean to rich along it’s total fuel curve, so you’re only going to help the lean spots, and hurt the rich spots. These units get sold with a lot of pooey about bikes always being lean, or “knowing load”. It’s cap, since we’ve seen hundreds of fuel curves off the dyno now that amply demonstrate that most VTXs run a little lean at low rpms, and rich in higher rpms (as a rule, but not always). This load nonsense is just that, since all of the available fuel controllers modify the signal produced by the stock ECU. Therefore all of them “know load”. Don’t get suckered by marketing malarkey. Frankly, while these units can reduce decel pop, I can’t see that as a goal worth achieving, and so I never recommend these units. I think you can do better, for slightly more money.

Power Commander III – This device also modifies the fuel output from the ECU, but does so based on a table of values referencing the throttle position sensor (TPS), and rpms. Since the position of the throttle plates directly affects the amount of air being allowed into the engine, the value of the TPS is a pretty good substitute for the “Load” value derived from the map sensor by the ECU. In effect, the PC3 produces an overlay curve of its own, and this can be manipulated to correct the de-tuning in the stock ECU. PC3’s can either lean or richen the mix, and so are capable of completely correcting the fuel curve produced by the stock ECU. Because the new curve is applied proportional to load, its effective regardless of the ambient air pressure – since the curves are proportional. Tuning the PC3 correctly requires a session on a dynometer, so these units are somewhat expensive – but set up correctly, they will produce optimum fuel curves under nearly all conditions. Of course, changing equipment on your bike will mean the curve is no longer a perfect match, and another adjustment session will be required in order to get a perfect fuel curve, so the PC3 is not without its gotchas. But for best performance, the PC3 comes out on top. It’s a very flexible and effective unit. Newer PC3’s also have functions that can emulate a tunable accelerator pump, or allow you to map each cylinder independently. Good stuff.

HPP – This unit adjusts the output fuel pulse by manipulating the value of the MAP sensor, based on calculations derived from reading the oxygen content of your exhaust with an O2 sensor. In theory, this results in a “closed loop” system, one that adjusts itself looking for optimal F/A ratio by reading the exhaust in real time. In practice, the O2 sensors are pretty inaccurate at higher RPMs, and so the device does some educated guessing, and therefore the fuel curve isn’t always perfectly optimized. However, experience has shown that properly set up, these units perform very well, and are an excellent alternative for someone who doesn’t want the hassle of fiddling with their fuel tables, or going to dyne sessions. They can also adapt immediately when new equipment is installed on the bike. So this is the unit to use when performance with convenience is your goal, and you are willing to give up a few percent of horsepower to gain that ease of use. Good stuff here too.

Alas, there is currently no unit on the market which can manipulate the timing tables, and that sucks. These big motors cry out for the ability to optimize timing tables to match configuration or changes to the fuel tables, and there’s no doubt that 4 or 5 horsepower lurk in the inability to manipulate the spark. Perhaps someday Dynojet will follow through on their promises to provide a timing module, but until then, the ability to manipulate spark effectively simply doesn’t exist. I yearn for a fully programmable ECU. I also yearn for a 20 year old hottie with great legs and big boobs. I’ll probably get my timing before I get the boobies though – more’s the pity.

Burn Baby Burn

The last thing I want to address today is the subject of deceleration backfire, or “popping”. This topic generates a lot of concern from inexperienced riders, or even from experienced guys who just hate the noise, so lets take a look at what causes it. But first things first, lets define the issue:

Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.

No ifs ands or buts, that’s what causes it. But the bigger question is how does gas get there in the first place, and that’s a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there’s a kicker, and something you should understand before we go any farther:

A motor in perfect tune will exhibit deceleration backfiring.

Therefore, just because your motor is banging it up, doesn’t mean there’s anything wrong. And consequently:

Getting rid of the noise means de-tuning your motor.

Yup. If you’ve jut got to eliminate that popping, you’ll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn’t necessarily a good thing. So lets talk about what causes the problem.

Ok, so you’re riding along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an “overrun” – that is, the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.

First, when you close the throttle, you are also closing the throttle plates. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. Remember I said earlier, that the A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire. In addition, Honda has added a device called a “programmed air injection valve” (Pair Valve) that actually injects some fresh air into the exhaust to help this process along – since fully burning the fuel results in cleaner exhaust. So the backfiring is not only a normal part of the engines operation, it’s also intentionally amplified by Honda! Of course, normally, that massive bazooka pipe Honda hangs on your bike hides most of the noise, but it’s there, even when you can’t hear it.

So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.

There are a few ways you can do this.

First, use the stock pipe. It will hide the sound, by absorbing it into mass, and masking it with the larger baffle space. Second, you can add more fuel during deceleration. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens. Lastly, you can remove the Pair valve, which reduces the amount of available oxygen in the pipe to burn the unburnt fuel.

Well, there you go. I hope this long piece has given you some information you can use, or clarified things a bit for you. I may ultimately add to this, and discuss valve timing, the effects of compression on performance, or other topics as they motivate me. In the meantime, feel free to criticize, correct, or just tell me how dumb I am. I’m still learning too, after all.
Texas X Riders

· Super Moderator
64,510 Posts
Discussion Starter · #5 ·
Explanation of motor vibrations in the VTX

Good Vibrations - An explanation of motor vibrations in the VTX and others

Good Vibrations

By: Dr. Peter Becker

An explanation of motor vibration using the VTX 1800 motor as an example. This article was posted on the German VTX forum at: and posted here by permission of the author. Dr. Becker can be reached on VTXOA under the username "peterb".

Translated from German by Kai Desler

Some time ago there was a discussion on this board about to what extent the vibrations of the 1800 engine are influenced by the use of two crankshaft pins. It was stated by someone that HONDA did not pick this particular design to keep vibrations under control, but rather to generate vibrations, because that is what a VTX rider paid for when he bought his motorcycle.

I marveled at both of these explanations, because they are wrong. I decided to go for a VTX 1800 because I was convinced by the design of this engine that - in contrast to many other motors that utilize a single crank pin - only vibrates noticeably when it has to work hard. Otherwise it runs rather smoothly and leaves my dental fillings in place.

In the following I would like to explain how the engine was designed by HONDA to achieve these characteristics. I will try to do so keeping in mind that people that do not have an intense background in engineering would like to understand what is going on, as well.

Everybody who is familiar with riding the 1800 VTX will acknowledge that we are dealing with two different kinds of vibration. First, the kind of mild vibration one experiences while cruising along at a constant speed of 50 mph in 5th gear. The second kind of vibration is very noticeable when one suddenly opens up the throttle wide at low rpms. Those vibrations are noticeable throughout our entire body and even visible when the whole tanks vibrates from side to side.

The first kind of vibration mentioned has to do with the forces of the masses that move around inside the motor, even if there is no internal combustions at all. The second kind is a consequence of the internal combustion, but these vibrations are actually also caused by moving masses and their associated inertia forces; they are only indirectly caused by the explosions happening inside the cylinders.

Part I: Forces generated by moving masses

To get a better understanding of the forces caused by moving masses we start out studying a single-cylinder engine, a thumper. The V2 engine can be, just like any other multi-cylinder motor, viewed as composed of a number of single-cylinder engines.

Let's imagine doing a little experiment and consider a thumper motor (without counter shafts or balancers), remove the head and camshaft, that it can be easily turned over. We put the open motor on a table and use a flexible shaft to connect the crank shaft to a drilling machine and let it rotate. What will happen?

The motor will start to bounce wildly on the table until it reaches an edge and falls to the ground. The reason are the forces created by the unbalanced masses of the crank mechanism. Even if we would very carefully balance the masses of the system consisting of the piston, connecting rod and crankshaft by attaching the correct weights to the crankshaft, we will always(!) end up having to deal with two kinds of resulting forces. Those forces act in the direction of the axis of the moving piston (up/down) whenever the crankshaft rotates. Strictly speaking, there are some additional forces and momenta caused by the moving masses, but those are small compared to the ones we just discussed and will be neglected in the following.

The two main forces in direction of the axis of the moving piston are shown in Fig. 1. Forces are denoted by arrows that point into the direction of the force; the longer the arrow the stronger is the force.

The stronger force of the two (blue in Fig. 1) changes its direction each time the piston moves up and down, i.e. its frequency is the frequency of the rotating crankshaft. The weaker force (in red) changes its direction twice as often; its frequency is therefore twice the frequency of the crankshaft. These forces are sometimes called first order and second order mass forces. Both forces are transmitted through the crankshaft bearings via the crankcase to the motor mounts and the frame, and, consequently, we can feel them.

Both mass forces are proportional to the square of the engine speed. That is, at 4000 rpm they are 4 times as strong, and at 6000 rpm they are 9 times as strong as at 2000 rpm. The absolute difference between the blue and the red force grows with increasing rpm, therefore we will mainly discuss the blue force in the following. (Anybody who has ever ridden a bike with a single cylinder motor without counter shafts knows how much the vibration increases with the engine speed.) Since the forces generated by rotating counter weights also increase quadratically with engine speed, they can compensate the blue and red mass forces.

To completely compensate the blue force of the single-cylinder engine one needs to have two counter shafts rotating with the same frequency as the crankshaft. Each counter shaft carries a weight with half the mass necessary for complete compensation. Both counter shafts have to rotate in opposite directions that the vertical components of their centrifugal forces add up and compensate the blue force. The horizontal components of their centrifugal forces are pointing in opposite directions and cancel each other; the reason is that the blue force doesn't have to a horizontal component that needs to be compensated.

To compensate the red force one needs a second set of two counter shafts that rotate with twice the frequency of the crankshaft. Thus, in total four counter shafts are required. If one takes into account that each shaft needs two bearings and that it has to be driven by some mechanism, it is obvious that the technical effort to build such a complex mechanism becomes enormous.

There is a simpler, more elegant solution to this problem: Let's take the single-cylinder engine and combine it with a second one such that the resulting two-cylinder engine takes care of eliminating unwanted mass force by itself. This can either be done such that all mass forces compensate each other completely. In a different approach the two-cylinder engine is designed such that the mass forces are not completely compensated, but that the remaining unwanted forces can be canceled very easily.

An example for the first case is the two cylinder boxer engine, see Fig. 2.

Here, the same (red and blue) forces are generated on either side of the engine, but they cancel each other because they always point to the opposite direction. The reason that boxers do vibrate, regardless, is that there is an axial offset between the two cylinders to accommodate the two connecting rods. We will come back to this problem in a minute when we will discuss the VTX motor.

The second case, as far as canceling the blues forces is concerned, can be solved by using at V-Twin engine with an angle of 90 degrees between the axes of both cylinders. Fig. 3 shows that the sum of the two blue forces (pointing up and downwards along the cylinder axes) is of constant absolute value and rotates with the crankshaft. Note that the tip of the resulting arrow describes a circular motion.

Why would that be desirable? The reason is that this force is identical to a force generated by an additional (unbalanced) mass on the crankshaft. Therefore, a counter shaft becomes unnecessary; all it takes is an extra weight on the crankshaft to cancel this force (or remove some material on the opposite side of the crankshaft, which has the same effect).

Other V-Twin designs make use of a singe crankshaft pin for both connecting rods, like e.g. Harley Davidson with a 45 degrees angle between the two cylinders or the 60 degrees design of the Aprilia motor. Fig 4. shows that the resulting blue force for these designs describes an ellipse. In this case additional weights on the crankshaft can only cancel this force partially, and some vibration will remain.

All this isn't new; it is rather known for 100 years. Not known for so long is the fact that the effect we just discussed can also be achieved with V-Twin engines that don't have a 90 degrees angle, if ones uses crankshaft pins that are off-set with respect to each other. This is shown at the following website,
where one also finds the nice animations I am using.

Which brings us to the VTX 1800 motor.

In the cases where the angle between the two cylinders and the angle between the crankshaft pins satisfies the condition

2*cylinder angle + crankpin angle = n*180; (n = 1, 2, 3...)

the tip of the blue arrow moves around in a perfect circle. This has the advantage mentioned above for the effortless canceling of the mass forces.

Let's try it: The angle between the two cylinders in our VTX 1800 motor is 52 degrees, and the angle between the crankshaft pins is 76 degrees. Therefore, we get 2 * 52 + 76 = 180.

This is shown in Fig. 5.

The condition mentioned above is, naturally, also satisfied for a 90 degrees V-Twin with a single crankshaft pin: 2 * 90 + 0 = 180, and for the boxer motor, as well: 2 * 180 + 180 = 3 * 180.

Now, one question arises: Why in the world does the VTX 1800 motor need a counter shaft, with weights attached to its ends, that are oriented opposite to each other, regardless of the fact that the condition above is satisfied, already? Answer: The argument stated above isn't the full truth, here. We only took forces into account that act in the top-bottom and back-forth directions.

Let's take another look at Fig. 5 that shows the motor from the side. The rotating arrow shows at every position the sum of the blue forces generated by the two cylinders positioned at 26 degrees to the left and 26 degrees to the right. Drawing not the sum but the individual forces would not resemble the shape of a circle. We are viewing the motor in the direction the crankshaft is pointing. Let's now imagine that we walk around the motor by 90 degrees such that the crankshaft is viewed from the side. Then, we notice there is an offset between the two forces! Their lines of action cross the center of their respective crankshaft pins.

This couldn't be seen in the side-view before. The blue (and the red) forces of each cylinder don't act on the crankshaft at the center between the two crank pins, but rather to the left and to the right of that center at the respective cylinder axis according to the offset between the two cylinders.

The rest is quite intuitive: The lateral offset of the forces generates an alternating momentum around the longitudinal axis of the motor (in the case of the VTX also the longitudinal axis of the bike). This momentum tries to tilt the motor to the left and to the right. Exactly this moment of tilt is compensated by the weights on the counter shafts. And this explains why the tank doesn't vibrate all the time.

By the way, the two cylinder boxer motor has the same problem generated by the lateral offset between the two cylinders. The moment of tilt usually acts around the vertical axis of the bike, depending on how the motor is oriented in the frame.

As stated above, everything we just mentioned, happens just because of the moving masses without any internal combustion happening.

Part II: Force generated by internal combustion

Whenever an explosion happens inside a cylinder it generates a pressure wave. The same happens in case of a motorcycle engine.

At least that is the phenomenon one observes. But it is not what actually happens physically during internal combustion in a motor.

Ideally, an internal combustion generates an instantaneous pressure increase in the entire combustion chamber. No matter how this volume may be shaped, the sum of all forces acting on the total inner surface of the volume (cylinder head, piston and the exposed parts of the cylinder wall) is zero.

This may be somewhat surprising. But if the sum of the forces wouldn't be zero, one could construct a hollow body that could be moved simply by raising the internal pressure e.g. by pumping air inside. Such a machine cannot be constructed.

So what is really happening? From the first section we know that the first order mass effects of our motor are compensated as long as it is freely spinning at constant rpm without internal combustion; the crank shaft drives the connecting rod which accelerates the piston upward and downward. As soon as we fire up the engine, the exploding air-fuel mixture gives the piston an additional downward acceleration which generates strong inertia forces caused by all accelerated components, the drive train and at last - although smoothened by various dampers - by the accelerated mass of the whole bike itself. The sum of these inertia forces is the reacting mass force, which causes the vibrations when you open the throttle.

The force gets transmitted via the crankshaft bearings and the motor mounts to the frame. And, since the cylinders are slightly off-set, one to the left, the other one to the right of the center of the bike, a moment of tilt is generated around the longitudinal axis, which makes the tank vibrate so nicely.

It should be noted that these kinds of vibrations cannot be compensated by the usage of counter shafts because they only depend on the pressure generated by the internal combustion, which in turn depends on the position of the throttle and, to a certain degree, of the torque versus rpm. The fact that the vibrations become more noticeable at higher engine speed is simply a consequence of the increasing frequency of the forces at higher rpm.

But one can diminish those vibrations by using rubber motor mounts that help to dampen them, if so desired. In my opinion Honda found an excellent compromise, and I just love to make the engine vibrate any time I like by simply opening the throttle.

Good to know that theory and practice agree with each other nicely.

Author: Peter

· Super Moderator
64,510 Posts
Discussion Starter · #6 ·
T-Rex on Engine Noise

T-Rex (an original VTX guru) on Engine Noise
"I'd really like to clear the air here. The reason cars have hydraulic valve lash adjusters is to get rid of noise so they will sell (and to reduce maintenance) and this is done at the expense of horsepower. So-called "solid" lifters, such as in 99% of motorcycles, including the VTX, make noise. Period. But they also result in more horsepower.

The same holds true for the decompression mechanism, which is the same technology and is inherently noisy, particularly if you set the idle too low, to be “cool”, which in turn keeps the decompressor fly weight from seating properly and, thus, makes the tapping noise.
Just ride your bike and enjoy it. The tapping, ticking, whirring, and whining is all part of making a high performance engine, and to get rid of those sounds, the engineers would have to sacrifice performance, such as in the case of hydraulic lifters. ("Hydraulics" require a milder cam, and heavier springs for the same lift, both of which cost you HP.)
The same is true of gear noise. As you change the mesh angle of the gear teeth, they get quieter, and quieter. BUT(!), this also consumes horsepower. Straighter cut gears make more noise and "give" you more HP at the rear wheel. Which would you prefer? Free horsepower or less noise? I think we both know the answer, and so does Honda, which is why your valves tap and your gears make noise/whine at certain speeds in certain gears. Just ride, and enjoy the bike. PS: I bought my VTX, broke it in for 600 miles, and then road coast-to-coast and up into Canada, over 10,000 miles, without any problems. NONE. Yes the valves tap. Yes the decompressor taps. That is what mechanical things do. It is a good bike. Enjoy it instead of nit-picking it."

· Registered
277 Posts
~^~Valve Adjust Feeler Gauge~^~

I was going to make these but the board will not allow me to sell these with out regestering as a sponser so here what I did and can share with you. Being a Machinist, I had some 12 inch long Sarrett feeler stock in .005 and .013 thickness. I bent the last 1 inch of the feeler stock / gauge about 15 degrees which allows the gauge to enter between the rocker and valve stem at the correct angle and leaves 11 inches to hold and manuver around the obsticles. Worked great in my mind and hope this helps you.

You can buy the .005 and .013 feeler stock at any machine shop supplier. ENCO, McMaster-Carr, Grainger, HSC, Carr-Lane, Starret tool suppliers.



· Super Moderator
64,510 Posts
Discussion Starter · #8 ·
Author: Dark Star

This is an easy way to remove the engine if you have the bike broke down to just frame and engine. Since all my buddies were out riding on this beautiful day, I had to find a way to get it done without help. After putting pipe thru the upper engine mounts and supporting the whole thing with jack stands, just disconnect the engine and walk the frame off the left side. I tied the frame off from the rafters to keep it from hitting the floor. I haven't decided yet if I'll use the same method to reinstall. Anyway, just thought I'd share yet another way to get the engine out with ya'll.
Use standard fence posts available at any hardware store, 1.75 inch diameter


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285 Posts
Rear Exhaust valve adjustment

After headaches trying to get at the valve adjustment lock nut with various tools, I found this fairly simple home made tool did the trick.

10mm box wrenches just didn't work for me on the rear. The angle was always wrong for a good seat on the lock nut, or they were too thick to fit around the 10mm lock nut. Not much clearance between the nut and the side of the opening. I even ground it down to thin it out.

My 1/4" ratchet & 10mm socket that worked fine on most of the other valves proved to have to much bulk to fit under the frame that goes over the rear exhaust valve (did move a coolant line out of the way and the tank gas line that was disconnect anyway after I removed the tank, and bowed out the brake line)

Finally..... I jammed a hex wrench into the 1/4" square opening of the 10mm socket. I thought it was jammed well, but the 6 sided hex would spin in the square opening of the socket when actually trying to loosen the 10mm valve lock nut.

I then did a few touches with the welder to lock it in there (I would guess you could grind/file down a larger hex wrench to a square to fit if u don't have a welder).

And ... coming in from the right side of the bike worked fine for the feeler gauge. A little work, but doable using some of the suggestions here.

All is good in VTX valve adjustment land now!

Thought I would share my magic tool for future searches on this task


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3,901 Posts
Valve Adjustment Notes for 1800

Extra Notes on 1800 VTX Valve Adjustment(2003 Retro w/ V & H Big Shots, Kury Air-box, and Pair Valve/Secondary Air removed)

Primary Notes and Video: Bare’s Site; Northstar Riders; VTXOA How To forum.

Additional Notes:
  • Specs: Intake: .005 +/- .001; Exhaust: .013 +/1 .001
  • Tools needed: .004 , .005, .006, .012, .013, and .014 Feeler Gauges; 4MM wrench; 8 MM wrench; 10 MM wrench; oil for lubing gasket rings and feeler gauges.
  • The 4 mm ignition wrench (available at Sears) is small. To elongate it, tie a long string to the end hole and pass it through a soft-plastic pen barrel (with ink filler removed). Jam the wrench end into the barrel to create a long-handled tool; the string thru the barrel will secure the wrench should it become dislodged from the barrel front. Each feeler gauge should be securely attached to a pop-stick or hemostat and string and should be bent to keep the gauge level and horizontal to the contact point (angled will give a false reading). Keeping the correct feeler gauge within the gap while the adjuster screw and locknut are lightly tightened can aid in setting the target gap (.005 or .013). Procedure:
  1. Be sure the engine is stone cold. Set bike in Neutral. Unscrew speedometer bezel (three lower screws), cover and tie up and out of the way. Remove seat and tank.
  2. Remove coil (green cylinder), then open and remove black zip-tie from frame, and pull wire harness and bow brake-lines away from frame.
  3. Remove and set aside the Timing Cover (use modified Allen wrench for hard to reach screw) and hole cover (17 mm).
  4. Remove the Front Right, Front Left and Rear Left (sitting on bike) spark plug covers and pull wires away from the tops of the jugs. Remove only the Front Right and Rear Left spark plugs.
  5. Remove the front Intake and Exhaust valve covers; single is the Exhaust valve and will be set at .013 +/- 1; the two in the rear of the front jug are the Intake valves and will be set at .005 +/- 1.
  6. With your thumb over the front, right spark-plug hole rotate the engine clockwise a few rotations getting a feel for the compression cycle; you’ll know the compression cycle when a strong push of air comes out from the sparkplug hole and pushes your thumb off. To ensure that the engine is NOT rotated counter-clockwise, use a ratchet wrench and 17 MM. socket. It will be difficult to rotate, so a block of wood or other support placed under the wrench (between wrench and floorboard) provides good leverage. (Removing an additional spark-plug does make rotating the engine is easier, but it will also make it more difficult to detect the push of air from the spark-plug hole being covered by the thumb).
  7. To set the engine, place thumb over spark-plug hole and turn it clockwise until the compression cycle (strong push of air on thumb); stop right away, as you’ll be almost at the index marks. You’ll know that you’re on the compression cycle if the valve rockers can be moved (they should clitter). Be sure to be in a good position, with plenty of light, to see the FT and index mark and continue turning clockwise until they align. Do all the valves on the front cylinder. Repeat this process for the rear cylinder, using the RT mark, instead.
  8. The front jug Exhaust Valve is most accessible from the left side of the bike, whereas the rear Exhaust valve is easier to access from the right side; the front and rear jug Intake Valves are accessible from either side, although moving from side to side seems to work best).
  9. If a valve needs adjustment, loosen the lock-nut by holding the 4 MM bolt steady and loosing the locking nut by turning counter-clockwise with a 10 MM. bent-elbow, box wrench. To open the gap, turn the 4 MM. to the left, to close the gap, to the right. Adjust with 4 MM wrench until a light drag on the feeler gauge. Hold the 4 MM on the adjuster and snug the lock-nut fairly tight (something less than 16 Ft/Lbs or 192 In/Lbs, by feel).
  10. The feeler gauge should be lubricated with oil to allow a smooth, scratch-free insertion between the gap. Tapping drag (shuddering when dragging) is ideal; the correct amount of drag on the gauge should be comparable to sliding the feeler in and out of a heavy book. Leave the feeler gauge in and re-check after snugging the lock-nut; then remove both wrenches leaving the gauge in. Check again and if not OK loosen lock-nut and move adjuster slightly either way until correct. Most give the 10 MM nut a good tighten and don't use a torque wrench.
  11. Rotate engine crank clockwise to TDC on compression stroke and check again. Same for all valves; never turn the engine backward to line up the TDC mark as damage can occur.

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36 Posts
Wooooow, this is absolutely amazing. I was getting ready to dig into my timing chain this weekend thinking that was causing the noise/ vibrations. What a load of knowledge you just dropped on me. I look forward to reading the rest of the info. I have yet to get to on this. Thanks for saving me hours in a garage and countless headaches to find out ""it was meant to do that"".
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