my thought has always been to balance.
seems like it would be easier on the crank bearings at least.

The only reason I want to comment further is to be sure that the reason behind my opinion is understood (and shared by at least one notable expert).

The crank and rod bearings are sized for the forces applied by combustion pressure. Compared to this force, the imbalance force is miniscule. If you spin the engine with a minor imbalance in the flywheel, the crank will not break. Period.

I am not arguing against balancing. Balanced is better.

I only stated that the imbalance will be unnoticable, and will not damage the engine.
Therefore, you should weigh the option of balancing with your budget and the availability of a balancing service in mind.

Man, I can't tell you how wrong you are on that one. Ask any engine builder what happens when an attached flywheel is not balanced and they will all tell you the same thing. These are higher order harmonics that you cannot feel, hear, smell, taste, etc. What you feel is one thing, it's the harmonics that you can't feel that cause the damage.

First of all, I do rotordynamics for a living.

Secondly, I did ask a very experienced race engine builder, who also does rotordynamics for a living, whom many people in the RC51 and VTR world know by name, if not personally. I've tossed his name around, but don't feel the need to do so here. His advice was the reason I stopped worrying about it (aftermarket pistons too, which was a bigger change to the dynamic balance).

Unbalance does not create harmonics. Go back to your vibrations book and look it up. It creates a 1X vibration vector that rotates with the shaft. The amplitude will peak when a nutural frequency is excited. The magnitude of that peak depends upon the amount of damping present. Since we have an oil film and proper bearing clearance, there is a lot of damping present.

One of the primary elements of making engineering judgements is the ability to break a problem down to it's fundamental elements. Let's look at a relative force comparison. The force of combustion is big enough to propel 600 lbs of man and motorcyle to the point of a shit-eating grin (is that acceleration, or are you just happy to be riding?). The other is a 5 mm dia. hole, 5mm deep, ~50mm radius form centerline. How big is that force, by comparison? If it could create harmonics, which it can't, they would be of lower magnitude than the primary, 1X frequency (more vibration fundamentals).

But I definitely recommend that you don't do it. Not worth losing sleep over.

I am with RCVTR on this, but I do have an example of an engine crapping itself.

BMW M3 racecar. It was not properly tuned, shook so bad from the preignition occuring on banks of daytona (Grand am cup car) that it tore the harmonic dampener clean off. Once this happened it destroyed the engine. It was not harmonics that broke it, but preignition. I am sure that this has nothing to do with this topic, but I thought it was funny that the harmonic balancer was bouncing down the track.

I was just coming back here to change the tone of the discussion and to talk about torsional harmonics. Torsional harmonics are another story and will break crankshafts. They are not caused by radial imbalance, however.

On an engine with a long crankshaft, such as a V8 or an inline 6 (even more so), we have torque input pulses coming from various positions along the shaft. These cause a twisting of the crank, especially at the front cylinder, since the reactive torque is always at the output end.

The crankshaft will probably have a torsional natural frequency somewhere in the RPM range. With radial vibrations, an oil film bearing has very good damping properties, which control the radial vibration amplitude. But with torsional vibrations, there is almost no damping whatsoever. The natural frequency can be exicted by perturbations at that frequency, or multiples (harmonics) of it.

If you excite a torsional natural frequency, the vibration amplitude can get dangerously high. That is the reason for the harmonic damper on engines with long crankshafts. I bet it was hard to tell whether the damper broke and took out the engine or the other way around, but I have a feeling the crank failed in torsion.

The VTR has a stubby crank. The torsional natural frequency is most likely far above the running speed range. But in any case, it can not be excited by radial imbalance. It has to be a torsional input.

Data aquisition showed the timing caused-preignition to be the cause. It was so violent that it tore the harmonic balancer, the crank then failed as a result. This happened on more than one engine thru the weekend and by the end of the weekend the engine builder who had "tuned" the FI was scarce and elusive.

One of the best ways to find the natural frequencies of a part, or structure is by "ringing" it. This is done by mounting vibration transducers, then literally striking it with a calibrated hammer. This instantaneaous step input will excite the natural frequency and the harmonics.

Detonation is the same. It is an instantaneous, very high amplitude spike. Especialy damaging because it can also cause a torque reversal. That may be what tore the damper loose. (Just a guess).

One more thing...

It's not the first natural frequency exciting the harmonics that cause the problem. It's the other way around. If you operate at a speed that excites a multiple (harmonic) of a natural frequency, and don't have much damping, it will reexcite the first natural frequency.

Thanks for finally agreeing with my original statement. The higher order harmonics are what cause the problems and a primary imbalance will excite them.

I don't agree with you.

A radial imbalance rotates with the crankshaft and doesn't even cause reversing stress, since it always points in the same direciton, relative to the crank. It squeezes the oil film slightly and transmits vibration to the engine case. Since there is no reversing stress, you can stop worrying about bending fatigue stress in the crank. And since the bearings are designed for much higher forces than the imbalance, you don't need to worry about vibration amplitude causing a rub in the bearings.

I'm talking about torsional natural frequency, in a long crankshaft, where there is very little damping. Primary balance of the counterweights and reciprocating mass can excite torsional vibrations. Radial imbalance on the flywheel can not.

I haven't heard of anybody breaking a VTR crank from installing aftermarket pistons either. I will eventually build an engine with Carillo rods. Then I will balance the crank. The engine is currently running JE pistons. The engine runs beautifully. Not going to lighten the RC51 flywheel.

I'm not qualified to comment on the physics of all this, but I will offer some anecdotal evidence that a perfectly balanced flywheel is not necessary for long and reliable engine performance.
I was discussing this with my local motorcycle mechanic, who I have known for 30 years or so. He has built race engines for lots of local racers with good success. He said that he used to lighten flywheels and remove material from the rotating magnet part of the assembly by using a hacksaw. He never balanced the flywheels and he says he never experienced crank failures or other problems associated with not having a perfectly balanced flywheel.
A massive imbalance I can see causing problems. A few grams here or there gets swamped by all the other forces at work.

killer5280, I writing this while you were writing. I'm going to try to explain the physics...

This situation is not going to be fully explained unless we go further into the basics of engine rotordynamics. I can't claim to be an expert, but I can apply what I know about rotordynamics to describe engine dynamics.

The work I do mostly involves collecting and analyzing vibration data from large rotating machines, with no reciprocating parts, so unbalance is always at 1X rotor speed. When I balance, I look at first and second bending modes, but do not normally concern myself with 2nd order imbalance.

In an engine, you have the pistons, wrist pins, rings and a portion of the connencting rods that stop and change directions twice for every revolution of the crankshaft. This causes a 2X, sinusoidal torque input to the crankshaft. It is a torque, because reciprocation of the pistons, etc. is translated to rotation of the crankshaft, which only effect 1X radial vibration, not the 2X torque input to the crank.

The main purpose of the flywheel is to provide inertia, to decrease the speed fluctuations caused by this 2X, sinusoidal torque. When you have an inline engine, with a flat-plane crankshaft, the entire reciprocating mass accelerates at the same time, so you have a large 2X vibration. Balance shafts are used on small automotive and motorcycle engines, to provide a counter-rotating 2X imbalance that has smoothing effect, so designers can get away with less flywheel inertia.

In a 90 degree V-twin engine, The two reciprocaing masses are 90 degrees out of phase, so when one reciprocating mass is stopped, the other one is at maximum speed. The inertia of the 2nd mass has a similar effect to the flywheel in minimizing the speed fluctuation of the crank and you end up with good secondary balance. This also allows for less flywheel inertia.

The crankshaft can only be truly balanced for first-order vibrations. In an inline 4 engine, with a flat-plane crankshaft, you can balance or lighten the crankshaft independantly of the rods, pistons, etc., because there are opposing counterweights. In a V-engine you must use the weight of the heavy ends of the rods and bearings (rotating weight) and a portion of the reciprocating weight to calculate an approximate balance weight (or bob weight), to give the counterwieghts some additional inertia, to improve secondary balance.

When you decrease the diameter of the flywheel, you are decreasing it's inertia. This will cause an increase in the 2nd order vibrations in the engine. This is much different than introducing a small radial imbalance.

I guess that you could compare it to a merry go round (here on out, m-g-r) with rusty bearings. Everytime you push, you accelerate the m-g-r, as soon as you stop your impulse (push) it deccelerates. If you throw a bunch of screeming kids on it, every time you push, that energy will be retained, so the speed will be retained better. It will take more pushes to get it up to speed, but it will keep spinning longer. Eventually a third grade will puke and half the m-g-r will clear off, and the other half will remain. It won't vibrate, but it will gain a slight bending momemt on the "crank". How great is that force, depends on the mass and radius as well as the speed. With a lightened flywheel you have achieved this inconsistent speed do to impulses and compression, but the ability to accelerate it is achieved. The bending moment on an unbalanced flywheel will depend on how imbalanced it is. The bending moment on my is going to be negligible at best due to the care I took in setup for machining as well as the fact that it was not balanced (or it was perfect) to begin with.

It took me a while to finally "grok" it (Robert Heinlein, "Stranger in a Strange Land"), but I finally visualized the secondary balance on a 90 degree V engine with a flat-plane crankshaft. There's nothing like a 40 minute commute to keep a nerd entertained...

This engine configuration has perfect secondary balance (no 2nd order vibrations). This is because half the reciprocating mass slows down at the same time the other half speeds up. So half the mass pulls on the journal while the other half pushes on it. So the two torques cancel each other out. I'd heard that 90 degree engines had perfect 2nd order balance, but had never thought about it enough to figure out why.

So, in that case the flywheel only acts to maintain momentum during the compression strokes.

Balancing the flywheel makes up for imperfections in manufacturing. If there is no mechnical runout, there will be no imbalance. So it pays to be careful, and minimize runout, checking with a dial indicator, radially and axially when mounting in the chuck on the lathe.

??? I'm confused on what is pulling on the journal. On the the upward strokes, you've got (1)compression (2) exhaust pressure. I think both pistons are always pushing against the crank journals ('cept for the intake stroke). They just alternate on which one is pushing harder.

I think the best way to visualize what I'm trying to say is to think about spinning the engine with the heads off. So take all of the work done on and by the gas in the cylinders out of the equation.

Then we are only looking at the dynamics of the crank, rods, pistons, rings, etc.

When one piston is at the top, or bottom of the stroke, it's speed is zero. The other one is at midstroke and it's speed is at a maximum. As the crank continues to rotate, it has to pull on one piston to accelerate it toward maximum speed, and has to push on the other one to slow it to zero speed. If the pistons have the same mass, the torques are equal and opposite.

So my next question is this:

If the secondary balance is determined by matching the reciprocating weights (in a 90 degree V), then piston weight should not effect the balance of the engine, as long as they are matched, right? And the primary balance is only effected by the rotating portion of the connecting rod weight and the counterweights on the crank.

So, in my mind, it seems like if you want to balance the engine, (neglecting flywheel balance for the moment), the bob weight is calculated from the rod wieghts only. But I was under the impression that, when balancing the crank, you provide the balancer with big and small end rod weights, as well as the weights of the reciprocating parts. I think I'm still missing something.

I just figured it out...

The reciprocating parts cause a 2X sinusoidal torque on the crank, but the reciprocation is 1X. The counterweights have to offset the 1X oscillation. Hence the need for piston, pin, ring weights in balancing. Complex stuff. Turbine rotors are easier...

I think that would accurately describe a pump, but not a motor. It sounds like you are deriving motion from the crank, not to the crank?

No input or output is defined. Only the physical description that would be used to define the equations of motion.

You could be turning it externally, to be used as a pump, or using the heat of compustion to use it as an engine. Either way, you are using the change in volume above the pistons to absorb or provide work. The equations of motion are the same. More descriptions and equations are required to define the purpose.

In fact, the engine is an air pump. But fuel combustion creates a net torque output.

Finally did my flywheel. Since I put it back on I have only ridden about half an hour due to freezing my ass off. The differences are subtler than I had hoped. I didn't have any trouble pulling away from a stop and the low end power seems about the same as before. Overall it seems a bit smoother and revs a bit quicker, but it's certainly not a night and day difference, although I didn't get a chance to really ride it hard. I'll give another report when the weather warms up and I have a chance to evaluate the changes more thoroughly.
The before weight was 8lbs. 12 oz. and the after weight is 7 lbs. 5 oz., for a weight reduction of 1 lb. 7 oz. Just for kicks, I took one full inch off the diameter. I did not have it balanced after the machining.

when you did this, do you disassemble the flywheel from the rest of the rotor? or is that even possible? I haven't looked at it closely before and its mounted on the bike currently.

The guy who machined mine left it attached. I didn't really look at removing it either.

You really cant remove it. The other issue is that the stator basket isn't really round. so getting that so that it has no radial and axial runout is a pain. You either need a four jaw or shims.

thanks both, glad i asked.

will be anxious to hear longer term impressions with the full 1inch off, and any effect on engine braking, stalling, etc

I separated the flywheel and alternator rotor. I marked the orientation, so it would go back the same way. I used red Loctite to put is back together. Separating them gave me a good purchase in the lathe. I used a 3 jaw chuck and bored some aluminum soft jaws, then checked runout with a dial indicator.

I believe that the flat torque of the VTR engine masks the improvement. It doesn't change the basic torque characteristic of the engine, so the improvement is not immediately noticable. It's there, though. I felt the same way at first about a heavily modified engine, because the basic character was wery similar, but soon realized there was more torque everywhere.

You can remove the starter sprague, but I thought that you can remove the weight from the basket. I must be thinking incorrect. I have done flywheels on a 125TTr, a yamaha 440srx, and the superhawk... can't remember what they all took to do.

I think the basket comes off when you remove the sprague. The bolts go through the basket and flywheel and thread in to the sprague carrier.

Ok.

Sorry about the late reply but I was unavailable the past few weeks.

Here is the VTR FlyWheel Mod Roger Ditchfield from http://www.revolutionuk.co.uk


http://users.fdn.com/~dward/SuperHaw...Flywheel-1.pdf

My image did not load

can you down load the PDF under the following link.

http://users.fdn.com/~dward/SuperHaw...Flywheel-1.pdf

That worked!

Bumping an old thread. I am curious what the consensus is for how much to remove? Should I remove 1/2" from the diameter or radius?

The general rule of thumb around here is to remove 1/2" from the diameter. Just for fun I had 1/2" removed from the radius. I have been living with this for about a year now and have to say that this is a great mod. The engine revs much quicker and is just much more lively and eager (if that makes any sense). Driveability has not suffered; it is not hard to pull away from a stop or anything like that. The lightened flywheel just helps everywhere. I think you could take more off without a problem.

How many miles have you put on that modified flywheel? I know there was a lot of discussion back and forth, but for me the proof is in the pudding.

I would guess 2-3000 miles. My initial impressions were that the effects were fairly subtle, but now I think that the engine is just much more responsive and eager to rev. Well worth the minimal effort and cost.

So has anyone done a 1/2" versus 1" removed comparison?

I doubt it. It's not that critical, but the lightened flywheel does make a noticeable and positive difference in the responsiveness of the engine. Like I said, I think you could remove more than 1" (from the diameter) and still see the benefits without negatives.

Well I know a lightened flywheel has benefits, but I am curious at what point you start seeing negative effects.

So I read up on the advantages of a heavy flywheel, because Honda isn't stupid and just slapped a giant metal thing in there, and found that it is better for drive-ability and better gas mileage. The last two are actually related, because the larger flywheel has more momentum. When you release the throttle on a lighter flywheel, you will have very rapid deceleration, even if you barely let go.

The advantage is that you rev up quicker, but you get a twitchier throttle, and possibly worse gas mileage(do we actually give a shit though?)

A flywheel is merely an energy storage device. Lightening the flywheel on the Super Hawk makes the engine more responsive and doesn't hurt driveability. Can the performance increase be measured? Probably not. It just makes the Hawk sportier and "funner."
Gas mileage? I don't care.