Forum bmwbike.com

I would like to start a discussion on lightened flywheels:

Lightened flywheels do not increase top engine speed
Lightened flywheels increase vibrations at idle speed (if stock cam profile is not changed).
Lightening factory cast iron flywheels is not recommended as drilling holes may cause failure of wheel.
Lightening flywheels increase accelaration....and are therefore very popular in the dragster scene!

OK, Pierre, I'll bite! I've had some experience with lightened flywheels on race bikes and race cars over the years, and here is what I understand about the subject, both theoretically and in practice.

1. You are right, a lightened flywheel will not increase top engine speed. The only way to increase top vehicle speed is to increase maximum RPMs or to change the top gear or final drive ratio.
2. You are right, a lightened flywheel will increase engine vibration at idle speed. The severity of this vibration increase is dependent upon how much the flywheel is lightened.
3. I have found in practice that it is generally safe to remove a small amount of mass from a factory cast iron flywheel, as long as you don't overdo it. This is practical for street applications. For
race applications you should change to an aluminum flywheel.
4. Lightening a flywheel does increase accelleration, and this is exactly the purpose for doing it. Here is how it works:

A gearbox is really just a complex mechanical lever. When an engine has a mass to accelerate (the total weight of the vehicle and occupants), it doesn't "see" the total weight, but rather the total weight
divided by the total gear ratio. For example, a BMW R26 weighs 347 pounds, and with a rider of 200 pounds the total weight would be 547 pounds. The first gear ratio is 5.33:1, and tht final drive ratio
is 4.16:1, so the overall gear ratio is 22.17:1. Therefore, the engine only "sees" about 24 pounds which it has to accelerate in first gear. If you can remove only 10% of the flywheel weight, or 2.4 pounds,
it has the same effect as removing 10% of the total vehicle weight, or almost 55 pounds! Of course the effect of a lightened flywheel is lessened as you move up through the gears. The answer to making this practical
on a street bike is to not go too far. If you remove a reasonable amount of weight you will receive a very noticeable increase in acceleration at the cost of only slightly
increased idle vibration and virtually no decrease in dependability. The solution to the idle problem is a longer period cam, which allows the valves to close later in the cycle so it doesn't take as much energy
to run the piston up the compression stroke. But again this isn't necessary if you keep the flywheel weight reduction to a reasonable amount.

Dave-
Thanks for the in depth analysis!

However, on the subject of vibration:

I always thought that engine vibration was in large part a factor of how out of balance
the pistons, rods, crank, clutch AND flywheel were. If your engine and lightened flywheel
are correctly (and finely) balanced, the idling vibration could actually be less than
stock.

Please either varify or dispute my comments.

We've been using the word "vibration" here, and I think that it's not so much more vibration as it is more roughness. The weight of the flywheel helps pull the engine through between power strokes and keeps it rotating at a steadier speed, making the engine feel smoother. Less flywheel weight lets you feel the power strokes more intensly which makes the engine seem rougher at idle, and it could be percieved as vibration. Of course this is with no other changes, and I'd bet that a good balancing job would help the situation. I've built a number of race engines with lightened flywheels over the years for small-bore sports cars (MGs, Austin Healeys) and motorcycles (Honda 4-cylinders) and I've always had everything rebalanced before assembly. I've never noticed a huge increase in engine roughness at idle, although these engines do get set up to idle faster than stock. The vibration or roughness issue disappears quickly as engine speed increases.

Thanks for biting Dave. So may I conclude in saying that if one is to ride his bike just to enjoy cruising along, it is a waste of time and money to install a lightened flywheel!

Yes you may, and I agree completely.

Pierre-

If you are building a sidecar rig, I would advise against using a lightened wheel. Everything
I've read seems to indicate that the stress on the crank caused by the extra weight of the
car would cause to too much for a lightned wheel to handle.

John,
I had no intention of lightening my flywheel. I just wanted to clear a question I had in the back of my mind, and Dave was good enough to complement my thoughts.I wouldn't even think of lightening my flywheel even if it was for solo riding.....I just enjoy cruising along.I am not in this high performance phase, even with a pre 1969 /2, S or otherwise. I prefer keeping the latter in their original design except for the electrics which I think are a bit too retro especially if you are going to use your motorcycle on a regular basis.