How do You Balance a Flywheel?


If the flange on the flywheel, or your car wheel, has a burr on it the location will be off centre. Even if there is no burr there is a margin for error when two cones get screwed together as to how they grip the item between them. My system uses a custom made mandrel for each type of flywheel which is a press fit into the central hole. Yes it’s a considerable degree of buggeration factor rather than relying on a universal mount but it means you get precise and repeatable results.

My colleague lightens and balances a lot of flywheels for performance customers but has no balancing system himself so he farms that out. I took my system down there to show him how to make one himself so he could save the £2000 to £3000 a year he spends on this third party service. The first thing we did was find a couple of standard flywheels and check how well they were balanced at the factory. As expected they were spot on because OE car makers use very expensive equipment tailored exactly for the engines they build. My system measures in terms of grimes at the flywheel rim and in both cases there was less than 1 grime error at about a 5 inch radius. Basically nothing measurable.

Then we looked at 2 flywheels he’d lightened but not yet sent away for balancing and 4 which had come back from the balancers but not yet been sent out to customers.

The 2 which had been machined but not yet balanced had errors of about 2 gramme at the rim which is pretty good. It shows that when machining the flywheels on the lathe he’d clocked them up concentric and removed metal evenly so they didn’t go out of balance much. They’d actually have been fine to just put back on the engine.

The 4 which had been balanced by this supposed specialist he farmed this work out to were a completely different story. The best was 3 grammes out and the worst was 8 grammes, enough to seriously upset a high rpm engine. About ten times as bad as OE factory balancing. In other words the balancing was actually making the flywheels worse than after he’d machined them but not balanced them.

As we checked each one his face fell further and further. It became apparent he’d been spending a lot of money for year after year to make his product worse rather than better. How a specialist in balancing could make flywheels so much worse than standard is debatable. Either a faulty machine or more likely a faulty mounting system with defective or worn cones.

There’s an old saying. If you want a job done properly do it yourself but of course if you could do that you wouldn’t need people like me. But what sort of standard of work are you getting when you entrust your engine to reputed engine builders in this country? By my measurements not a very good one.

I could take the opportunity here to add a bit about methods of balancing flywheels. Contrary to popular belief it’s certainly not necessary, or even always helpful, to balance a flywheel on the crankshaft it’s going to be used on except perhaps for extraordinarily high revving and delicate race engines. Unless the flywheel is tightly dowelled to the crank it’s going to sit in a slightly different place every time it’s fitted anyway. There will always be a small amount of play between the crank spigot the flywheel sits on to centralise it and also between the flywheel bolts and their holes in the flywheel.

Now we’re only talking a thou or so of possible movement but that’s enough to make it pointless trying to achieve any better level of accuracy than by balancing the flywheel on a proper mandrel in its central hole. At least you then know the flywheel itself is in perfect balance and it will run very close to that on any crank it’s then fitted to. That’s certainly the way all production flywheels are balanced anyway.

My own balancing system is accurate enough to detect the effects of tiny offsets like that such as a flywheel sitting a thou off its true centre. It makes less than a gramme of difference and frankly it’s not worth fretting about. In fact none of the flywheels that were being balanced incorrectly by the specialist my colleague above was using ever caused a customer to note any engine vibration so I think you can conclude that most engines simply aren’t that sensitive to a few grammes error in balance, not of course that it’s an ideal situation.

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