Lightening a 35kg flywheel.

[Mouse]

I'm still considering getting the Motorenwerk Cunewalde 2VD 8/8 - 2SVL from a few posts below into a BMW R80 frame. It weighs 124 kg so I've been stripping off all the auxiliary stuff like the tank, cooling fan, dynamo and all the over-engineered cast bracketery with the hope of making it a bit lighter and there's the real chance I can get it under 100kg.

Today I removed the flywheel and expected it to be a heavy lump but not expecting it to be as heavy as 35kg (Molly my rottweiler weighs 34kg!) . So this leave me wondering if I can remove some of this weight and lighten the whole thing up a bit and also improve performance. It's a huge cast thing with plenty of scope for cutting some excess off without making the centre part thin and weakening it. I'm thinking of removing about 10 KG from the original weight but also adding a BMW R80 clutch (2kg) so that's about 12kg to shave off. I'm considering being reckless and attempting to lighten it by 50% but need a bit of feedback from people who have done a bit of flywheel lightening before I embark on something as ambitions as that.

I know this has been discussed a bit before but not for a flywheel so heavy as the monster on mine.

It would also be interesting if anyone knew the weight of other V twin diesels to give me a barn door to aim at.

[Sphere]

I can't offer any technical insight, but wouldn't it be easier to treat yourself to a variety of flywheels, modify them to see if your engine will still run properly? I'm assuming that the 12kg you seek to drop will get you the coveted 100 kgs. Perhaps find a 25kg flywheel somewhere?

[coachgeo]

just posting a cross reference. Got a thread in the calculations forum asking questions about the science behind flywheel weight and trying to figure out how much we can/cant lighten them.

https://www.dieselbikeforum.com/view ... 7890#p7890

[Crazymanneil]

Wish I knew the answer to this one too. I'd go with Spheres idea of getting a couple of flywheels to play with and experiment if you could.

I do know that generators tend to have big flywheels because the set needs to run at a constant rpm and the big mass means that variations in load tend to change engine speed more slowly, thus giving the set a chance to compensate with throttle. If you were using a generator engine with flywheel its probably the case that it could be lightened a fair bit?

The flywheel effect is to do with the mass and the speed at which it is moving. Therefore the mass at the outside of the flywheel has more effect than that near the middle as it is turning faster. If you reduce the weight from the middle, the overall effect is not as great as removing the same weight from the outside. However, remove too much from the middle and you risk the flywheel pulling itself apart at high rpms which could really ruin your day...

This is all general stuff I know, just my 2p

n

[Sphere]

He has already put it in the lathe, see https://www.dieselbikeforum.com/view ... =31&t=1242

[mark_in_manchester]

I used a Ural flywheel to replace the Daihatsu unit on my bike, because I wanted to retain the Ural clutch and this was the easiest way to do it. The flywheel was at least 30% lighter, and there was much less mass at the rim (where it matters, for rotational inertia). I was worried like you, but the bike is finished, and ticks over fine.

The flywheel effect is directly proportional to the speed of rotation, the rotating mass and the effective 'distance' of that mass from the axis of rotation. Taking the first term - this means that at low speed (tickover) if your lightened flywheel works OK, then it will be OK for all revs...so long as you have not weakened it such that centrifugal forces cause it to explode at higher revs!

I just built the bike, and when it ticked over one winter when it first ran (with ambient temp at about 0° C) I knew I was going to be OK. If I was you, I'd acquire a scrap flywheel from a car, machine it to fit, and turn stuff off it until the engine tends to stall on tickover. Then make sure you don't take as much off the real one!

There's a maths route to answering this question which involves calculus (integration) and a fairly detailed knowledge of the thermodynamics of your engine. I'd suck it and see.

MRA

[IgorVigor]

You said the flywheel is cast?

if its cast iron, DO NOT try and lighten it...theres a high chance it will cause micro-cracks...
and then it will disintegrate (see explode/tear apart/cause lots of damage)

if its cast steel, take it to machine shop and see if they can do it for you...

if the fly wheel isnt perfectly balanced, it could tear the engine apart, or if your lucky, rapidly wear out the bearings...

trying a different flywheel will be your safest option...

[mark_in_manchester]

I'd say a cast iron flywheel would have been cast and then machined (certainly as regards the face against which the clutch plates bite) already, at the factory - so a bit of additional turning off the circumference shouldn't do any harm. Cast iron is a bit dirty to turn - the 'swarf' comes off as black powder - but normally turns nicely with a good finish. I can't see how this is dangerous, so long as the 'web' of the wheel is not skinned-down. And I'd say that one's bum on the saddle is a reasonable accelerometer as far as out-of-balance forces go - no engine is in 'perfect' balance anyway, since primary and secondary imbalances can't be simultaneously addressed. I'd say go for it, IMHO...

M.

[IgorVigor]

The jag v12 might be only 100% balanced engine

if tuned correctly, you can sit a coin on the top of the engine and rev it, and the coin wont fall over...

Beautiful engine

[Mouse]

so a bit of additional turning off the circumference shouldn't do any harm.

I suppose I should have made it clearer that the mass I wanted to remove was the large 'lip' on the outer periphery of the flywheel and not the inner thin web, maybe there would be less of the naysaying?.


Looking at the cross section it is obvious that I may have 'strengthened' it by removing a lot of the rotating mass + stress from the outer part.

Cast iron is a bit dirty to turn

Yea, turning it down was fun. In all I was in a friends shed with his lathe for about nine hours making the black granular stuff. There was about 12 kg (err 24lb) of granules after lightening. There was a moment during tea breaks where we both looked at the accumulating pile of uniform black granules and wondered if there was any use for them I think the lathe owner has kept a jar of them just in case an idea crops up in the future.

I like the idea of butt-wobble to gauge flywheel balance

[Crazymanneil]

Isn't it iron filings that those sparklers you get at halloween are made of? ..........

N

[smokyjoe]

Yup, mix it with some aluminum powder and Voila! Thermite! That's what they use to flash-weld railroad track. Also handy if you want to blow a nice hole through the top of your neighbor's car engine right through to the bottom in milliseconds. Just takes some heat to get it started... Also handy if you want to get rid of all those incriminating records in the file cabinet before the tax man comes, set a can full on top, light it and the molten stuff will burn all the way to the bottom. A very widely used demolition charge. There was a video on YouTube of a tank being destroyed in Iraq with a thermite charge- pretty spectacular especially when it hits the magazine. Powdered iron burns pretty well on it's own, someone local burned down their shop with a pile of iron powder when a grinding wheel set them a'glow. Also can be mixed with cement to make an expanding quick setting mortar. Lots of uses, save that stuff but keep dry, just the exposure to air will rust it pretty quick. Stay safe!

[Mouse]

Got it down to 12kg now. A total reduction of 23kg!!!
It rides a lot better without after removing the second lot of mass. There is less torque reaction when the engine revvs and there is no shock loading to the transmission if the revvs are not matched on a gear change and the bike alters speed to match it.
It also ticks over with no problems and still takes many revolutions for the engine to come to a standstill after stopping it from a tickover suggesting it could be even lighter and still do the job.


A big difference from original.