Flywheel rebalancing - NW / Lancashire area

[Rosewood Red]

I'm a long time lurker, relatively new member and don't have many posts, so bear with me


I've got a bit of an odd one here - I'm doing a 3 cylinder Polo TDI build, however no (readily available) aftermarket clutches exist for the stock 216mm flywheel. I'm swapping over to a 4 cylinder transmission setup that uses a slightly stronger gearbox and a 228mm flywheel, with lots of readily available clutch options. My issue is that the stock (solid mass) flywheel is balanced specifically for this engine (ie off-centre; see the illustration I've attached from a VW technical document). The 4 cylinder flywheel (I'm actually planning on using a solid Golf Mk2 G60 flywheel, as opposed to a 1.9 TDI DMF...)on the other hand is balanced "normally" if that makes sense.



Here is an image I managed to get hold of showing the crank-facing side of the 3 cylinder flywheel:



And here's the crank-facing side of the G60 flywheel:



It's obvious where there's a "missing chunk" on the 3 cylinder flywheel. My question is whether it would be able to rebalance the 4 cylinder flywheel so that it's balanced "off-centre" in accordance with the stock 3 cylinder one. Are there any companies in the Northwest that you'd recommend? I've sent a few email out to so-called engine balancing specialists, but no-one's bitten so far.

Thanks,
Ash

Edited by Rosewood Red on Monday 2nd April 00:25

[stevieturbo]

Bit of a weird one that !

I'd imagine anyone taking it on would want the entire rotating assembly for balancing. It'll be a git risk otherwise.

[davepoth]

+1, I guess they'll need the crankshaft to do it properly.

[Tango13]

Weird answer for a strange question. Try speaking to Triumph Motorcycles in Hinckley, they used to build both 3 and 4 cylinder bikes, they might be able to advise you as to any balancing issues.

[GavinPearson]

This isn't as simple as you might think.

You need to know the level of imbalance the existing part provides and then replicate that on the new part.

I would take detailed measurements of the existing flywheel so that you can work out how much mass is missing from the removed section and the level of offset and angle it has.

Translate that to a mass the new flywheel can be machined to.

Potentially externally balance the whole assembly using studbolts to hold the clutch cover on, then add mass to fine tune.

[Pumaracing]

GavinPearson said:

This isn't as simple as you might think.

It's actually pretty straightforward provided there is sufficient metal available to remove on the G60 flywheel and I can't tell that from a photo of just one side. The OE flywheel has the big rim to allow this to be done. If there's no similar rim thickness on the new flywheel then it's a no no.

Essentially you mount the crank, pulley and flywheel together on the balancer between the end two main bearings and just let it rotate under its own weight to find the heavy spot on the flywheel. Then keep milling off metal around the high spot until the assembly will rest in any position without turning. By then the balance will be close enough to let you spin the crank up slowly without it flying off across the workshop, drill further balance holes until finally you can raise the balancer to full speed and make any last adjustments.

Edited by Pumaracing on Thursday 9th February 07:52

[stevieturbo]

I guess if there wasnt enough material to be removed from one side, heavy metal could be added to the opposite side instead ?

Perhaps this place ? Although never used them myself.

http://www.vibrationfree.co.uk/motorsport.htm

[Rosewood Red]

Thanks for the replies folks

So is it impossible to get the flywheel balanced without the crank and pulley? I was under the impression that since the 3 cylinder flywheel was balanced in accordance with the crank, balancing the G60 flywheel in accordance to the stock flywheel would even things out, ie if a = b and b = c, a = c. Take it things aren't so simple? Taking the crank out etc will be a major pita as it's a daily driver. Maybe I should go for a dual mass setup instead...unless anyone has got other suggestions? I'll post a few more pics of the g60 flywheel when I'm home which may show whether some material can be taken off.

Thanks again.
Ash

[stevieturbo]

Rosewood Red said:

Thanks for the replies folks

So is it impossible to get the flywheel balanced without the crank and pulley? I was under the impression that since the 3 cylinder flywheel was balanced in accordance with the crank, balancing the G60 flywheel in accordance to the stock flywheel would even things out, ie if a = b and b = c, a = c. Take it things aren't so simple? Taking the crank out etc will be a major pita as it's a daily driver. Maybe I should go for a dual mass setup instead...unless anyone has got other suggestions? I'll post a few more pics of the g60 flywheel when I'm home which may show whether some material can be taken off.

Thanks again.
Ash

You arent asking for a normal balance though. Most rotating parts will be neutrally balanced. You want something that is off balance.

Someone may be able to replicate that, but will they take the risk ? I doubt it.

[GavinPearson]

Pumaracing said:

GavinPearson said:

This isn't as simple as you might think.

It's actually pretty straightforward provided there is sufficient metal available to remove on the G60 flywheel and I can't tell that from a photo of just one side. The OE flywheel has the big rim to allow this to be done. If there's no similar rim thickness on the new flywheel then it's a no no.

Essentially you mount the crank, pulley and flywheel together on the balancer between the end two main bearings and just let it rotate under its own weight to find the heavy spot on the flywheel. Then keep milling off metal around the high spot until the assembly will rest in any position without turning. By then the balance will be close enough to let you spin the crank up slowly without it flying off across the workshop, drill further balance holes until finally you can raise the balancer to full speed and make any last adjustments.

I'll put it in Dave Vizard terms:
It's like balancing an externally balanced big block Ford V8 without knowing how much the pistons or rods weigh and therefore having to assume the flexplate imbalance is the target imbalance.

Easy for experts, less so for the majority.

[Pumaracing]

Rosewood Red said:

Thanks for the replies folks

So is it impossible to get the flywheel balanced without the crank and pulley? I was under the impression that since the 3 cylinder flywheel was balanced in accordance with the crank, balancing the G60 flywheel in accordance to the stock flywheel would even things out.

But how do you think anyone could mount the two flywheels together exactly 180 degrees apart on some sort of custom made and precisely oriented jig to "match them up" in any way that's not much more complex and expensive to make than just putting the new flywheel on the original crank in the first place?

I don't know what the factory recommend anyone to do when the original flywheel needs replacing due to wear but as far as I can see each crank will be unique because they will all come out of the forging and machining process a bit different and can't be balanced on their own. Until a flywheel and pulley are attached none of it can be balanced and every bottom end then becomes a unique assembly. Putting the flywheel from one engine onto any other one would end up with an imbalance. Maybe not enough to greatly upset a low revving engine but certainly not ideal.

If anyone can advise you fully it'll be Vibration Free but I know what I'd do if it were my problem to solve and that would involve balancing the entire assembly as a unit.

[CarsGoVroom]

I know from my work, that OEMs can and do balance the cranks to within a certain inertia tolerance. They drill into the outer circumference of the counterweights and re-measure. 100% of parts in the case I saw.

Sure, it is definitely benefitial to find matched pairs of flywheel/crank/damper pulley/clutch etc. But it's a lot cheaper in mass production to design all within a tolerance that the engine and bearings can cope with for durability.

[Fastdruid]

I've not tried this and I'm no expert on the subject but one suggestion I saw was to create a stub with the exact same end as the crankshaft (same PCD, bore etc). Balance this stub to the original flywheel for the engine, that gives you a stub which should have the same balance as the original engine.

Now add your new flywheel to the stub and balance the flywheel to the stub, it should now match the balance of the original flywheel. This may not be ideal as it will still only every be balanced as well as the original flywheel but should put you in the right ballpark.

[Pumaracing]

Fastdruid said:

I've not tried this and I'm no expert on the subject but one suggestion I saw was to create a stub with the exact same end as the crankshaft (same PCD, bore etc). Balance this stub to the original flywheel for the engine, that gives you a stub which should have the same balance as the original engine.

Now add your new flywheel to the stub and balance the flywheel to the stub, it should now match the balance of the original flywheel. This may not be ideal as it will still only every be balanced as well as the original flywheel but should put you in the right ballpark.

F**ks sake. Just exactly how much time and money do you think it would take someone to machine a sodding great crankshaft substitute out of probably 100 quids worth of metal, christ knows how many hours of labour, balance that to the old flywheel and then balance the new one to the stub rather than just using the original crankshaft?

Does anyone in here have even the tiniest bit of common sense?

[anonymous-user]

You need to understand the concept of "Internal" and "External" balance!

imagine you have 2 cylindrical metal masses. If each is mounted in centres and balanced on their own, when bolted together (around the same axis as they were balanced around) they will still be in balance. This is because each part is internally balanced.

Now immagine those two masses bolted together with an offset between their neutal axis. They can still be balanced as an overall lump, but if split they will no longer be internally balanced.


On a conventional 4 cyl engine, the crankshaft counter weights exactly balance the piston and road loadings, and conviniently, the 180deg firing angle also results in a crank that has a natural internal balance on its own (2webs up, whilst 2 webs are down, seperated by 180deg)

On this 3 cyl engine the crank does not have a natural balance (the original info suggests insufficent room to package a naturally balanced system. In this case the flywheel is deliberately "unbalanced" so that the entire system is balanced when assembled.

The issue with this approach is that internal loadings within the rotation systems start to appear (imagine two different sized rocks taped to a length tube, if you whirl them around your head you will be able to find the point around which the system will spin smoothly, but the tube in the middle is actively taking load to maintain that balance)

If you take an internally balanced object you can spin it around at any speed and it will only fail when the material strength is insufficent to withstand the CF, but an externally balance assembly of components will fail much much sooner due to the internal forces between the distributed mass centres.

VW has designed the std 3cyl crank and flywheel assy to maintain a suitable balance tollerance and to withstand the necessary maximum speed required during operation. If change anything then that is no longer necessearily the case.
If you use another flywheel that has chunks machined out of it you may well be able to get a suitable overall balance tollerance, but the overall system may still fail at high speed due to the internal forces exceeding the material properties (especially at any stress concentration points etc)