Crankshaft lightening

[Rusty]

Perhaps they have ultra-light flywheels and make the crank heavy for strength but maintain a low inertia for the whole system of cranks, pulleys, flywheel and clutch. Don't forget that the mass of th reciprocating components, pistons and rods, have an affect too. I'd guess you want these as light as possible.

[stiglet]

Inertia of the engine (flywheel, clutch, crank mainly) is multiplied by the square of the ratio of the G/B gears and the final drive. this means that the rotational inertia seen at the wheels is HUGE in first and second gears, then declines with higher gears. In 5th (6th for some) it is next to nothing. However, in 1st gear this can increase the "effective" mass of the vehicle by a third. If your car has a stationary weight of 1500kg, in first gear you are accelerating an effective mass of 2000kg (based on typical engine inertias). In second gear its more like 20%.

So, any weight off the crank is a very good thing.

Same goes for prop shafts, however this is only multiplied by the square of the final drive ratio and not the G/B ratio as its after the gearbox.

I'm a bit rusty on this stuff so i could be wrong on the theory, but i'm sure a typical value for effective mass in 1st is 33%.

I don't know the exact inertia values of individual components but i'm assuming the crank is high up there. Its heavy but the mass is nearer the rotational centre. The flywheel is the biggest culprit.

BTW, on a completely different subject, ABY owners: i have a few 7A flywheels going spare....

No extra torque, no less torque, but better acceleration. How exactly does this give the synchros a harder time? i wasn't aware of this?

[mcandmar]

I rang BDB this morning about the pros/cons of a lightened crank. Interesting point he mentioned was the stock engine uses a 50% balance, and relies on the flywheel weight and front pulley for the overall balance. With a lightened crank he uses a 100% balance, which if i understood him correctly balances each part on its own. He completely lost me after that but the point is, a stock 3B flywheel/pulley would need a sizable amount of weight machined off to be able to work with a lightened crank. So it’s not a runner basically.

[Rusty]

If I got a lightened crank then everything else would be done at the same time but that's the way i do things.

[stiglet]

It is true that the assembly is balance by audi in one go. However, each of the individual components are in harmony individually to some extent too. For example, many people here are using 7A flywheels with their ABY cranks without any problems, which effectively is the same as lightening the crank.

I reckon you could get away with it, or use a fidanza ally flywheel from 034.

[4WheelDrifter]

Are you guys using a lightweight flywheel in conjunction with the lightened crank?

The further the mass is from the centre of rotation the more influence it has. So using a stock flywheel with a lightened crank would be a waste of time really, it would be better the other way round.

I have a stock crank lightened by 1.4kg and a 6kg steel flywheel. Because my crank is internally balanced I can swap things like the front pulley for example, and it won't upset the balance, as long as that part is also balanced of course.

[stiglet]

I have a stock crank lightened by 1.4kg and a 6kg steel flywheel. Because the crank is internally balanced I can swap things like the front pulley for example, and it won't upset the balance, as long as that part is also balanced of course.

Did you get the crank internally balanced or are you saying it is anyway? I wasn't sure if it was possible (being a 5-pot) since audi didn't do it in the first place?

[4WheelDrifter]

Did you get the crank internally balanced or are you saying it is anyway? I wasn't sure if it was possible (being a 5-pot) since audi didn't do it in the first place?

AFAIK Audi balanced the cranks externally, crank, flywheel, front damper together.

I've now edited my post.

[stiglet]

Cheers. I only ask cos i wanted to know if anyone in the UK could balance the cranks seperately. Would be useful for swapping bits like you say

[Bram]

For example, many people here are using 7A flywheels with their ABY cranks without any problems, which effectively is the same as lightening the crank.

Any info on this mod? Can I simply replace a DM ABY FW with a 7A one? What clutch type to use? A ABY or 3B one?

[stiglet]

I think Andy Casey has done this mod to his ABY?

[4WheelDrifter]

3B and 7A flywheels are interchangable. Ideally you should get the whole lot re-balanced, but I guess because the stock 7A/3B flywheels are still pretty heavy you can get away with swapping the ABY item.

[Rusty]

You are correct , I have done this as have a few others on here a few years ago but my conversion caused a few issues and was nly back in December so it's been discussed recently. There are a couple of threads about it in the transmission section, 3B is the one to go for with 3B suitable clutch.

I've got some interesting vibrations at low RPM but I've got a few things to sort with the exhaust and propshaft so can't say if it's due to the flywheel. It's only at 1500rpm, everywhere else is fine. I was thinking that I should fit a 3B front pulley as it's lighter than the ABY one and is designed for use with the 3B flywheel. Hopefully I'll get everything lightened an d balanced at some point. I would say that it's worth getting an old 3B item refurbished as I didn't bother and I have a slight judder when warm but I'm sure that a few 1/4 mile runs should sort that out :mischeif:

[mcandmar]

@Bram: 7A/3B flywheels as the same, only difference is approx 1kg of extra weight on the 7A flywheel. You will need to use a 3B clutch disk and pressure plate.

@Stiglet, BDB can balance just a crank, that’s the process they use with lightened cranks for example. Or internally balanced instead of externally balanced. Best to give him a call, nice guy to talk to.

Don’t forget the front vibration damper/pulley is heavier on dual mass engines, ideally you should change to a 3B front pulley with the 3B flywheel IMO.

Interesting observation i noticed weighing parts this weekend, the crank is approx 44lbs, and the combined weight of the 3B flywheel, pressure plate, and front pulley is ~43lbs. Guess there is some relationship in weights, and would explain why a lightened crank can’t work with a 3B flywheel without removing the equivalent amount of weight from it.

[Rusty]

3B and 7A flywheels are interchangable. Ideally you should get the whole lot re-balanced, but I guess because the stock 7A/3B flywheels are still pretty heavy you can get away with swapping the ABY item.

Ideally, yes but it seems fine. If I still have vibrations when I've sorted the exhaust and prop then that's the next step.