Flywheel design options for multi plate clutch

[Boyracer]

Hi again!

I have Tilton OT-II 7.25" three disc clutch which I want to use in my BMW M42 project (1,8 liter I4 16V 8500 rpm). Now I need to design as light as possible flywheel for it while maintaining good reliability and safety. I think this discussion might be interesting and helpfull to other members with different engines as well, I guess flywheel design is pretty universal.

Options so far:

1. Modifying stock flywheel (I have two of these) by removing material from edges. Stock weight is 9,9 kg and there is about 25 x 25 mm rim on the edge of flywheel which is only for increasing flywheel mass and ineria. This could be machined down but I think flywheel would still weight about 5-6 kg. Also mounting holes etc for multi plate cluch cover must be machined.

By the way, is there sure way of testing if flywheel is cast or something more durable material?

2. Use automatic transmission flywheel (I have one) with separate custom made center (maybe even made from stock flywheel?) that only acts as mounting to clutch cover. Automatic flywheel seems to be just a holder for started ring, stock weight is 2,5 kg which should go down to 2 kg by machining few large holes. Center boss (not sure if this is good word for it) would have diameter just enough to mount cluch cover and act as surface for clutch disc to press against. Total mass should be well less than modified stock flywheel.

3. Get fully custom made flywheel. This might be the easiest and perhaps safest but also most costly option and funds are limited...

Since I already have parts required for options 1. and 2. I would of course like to go with either of them. But my knowledge is limited in this field so I would like to have expert opinion on this matter.

I think option 2 might just work? Automatic flywheel can be made quite light so it does not store much energy so it is quite lightly stressed. Center boss has outer diameter of maybe 200 mm and it also is quite light and most of the mass is centered near the axis so stresses should be lowish. But this would be two piece design, only held together by flywheel bolts and dowels, how about the vibration? Or maybe automatic flywheel could have it's center removed and then the outer section is welded to center boss?

[pastaroni34]

Which type of clutch do you have? Tilton primarily makes two types: a) the type made for a step (0.0985") or b) the type which has a friction surface as the bottom plate on the clutch, which requires no step and no machined friction surface.

I ask because if you have the step type, the friction surface on the stock m42 single mass flywheel is not sufficiently large for the Tilton clutch disk to get full engagement. Do you have a single or dual mass flywheel? In the states we got mostly dual mass ones.

Stock flywheel material will have to be determined by lab testing to be certain. A good machinist will be able to tell a good bit about the material by cutting on it though. I doubt, however, that it is anything more than a ductile cast steel. More than likely it is a gray cast steel which is more brittle. I can certainly say it is some sort of casting though, as alternative manufacturing processes are more expensive.

As for the automatic flywheel, I have not seen an automatic flywheel for a m42. If it is flat similar to a DOHC Fiat variety, you might have a problem of finding room for the flywheel to crank bolt heads between the flywheel and the clutch disks. (I think you already understand this from your description.. just making sure) These are usually a welded assembly with a center boss, a round plate, and ring gear. From what I've seen the center is usually a punched low grade cold rolled steel. Be careful how much material you remove from it.

Finally there is your last choice, a custom made solution, which is like you said, probably the easiest solution but the most expensive. If constructed correctly it is also your safest solution, which takes top priority in my book.

[Boyracer]

Thank you for answering!

There is part number on cover which is 1106-0037, not sure which type it is. I attach few photos of the actual unit and maybe it is apparent from those. I will try to take photos of stock flywheel and automatic flywheel later.

Most propably I will not use the cluch in this three disc configuration because my puny little 1,8 liter does not produce that much torque and also I have already sold the cllutch discs because they had wrong spline for BMW gearbox. I will either get similar discs with correct spline or replace them alltogether with one thick paddle type clutch disc.

Stock flywheel is single mass design and it is common part with M40 engine (I4 8v design, not sold in US I think) where they were usually mounted. M42 usually had dual mass units, especially when mounted to E36 body. Automatic flywheel is one piece design for M40 engine and it is machined to be about 5 mm thick flat disc, not sure what exact material it is.

I would think modifying cast flywheel (which the single mass unit seems most certainly to be) is not good idea for engine that revs up to 8500. Then the question is can modified (reduced in diameter and machined to accept the clutch) cast flywheel be used as mounting for clutch and then can the automatic flywheel be modified (lightened and cut to fit the center piece) to act as holder to ring gear? This way stock flywheel would take stresses caused by the clutch and automatic flywheel would just take stresses of holding ring gear.

[Boyracer]

Here are promised photos of flywheels. This was the first time I had clutch and flywheels in the same place so I could do bit of test fittings to see how the clutch cover would go with the flywheels.

From the photos you can see how ridiculously heavy the stock single mass flywheel is (dual mass flywheel is 2,5 kg heavier still!) and most of the mass is centered on the outer edge of flywheel. I would think intuitively that removing bulk of that mass and leaving just enough to support started ring while leaving the center alone, flywheel should be able to withstand high revs better because center (which I think is the critical area) is subjected to less stress caused by centrifugal forces. It should be quite easy to reduce weight from 9,9 kg down to maybe 4 kg, almost all of it from outer edge.

But then there is question of stress caused by vibration etc from which I have zero knowledge :(

[Guy Croft]

I'm back..

First thing I want to point out is that using a triple plate is a bit 'over the top'. What is the torque rating of that item? Depending on the springs in the cover even a single plate would generally (based on Helix data) cope with 170-360lbf ft.That said, one would quite naturally expect a triple to longer because, there are, er, three friction plates..
However the pressure (in the axial direction) to actuate a heavy clutch like yours might well put an excessive load on a cast iron FW and I would also judge that really, having such an item on a cast iron FW at all is a bit of a mismatch. My advice is go steel and I imagine Jason (above) might well be able to put one together for you.

GC

[Boyracer]

Thank you for your response!

After posting this questions I have already ordered a clutch disc, it will be sintered three paddle design which should be more than enough for my measly torque output. Once it arrives and I can measure it I can also design and get suitable flywheel made. I think I will go for new steel because it would not cost much more than modifying the old one. And I think it is wise to spend some "extra" for little peace of mind.

Altough I started thinking about getting flywheel made out of aluminium with screw in friction surface but that might be taking it to extreme on a car that is not going to be raced!

[Guy Croft]

Yes, wise to avoid aluminium FW: you have to be careful with those on any engine - they can exhibit embedding of the FW bolts and that can lead to loss of bolt preload & disaster. My advice on them is pure-race only and check after every event. Anyone who thinks (or does) otherwise - good luck!

GC

[Boyracer]

Just to post a closing to this thread...

Just got this flywheel made. It's machined from billet steel (not sure about exact material name) and guy who made it said it to weight around 7,6 kg with pressure plate and starter ring, altough I am quite sure it is bit more. Significant improvement if you compare it to stock flywheel that weights 9,5 kg alone and most of the mass is in the outer edge.

Could have been made bit lighter but this is no competition engine and price was only 200€. Next I need to take it to balancing with vibration damper and crankshaft :)

[Guy Croft]

Very good! Be sure to get the assembly balanced (I guess you know that).

Care to share the maker's details with us? Always useful to know.

Thanks,

G

[Boyracer]

Very good! Be sure to get the assembly balanced (I guess you know that).

Care to share the maker's details with us? Always useful to know.

Thanks,

G

Got the flywheel weighted today. Flywheel alone is 4,6 kg and with pressure plate (without bolts) 6,5 kg.

Next I will have the flywheel/pressureplate, crankshaft and vibration damper balanced. I think first flywheel/pressure plate are balanced together as are crankshaft/vibration damper. Then they are put together and balanced again. Is there some kind of value with limit that balancing should aim for?

Flywheel was made by a local guy here in Tampere, Finland, so I it is some distance to UK. But if anyone is interested his contact info can be asked from me :)