Flywheel/pressure plate pin question

[LA]

I should keep quiet too but...

We weighed the PP on the e30 we are working on but I don't remember the number, bigger than you think and close to the FW. In fact my son is probably working today on balancing the whole assembly and we hope to put it back in again this weekend with the trans. We will weight it again for fun. My point is there is a lot of talk about the flywheel as if it's the only thing in there storing energy when in fact the whole rotating assembly including the crank, front hub, FW and PP all store this energy. So changing the FW alone 4lbs is nothing like a 4/22 change and probably more like a 4/50 change. So there is maybe less than 10% difference in the storage/discharge with a lighter FW.

I don't think you should keep quiet. I don't dissagree with anything you said; you clearly understand. I would have estimated less than 10% but maybe it is that much. Certainly can be calculated and can also be measured.

People lighten rods, pistons and cranks all the time - or make them heavy if you need low RPM high torque. What is right depends on the application.

[turbodan]

It must be impossible to stall my engine with the stock 18lb flywheel installed right? Because it contains so much rotational momentum?

A heavy flywheel subdues power delivery, which would make dragstrip launches easier. A 50 pound flywheel, being substantially heavier, would certainly store more energy than the ones used in the e28. Its still not going to push a car up a hill though.

Your analogy is really quite ridiculous. Grab a bicycle tire spinning at 300 RPM and it too will rip your hands off. Not that its a very scientific measurement. The fact that your engine does not continue to turn for several minutes after the ignition is turned off isn't either, but it does give you a good idea of just how much energy the flywheel really stores.

[LA]

It must be impossible to stall my engine with the stock 18lb flywheel installed right? Because it contains so much rotational momentum?

Not at all, but it is harder to stall than if it had a lighter flywheel.

A heavy flywheel subdues power delivery, which would make dragstrip launches easier. A 50 pound flywheel, being substantially heavier, would certainly store more energy than the ones used in the e28. Its still not going to push a car up a hill though.

Maybe the confusion is because at some point curves cross and there are many variables. You seem to agree the heavy FW will have more energy stored at the bottom of the hill - I wonder where the energy goes if it does not contribute to climbing the hill? On the opposite end of the spectrum - double the inertia of your turbo rotor and see how it runs. I think you will find it does matter.

Your analogy is really quite ridiculous. Grab a bicycle tire spinning at 300 RPM and it too will rip your hands off. Not that its a very scientific measurement.

That was not my analogy but it is sure not ridiuculous. Someone else offered that analogy to try to illustrate why there is more torque available with the slick than with the bicycle wheel spinning at the same RPM. Torque is force X distance. Distance is the same - force is higher - you can figure it from there. I thought the guy's analogy was very insightful.

The fact that your engine does not continue to turn for several minutes after the ignition is turned off isn't either, but it does give you a good idea of just how much energy the flywheel really stores.

Several MINUTES! Come on, it was pointed out the difference is small. But the difference in spool down time with significantly different FW weights could be measured, probably in miliseconds.

Tammer - I've been watching a transmission for several hours now and it has not made a single in-oz of torque. I know you meant 'multiplies' rather than 'makes.' You are right on about this issue being a matter of conservation of energy.

I've no more time for this. Just go get a good text on rotating machinery design.

[turbodan]

As I stated earlier, the difference between a 12lb flywheel and a 24lb flywheel when it comes to climbing a hill is negligible. If you were going to have to downshift with the lightweight flywheel, you were going to have to downshift anyway.

Noone is debating wether or not rotational momentum exists. I'm saying that it makes no difference in this application other than quicker rev matching and making the car somewhat more difficult to stall.

[Tammer in Philly]

"Flywheels just store up energy and release it later--they make nothing."

Bingo! The bigger one has more to release, and yes of course it took more fuel (energy) to bring the heavy one up to speed. That was not the issue. It is the release of stored energy that helps you up the hill.

Do you coast up hill?

-tammer

[Tammer in Philly]

But the difference in spool down time with significantly different FW weights could be measured, probably in miliseconds.

So in practice, with a heavier FW you might get to wait a few milliseconds to open the throttle when climbing a grade.

Why do you continue to insist that this is some kind of useful point to make? No lightweight flywheel on the market is going to make the car undriveable. The feel might change, but in practice it's all the same thing, and a heavier FW will not make a car accelerate faster because it will not "make more torque." As long as the engine can idle without stalling and the revs can drop to idle without stalling, there is enough rotational inertia in the flywheel to get the job done.

-tammer

[LA]

But the difference in spool down time with significantly different FW weights could be measured, probably in miliseconds.

So in practice, with a heavier FW you might get to wait a few milliseconds to open the throttle when climbing a grade.

Why do you continue to insist that this is some kind of useful point to make? No lightweight flywheel on the market is going to make the car undriveable. The feel might change, but in practice it's all the same thing, and a heavier FW will not make a car accelerate faster because it will not "make more torque." As long as the engine can idle without stalling and the revs can drop to idle without stalling, there is enough rotational inertia in the flywheel to get the job done.

-tammer

Actually, in practice for your scenario it would be more than milliseconds because the engine continues to add energy to that being released by the decelerating FW. Milliseconds was for the 'turn off the car' scenario with no energy addition other than from the decelerating rotating assembly.

I have never suggested a heavy FW increases acceleration, exactly the opposite is true; it slows acceleration.

I do not insist this is a point at all, significant or otherwise. When someone makes a statement that is not supported by physics, it seems appropriate to clarify the point. Besides, some of the ideas in these threads are so humorous they are entertaining...up to a point. I am not refering to you tammer - our posts have not been contridictory other than the 'millisecond' statement above. Well - and the clairification about transmissions. Mine still has not made any torque.

[turbodan]

When someone makes a statement that is not supported by physics, it seems appropriate to clarify the point.

It certainly does. I checked again on my lunch break, BTW. Even with the 18lb flywheel my engine stops rotating almost instanly when the ignition is turned off. So much for that tremendous amount of stored energy.

[LA]

"Flywheels just store up energy and release it later--they make nothing."

Bingo! The bigger one has more to release, and yes of course it took more fuel (energy) to bring the heavy one up to speed. That was not the issue. It is the release of stored energy that helps you up the hill.

Do you coast up hill?

-tammer

I think you know as well as I do that theoretically with a heavy enough FW one could indeed coast up a hill by using the energy input used in spinning the FW up before the hill.

[LA]

When someone makes a statement that is not supported by physics, it seems appropriate to clarify the point.

It certainly does. I checked again on my lunch break, BTW. Even with the 18lb flywheel my engine stops rotating almost instanly when the ignition is turned off. So much for that tremendous amount of stored energy.

'...almost Instantly' The 'almost' proves my point. I never used the word tremendous; as discussed previously, the effect is small.

[turbodan]

'...almost Instantly' The 'almost' proves my point. I never used the word tremendous; as discussed previously, the effect is small.

Correct. So small, in fact, that it will have a negligible effect on nearly every aspect of driveability and performance other than rev matching and starting off.

Glad we agree.

[LA]

'...almost Instantly' The 'almost' proves my point. I never used the word tremendous; as discussed previously, the effect is small.

Correct. So small, in fact, that it will have a negligible effect on nearly every aspect of driveability and performance other than rev matching and starting off.

Glad we agree.

Me too, and I know you understand how things work. Possibly you were reading more into what I was saying than I intended. I even said 'in theory' and 'theoretically' a couple of times. This is a case of practical considerations precluding what is theoretically possible; otherwise the difference could be TREMENDOUS!

Now we can debate what 'negligible is!

[turbodan]

Your statement about having to downshift due to a lighter flywheel implies that it has a greater than negligible effect under those conditions. Can't say I read anything into that. You made it pretty clear.

[LA]

Your statement about having to downshift due to a lighter flywheel implies that it has a greater than negligible effect under those conditions. Can't say I read anything into that. You made it pretty clear.

So you do want to debate negligible...

I stand by what I said. Remember I have a 70 pounder on my jeep, which I also drove with one around 20. That’s not negligible, or maybe it is. It is certainly noticeable on hill climbs. Again, lots of variables involved.

Did I imply or did you infer?

[tn535i]

70 vs 20 does not sound neglible. It would ruin an e28 for all but steady cruising. But you might be able to coast over a speed bump.

[a]

I'm interested. I am gonna be doing a clutch R+R on my Ranger. The present clutch is slipping badly and I expect the flywheel is gonna need machining. I was under the impression that a shim was used between the crank hub and the flywheel to offset the material machined off the clutch surface of the flywheel. So that the stock distance of throw is preserved and stock clamping pressure is maintained. Also could I get a ballpark price for the job? TIA

[LA]

The following from the jokes thread reminds me of myself in this thread about FW weight:

You must be an engineer" says the balloonist.

"I am" replies the man. "How did you know."

"Well," says the balloonist, "everything you have told me is technically correct, but it's no use to anyone."