mx5cartalk.com

Pulled a turbo BP down and found its lightweight CrMo flywheel 4.5 Kg has had a really hot time :



The thickness of the metal at the clutch face is minimal with these exquisite Japanese Tuner CrMo flywheels explaining their extreme lightness.
But lightness comes at a price shown here as sensitivity to clutch slip.

This photo is evidence that a light clutch pedal, a light flywheel, a heavy foot and high gear ratios in a turbo car can combine to a failure mode waiting to happen.
If there is one, I do not know the catastrophic failure mode of these flywheels under these circumstances. But there is enough evidence to ask how the risk is being managed.
There is also enough evidence to ask for awareness that if your driving involves a heavy foot and a slipping clutch, to reassess going too light weight as such units are unfit for the purpose.

The likelihood of flywheel rupture are unknown. The consequences in the cabin are not good when the bell housing is only made of aluminium.
The risk then is based on how much is known of the likelihood ?
That is, what is the ultimate mode of failure that a left and right foot in rhythm with each other can inflict on that specific material and design of flywheel ?
If no one has the experience, is it ok to settle on an educated guesstimate ?

Having posed that question, another approach is to avoid the issue by eliminating the risk of clutch slip altogether.
I think I have done that with my 1.6 CrMo flywheel and standard clutch behind a 1.8 using a very low ratio diff, a 6 speed box and proper rhythm between the left and right feet.
Having said that I do like a lot of gear changing.

need a tin hat mate

What he said.

Tin hat and chain mail scrotal guard .....

I'd say you need something with a bit more meat behind the clutch face area for what you're doing. Thin metal simply can't dissipate the heat fast enough.

changing gear isn't the problem. slipping and riding the clutch pedal is. depending on the application, a cast iron wheel might be better.

as for failure. considering the heat damage is located in the middle radi of the plate. if and i say if with a huge amount of if, the clutch would failure in a complete 360 degree at roughly the same radi all the way around. the part failing will be going in a circular motion and you'd probably kill a behousing but not get through a fire wall or exhaust/shaft tunnel.

I think the flywheel would warp to a point that would render it almost undriveable before it failed.

zossy1 wrote:I think the flywheel would warp to a point that would render it almost undriveable before it failed.

I think I will pass on the PPE thanks (tin hats and scrotal guards) and agree with zossy1 that it will give out warning.

project.r.racing wrote:changing gear isn't the problem. slipping and riding the clutch pedal is.

That's great and means I can continue unfettered with my gear changing.


BTW I am innocent of flywheel abuse. The flywheel came from a BPT engine from some one else's car !
But that's irreverent; simply hard for innocent to escape crucifixion this time of year.

I think what zossy is saying is that the clutch wouldnt work and leave you immobile before it got to a point of flywheel failure. Even with a ceramic disc, the clutch should let go well before the flywheel.

Has anyone done an FMEA on this set up- probably not and maybe just the installer decides on what to mix and match. To an untrained eye I would suggest that the flywheel is a suspect component. PRR is right of course- the idea with a manual car is changing gears and the life of components will be affected by abuse which includes poor technique. This gets us no where but as 97 said "But that's irreverent";

Looking at "static" load cases, ie centrifugal inertia force and driving torque this is really no problem as the stress at the radius of the hot spots is quite low....Happy Days

zossy1 wrote:I think the flywheel would warp to a point that would render it almost undriveable before it failed.

Given that the centre of mass is offset, agree warping/buckling of the heated thin disc flywheel would be a failure mode very happy to clearly advertise its presence....Happy Days

Its propensity to warp refutes the possibility of an insidious failure modes such as possible resonance modes to consider for possible accelerated crack propagation in a thin disc.
In any case, given it is a massively stiffened by a clutch at its periphery, resonance will likely not be an issue despite the horrible vibrations that an inline 4 generates ....again Happy Days

Look at the lovely stuff it is made of....pure Japanese CrMo steel. CrMo hardens nicely. It is not like cast iron full of defects to begin with.
So with low maximum stresses and low stress ranges and no defects to concentrate stress, so what if the hardness gets a bit high ?.....Happy Days

Finally there is the absence of Google search of failed overheated CrMo flywheels.....Happy Days

So that leaves us just one thing to do......find a Crash Test Dummy.....Happy Days

Someone blind and fearless enough to weigh Up the facts.....Happy Days

And if we lose the dummy who else is there blind and fearless enough to weigh up the facts ?.....Happy Days handling the soap in the shower.

Hi,
You need a scatter shield for the bellhousing. Should be mandatory if you are competing on circuits. We always used them and started using them when racing minis and cortinas after seeing the results of a major mini failure.

Flywheels can fail, and do often on drag cars.


It's not so much the heat at the time causing it to fail, it's the heat cycling over time that can fatigue it (plus warping etc putting odd forces into it)

Basically, flywheels can let go, and when they do it's usually pretty dramatic (at 7000rpm there's a lot of centrifugal force on them)

Personally, I wouldn't use that flywheel for anything more than decoration, it's probably fine... but I wouldn't want to be the one to find out.
The bellhousing won't stop it, and in a lot of cases the trans tunnel does stuff all too. (at least in an MX5 the clutch is in front of your feet)

zossy1 wrote:I think the flywheel would warp to a point that would render it almost undriveable before it failed.

Without a reasonable analysis of stress due to centrifugal force, versus strength of material at the temperature it was at when the stress was applied, this could be a dangerous assumption.

If the metal is so hot that it has either a) lost enough strength to start warping, or b) undergone enough of a differential temperature gradient to impart internal stresses great enough to start warping, it may well have lost enough strength to hold together at all speeds the engine is capable of.

Having said that, I did some analysis some time ago to determine the stress V strength of a standard flywheel, and there was lots of safety built in. So much that I couldn't see a standard flywheel ever failing unless there was some problem with it. Could localised high temperature be such a problem??

I'd like to know how hot the CrMo Steel was to cause the discolouring, and whether that 'heat treatment' negatively affected the strength of the steel. In saying that, the steel should be strong enough once back to normal temperature. My main concern would be the strength of the steel when *at* high temperature. So, straight after a heap of clutch slip, revving the engine to the redline would be the danger point.

I'd also like Dr. Guran's opinion on this, if by any chance he sees this.

The flywheel hasn't even been drilled. So most of the meat is still there in a continuous disc. I can't really see how the heated areas would bring up enough internal stresses to cause it to fail. If it did it would have failed / warped at temp. And besides, the stresses on a 4140 flywheel are less than a steel flywheel since the mass is lower. Plus it's a billet part I believe.



I'd run it no problems. The 4140 mazdaspeed flywheel in my 1.6 is awesome. I wouldn't rev it past 10k though

madjak wrote:And besides, the stresses on a 4140 flywheel are less than a steel flywheel since the mass is lower.

The reason the CrMo steel flywheel is lighter is a) because it is thinner and b) it doesn't have a thick band of steel near the periphery as the standard B6/BP flywheels do. CrMo steel (e.g. 4140) has the same density as other steels, and therefore the same weight for the same size.

Madjak's comment is probably true due to the lack of that thick band, (b), and for no other reason. For two flywheels that are both cylindrical in shape, the same diameter, any thickness, one made from CrMo Steel and one made from any other steel, and both spinning at the same speed, they will both endure the same stress. Thinner means less weight, *and* less material to endure loads.

The real advantage of the 4140 is that it is stronger than the steel in a standard flywheel, and therefore less likely to fail. But the main reason that 4140 is stronger is that it is heat treatable and heat treated. You really don't want to weaken the steel by undoing any heat treating.