m73m95

The reason for using stainless isn't only for corrosion resistance. There are many many machines, and tooling made of stainless for its specific properties.

Actually, the specific strength and yield of normalized 4130 and stainless (Depending on the alloy) are within a few thousand psi of each other. Some alloys of stainless are higher, some lower. 304, which I would say is the most common, is actually about 10k psi lower in specific strength than 4130 steel (It can be worked harder though). 10k sounds like a lot, but when "regular" mild steel's strength is ~45k psi, and 4130 is 85k psi, the 75k psi of 304 isn't too shabby. Where stainless earns its money is in elasticity. The elongation of 4130 is ~25%. The elongation of 304 is over 50%. As I said before, I believe the problem with the stock fork is that it is too brittle. I couldn't find a specific elongation value for forged steel (probably because there are so many differences to the forging process and materials used). I know the specific strength would be in the 90k-110k psi range, but I cannot imagine the elongation % is very high. Anyway, that means that stainless would flex farther before breaking than 4130 would. Strength not being the main issue (IMHO again), because with either 4130, or stainless, we are going to lose specific strength over the forged steel fork, we are going to gain ductility.

This does NOT mean that the fork will bend easier. It means that when the 80k psi is reached, the metal will give before breaking. Stainless with give 50%, and 4130 will give 25%. With the fork being loaded and unloaded a few thousand times, it is getting work hardened, making it more brittle. A more ductile material would fight the effects of that.

So, if we want to talk in specific terms, 304 stainless is, indeed, weaker than 4130 steel, but it is MUCH more ductile.

The other problem with the forks might not be the fork at all. It might be the bell housings. The few forks I have seen, and even the pictures posted in the last few days show uneven wear on the throwout bearing pads. Doubling the load on one side of the fork would make it break much faster.

If you guys are really serious about making up some forks, I could do more research, and choose the best material for the job. Getting the pressure of the KEP 2 plate is just one phone call away, as well. I have the ability, and the tools required to work with 4130, as well as a few alloys of stainless.

ehall

hhmmmm I'm going to have to watch this thread.

m73m95

4140 is indeed stronger, but I cannot work with it. I believe it can only be welded in its annealed state, then would have to be properly heat treated to get back to its desired strength.

4130 is easier to work with, and should be strong enough.


The proper way to go about this would be to get the exact specs of the PP from KEP. That way we would know what we're dealing with, and not just throw out what we think the strongest metals known to man are...

"Strength" is a term being thrown around (I do it too), but its not the important factor here. I've said a few times now, the strength of the stock fork isn't in question. Forged steel is unbelievably "strong". What it is not, is flexible (ductile). I think we could give up some actual strength for a material that wouldn't become work hardened, and wouldn't snap from the pressure. What good is something that has the strength of a million tons, but would break into a million pieces if you dropped it on the ground?

The forks that have been welded I think work, not so much because they are thicker, but because they are more ductile. 7018 welding rod is 70k psi. That's the 70, in the 7018. So, it is weaker than the forged steel it is welding....but it is MUCH more ductile. It survives the constant "on-off" pressure of the clutch better than the "stronger" forging.

m73m95

I've just returned from my "thinking chair"...

All of this might be much ado about nothing. The proper answer might just be to heat treat the stock fork. Not to make it stronger, but to make it weaker...and more ductile...

100k psi (semi-educated guess) of the original forging is tough as hell. I can't see even the KEP 2 needing that much force. That's why the forks don't break on the first application of the pedal. It takes years for it to snap, because it becomes more brittle from using the clutch (Work hardening). If we could reduce the specific strength to 80-90k psi, and make it more ductile, it might better withstand the work hardening, and last for a good long time.

Just a thought...

Oddjob

Probably worth looking at multiple broken forks to determine the common
failure mode. Is it brittle fracture or fatigue? I would guess its
fatigue.

If you were to mention something like 17-4, 15-5, or previously mentioned
2205 Stainless, then it would be worth comparing to a good carbon steel (although
the cost and machineability of those SS alloys makes them impractical for
a clutch fork).

304 is about the weakest SS available, so I would recommend that you do
some more research on engineered material properties.

Don't compare UTS numbers of the materials. Not really what we care about
for a cyclic loaded part. Single test sample pulled until it breaks,
which is long past the yield point (plastic deformation).

Certainly do want to compare yield strength, shear strength and
stress/strain curves. The cyclic bending moment is what causes the forks
to break.

304 has about half the yield strength of a good carbon steel. So for a SS
part with high UTS and low yield, it will just bend under load and and
hold the bent shape.

And the ductility of 304 is too high for good wear surface resistance.
The tips of the fork in contact with the throw-out bearing would gall,
probably terribly bad.

There is a reason you don't see head studs and spindles made out of 304.
I doubt you will find very many ME's this side of China that think 304 is
good for high stress applications. Its structurally crap material.

m73m95

That's absolutely right. I used 304 in my examples because it is the most commonly known stainless alloy. Its easiest to find information on, and it shows the differences between stainless properties and steel properties to illustrate my points. The "crap" stainless alloy is comparable to a "great" steel alloy in strength, but you're right. It is too soft to not only be used in a working application, but also in a high wear application.

The fork are indeed breaking due to fatigue from years of work (I thought I said that). The initial strength of the fork is not a problem. After years of working pressure, they become hardened, and break. Stainless alloys in general, usually have less tendency to become work hardened, than steels do. That is the reason I think stainless would be the "ideal" material to make a fork out of.

Bri Bro

So, would it be in order to heat treat a used fork before putting it back on or is the damage already done?

m73m95

I don't see any reason to even worry about the clutch fork, unless you are using a very stiff pressure plate.

If you are using one of those PPs, then I think heat treating is a good step. However, don't heat treat it to make it stronger. Heat treat it to make it weaker, and more ductile. It won't get work hardened as quickly, and depending on how often you drive you car (Track cars), it might never break.

samluke

I say leave it to Lindsey to figure out. Mike has the resources and potential sales volumes to make the right decision on materials and specs. I am sure it will be better than stock.

I just bought a spare, just in case I need one.

333pg333

Pretty sure Markus B advertises his modified stock forks as also being heat treated.