DefSport

Do you think the people who are aftermarket powder coating their wheels are going to perform a statistical study and control all variables? That's a research project, not engineering, so rarely do you have that luxury.

As for my credibility, I'm a mechanical engineer who deals in this area enough to put two and two together when you see people developing cracks in their wheels at what I'd call "lower than normal number of loading cycles."


Keep in mind that *ALL* aluminum wheels on track are living on borrowed time, it's only a matter of time before they develop fatigue cracks, or bend/wobble due to plastic deformation. For some wheels it might be around 30-40 days, others might have such a robust (and heavy) design that they can last 100,000 track days.

Fatigue is hard for most people to conceptualize, because it takes a relatively large number of cycles (1,000,000+ typically in a decent design that has a margin to yield) before a failure is seen. Some people also want to think that crack is the result of a large single load that went into the wheel, but that's usually fairly obvious, and causes a lot more damage than just a single spoke or two starting to form cracks at an edge and propagating through.


And again, keep in mind I'm not saying that it's impossible to powdercoat wheels. You totally can... if you go back through the heat treatment process, which for 6061-T6 (similar to alloys used in cast or forged wheels), you're looking at a controlled soak at a temperature (the aging) AFTER solution heat treatment. Aluminum Workshop: Achieving T6 designation for 6061 - The Fabricator

If you just put a fully heat treated part in an oven above 350 F, you'll start affecting the temper to some degree. For instance, one area that I know this is a common problem is on fighter jets. All aluminum pieces (which are heat treated for their higher strength/weight) are forbidden to go above 350 F due to this exact problem. Anything with service temps regularly over 300 F (to give margin) is made from titanium as a result.Your S-N curve is affected (shifted down), so you can take less cycles at the same stress range of loading. To put in it broad terms, instead of your wheels lasting say, 60 track days, now maybe they only last 20-30.

TL;DR - You get a different S-N curve after you affect the temper, and will see your lower cycle fatigue performance diminish, which reduces your allowable fatigue life.

Cloudcapp

Also as a mechanical engineer with specialty in fatigue and fracture mechanics, I agree with DefSport on reduction of capability. Whether you're going to fail your wheels or not will depend on number of cycles and severity of those cycles and how much margin there was to begin with.

rpg51

I take this to mean that there is some impact if you go over the indicated temps and I accept that. As far as the actual impact on failure rate in the real world - my take away is we have no clue without more information, most importantly, the margin built into the original design but also the number and severity of cycles that the wheels are subjected to.

DefSport

We do have a good idea of the impact of the reduction in fatigue life, it's significant (from the standpoint of this is a fatigue limited structure, and its fatigue capacity has been reduced through partial annealing).

What that means for the actual life of each individual part is up to its loading history - just as any factory wheel has a life dictated by its loading history.


Don't get so caught up in the exact number of cycles, that's a black art even with the best data, listen to what we're saying on the relative change in fatigue performance. That paints as accurate as a picture as you'll get for something like this - even if you had a hundred thousand dollars to instrument and test 50 wheels for a statistically significant measure. The error bars will be large enough that the general "it's significant, and the failure will still be fatigue driven" is good enough.

rpg51

Is the knowledge re impact on fatigue life of an alloy wheel based on experimentation? Are there studies? Published?

DefSport

Yes, it's based on lots of mechanical fatigue testing of aluminum alloys. Doesn't matter if they're a wheel or not (the testing is not "product or usage specific").

You're free to read up on fatigue strength reduction of different tempers of aluminum alloys, as there is tons of published data out there. The alloys used in wheels are close to 6061-T6, and if you over heat the aluminum it can drop down somewhere in the T4 to T0 range. Look at S-N curves for those, and read up on it if you really want to know more.

Schneck

This has been fun to read.

I recently had an alignment shop scratch all 4 of my wheels with their sensor. They agreed to repair them however I saw fit. They were RPF1s with soft paint, and I thought, cool let’s get them powder coated.

I went and talked to the “Wheel Doctor.” He said, after 20 years is repairing, that he doesn’t recommend powder coating for wheels.

Powder coating isn’t flexible in the same way paint is. Powder coating is ideal for a roll bar, subframe, etc, but nothing that moves.

PROPER wheel paint is actually more of an epoxy, and more like what Brembo uses to finish brake calipers.... Strong but flexible, can withstand some abuse.... like a male cheerleader.

For the “data” requested above, I found the same argument on the BMW M2 forum, and they’ve pretty much proven (to me anyway), that powder coating isn’t good for a forged structure.

https://f87.bimmerpost.com/forums/sh....php?t=1535015


For the SAME money, less risk, and the ability to touch up if necessary, who the hell is still powder coating wheels?

c32b

it seems to me that you are dead set on PC. I think people tell you about the failure potential simply because they are looking out for you but it's up to you.
Wheels need strength not only because they are tracked, retaining the original wheel may mean the difference between a failure that allows you to stop safely after a pothole or wheel impact rather than a catastrophic failure.
Wheel shops owners tell me that they see bent wheels all the time, both forged and flow formed, but PC may alter the metallurgy to the point that the wheel just cracks in a brittle way rather than bends.

It may be useful to look into the individual PC shop's process, inspect the PC wheel regularly (although I have no idea how you will able to see small cracks) and be sensitive to how the car feels.

YMMV