[QUOTE=Neutered Sputniks,Nov 25 2009, 05:11 AM]Triple H in Upstate NY has powdercoated his wheels many times - and he's a track junkie.

ETA:

 

There's a bit of a difference.

You're implying it's inevitable my wheels will break or I'll have crankwalk.

ETA: You're also comparing a majority to a minority (majority of smokers have/will have bad health, minority of PC'd wheels have failed, minority of ACT clutches have been on cars with crankwalk).

 

I have heard/read about PCed rims failing. I just want to relate my experience racing a PCed wheel, Axis Maglite (four spoke) in 15x7 size.

These wheels are cheap, light and strong. I have had them PCed in Chevy orange and use them on my black SE-R.

I have had zero problems with them. I have even hit a tire wall at BW on the right side of the car with the rear wheel absorbing some of the impact. Nothing happened to the wheel. Nothing.

I just monuted a set of NT01 tires on these wheels and only one required balancing weights. Still very round and very strong.

I have another set of Axis Maglites that I have PCed, but I have yet to put these to the test.

This is just my experience. I am not saying that PCing will not weaken the wheel. Just sharing after 10 years of hotlapping/racing.

 

Who cares!!! You guys can do what ever you want with your wheels.


This is the way I see it.

First of all we are talking about track wheels. Who the hell cares what they look like. They are gonna look like crap after a while anyways. Secondly, its totally plausible that aftermarket PCing a wheel (despite the process) can lead to these problems (I think we can agree on that at the very least). In a sport where we buy fire suites, head and neck restraints, roll cages and many other safety features to preserve our life and those of others that you are sharing the track with, why would anyone knowingly create a possible safety issue?

I never use questionable parts, processes, or exceed manufactures recommendations. Period.




Now back to the actual subject. I still want to know if this wheel was powder coated

-Rob

 

He simply constructed a sentence with the same logical structure as yours, demonstrating that you can't prove a universal negative statement with a handful of negative examples.

It has been established that there is a mechanism by which powder coating can weaken the aluminum of the wheel, which depends on the specifics of the powder coating process and the particular alloy and heat treatment used in the wheel.

Until someone does some physical failure analysis on this particular case, no one can say for certain what caused the failure. People can say "I bet it's this", and they have every right to do so.

 

[QUOTE=Orthonormal,Nov 26 2009, 08:23 AM] He simply constructed a sentence with the same logical structure as yours, demonstrating that you can't prove a universal negative statement with a handful of negative examples.

It has been established that there is a mechanism by which powder coating can weaken the aluminum of the wheel, which depends on the specifics of the powder coating process and the particular alloy and heat treatment used in the wheel.

Until someone does some physical failure analysis on this particular case, no one can say for certain what caused the failure.

 

/\/\
Agreed.

PC may degrade properties--I haven't been involved enough in metallurgy to comment. But that's not the only issue.

I'm guessing without any proof that this is a fatigue failure, so here's a bit more info on that.

Asura had a good point on the process--forged vs. cast. Forged wheels have compressive residual surface stresses. Cast wheels have a porous surface structure since you can't get rid of all the bubbles. Sanding/etc helps the surface, but you are always stuck with them inside the spoke. Those bubbles / pits / voids are great sites for crack initiation. So the fatigue resistance is much lower for cast wheels than forged wheels, ASSUMING THE SAME ALLOY. Look through MIL-HDBK-5J (www.everyspec.com has it free) and you should see trends. Also, two big drivers: peak stress level and the degree of load reversal (large tension --> ~0 load --> large tension as a cycle vs. large tension --> large compression --> large tension).

I tried to verify the spec's contents before posting, but everybody just wants to sell me the spec. If anybody has access and could check that, SAE J1095 is the US standard for evaluating fatigue performance of rims. Let us know what you find. I have no idea what the comparable standard would be in Japan.

So the point: According to my fatigue professor (I'm working on my MSME, and Dr. Hoeppner has written hundreds of papers on the topic--google "hoeppner fatigue"), the root problem is that the SAE fatigue test has NO LATERAL LOADING as part of their standard test procedure. Load wheel, spin wheel many times under load, wheel fails. There are no sideward loads. That probably works great for semis and ok for Camrys, but not so great for road racing. As anybody would guess, when your wheels are loaded differently from the tested conditions, your fatigue tests are invalid.

 

Your logic is flawed, sir.

His sentence compares denial of a medically/scientifically proven, guaranteed eventual outcome to my statement.

The overwhelming majority of smokers will have related health issues. The overwhelming minority of PC'd wheels will break/the overwhelming minority of ACT clutches will be on cars that develop crankwalk.

He was stating, implicitly, that PC'ing wheels has a proven, guaranteed eventual outcome - and that's just not the case with the majority of PC'd wheels. We haven't even proven that the PC process is what is responsible for the failures.

Correlation =/= Causation.

Right now, all we have is correlation. We don't know the specifics of the processes used to prep/cure any of the wheels that have cracked under the extreme stresses of track duty compared to those that haven't cracked. Until we know these specifics and can sift through and analyze that data, we are merely speculating as to the cause of the wheel break.

Hell, in the OP, there was only 1 out of 4 wheels that cracked. Not that I'm implying or stating that since it was only 1 out of the 4, it's ok. If all 4 went through the same process, maybe the damaged wheel had inherent weaknesses from the manufacturing process. We simply don't know. And wild speculation helps nobody.

 

Virtually all wheels marketed as race wheels in the US use either Japanese VIA standard or sometimes the more stringent TUV German standard. If you don't see a VIA stamp from an Asian wheel, know that its likely built to the weak SAE standard.

Test values are determined by the diameter and PCD.

VIA has, among other things:
Radial fatigue. This is max load the wheel is rated for, with a tire inflated to max psi then run at very high speed. In the case of say, the 17" 6ULR with 5x114.3 pcd, that's 690kg (1518lbs), at 44psi tire inflation and 300km/h (186mph).

Radial impact. Same tire and load but they drop it on a mandrel from a specified height

Cornering fatigue. You guessed it, max load then cornering load on a drum fixture for a specified time.

I don't have the info on my laptop here at home but I think the test parameters can be found online somewhere. TUV OTOH, is quite a bit more stringent than even VIA. TUV wheels end up being quite heavy, no matter what you do.

So yes, SAE criteria is worthless.

 

This chart must initiate with yield strength prior to age-hardening, not in T6 state (which is the state that as-delivered forged wheels would be in).
Yield strength for 6061-T6 is ~220-240 MPa, well above the 147MPa that these curves initiate at.
Normal procedure is age hardening at 350F (177C) for 8 hours to produce T6 properties, looks like this chart is illustrating different methods for age-hardening, perhaps? Anyway, it doesn't look like it represents what happens to material that's already in the T6 state when further exposed to highish temperatures, though it would seem to imply that temperatures below 350F for times of less than many hours shouldn't be a problem based on the slope of the 170C curve at 8 hours.

It would be good to see a similar chart with the starting point being 6061-T6, not 6061 prior to age-hardening which is apparently what this chart is showing.

Please note that I am NOT saying that powdercoating of wheels is always bad, or that there aren't other mechanisms that might initiate cracking. Just saying that elevated temps can and will affect the strength properties of heat-treated wheels. cast A356-T6 more so than forged 6061-T6.