Cal Club Autocross Forums

CAUTION -

Darn near catastrophic fauilure happend to me this weekend of a Factory Honda wheel that was secondarily powder coated by the previous owner.

THANK YOU AARON GOLDSMITH for spotting this in grid and stopping me from going out again.

BTW, I realized later that I did just finish my last run for that session. Doubly lucky.

Pictures Link.... http://picasaweb.google.com/moriuntensh ... directlink" onclick="window.open(this.href);return false;

PHEW.....

Related engineering-esque thread, some think it will weaken with 400F temps...

http://www.eng-tips.com/viewthread.cfm? ... 053&page=7" onclick="window.open(this.href);return false;

I couldn't believe my eyes when I saw that, baaaaad news.

Forget AX, you're lucky you didn't have catastrophic failure on the freeway!

Does it matter if the wheel was cast or forged?

Factory wheels are typically forged ... so it's pretty amazing that they broke so relatively easily. Pete HAD done some R tire runs with those wheels also.

I had a SSR wheel from my WRX crack:

The Tire Rack Wheel Tech article on using wheels in track/competition events ... and wheel fatigue:

http://www.tirerack.com/wheels/tech/tec ... techid=110" onclick="window.open(this.href);return false;

Mako Koiwai wrote:Factory wheels are typically forged ... so it's pretty amazing that they broke so relatively easily. Pete HAD done some R tire runs with those wheels also.

I had a SSR wheel from my WRX crack:

Actually most OE wheels are low pressure or gravity cast.

Mako Koiwai wrote:The Tire Rack Wheel Tech article on using wheels in track/competition events ... and wheel fatigue:

http://www.tirerack.com/wheels/tech/tec ... techid=110" onclick="window.open(this.href);return false;

The May 2009 SportsCar also has some great wheel info... Including a tip from one manufacturer about not powder coating wheels after the fact. ;)

Jason Isley BS RX8 wrote:

Mako Koiwai wrote:Factory wheels are typically forged ... so it's pretty amazing that they broke so relatively easily. Pete HAD done some R tire runs with those wheels also.

I had a SSR wheel from my WRX crack:

Actually most OE wheels are low pressure or gravity cast.

Agreed, factory forged wheels would be pretty rare.

Aaron Goldsmith wrote: Agreed, factory forged wheels would be pretty rare.

Even a lot of the very expensive, and fast, cars use a cast wheel - although they typically step up to the spun cast versions.

I am not a fan of powdercoating wheels, but I have seen quite a few NON powdercoated wheels and even some forged wheels break the same way.

I must say, that one was very far gone.

Lucky.

I was thinking for more powerful sporty cars ... STI's, M3's etc. the cars that typically interest us ... forged wheels.

i've always heard forged are a big no no, but i'm starting to think anything other than steel isn't a good idea

Powder coating involves some heat. How much does that heat affect the wheel I do not know. What I do know is that the coating acts like an insulating barrier and when the brakes transfer heat to the wheel, dissipating all this energy gets even harder.

Chrome plating is even worse(Nissan has issued TSBs about it because of failures). The plating gets inside any casting voids and when the wheel gets hot, the plating expands at a different rate than the aluminum and ends up splitting the wheel in extreme cases.

Lastly, polishing a wheel makes it actually stronger as it removes sharp edges and reduces stress concentration. The OEM polished wheels on 1994 R Pkg Miatas were lighter than the non-polished regular wheels. They looked identical from the outside but they actually were a different casting with more material missing from the backside of the spokes. That was back when lightness and unsprung weight were kings.

GD

George Doganis wrote:Powder coating involves some heat. How much does that heat affect the wheel I do not know. What I do know is that the coating acts like an insulating barrier and when the brakes transfer heat to the wheel, dissipating all this energy gets even harder.

Chrome plating is even worse(Nissan has issued TSBs about it because of failures). The plating gets inside any casting voids and when the wheel gets hot, the plating expands at a different rate than the aluminum and ends up splitting the wheel in extreme cases.

Lastly, polishing a wheel makes it actually stronger as it removes sharp edges and reduces stress concentration. The OEM polished wheels on 1994 R Pkg Miatas were lighter than the non-polished regular wheels. They looked identical from the outside but they actually were a different casting with more material missing from the backside of the spokes. That was back when lightness and unsprung weight were kings.

GD

Yes the FD RX-7 of the same era had thin and thick spoke version wheels. Some may remember I broke one off during a run at the event at the Big "A". The wheel was an earlier thin spoke model that the original owner had chromed.

Rick

I'm going to geek out now...

If you look in MIL-HDBK5 "Metallic Materials and Elements for Aerospace Vehicle Structures", you can see what heat does to different Aluminum alloys yield strength over time.

For example, for common 6061-T6:


And for a 7075-T6:


For the 6061-T6, even a long 1 hour cure at 400F only drops yield strength ~10%. For the 7075-T6, you lose ~20% (find the intersection of 1.0 hours and the 400F exposure line, read up to the 80F test line). No data is given for tempered casting strength as a function of temperature exposure, but they do list data at room temperature for T7 condition castings which I would imagine would be affected by exposure to 400F in some way.

What happened to Petes wheels clearly a fatigue failure, perhaps exacerbated by a *slight* change in material properties due to the powder coat. It seems more likely to me that the wheels were a casting with shoddy QA and potential voids which allowed cracking to start... But the handbook doesn't have any information on fatigue life as a function of temperature exposure, so who knows. It's possible a "small" difference in yield strength of 10-20% might bring with it an order-of-magnitude difference in fatigue life, which is difficult to estimate in the first place short of doing tests on materials processed in exactly the same way. I'm not a metallurgist, I have materials people to answer the tough questions for me at work ;)

As you can see from the curves above, things can start to change very quickly if the temperature and/or exposure duration are increased. The curves may be different/more extreme for other alloys. Also, there may be effects related to very sudden cool down/heat up ramp rates that aren't captured in those charts. That's why it's important to have powder coating work done by someone who knows what will happen to the metal, or who has the ability (and the understanding of potential problems) to consult with someone who knows. If you're taking stress-critical Aluminum alloy parts to them and they brush the issue of changes in metallurgy off, it's probably better to find another place to do the work.

My vote is for get rims in the color you want, and save powder coating for parts where stress and fatigue life are not critical.

Agree with Andrew here.....the crack(s) probably began due to a void or some imperfection in the wheel....rather than the heat....or some other stress concentration (as per George). Sharp edges are a no-no....and notably these AP2 wheels have plenty of "sharp" edges which probably does not help.

Pete, you might want to check for any flaws along those cracks for interest's sake ... maybe even finish breaking off the wheel so you can see inside the castings/spokes/cracks.

However an interesting note about aluminium is that no matter how strong/big you make a part, it will eventually fail due to the properties of the metal. It does not exhibit infinite strength characteristics such as steel when subjected to infinite cyclic loading - no matter how small a load or how big you make the part.

Ever wonder about all those aluminium planes in the sky? I do.

Reijo

Chuck Fowler wrote:i've always heard forged are a big no no, but i'm starting to think anything other than steel isn't a good idea

For powder coating? It would seem that a forged wheel would be OK to powder coat because it's stronger; but it could be its heat dissipation properties say otherwise.

Chrome plating and powder coating add weight, which is reason enough for me to take a pass.

I've heard that powder coating masks cracks as well. Because it's so thick and flexible, cracks begin long before they break through the coating. I don't know if that's true, but it seems that a thinner coating (or a clear one, or none at all) would facilitate inspection.

Mako Koiwai wrote:I was thinking for more powerful sporty cars ... STI's, M3's etc. the cars that typically interest us ... forged wheels.

Check it out... You will be surprised by how rare it is in OE use. I expect you wont find it as an OE application on a modern car that is much under six figures. The spun and high pressure cast stuff is so good now they don't need to go to the expense of forged wheels. Even BBS uses cast wheels in some of its racing applications.

Reijo Silvennoinen wrote:Agree with Andrew here.....the crack(s) probably began due to a void or some imperfection in the wheel....rather than the heat....or some other stress concentration (as per George). Sharp edges are a no-no....and notably these AP2 wheels have plenty of "sharp" edges which probably does not help.

Pete, you might want to check for any flaws along those cracks for interest's sake ... maybe even finish breaking off the wheel so you can see inside the castings/spokes/cracks.

However an interesting note about aluminium is that no matter how strong/big you make a part, it will eventually fail due to the properties of the metal. It does not exhibit infinite strength characteristics such as steel when subjected to infinite cyclic loading - no matter how small a load or how big you make the part.

Ever wonder about all those aluminium planes in the sky? I do.

Reijo

It was 3 wheels, not one. Sounds like the math said it was time to fail.. though why...

You mean like a planes flying to hawaii?

Bob Beamesderfer wrote:

Chuck Fowler wrote:i've always heard forged are a big no no, but i'm starting to think anything other than steel isn't a good idea

For powder coating? It would seem that a forged wheel would be OK to powder coat because it's stronger; but it could be its heat dissipation properties say otherwise.

Chrome plating and powder coating add weight, which is reason enough for me to take a pass.

I don't think that's a reasonable conclusion at all - the combination of alloy and temper of a forged wheel may result in a base material with a higher yield strength, but if the person designing the wheel takes advantage of that strength to build a lighter wheel using less material the strength of the wheel itself may not be any higher then a heavier cast wheel....

And none of the references I can find easily talk about the effect of temperature exposure on fatigue life, which is important.

Andrew Bingham wrote: And none of the references I can find easily talk about the effect of temperature exposure on fatigue life, which is important.

you can find charts of yield strength vs temp and then apply that to fatigue vs yield strength in the MIL-HDBK.

long story short, aluminum becomes mush at elevated temperatures.

I've got my old "University Physics" textbook. I'm trying to find "mush" in the glossary.

Andrew Bingham wrote:

Bob Beamesderfer wrote: For powder coating? It would seem that a forged wheel would be OK to powder coat because it's stronger; but it could be its heat dissipation properties say otherwise.

I don't think that's a reasonable conclusion at all - the combination of alloy and temper of a forged wheel may result in a base material with a higher yield strength, but if the person designing the wheel takes advantage of that strength to build a lighter wheel using less material the strength of the wheel itself may not be any higher then a heavier cast wheel....

And none of the references I can find easily talk about the effect of temperature exposure on fatigue life, which is important.

I didn't draw any conclusion, only stated a possibility. Taking the "forged wheel/less material" discussion further, wouldn't less material to absorb/dissipate heat compromise resistance to fatigue?

Of course, Marshal's point is the overall issue with aluminum vs. heat.

i thought it just plain screwed up the grain structure the forging so nicely hammered into place