RonCT

We had our CVR PCA meeting last night and the Northeast regional BBS representative gave a presentation. Of particular note was the damage that powder coating does to a wheel if it's done outside of the initial manufacturing process. In other words, if adding clear coat, anodizing, or even powder coating is factored in when the wheel is made such that the chemistry is planned out, then it's not a problem. But people that take delivery of a wheel and then send it out for powder coating are going to see a 30% degradation in the structural integrity of the wheel. The way I understood what he said - it was like heat treating something in the manufacture and then "undoing" it by baking on the powder coat. I didn't quite understand how that differs from heat cycling the wheel on the track - I just don't know much about the powder coating process.

Anyway - I had never before heard of negatives of powder coating metal and even wonder if this applies to things like roll bars. Just a heads-up to all...

P.S. He rolled his eyes and said "Honestly, I don't know why anyone wants black wheels in the first place - if it's to make the car look racy, it isn't working because all race cars have silver / aluminum colored wheels..."

Geoffrey

I know someone who had an original 17" magnesium cup 1 wheel set powder coated and they cracked from the heat.

Willard Bridgham 3

Paint any surface and it serves as insulation and cuts down on heat transfer....

Local hot spots are not good for heat-treated alloys.

Gary R.

Bah.
Black (sprayed on enamel) will look HOT with my Signal Orange 930!

TheOtherEric

It's been a while since my undergrad heat transfer class, but I think it's safe to say that's not always so. Flat black paint, for example, can increase radiative heat loss that more than offsets the amount of added "insulation." But this is all basically moot for what we're talking about anyway.

Mark in Baltimore

I love sweeping generalizations...

doc2s

maybe it was cooled down too quickly after baking ad hence thermal fracture.

Darren

I think magnesium is a special case, the real question is about what the effects are to aluminum wheels. Wheels are powder coated not just for people wanting cool black wheels -- autobody shops also very frequently send out wheels for street cars to be powder coated. The insurance companies force them to do this rather than to replace the wheels in many cases.

The powder coating process involves heating the piece to about 400 degrees, and keeping it at that temperature for 20 minutes or so. I'd like to understand why that affects the integrety of the wheel. I'm no engineer (I'm sure some are lurking about) -- wouldn't the process of heat treating the wheel from the factory be at a much higher temperature? If a local hotspot is an issue, why isn't this an issue with the center of the wheels while on the track? How hot do wheel centers get when they are attached to smoking hot rotors?

As a cynics view -- maybe BBS just wants to sell more wheels

TheOtherEric

+1. 400F doesn't seem all that hot. Sounds a little fishy.

Any Materials experts in da house?

JayP

I kinda get that- metals have a crystalline structure when treated properly are strong. The structure can/will change when heated under powder coating after the initial treatment.

Now heavy braking can get very high temps, but it's localized to the thick hub and for the most part the wheel is getting cooled. Powder coating to 400deg, that's thru and thru. The thinner spokes will heat and cool faster.

In my early DE days, I'd seen several OEM wheels fail on track, all powder coated. Never seen a non-coated wheel fail. Ever.

Whether or not there is a way to powercoat wheels and maintain the original metallic structure, I don't know. I'll stick to ugly wheels.

I'll qualify my material expertise- I made a "B" in the class but that was 15 years ago.

kurt M

Google Al heat treating. I am all geek for this kind of stuff. You will find that there is a lot that goes into it. Al wheels are made for all kinds of Al alloys and blanket statments about running an alloy wheel to 400 deg can't be made but.... there is a LOT that goes into making strong Al and some can be changed at 400 deg for 20 min and the cool down is also part of the formula. Powder coaters cool the parts very slowly. Metal part makers quench them quickly. Often times the powder coaters will burn off the old coat adn they do so at temps far higher than 400 to do so. They might get into the orginal treatment temp range doing this and if so they will profoundly change the propertys of the metal for the worse.

Again you can't make a blanket statment that 400 for 20 min will alter the metal. It can in some cases and it won't do jack in others. You have to know the metal used and the process that was used to form and treat the part. Anything less is a crap shoot.
It's solvent paint for my rims thanks. No chance of messing with the structure.

}{arlequin

i think the problem starts w/ taking advice from a grown man that rolls his eyes... other than that, i go w/ krylon semi-gloss. helluva cheaper than powdercoating and it doesn't affect jack as far as wheel integrity. does it chip and not look as pretty as p/c'd wheels? (now it's ME rolling my eyes...) it's a track wheel fool! who cares how pretty it looks...and the beauty of krylon is that you just rinse-and-repeat...

500

I work in product development for a constrution equipment company that makes shoring and forming equipment predominantly made from stuctural wrought aluminum and (to a limited degree) cast aluminum.

First, park everything you know about steel heat treating when it comes to aluminum. As mentioned above, the actual alloy of aluminum will dictate the heat treatment regime. For example, the 6000-series magnesium/silicon alloys are normally strengthened by a quenching operation (from around 980 degrees F) and then baking for eight hours typically at 350 degrees F. Basically first heating and queching phase created a supersaturation of magnesium and silicon in the crystal structure. The post-heating phase is very important as it then causes Mg(2)Si to percipitate and form coherent clusters which strengthen the material by creating obstructions to the movement of dislocations. One of the important factors in this regime is the final bake time. If the aluminum is baked too long, then the soluble elements will over-perciptate and create fewer, larger clusters, which have a reduced effect on strengthening the material. So, baking this alloy at 400 degrees F can definitely have an effect on the properties. Cast alloys are heat treated using similar principles and processes. The BBS guy was probably not wrong.

Having said all this, there are two important caveats:

1) I don't know what alloys BBS uses in wheels, so the above example is not necessarily representative.
2) The effects may be relatively small for a street driven car and not an issue there, but only under the stresses of DE/racing.

Larry Herman

I've seen plenty of untouched wheels fail. Powder coating may hasten the failure on some, but it is not necessarily the deathnell for all wheels. That being said, I would not do it to my wheels, but the P.O. did on some of them.

bleucamaro

This topic has been covered here before, please read: https://rennlist.com/forums/924-931-944-951-968-forum/262853-powder-coating-wheels.html