1) How is it that the sandblasting cracks the wheels? I'm admittedly ignorant on the finer points of metallurgy but that sounds counter intuitive.

2) I am on the verge of buying a set of used 1st gen 6ULs (15x9) that have been raced with r-comps for a good while. How worried should I be that they will crack? If they are cracked, will it be visible to the naked eye?

3) How is one supposed to change the color of powdercoated wheels if you can't sandblast them?

Thanks.

That looks pretty fucking bad. Now my butthole is getting all puckered at the thought of buying used wheels.

1. Sandblasting or sanding cuts the surface of the metal leaving sharp edges on a microscopic level. These sharp edges act like stress risers and can become the starting point for cracks. It is similar to drilling holes in your brake disks . . . great way to start cracks.

What is interesting is that a similar process, shot peening, has the opposite effect. The difference is the media. Sandblasting cuts while the steel shot used for shot peening "dents." The "denting" hardens the surface. A lot of internal items in the MSM are basic NB parts that have been shot peened. Go figure!

2. I'd be worried. Defer to Emilio's write-up on how likely/visible it is. Based upon this thread, it looks like it would be pretty visible. BTW, what's your life worth?

3. Dunno. I personally buy new wheels in the color I want. I like Silver which makes things really easy.

6uls have never been powder coated. They've always been painted as far as I know.

Thanks for the info. I'm not necessarily looking to paint the used 6ULs I'm planning on buying. The reason I'm buying used is because it seems like every time I'm ready to buy new, I can't find the ones I want. Also, the vendors who sell these wheels don't exactly have a "fall all over themselves to help customers" kind of attitude.

So if sandblasting causes the aforementioned problems, how can they hold up to any type of curbing or catching a rock, etc.?

I have faith that you guys have accurate info (read: I'm not saying you're wrong) but I just can't wrap my mind around the idea that you can have a wheel that is perfectly fine, hit it with some sand, and then it cracks like that.

I'll let Emilio speak up with the proper terminology, as it's been a few years since I last heat treated something. But basically it has to do with the sand blasting rearranging the particles of the metal, and the fact that it covers the entire surface, vs. a single rock or two.

A search on the interwebz didn't turn anything up. I'd love to know more about it from Emilio, but I strongly suspect that his explanation will be as painfully concise as all of his other responses. :-)

Here are some facts for you on cracked/replaced wheels, Mr.E.G.:

Read mattyj's post:
NWP4LIFE: ***949Racing Wheel Thread***

Basically has to do with how "sharp" the cut is. In fact, common practice to stop a crack in sheet aluminum is to drill a 1/8" hole at the end of the crack. That's enough to spread the load and stop the crack.

I would think if you caught a hard/sharp enough rock with a spoke, it could create a stress riser that could eventually lead to a crack.

Curbing usually nails the outer edge of the wheel rim. This part is not as highly stressed as the spokes or where the spokes connect to the rim.

Incidentally, another type of cracking that affects aluminum in particular is fatigue. Fatigue is all about cycles. A spinning sheel negotiating a curve is an ideal stress cycler (stress one way when the spoke is near the ground, the other way when the spoke is at the top of the fender, etc.). Fatigue cracks happen even to wheels that are otherwise completely undamaged.

yeah, you have probably done this many times with a paperclip, bending it one way then the other untill the metal work hardens and becomes brittle and cracks and breaks in two

Fatigue in aluminum is different than this. Steel doesn't fatigue.

What you are doing with the steel paperclip is cycle stressing beyond the elastic deformation range into the plastic deformation range (fancy way of saying bending, LOL).

With aluminum, even if you stay well within the elastic deformation range (i.e., put stress on it but far below the stress that causes bending), given enough cycles . . . POP . . . failure. Look up the DeHavilland Comet on Wikipedia. That will show you some really catastrophic aluminum fatigue failures.

Steel will fatigue if you stress it too much. What you can do with steel is design the part such that it doesnt seen enough stress to fatigue fail, ever. Where as with an aluminum part it will fatigue fail no matter how much you over build it, though it might just take a really really long time.

I see what you're saying, but it's not technically correct. Still does fatigue, but there is a point in the design size of steel where the fatigue like becomes infinite. Aluminum can approach that, but can never actually reach it.

I doubt that makes sense, but hopefully it does.

Edit: leafy said it better.

Agree. Leafy said it better. What I really wanted to get across was that AL has this unique property and we need to be cautious with AL track wheels that have a lot of time on them.

Okay, we all stop posting, send Leafy our comments and let him re-word them to be more clear and concise. Agreed?

lol, yall.

Partially related:

What were the offsets available on the first-gen 6ULs? I seem to recall two options, but I don't remember what they were.

+40 and +36, IIRC.