HotMess

I wasn't able to see what forgestar makes their wheels from on their site, but it appears that they're all flow-formed wheels. Presumably, they're A356 or something similar. The interesting thing about the flow-forming technique is that it will have essentially no effect on the strength of the wheel in areas where they don't deform it (the hubs/spokes/face of the wheel all appear to be initially cast). the strengthened area will be limited to the barrel. This is dramatically different to machining wheels from forged blanks (as is done with the higher end HRE wheels), where the whole wheel will be relatively homogeneously strong.

From what i can tell in literature, heat effects on A356 alloys isn't actually as big a deal on fatigue performance as you'd expect. The 'runout' stress (stress at which the material fails at 10,000,000 loading/unloading cycles) is essentially the same in the as-cast and the heat treated condition. This alloy IS heat treatable, but fatigue life is really hurt by other things like casting defects.

yessamgerg

as-cast and heat treated =/= strength AT an elevated tempature

Just because the material does not have a large fatigue life delta between heat treated and annealed condition does not mean that its fatigue life at the elevated temperature is not much lower than at room temp. The issue at play here IMO is that the very hot rotors are heating the material past it's usuable limit. That also explains why many heavier cars have used F14 successfully as they have much larger, cooler brakes.

HotMess

That's true, although I wouldn't expect the delta to change that much. That's beyond my area of expertise for now though. From what i read, it was just the rear brakes that run really hot, whereas the fronts run cooler (I don't know never tracked my car). The fact that a front wheel failed made it seemed like it wasn't an elevated operating temperature issue.

Edit: nevermind. Brain fart. If the damage was done when it was on the rear, then swapping the wheel to the front could cover up a rear brake temp issue. So, who's going to measure the rotor/wheel temps for us on a hot stock rear setup?

JJ7

Seems like there are more than a few s2k track cars with similar set ups driving under similar conditions on these wheels now. So does this mean the expectation of these wheels is less than 15 track days? Could it be a casting defect even thoug all 4 wheels showed cracks? I can't think of any situation that would lead to me buying these. I never even bent my RPF1s and they had about 30 track days. I understand wheels can fail but this seems beyond reason. If I'm wrong help me understand why...

EDIT: IF CHRIS' SUSPENSION WAS STILL BOTTOMING OUT THAN ITS REASONABLE THAT ANY WHEEL WOULD FAIL.

HotMess

A casting defect isn't really one single flaw, but rather the sum of a bunch of things that are inherent to casting. micropores, segregation, entrapped oxides, etc... Presuming the design of his wheel was flawless, what could have happened was that this particular heat of aluminum was not to spec chemically, or procedurally something went wrong. Given that RPF1's are made in (from what i can tell) virtually the same process, and that forgestar probably outsourced the casting to a company with the knowhow, i'd doubt this is the issue.

yessamgerg

From the below paper, looks like there is a knee point at 200 degC (392 degF) where A356 looses a lot of it's strength. This paper only has yield/ultimate strength but fatigue will follow a similar trend. 400 degF seems pretty hot to me to be able to due to brake heating. I suppose between the tires @ 175-200 heating up the outer rim and the brakes heating up the hub it is feasible though. I've seen stock rear rotors around 1000 degF so I wouldn't be completely shocked if this was due to heat. the fronts actually could have more ability to heat up a wheel as even though they are generally a bit cooler they have 2-3x more energy going into them. Either way this is a great argument for running a two piece rear (and front) rotor.

A356 Material Properties

s2000ellier

Any word from the manufacturer regarding the failure?

HotMess

If heating the wheel through the brake/hub was the problem, it might be cheaper to put some sort of thermal barrier between them. A stainless steel spacer or something like that.

takchi

Just a quick opinion. I suspect it's a design of the wheel, quality of the material, quality of the casting process, and possibly how it was powdercoated (prepping is important).

On a more controversial note, maybe thepoi is so fast that he's putting too much stress for these wheels to handle.

duffman13

Or even better, a 1-2mm titanium spacer so it doesn't transfer heat as well.