Having trouble finding anything besides discussions of the GRM test from my parents computer. Worst case I can scan/take pics of the article.. it was in GRM in June.

Ok I have the magazine in front of me. I'll have to take some high res pics then post links but here's some quick info and their test results. Test car is their NC Miata project car.

Wheels:
17x9 Enkei RPF1: 15.9lbs
17x9 TRMotorsports C3: 19.6lbs

Lapt time on tire rack test track (averaged):
RPF1: 53.65sec
C3: 53.56sec

Chassis Dyno:
RPF1: 141.72hp
C3: 142.61hp

They also tested different size Star Specs on the same wheel to see what difference a smaller tire would make (this one is pretty obvious though):
255/40/17: 53.65sec
245/40/17: 53.13sec

I'll take pics of the pages when I have energy. Turkey day food making me even lazier than normal.

As a sidebar they mention this isn't the first time they did this testing. Back in 2003 they did so with a NA Miata, the lighter wheel was SLIGHTLY faster, in that case the lighter wheel was 13.5lb K1s vs 22lb MSW Type 33s and the time difference was 2/10ths on their autocross course they used for testing.

I remember a lot of discussion on racing and autocross forums when this article came out and many people (including wheel manufacturers) commented on it. From their comments they basically focused on the strength of lightweight wheels not being enough to prevent twisting and flexing from the forces being placed on the wheel, where a heavier wheel had more strength and mass to deal with it. edit: one statement removed as it turned out to be an invalid point as discussed below.

I still run light wheels on my cars, they are easier to move around and deal with. I don't look forward to lifting and lining up a 45lb tire and wheel combo onto a car on a lift, or even dealing with one on jack stands. My race car uses stock wheels (cheap as hell for replacements) and it's noticable moving them around and putting them on the car, etc compared to my street wheels.

I'll also note that I personally haven't noticed a difference tracking and autocrossing with light forged vs heavy OEM cast wheels but I've always attributed that to my general lack of paying attention to minor details in competition.. I just drive the car and don't let the small stuff bother me.

In a straight line, surely not. Around a race track or autocross course... Maybe. It's not so much that heavier is faster, it's that wheel stiffness is just as important (if not more so) than wheel lightness. Everything bends/flexes/stretches under load, it's the way matter works. When that bending and flexing is happening in your wheels as you make your way through a turn at speed, the wheel bends in the direction of positive camber. That means that if a lightweight wheel has given up too much stiffness in exchange for weight it could very well lose more time in the corners due to compliance than it gains in the straights by being light.

Cast vs. Forged makes no difference to stiffness, only to yield strength and fatigue life. Stiffness is all about geometry and modulus of elasticity.

I'm no expert for sure but wouldn't the increased strength of the compressed alloy add to the stiffness? Stress-Strain diagrams I was just looking at show forged as significantly better, and that's based on (I believe) the elongation of the metal based on various loads, which to me at least means stiffness?

The stiffness of the material itself is only in the very first linear portion of a stress-strain diagram, and it's the slope of the line not how high up it goes that matters. The point where the line stops being linear is the point where your wheel has permanently bent, so all else being equal the forged wheel will take more abuse to ruin. Thing is, manufacturers of forged wheels often use that increased strength to make a wheel that is equally as abuse-resistant as a cast wheel but with less material (which means it's lighter, but probably less stiff).

Ok, so stiffness is the slope of the linear-elastic portion. Yield strength is the point when the line starts to curve and the material is no longer quite the same shape as it used to be. Ultimate tensile strength is the highest spot on the line and it is the point of no return, when the material will soon be in two pieces. Resilience is the area under the linear elastic portion of the chart, it's how much kinetic energy the material can store before it bends permanently. Toughness is the area under the entire line, it's how much kinetic energy the material can turn into heat as you tear the poor thing in half.

BUT the strength and stiffness of a part made from a certain material is much more dependent on the shape and size of that part than on the characteristics of the material it's made of. I could make a doodad from cast Chinesium that would be 100 times stronger/stiffer than a doodad made from forged Unobtanium. It would weigh a whole lot more, but it would be stronger.

Oh, and forging doesn't compress the metal, it forces the grain structure of the metal to align with the shape that the metal is being forced into. This doesn't technically make the part any stronger, but it minimizes the chances that a flaw in the material will cause a catastrophic failure under load. Casting on the other hand has to be done with alloys that are suited to casting, none of which are particularly strong, while forging can be done on cool stuff like 7075, 2024 or Ti6Al4V.

Thanks :thumbup:

Your explanation makes the graphs I was looking at make more sense :)

So my point about those wheels being cast was pointless. :thumbup: I'll edit it in case people don't read past it to this part.

One of the conclusion you can make from this thread is to use GOOD aftermarket wheels.
Stick with companies that use good quality steel and that have a competent R&D department. Basically any company that make wheels for high level motorsport is ok:
BBS, OZ, Volk, Speedline...

I spoke to one fo the guys at OZ about the differences between the Superleggera, the Ultraleggera and the Alleggerita, it's amazing how much work and testing go into these products.They even do very long fatigue testing..I don't know how many company can do this.
From the Superleggera to the ultra they reduced the weight but increased the strenght and the wheel is also more rigid. The Alleggerita is even better but it looks smaller so they're selling less of them, sadly.

Enkei makes very good products including a lot of oem wheels and they were slower in that testing than a cheap arse wheel manufacturer :)

What you can learn is probably to pick wheels with big thick spokes intend of a bunch of little ones as thick spokes would be harder to bend or twist I'd think. Old Greg may know.

I don't really believe in those laptimes. So many factor are involved in a laptime, the wheels are less important than for example how much the tyres are worn out or their temperature.

I judge wheels by (not necessarily in this order)
-Shape
-Weight
-Manifacturer (for reliability and availability in case I break one)

GRM accounts for that. They are averaged and they run A-B-A tests to cover variances in track temp/tire wear. When I get less lazy I'll snap pics of the article.

However if lighter wheels are truely THAT much better the difference would edge in the light wheels case anyways. 1/10th is really a worthless difference regardless but with the big focus people put on wheel weights I'm sure most believe it's more than that.. and in this case that 1/10th was in the wrong place anyways. Also manufacturers commented that this was not surprising at all and some of the honest ones explained the reasons. GRM used dataloggers so it wasn't a guy with a stop watch either. All of their drivers felt that the car was faster with the lighter wheels, but it wasn't.

Big +1 on the GRM testing procedures. They are serious about that stuff, being autocross focused and how close some autox wins/losses can be.

Since we are taking about unsprung weight, is it possible that the drivers were sensing a great responsiveness?

Just wondering.

It depends. A few big chunky spokes are likely to be stiffer than a bunch of little spindly ones, but the big chunky spokes don't support the wheel rim as well. So if the rim is thin and lightweight you may end up with more compliance anyway.

In general though, the depth of the spoke (from the outboard face to the inboard face) is much more important than the width. And the depth near the hub is more important than the depth at the rim. Hollow spokes are the best in terms of stiffness/weight if you can find them, but you can't do that with a forging, only with a casting. Oh, and three piece wheels are worse than single piece wheels, their only benefit is in having many available offsets/widths and being easier to repair (other than that, they suck).

Really, the best option is to just pick the wheels you like the most and not worry too much about the weight or stiffness. Because unless you're doing physical testing of several different alternatives all you can really do is guess. And unless you're racing competitively (ie. Solo Nationals, World Time Attack, SCCA Pro Racing, ALMS, etc) the difference in performance isn't worth worrying about.

AHHHH!!!!

My thread turned into a wheel thread while I was gone :mad0260:
























:scared0016:

LOL!!!!