932 series rim weights

[warsch]

What are the 17 inch standard rim weights for 932 series?

I have a 156 sport pack with multispoke 17 inch rims. And as I'm planning on taking it to some track days I'm thinking of putting some lighter rims on it. OZ super and ultraleggera are a bit pricey but I've heard that 17 Ti rims are few kilos lighter than other rims. How true that is?

As per attached picture, how lighter would the ones on the left be compared to the ones on the right?

I'm also curios about weights of other 17 inch designs as well as other sizes.

[Dna Dave]

Cant answer the question on weight, BUT, especially for track use, i would recommend not using the Ti rims, they are very weak and buckle easily, i have used the ultraleggera on a track car and never had an issue.

Good luck

David

[warsch]

Tracks around Brisbane seem to be rather flat so I didn't think I need to worry about buckles.

How much do 17 inch ultraleggeras cost?

[Dna Dave]

Maybe we hit the kerb too hard down here  😜

[warsch]

Or maybe not given how slow I am. But either way, as far as I can see, one OZ rim costs about the same as a set of used Ti wheels. So a set of OZ will exceed my budget, unfortunately. I will definitely consider Oz rims in the future.

[johnl]

It's not about a bumpy vs smooth ("flat") track surface. If you're using slicks, or even 'R spec' road tyres, then it is at least possible to generate enough loading to deform a particularly weak wheel (i.e. bend, warp, buckle, twist, etc), without ever hitting an actual bump. I've seen photos that would frighten you, but they were all aftermarket wheels, not OE.

I don't know how weak the Ti wheels really are, I hope not all that bad because my car has them... They do look a bit 'spidery', but mine have lasted OK on less then great rural roads without damage (*still not warped when spun).

For a track day toy I think you are wasting your time trying to find the lightest wheels you can, and would be better off trying to find the stiffest wheels you can. Any weight difference between wheel X and wheel Y is not going to be much (best guess maybe 2kg or less), and certainly a very small fraction of the totality of the unsprung weight (i.e. 100% mass of tyre, wheel, brake, hub, suspension upright, damper body, about 50% driveshaft, wishbones, spring, ARB, etc.). I would be quite surprised if you can save enough weight in the wheel to make a significant (if any) difference to the lap time, or to how the car feels to drive.

Regards,
John.

* When I wheel aligned my 147 for the first time I rotated each wheel against a pointer held near the rim edge (checking for straightness). This showed warping at each rim edge of about 0.5 to 0.75mm off true. I needed to use the rim edges as the measurement points for alignment, so machined the rim edges by slowly rotating them against a sharp chisel blade (held in a jig) until each rim edge was straight. Since then I've checked the wheel runout a couple of times (when preparing for alignment), and the rim edges are still true (despite bumpy dirt and tar local roads).

Not all OE wheels are as robust as we might want them to be. The wheels on our Saab 95 were also a little bit 'warped' when we got the car, but worse still a couple of them cracked (warning symptom being slow leaks). I had these 'professionally' repaired, only to have one re-crack at the repair. By chance I found a set of near new Holden Vectra wheels at a local wrecker for a very good price, and these bolted straight on (Vectra basically being Saab 95). These are still going strong and as an added bonus look nicer than the kosher Saab wheels...

[warsch]

Fair enough but there's even less information about how stiff the wheels are. Thus I can't even know how to begin looking for stiff wheels. My guess is forged alloy wheel would be good. And I will buy them eventually.

This year I will be taking part in street classes only, so no semi-slicks for me, only road legal tires which means less stress on the wheels. And either way I need a set of rims so why not save a few kilos.

[johnl]

My understanding is as follows:

I would expect the stiffer aluminium wheels to be those that have the most 'meat' nearer the wheel centre, whether forged or cast. That is, the greater the number of spokes (assuming spokes of roughly the same thickness) and / or the thicker the spokes, the stiffer the wheel is likely to be. Wheels with fewer but very thick spokes may well be stiffer than wheels with more but thinner spokes, or maybe not, depending. The loadings increase toward the wheel hub, so this is where you want to see more metal, i.e. more metal the closer to the hub, and where the spokes join the hub.

For 147 / 156 stock alloy wheels, I think the ones with fewer but thicker spokes at least look as if they are more likley to be stiffer than the ones with more but thinner spokes. This is merely based upon how they appear visually to my eye, no data involved. My 147 has the thin spoked Ti wheels, if I had the option I would have chosen the thicker spoked variety.

Forged wheels are likely to be 'tougher' than cast wheels (all else being equal), but not necessarily stiffer. That is, forged aluminium will most probably have a higher elastic limit / yield strength than cast aluminium, so a forged wheel will most likely deflect further before it either permanently deforms (bends, warps etc), or cracks. Forged should also deflect to or near to the elastic limit more times before starting to develop metal fatigue. Beneath the elastic limit, the stiffness difference between forged and cast wheels (of the same physical dimensions) is probably minimal to non existant, since 'Youngs Modulus' is much the same for forged or cast aluminium, and regardless of alloy or heat treatment.

Having said all this, forged aluminium is superior to cast aluminium for wheels, due to greater fatigue resistance and ultimate strength / toughness. So, the choice is easy, except that forged wheels are also much rarer and more expensive. I have no idea which stock AR wheels are forged and which are cast, but I would expect many more to be cast than are forged...(?).

Regardless of yield strength, IMO you want a stiffer wheel for handling and steering response / precision because the stiffer wheel simply deflects less for X loading (i.e. greater camber, toe and steering input fidelity, and faster lateral load transfers). Two aluminium wheels of X stiffness and similar physical dimensions will weigh more or less the same.

Very few aluminium wheels will be as tough as steel wheels, but steel wheels will generally be significantly less stiff than aluminium wheels. Despite common assumption, there is generally little weight difference between steel and alloy road car wheels because a lot more aluminium than steel is needed to make the wheel tough enough.

It would be entirely possible to make even a forged wheel that was inadequately stiff and not strong enough, if the designer got carried away with reducing weight, or wasn't good enough at maths, or made some bad assumptions...

Regards,
John.