yakmastermax

I am considering putting C5 magnesium wheels on my 1991. Found a solid deal locally for a set of six C5 Magnesium wheels for $600. Four wheels are the rear spec, so 18x9.5 ET65mm offset, and two wheels are the front spec, 17x8.5" ET60mm

Plan would be to sell the 17x8.5"s front wheels and just run a square setup with the four 18x9.5" wheels.

Factory GM 1991 wheel specs are 17x9.5" ET56mm front and rear, that means without spacers these C5 wheels would fit 9mm inboard relative to factory?

I have extended studs already. Could I run a 15mm or even 20mm spacer? Is that safe to do for a track car? Are there correct hubcentric spacers for this application, C5 wheels on a C4?

Are there good performance tires in the correct size for this setup? Bonus if they are lighter tires.

Even with 20mm spacers, I stand to lose around 10lbs per corner just on the wheels alone. I think the smaller tires would also shave another 5lbs or so.

What do yall think?

WVZR-1

Which of the C5 wheels were prone to 'fractures'? What branding are these wheels?

yakmastermax

Not sure about branding or cracks/fractures.

The wheels I am considering were offered OE GM as options for the C5. Italian made magnesium wheels. N73 option I believe.

More questions:

Are properly spec'd and fitted wheel spacers with extended studs safe for track use?

Can one knock the studs out of a wheel adapter to make a thicker spacer, studs lengthened and wheel adapter spec'd correctly.

Are there any good 18x9.5" or wider lightweight wheel options that are correct for our cars? 35mm to 55mm offset or so, 70.3mm center bore, 5x4.75" lug pattern?

Shooting for around 20lbs per wheel.

Thanks!

WVZR-1

Have you had an 'eyes on' confirmation of the wheels? I believe you take all of the casting information to the C5 group and ask the fracture/crack questions. Without an 'eyes on' most of what you mention might just be a WAG?

There are hub-centric spacers likely available for any thickness with appropriate center bore

QCVette

The C5 cracking was on the thin spokes. I have never heard of it on the magnesiums.

I am not sure you would get 10lbs per wheel reduction. The magnesiums weigh about 17 lbs front and about 20lbs rear. For comparison the C5 Z06 wheels are 19.5 lbs front and 21.4 lbs rear.

I had a set of the magnesiums and switched to the Z06 wheels. They are wider and only a lb or two heavier. Also note that the magnesiums are more difficult to maintain. Especially the finish since it chips easily.

I also question your choice of thickness for the spacer/adapters. If your stock wheels were 56mm offset and the new ones are 65mm offset there is only a 9mm difference. If you are wanting a 15mm or 20mm spacer then you would be moving the wheel outboard (which may be what you wanted).

Good luck.

FAUEE

I wouldn't run the mag wheels on track, nor would I run spacers. I also wouldn't waste money on an 18" square setup that wasn't either c5z rears, or c6 gs/z06 fronts. That way you can run some 305s, and tuck in c6z brakes.

I recall a few c5 racers having negative things to say about the mag wheels as a track wheel..I looked into this for my c4 at one point, and they recommended against it, and said to go with c5z rears. I think the mag wheels are easy to bend, and so damage from going off track, or from a tire shop.

VikingTrad3r

c6 z06 fronts? fit?

FAUEE

Never tried but as I recall I saw people running them on the c4. Looked goofy with our stock brakes being too small.

MatthewMiller

Yak, I can't comment at all on whether these OE mag wheels are safe for track use. I don't know. However, with longer studs to get adequate thread capture with the lug nuts, plain "pass-through" spacers are absolutely safe for track use. I always refer people to the Maximum Motorsports "Mustang Wheel Spacer Tech" writeup to understand the physics of what actually locates and holds the wheel on the hub. Once you understand that, you will know that all the warnings about stressing the studs and needing hubcentric wheels and spacers is bullshit. Assuming you're using the stock stud threads, then as long as you can capture six turns of threads and the spacers are really flat, then you will be safe with spacers.

As others noted, you really only need 9mm of spacing to get back to stock fitment. 10mm spacers will be obviously easier to find. You could certainly go with 1/2" (13mm). I wouldn't go further than that because you can start to add extra stress to the already-fragile wheel bearings and you start to change scrub radius and suspension geometry.

For 18x9.5 wheels, you can run 275/35/18 tires and there should be a good selection in that size. You could also run 245/40/18, which would be narrower but the same diameter (both are the factory 25.7" diameter). 255/35/18 would also work, being a bit shorter in diameter. That will look a bit odd but perform very well. 255/40/18 would also work, being just a touch taller than stock diameter. For track tires, the Continental EC Force is a great option and available in the first two sizes. The Goodyear Supercar 3 (not 3R) is also a killer track day tirethat is reasonably priced and lasts really long, and it's available in three of these four sizes as well.

yakmastermax

Oof dang I didn't think of the extra offset changing the bearing load and suspension and steering geometry... Really good point man.

That is reason enough to not go much thicker than 13mm, heard chef heard.

On my 85 though with factory offsets are around 38mm-45mm IIRC, then a thicker spacer would be warranted? The goal is to get the plane that bisects the wheel to be at the original brake rotor hat top face?

Thanks as always!

MatthewMiller

Yep, I think of it as "net offset." Whatever the wheel's built-in offset is, a spacer will subtract its thickness from it. And the goal is keep the wheel's centerline (the line that evenly bisects the wheel's rim width) wherever it was from the factory, which is not necessarily coincident to the hub face or rotor had face. IOW, keep the net offsets close to the stock wheel offsets. As long as you do that, you don't have to sweat the bearing loads and suspension geometry. So for those 65mm-offset mag wheels, using a 10mm spacer gives you a net offset of 55mm (only 1mm off the later C4's stock 56mm offset). And yes, for the 85, the factory 16x9.5 had 38mm offset*, so to match that you'd need 27mm for the C5 mags. A 1" spacer would get within 1.6mm, and that's good enough. I doubt there are any 27mm spacers out there.

*Corrected per WVZR-1's post below.

WVZR-1

Early 16 X 9.5 are 38mm offset

Do you actually have these wheels yet? Have you confirmed/measured for offset?

MatthewMiller

Dammit, I got them confused with the 16x8.5. Thanks for catching that! I will edit above so as not to spread misinformation.

70T/A400

I have had C5 Magnesium wheels, 18x9.5" with 65 mm offset on my F-Body for years with the use of a 1/2" adapter. Never had issues with wheel bearings or cracks.

They are very light wheels. I run 275/35/18 Michelins or Continentals. Car has almost 180K miles. Great wheel.

yakmastermax

Looking for C4 wheel stud recommendations that add 1" or so to the factory length. Thanks!


@MatthewMiller
I just read that tech article you linked about clamping force.

Does this mean that the hub centric ring fitment is more about just locating things properly to ensure flat flexless mounting occurs without installer error rather than actually bearing any load? Same with the tolerance of the holes in the spacer for the studs to pass through?

Am I understanding this correctly?

I ask because there are some nice AMT C5/C6 hubcentric spacers in 19mm and 25mm, but they are easily $550 per set of four...

On the other hand, I could get some cheaper ebay adapters in 20mm or 25mm, or spacers in 12mm, (for my 85 and 91 cars respectively) and knock the studs out of the adapters and run my longer studs through them, and spend less than 1/5th of going with the AMT spacers.

What do you think about that? My concern with that was the tolerances of the stud holes.

MatthewMiller

The standard go-to is ARP. My C4 had them, and I think they were 1" longer than stock. Zero issues.

Yes, exactly. Hubcentricity is a massively overblown thing. Car manufacturers use it because it makes assembly easier/faster. That's it. Millions of vehicles have been sold without hubcentric fitment at all. Our wheels are located only by the tapered lug nuts and seats. Then the clamping force created by the torqued lug nuts (literally thousands of pounds) holds the wheel in that location. Period. The hub pilot doesn't bear any loads...or at least it should. If it does, then the lug nuts aren't tight. And yeah, the spacer also doesn't locate anything radially along the hub face. The holds for the studs don't have to be very exact. The main thing is that the spacer is flat and has similar surface area as the hub. That's it. That's all that matters here. So spending $550 for a set of plain spacers is insanity.

If the holes that are left after you knock the studs out are big enough to all your hub's studs to pass through (they should be), then I guess it would work. That said, you'll still have less surface area because of the "wells" to bolt the adapters to the hub's studs. I would think a plain spacer that has only five holes for the hub studs would be preferable. I'm not totally sure of that, because in theory the friction force should be the same regardless of surface area if the coefficient of friction remains constant. Surely you can find 25mm plain spacers out there for a reasonable price?

yakmastermax

Yes exactly my concern after the "locating" "issue" (not an issue) was the "wells" left as you put it!

The $550 plain 25mm spacers from AMT have lightening cuts done to remove material and weight, but have a reduced surface area as a result, so perhaps as you and physics says (to first order) the surface area doesn't matter that much...


Okay ARPs thanks!

Super excited to get on some real tires on 18"s
Not to mention some cool Italian made magnesium alloy wheels..

As you said a while back, the Nittos have ended up being pretty terrible....

MatthewMiller

Or maybe it does and AMT has no clue what they're doing (entirely possible). I think lightening holes in a spacer are a bad idea, even if they don't reduce the friction. I'd rather have more material to resist "crush" and a more consistent surface...not to mention it costs them money to cut those pointless holes. Saving a couple ounces of unsprung mass isn't worth it when you are going to be using wheels and tires the combine for more than 40lbs.

Well, it's good to know they are consistent!

yakmastermax

Yeah my Nittos (both sets of 4, a set of 555s and a set of NT05) all have some weird vibrations that are probably flat spots since they don't balance out.


Anyway, ordered some ARP 3.25" UHL M12x1.5 wheel studs from Jegs. Set of 20 total for $165 shipped. They should do the trick. Part number 100-7713

Now I gotta try to snag some 25mm adapters to convert into spacers. That'll bring the 65mm offset C5 wheels to a net offset of 40mm which is just a hair (2mm) inboard of the 38mm factory offset.

Oh and some steel open ended lug nuts.

WVZR-1

You might rethink these adapters. All of the vendors use whatever they might like to use for their studs. To do as you want you would have to have the spacer/adapter first, press out a stud and then match the retention knurled diameter end to use for replacements. Another issue using adapters is spinning a stud in the adapter when trying to remove a wheel nut to remove a wheel/tire from the car. For one fellow close here each wheel stud was drilled and a roll pin inserted to 'lock' the studs. That operation X 20 doing all 4 would be quite an effort. Is it required? I don't know of another method to 'lock' the studs in the adapters.

I'd rethink this and maybe cancel if you can the studs from Jegs until you sort it.

**** If you insist on adapter use you can get them fabricated to your specifications.