I have been reading everything I can on this but still do not see the picture clearly...

I have new wheels for my '91 Z28 ready to go on, as I prepare to start autocrossing this spring. They are 17x9.5 square ZR1 corvette wheels from OE wheels. I need to take a few more measurements but I suspect I need about 2.25" thick spacers in the front, and 1.75" thick in the rear.

I do have another thread below with more details on the wheels:
(Not So) Bare Minimum to Give Autox or Track Racing a Shot? - Third Generation F-Body Message Boards





The wheels I have on the car currently are Centerline Telstars. These are not hub centric wheels. the center hole does not fit tightly over the center of the hub. I use acorn nuts and plan to reuse the same nuts on the new wheels. I have been driving these wheels, including drag racing it, for the last 15 years or so with absolutely no issues or excessive vibration. I believe I have heard that the factory wheels from our cars are not even hub centric, they are only lug centric. If this is the case, than why does everyone talk about hub centric spacers being a better choice than the standard lug centric ones?

I am actually a design engineer and while I don't claim to know everything, I do know if you don't have a close tolerance machined mating surface you are not piloting (tightly locating) on anything. The adapters claim the hub size is 70.3mm but the wheels hub size is listed as 70.7, so that is 0.4mm clearance...When you properly tighten down then torque the lugs, the OD of the hub and ID of the wheel should be concentric, and not even making contact. I suppose it gives some protection IF the lugs begin to loosen, but I'm not sure that is enough reason to go hub centric, since a properly torqued adapter, or wheel, should not come loose... It is clear that the ID of the bore on my new wheels is painted, you don't do that if it is intended to be a piloting surface. The factory wheels appear to be the same.

Factory Wheel Specs:


I can get lug centric only adapters for only $180 shipped: Wheel Adapters 5x4.75 | 5x120.7 Spacers for Chevy Corvette El Camino (ezaccessory.com)


The hub centric options would possibly be these for about $350, these advertise my vehicle's hub bore is 70.3mm (The wheel hub is advertised as 70.7MM): Wheel Adapters, Wheel Spacers, Hub Rings for your car! | Motorsport Tech (motorsport-tech.com)


Honestly the price difference doesn't even make that much of a difference to me, but I just want to understand what is going on here! -Thanks

 

Really? No one has anything to contribute on this topic???

 

I wish I could. I’ve been fighting a rough vibration for awhile. Soon as I have time my front wheels and spacers are coming off and replaced with some known good wheels without spacers. So hearing the consensus on hub vs lug centric would be nice. I figure lug centric would be easiest, but just a guess.

 

Hub centric is best, so the adapters mount onto the car's hub the same way the wheels did.

And, ideally, you'd also want a wheel centric ring on the face of the adapter, so the wheels will mount onto the adapters the same way they used to get mounted onto the car's hub.

It's those hub centric and wheel centric rings that help prevent vibrations by maintaining centered wheel mountings.

Lug centric, being where there is no ring, and the wheels are centered simply by the lug nuts on the studs, can cause vibrations, due to the possibility of not getting the wheel centered when tightening the lug nuts, and one or more of the nuts doesn't get installed perfectly. More opportunity for human error during installation.

You can always get wheel centric rings that slip over the wheel hub if the adapters don't have rings. That might be best for hubs like 3rdgens have, where the adapters have to fit over a long snout, and the inside of the adapters might not be bored completely through to accommodate such a condition.

So in a nutshell, centering the wheel hub centricly is always going to be better and less risky than relying purely on the lug nuts only.

 

I recently used hub-centric spacers on one of my builds.

The bolt-on spacer adds a second source of accumulated runout.
In this case, why not get hub-centric spacers to be safe?

I actually bought mine off of amazon, for a very reasonable cost.
I got the OCPTY brand for under $100 per pair & they fit with very close tolerances.
I have no vibrations or issues with retaining the lug torque.

You can also get a metal set of hub centric rings if you cannot get a set of bolt-on spacers with different hub sizes.

 

Just an alternate viewpoint. I used to run Amazon adapters, but now I am running American made custom set from info@motorsport-tech.com

For my LS1 swap I found the 2 inch spacers were just barely not enough with my 4th gen style ZR1s and the tie rods would hit the inner rim lip. I have also upgraded to PowerStop front rotors. Motorsport tech was able to make me perfectly fitting hub and wheel centric adapters using my measurements and everything fits super tight. And now I don't have vibration anymore. It was more expensive, but in my mind worth it to have an American made part that reduced vibration.

They also have really good tech support if you need help, they actually pick up the phone.

 

Sounds like you have very similar wheels to me. Also sounds like the reading I have done sounds correct saying you need just a little more than 2" spacers, so 2.25", to fit those without contacting the tie rods. I don't think those Powerstop rotors should have changed wheel positioning at all right? Do you know your back spacing for those wheels. Would be nice to know if it is the same as mine.

I am actually planning to order my spacers from Motorsport-Tech, tried to call them Friday, but we kept missing each other. I'll try calling them again tomorrow to make sure I understand everything correctly before taking final measurements and ordering the spacers.

 

Honestly I've never measured the backspacing on the ZR1 rims. This thread might have the info you want: ZR1 17x11 backspacing? - LS1TECH - Camaro and Firebird Forum Discussion

If you have Powerstop rotors you need to tell them the correct dimension, the stock brake rotor is a different shape and the hub centric hole will be off. Ask to talk to Taylor, he is the guy that helped out. I highly recommend you take a caliper and measure your rotor yourself, but what I had was a raised lip on the rotor side that was 12mm deep and 71mm wide. The stock rotor hub did not have this lip.

 

I tighten lug nuts as if I were pulling down the bolts of an intake manifold or something. That is, keep the wheel in the air at first, and go around in a pattern little by little, manually holding and centering the wheel as you draw down the lugs which also helps to center it as they get tighter. Use the pattern and torque in small increments like you are trying to seal and smooth a gasket. Without the weight of the vehicle it is alot easier to center the wheel during this time.
Then, I set the wheel down a little bit. A tiny bit of vehicle weight to help keep it from turning while you make the lugs slightly tighter. But still not full vehicle weight. Enough torque on the lugs should spin the wheel even though its touching the ground at this point.
Then finally you can set it down without worrying that the weight of the vehicle will UN-center the wheel. Now you can fully tight the lugs.

 

So I think I got to the bottom of it...

To start with I verified the wheels I bought from OE wheels are modeled after the C4 Grand Sport (Z07 Package) wheels as specified below:


This still calls out the standard 70.3 hub diameter that all Corvettes and Camaros are supposed to be so I looked OE Wheels website and all the corvette wheels except the ZR1 wheels I ordered are listed as having the correct 70.3 hub diameter. So, I called them, and was able to have one of their engineers confirm, their listed hub diameter of 70.7 for the ZR1 wheels is wrong, on the website and should be 70.3 mm like all the other wheels...

With that information I also called and talked to a few people at spacer companies, and came to the following conclusion. All factory wheels are designed to be hub centric hence having an advertised diameter. The aftermarket reproduction wheels should have the same specs as the factory wheels. When you see 70.3 mm that is a nominal dimension. So the actual bore in the wheel is going to have a tolerance on it to make it slightly larger, and the actual tolerance on the hub will make it slightly smaller. One of the spacer manufacturers mentioned that the total clearance between hub and bore should be around 0.005" so 0.127 mm. This makes much more sense. Enough room to accurately position the wheel, but enough clearance to allow it to easily slide on and off. The rep also explained the reasoning for the pilot as follows, it is there to help position the wheel correctly prior to tightening the lugs. By centering the wheel you eliminate the likelihood of vibration from having the wheel off center and flexing the studs off axis. Once you properly torque down the lugs, all of the holding force is coming from the nut forcing the wheel hub against the axle hub. There is no longer any load on the ring/pilot once the lugs are properly torqued.

This all said, I do agree that hub centric wheels are probably a better option. That said, I have an autocross training event coming up on April 2nd, so about 2 and a half weeks, and I'm not finding out while talking to the vendors, the custom spacers could take 2-3 weeks to even manufacture! So, now I have to figure out what I am going to do for the time being. As I think I mentioned I still need to take final measurements off the car which I'm bringing home tomorrow, then I can place orders. I can get the custom proper rings, and hope they come on time. Or I have the following options:
1. Just get lug centric spacers which I can buy pre made online in the slightly uncommon 2.25 thickness.
2. Get a standard size spacer with the hub centric ring in a standard size 2.0" then get a 0.25 thick spacer to put between it and the axle hub. This could require longer studs, which could be a possibility...
These

Plus These


OR...
3. Get the custom hub centric spacers, and pay the $100 expediting fee...

Which do you guys think is a better option?

 

Ok. Got the car home took the wheels off and took lots of measurements. Strangely the rear hub Dia on the axle measured 70.40 mm, and the one on the fronts measured 70.56 mm. Doesn't really make any sense to be given a nominal of 70.3...

That said given the lead time of the custom spacers i ordered a bunch of different ones off Amazon with free returns to take measurements on, and if they fit, use temporary or perminantly. I got two different hub centric 2" thick spacers one that advertised a 70.5 hub dia and one that advertised 70.3 hub Dia, granted it had low ratings than the other. The 70.5 ended fitting the front and rear hubs on the car, and on the wheels. The rear spacers went on great, with the new rear wheels! Only problem is it contacted the bearing housing on the frond discs. I also got .25" spacers because my measurements showed I would need 2" of spacer in the back but 2.25" in the front. The pilot on the disk is long enough, that even with the thickness of the .25 spacer and the chamfer on the 2" spacer, I still get engagement for centering, and the extra .25" allows the clearance for the 2" spacer. I'm still getting about 7.5 full rotations of the lugnuts and I've read animus of between 6 and 6.5 turns, so I'd say I'm good. Only problem is centering the .25 spacer so I don't get any vibration...

Thoughts?

 

So, she's all together for now. I ended up only having about 12.25 mm of stud to lug engagement through the two spacers so I changed out the studs for longer ones. I also found on Amazon some cheap aluminum tubing with an id of 12mm and od of 14mm, I took a tiny amount off the od to allow them to slide over the studs and inside the spacers to allow me to center the .25" spacers. Once I got everything positioned with two of the aluminum tube spacers, I tightened three of the lugs, then removed the tubes and torqued the rest of the lugs. Using blue loctite bolting the spacers to the hub. I suspect I'll eventually upgrade my front brakes at which time I'll just remove the .25" spacers.

 

That's a beautiful car you've got there!
Garage door is shaving it kind of close don't you think?

 

Thanks! She looks great from afar, but up close she is far from great... I do love the look of late third gens, and I really think the new wheels look perfect.

You think that is close... My DDis a Druango R/T. and it gets to live in the garage over the winter. That leaves about an inch behind and 4 inches in front to my work bench. That is tight!

 

Just wanted to post an update... I made it through my first 2 day autocross event using the above spacer configuration, and everything went great! Nothing loosened up, or vibrated as far as I could tell.

Also, with my ride height and even under racing conditions there was no problematic rubbing anywhere. Only, the tiniest bit with the front drivers side against the plastic inner fender in the front, and only at nearly full right hand lock of the wheel.

 

What Brand of wheel spacer did you end up using?

 

I went with these off amazon, but mine are 2" thick and are anodized black. Studs and nuts are still plated. They don't seem to have any more in stock at the moment...

3 inch 5x4.75 to 5x4.75 Hubcentric Wheel Spacers with Lip (70.5mm Bore, Metric 12x1.5 Studs Nuts) Compatible with Chevy Blazer Camaro Corvette S10 GMC S15 Jimmy - Silver Adapters, Pack of 2 https://a.co/d/8dOnpyy