Off set / Spacers / adapters / and calculators
Thread Starter
Melting Slicks
Joined: Jan 2004
Posts: 2,659
Likes: 3
From: Oxford CT
Off set / Spacers / adapters / and calculators
Ok, well i have been trying to figure out how to determine the needed off set on a rim for a while now.
I have figured it out !! (for the most part)
I know that if my 85 c4 was a 32mm (stock) and i want to put a 79mm off set rim on then i would need a spacer / adapter to make the difference.
I belive the twist is that if the 32mm was a 8.5 wide rim and the 79mm is a 10" wide rim then there is some more calculations involved.
In an effort to make this easier. I have the new rims.
I was thinking just put them on the car. If it looks like the tire does not sit in a nice spot (like stock). or rubs.
Then instead of the calculations, I will put a certain amount of washers on the lug bolts (until i get the rim where i want it (exactly)
Then i would mesure it. ???(how would i measure the mm to be the same as they would a spacer)????
Then i can double check where i want my rim to sit with the proper math of a stock vehicle.
If this makes any sense give me some feed back.
Dan
PS this can aid in seeing how it will sit before spending the money and hoping you like where it sits
and hoping that you figured it out properly.
I have figured it out !! (for the most part)
I know that if my 85 c4 was a 32mm (stock) and i want to put a 79mm off set rim on then i would need a spacer / adapter to make the difference.
I belive the twist is that if the 32mm was a 8.5 wide rim and the 79mm is a 10" wide rim then there is some more calculations involved.
In an effort to make this easier. I have the new rims.
I was thinking just put them on the car. If it looks like the tire does not sit in a nice spot (like stock). or rubs.
Then instead of the calculations, I will put a certain amount of washers on the lug bolts (until i get the rim where i want it (exactly)
Then i would mesure it. ???(how would i measure the mm to be the same as they would a spacer)????
Then i can double check where i want my rim to sit with the proper math of a stock vehicle.
If this makes any sense give me some feed back.
Dan
PS this can aid in seeing how it will sit before spending the money and hoping you like where it sits
and hoping that you figured it out properly.
Race Director
Joined: Sep 2003
Posts: 16,838
Likes: 341
From: Database Error Indiana
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
The 88 and later wheels had a 56mm offset. So the wheels you are looking at with a 79mm offset
are going to be off at least an inch if you use adapters for the later C4 offset.
Wheel offset is the distance from the wheel mounting flange to the center of the wheel. It really doesn't make any difference how wide a wheel when measuring offset. All of the 84-87 C4 9.5" wide wheels had a 38mm offset. Only the early 8.5" wide 16" C4 wheels were 32mm.
The OEM ZR-1 11" wide rear wheels used a 36mm offset and those are the only later factory C4 wheels that fit on the early cars without an adapter.
Backspacing is another story. Backspacing is the distance from the wheel mounting flange to the inside edge of the wheel.
IMHO, this wheel with the 79mm offset is not going to work.
are going to be off at least an inch if you use adapters for the later C4 offset.Wheel offset is the distance from the wheel mounting flange to the center of the wheel. It really doesn't make any difference how wide a wheel when measuring offset. All of the 84-87 C4 9.5" wide wheels had a 38mm offset. Only the early 8.5" wide 16" C4 wheels were 32mm.
The OEM ZR-1 11" wide rear wheels used a 36mm offset and those are the only later factory C4 wheels that fit on the early cars without an adapter.
Backspacing is another story. Backspacing is the distance from the wheel mounting flange to the inside edge of the wheel.
IMHO, this wheel with the 79mm offset is not going to work.
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
If the new rim is wider, subtract half the difference from the offset difference.
Or just look on the website.
Also note that Z51 wheels stuck out a 1/4" more than base 8.5" wheels from the factory.
Or just look on the website.
Also note that Z51 wheels stuck out a 1/4" more than base 8.5" wheels from the factory.
Melting Slicks
Joined: Mar 2001
Posts: 2,150
Likes: 304
From: Timra, Sweden
Your 1985 had two rim options.
16" x 8.5" (216mm) wide with 32mm offset. (76mm out and 140mm in)
16" x 9.5" (242mm) wide with 38mm offset. (83mm out and 159mm in)
Another rim that works on early 1984-1987 Corvettes is the rear ZR1 rim.
17" x 11" (280mm) wide with 36mm offset. (104mm out and 176mm in)
You could use the same offset as the stock 9.5" wide rim. It is the closest match for your new 10" wide rim.
10" is 254mm.
38mm offset will give you 89mm out and 165mm in. (6mm outside stock 9.5")
44mm offset will give you 83mm out and 171mm in. (Outside like stock 9.5")
If the rim has 79mm offset then:
you need a 41mm (1.6") spacer to get 38mm offset.
you need a 35mm (1.38") spacer to get 44mm offset.
I vote for a 1.5" spacer. The rim will stick out about 0.1" more than the stock 9.5" wide rim.
Last edited by JoBy; Feb 5, 2007 at 03:20 AM.
Thread Starter
Melting Slicks
Joined: Jan 2004
Posts: 2,659
Likes: 3
From: Oxford CT
Yes, rim width affects the offset you need.
Your 1985 had two rim options.
16" x 8.5" (216mm) wide with 32mm offset. (76mm out and 140mm in)
16" x 9.5" (242mm) wide with 38mm offset. (83mm out and 159mm in)
Another rim that works on early 1984-1987 Corvettes is the rear ZR1 rim.
17" x 11" (280mm) wide with 36mm offset. (104mm out and 176mm in)
You could use the same offset as the stock 9.5" wide rim. It is the closest match for your new 10" wide rim.
10" is 254mm.
38mm offset will give you 89mm out and 165mm in. (6mm outside stock 9.5")
44mm offset will give you 83mm out and 171mm in. (Outside like stock 9.5")
If the rim has 79mm offset then:
you need a 41mm (1.6") spacer to get 38mm offset.
you need a 35mm (1.38") spacer to get 44mm offset.
I vote for a 1.5" spacer. The rim will stick out about 0.1" more than the stock 9.5" wide rim.
Your 1985 had two rim options.
16" x 8.5" (216mm) wide with 32mm offset. (76mm out and 140mm in)
16" x 9.5" (242mm) wide with 38mm offset. (83mm out and 159mm in)
Another rim that works on early 1984-1987 Corvettes is the rear ZR1 rim.
17" x 11" (280mm) wide with 36mm offset. (104mm out and 176mm in)
You could use the same offset as the stock 9.5" wide rim. It is the closest match for your new 10" wide rim.
10" is 254mm.
38mm offset will give you 89mm out and 165mm in. (6mm outside stock 9.5")
44mm offset will give you 83mm out and 171mm in. (Outside like stock 9.5")
If the rim has 79mm offset then:
you need a 41mm (1.6") spacer to get 38mm offset.
you need a 35mm (1.38") spacer to get 44mm offset.
I vote for a 1.5" spacer. The rim will stick out about 0.1" more than the stock 9.5" wide rim.
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