1/2 inch backspace rallys - Team Camaro Tech
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post #1 of 18 (permalink) Old Feb 11th, 13, 04:16 PM Thread Starter
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1/2 inch backspace rallys

I am running 15 x 8 Corvette Rallys all the around my 69 Camaro. P255/60 in back (rolled inner fender lip so no rub), P245/60 in front. Want to lower front one inch lower than stock. I know 1/2 inch backspacing is needed but how does that affect the look of the center derby cap in the center. Seems to me it might be noticable sticking out a 1/2 inch further in the front than the backs do. Anybody come across this? Was it noticable to the eye? Could go 1/4 inch backspace with a P225/60. Thoughts, please.
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post #2 of 18 (permalink) Old Feb 11th, 13, 04:46 PM
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Re: 1/2 inch backspace rallys

You lost me Chet, 1/2" backspacing would be a very deep dish pushing the tire 4-5" outside the fender lip. If you want to fit fat tires shoot for a +6 offset no matter how wide your wheel choice is. On a 15x8 a +6 offset equals 4.75" of back spacing. That would just about center your 245/60 in the fender well.

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post #3 of 18 (permalink) Old Feb 12th, 13, 02:17 AM
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Re: 1/2 inch backspace rallys

Chet.... the offset in the 1/2 inch backspacing will have no effect on the center cap. That remains the same depth from the hub or drum. The only difference you may see is between the rim and inner trim ring. My 15x8's have a 3" deep trim ring and still have about a 3/4 inch space away from the inner rim.


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post #4 of 18 (permalink) Old Feb 13th, 13, 03:31 AM
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Re: 1/2 inch backspace rallys

Info....

Offset is measured from the centerline of the barrel of the wheel to the hub mounting surface. If that hub mounting surface were in the exact center of the barrel of the wheel the offset would be “0". If the mounting surface is away from the center of the wheel the offset is POSITIVE, if the mounting surface is towards the center, the offset is NEGITIVE. Offset is measured in millimeters. There are 25.4 millimeters to an inch. So if the mounting surface is 12mm from the centerline of the wheel it has a 12mm offset. That would also mean that it is about ½ inch. If the mounting surface is moved away from the vehicle that is a positive offset. The picture shows a wheel with the positive offset. A positive offset will cause the wheel to set in or tuck into the vehicle. Originally you found positive offsets on just front wheel vehicles. Due to that some people refer to positive offset wheels as front wheel drive offsets. Currently there are many rear wheel drive cars and trucks with positive offsets. The higher the positive offset the less it sticks out from the vehicle and it will have a higher Backspace. If the mounting surface is moved in toward the vehicle past the centerline, that is a negative offset. A high negative offset will produce a lower Backspace. Once again the measurement in millimeters is how far away from the centerline the mounting surface is. A -24mm offset means that the mounting surface is located 24mm or 1 inch from the centerline toward the vehicle. The wheel will come out farther from the vehicle and will have that deep dish look.

Width

The width of a wheel is measured inside the beads which are usually ½ inches. If you measure on the outside of the beads an 8 inch wheel will measure 9 inches

Backspace

Back spacing is measured from the inner edge of the wheel to the hub mounting surface. It is a convenient measurement in that, as long as the back spacing remains the same, the clearance to the suspension also remains the same. If you know the width of the wheel and the offset you can compute the backspacing. For example if you have an 8 inch wheel with a +24mm offset. An 8 inch wheel is actually 9 inches wide so if the offset were 0 the mounting surface would be right on the centerline and the backspacing would be 4.5 inches. With a positive offset the mounting surface moves off center 24mm or 1 inch toward the outer edge of the wheel. This will make the inner edge go more inside which will result in a 5.5 inch backspacing.

Centerbore

This is the diameter of the center of the wheel. The centerbore must be equal to or larger than the hub of the vehicle for the wheel to seat. If the wheels centerbore is the same as the hub on the wheel then that wheel is hub-centric and uses the hub to center the wheels. If the centerbore is larger than the hub then the lugs will center the wheel and that is lug-centric. When using a lug-centric wheel be sure and snug the lugs up slowly and tighten them in an opposite or star pattern.
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post #5 of 18 (permalink) Old Feb 13th, 13, 04:00 AM
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Re: 1/2 inch backspace rallys

Tried to add a pic of my wheel/tire and Admin's new rule would not allow me to add or change anything.

Anyway,
This is my rear rim/tire. Its a 15x8 "0" offset Rally Wheel with 4 1/4 backspace. As you can see there is space behind the trim ring for the back space to be changed and still use the trim ring. If the backspace was made to a 5 " using this rim...factory trim ring will be just fine. With no changes to the "offset" the center cap will be fine, just will stick out 3/4 more than original.

Kev
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post #6 of 18 (permalink) Old Feb 13th, 13, 05:01 AM Thread Starter
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Re: 1/2 inch backspace rallys

That re-affirms what I was thinking, thanks. I was wondering if it was noticable to the eye if the center cap was sticking out an extra half inch.
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post #7 of 18 (permalink) Old Feb 14th, 13, 05:05 AM
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Re: 1/2 inch backspace rallys

Chet....

You could try a 15x7 Rally Wheel "0" offset with 4 1/2 backspacing with a 225-60-15 tire. The look will be about the same and give you the needed backspacing. I am sure someone on this site has used the 15x7 4 1/2 " bs front and 15x8 4 " rear.


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post #8 of 18 (permalink) Old Feb 14th, 13, 06:37 PM Thread Starter
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Re: 1/2 inch backspace rallys

Does anybody know if the center 'derby' caps are the same height for 15 x 7 rallys and 15 x8 rallys.
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post #9 of 18 (permalink) Old Feb 14th, 13, 06:56 PM
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Re: 1/2 inch backspace rallys

I am so confused on off set and backspacing. I will be needing rims and tires very shortly on my build. Grrr...
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post #10 of 18 (permalink) Old Feb 14th, 13, 09:01 PM
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Re: 1/2 inch backspace rallys

Hey Kev, 0 offset on a 7" rim is only 4" backspace. These rub on turns under compression on a factory disc car with a 235. I bought a car that had some of those with Year One rally package on it already. Never had a problem with the 4.25" (+6) offset 7" rim at least on a drum car.

Chet, the "high top" derby cap is the same for either rim.


Here's about everything you'd want to know about offset and backspace.
https://www.rsracing.com/tech-wheel.html

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post #11 of 18 (permalink) Old Feb 15th, 13, 03:11 AM
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Re: 1/2 inch backspace rallys

Hey Steiner....

I know info on these rims are all over and unknown what is correct without some input from members like you who have used them. I know that some of the Year One listings were incorrect a while back. Check the listings at Summit and see what aftermarket Rally Wheel size and backspace are available.

As I understand there were 15x7 4 1 /2 " backspace rally rims with the standard offset. There were Monte-Carlo rims. I may have confused the issue with the standard vs zero offset. What that actual " standard " offset was is not listed as far as I could find. I know many places now sell these wheels with up to 5 1/2 " backspace listings however no offset listing.

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post #12 of 18 (permalink) Old Feb 15th, 13, 03:13 AM
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Re: 1/2 inch backspace rallys

Quote:
Originally Posted by 69&76 View Post
I am so confused on off set and backspacing. I will be needing rims and tires very shortly on my build. Grrr...
Ty...

Post what year and rim size you woould like and alo read the info listed above in the Topic about Tires and Wheels for 67, 68, 69 Camaros


Kev
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post #13 of 18 (permalink) Old Feb 15th, 13, 03:17 AM
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Re: 1/2 inch backspace rallys

Quote:
Originally Posted by cool rocking daddy View Post
Does anybody know if the center 'derby' caps are the same height for 15 x 7 rallys and 15 x8 rallys.
Chet, The turbine derby caps are all the same however flat caps will fit also but ....

Kev
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post #14 of 18 (permalink) Old Feb 15th, 13, 05:29 AM
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Re: 1/2 inch backspace rallys

You're right Kev it's all over the place. Throw in that lots call everything "Corvette rally" and it gets even more confused.

I never tried the FW rims on a disc car but from what I recall with the drums they should tuck OK.

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post #15 of 18 (permalink) Old Feb 15th, 13, 07:14 AM
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Re: 1/2 inch backspace rallys

Quote:
Originally Posted by SixtyAte View Post
Chet, The turbine derby caps are all the same however flat caps will fit also but ....
Kev
The original owner of my '69 Corvette (15x8 rallyes) shortened the derby caps which make the wheels seem deeper. I like it. Until this morning, I'd never actually measured how much shorter they are, but after reading this thread and decided to reply, I did. The shortened amount is subtle, but definitely makes the wheels look wider/deeper. The 'black ribbed' portion was shortened from 2-1/8" to 1-3/8". To shorten, remove the extension from the cap, saw off 0.5" to 0.75" of the extension at the back, and then reinstall to the cap. I made photos this morning if anyone is interested, I just can't remember how to post them here..

Gary
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