Alright guys and gals, you helped me decide in an earlier thread between 17 and 18 rear wheels and the appropriate offset in the rear. Now I am looking for some help on the front wheel offset. The back is easy to measure for offset but the front not so much. Again I have the factory KH sbp front discs and am going with a 17 x 7 inch showwheels streeter wheel and am planning on mounting a 215/50R17 tire on the front. My question is should I go with a +6 or +12mm offset. It works out to be 4.25 or 4.5 inch backspacing. I talked to showwheels and they can machine the back of the stock 4.25 inch backspace wheel to achieve either back spacing. I had originally figured using the 17x7 with 4.25 back spacing front and rear but when they said they can machine the back of the wheel to add an extra 1/4 inch (6mm) of back space, I decided to go with an 17x8 machined by the 1/4 inch to get it to have a backspace of 4.75 inches in the rear. Now I am wondering if I should have them machine the fronts too to get the extra backspace. Thanks for the help and have a Happy Thanksgiving!
Front wheel offset
- Thread starter BGTLS
- Start date
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69_340_GTS
Well-Known Member
With a 7" wheel you want around 4 1/2" backspacing.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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For the least amount of highway wandering due to scrub radius interference, you need to keep the scrub the same or very similar to what the front end was designed around.
This means that if your tires are the same installed height, then the offset needs to be the same as stock. If your wheel assemblies are taller,you can move it outboard. Your 215 calculates out to 25.5 tall. This is about 1 inch taller than factory 14s.I have found on my 68 FormulaS, that running as little as 1/2 inch off the target shows up in the steering wheel as wander at highway speeds.
So a 7 inch wheel has about an 8 inch overall width. A 4 inch backspace would thus be a Zero offset wheel. To move the wheel outboard to play nice with the scrub radius, I suppose about .25 less backspace might be a good target. That then gives you a range of 3.5 to 4.0; with a 3.75 as the target.
Of course with your chosen tire width, this might not be a big issue. A wide footprint is more sensitive, as is a sticky compound.
If it were mine, I suppose I'd throw them on with the 4 inch BS, and if it wanders I could put 1/4 " spacers on and see if that helps. But if your tires get into the fenders, IDK. My head hurts. Too many what-ifs.
This means that if your tires are the same installed height, then the offset needs to be the same as stock. If your wheel assemblies are taller,you can move it outboard. Your 215 calculates out to 25.5 tall. This is about 1 inch taller than factory 14s.I have found on my 68 FormulaS, that running as little as 1/2 inch off the target shows up in the steering wheel as wander at highway speeds.
So a 7 inch wheel has about an 8 inch overall width. A 4 inch backspace would thus be a Zero offset wheel. To move the wheel outboard to play nice with the scrub radius, I suppose about .25 less backspace might be a good target. That then gives you a range of 3.5 to 4.0; with a 3.75 as the target.
Of course with your chosen tire width, this might not be a big issue. A wide footprint is more sensitive, as is a sticky compound.
If it were mine, I suppose I'd throw them on with the 4 inch BS, and if it wanders I could put 1/4 " spacers on and see if that helps. But if your tires get into the fenders, IDK. My head hurts. Too many what-ifs.
Thanks for the thoughts and I originally thought the same thing and was hesitant about having the backs machined but they do it on all of the 5 on 4.5 pattern wheels. Apparently they just machine the back before drilling the bolt pattern and then they don't drill the pattern as deep into the front of the wheel mounting flange. This way there is still plenty of metal between the lugs and hub. I really need the extra backspace on the rears so that the tire stays off of the wheel lip but it is the fronts I am wondering about. If I leave them at 4.25 inch of back space they will have the same offset as the rears and the track width should not change much over what it is now. I think the factory back space on the 14x5.5 inch rally wheels (wheels currently on the car) was 3.5 inches so that mean a 1/4 inch positive offset. Some one please correct me if I am wrong.
69_340_GTS
Well-Known Member
Nope. 1/2".
GMachineDartGT
Senior Member
All mopar wheels had 4.25 backspace. The additional widths were added to the outside.
69_340_GTS
Well-Known Member
That might be true, but he said he had a 5.5" wheel with 3.5" backspacing, and wanted to know what the offset was. If that's what he has, that wheel has 0.5" offset.
GMachineDartGT
Senior Member
Why not run the 8" up front too? Running the same size tires all around with optimize handling. I run 9" wheels.
How was the half inch figured out, I was always told 5.5 inch rim means 6.5 inch total width of the wheel. Cut that in half and get 3.25 inches, that would be a zero offset. Because the wheel has a backspace of 3.5 inches, a quarter inch more than the 3.25 inches, it has a .25 inch positive offset. This is just how I was taught to figure it out, It makes sense to me but maybe I have been wrong all these years.
69_340_GTS
Well-Known Member
I stand corrected. Your calculations are perfect. Sorry about that. Should not do math after a Thanksgiving dinner!
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