8.5” rear wheel offset question to keep it flush with fenders
#1
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8.5” rear wheel offset question to keep it flush with fenders
i am currently running cup 1rims with 225 in front and 255 in the rear. I am considering getting some custom wheels made for me. I would like the rims to be as flush with the fenders as possible without having any rubbing issues. Also I want them to weigh way less than the aluminum cup 1s. My fenders are rolled from a previous owner. For the fronts I was going to go 17x8 52et. That seems like a fairly safe size from the reading I have done on the forums.
Here is is my actual question. For the rears I was thinking of having a 17x8.5 wheel. The 8.5 inch wheel should be a bit lighter than a 9 or 9.5 inch and the 255 tire still fits. My issue is like I said I want to go flush with the fender without rubbing and I cannot find much info on 8.5inch rear wheels and offsets used. The RS Cup car is a 9” with a 55et. If you want to leave the outside lip in the same position on an 8.5 inch wheel it would be approx equivilant to an 8.5 49et. I’m pretty sure those are not flush with the fender. I saw that people are running 9.5 et68 with 21mm spacers which is equal to 9.5 et47. If I use an approx equivilant to keep the same position of the outter lip that would be an 8.5 et35. Can anyone confirm if I am correct and if that would work. My other concern is maybe the wheel won’t fit over the calipers. Maybe that’s why the high offset wheel with the 21mm spacer is used on the 9.5 wide wheel. Is there anything else that I should be concerned about?
Here is is my actual question. For the rears I was thinking of having a 17x8.5 wheel. The 8.5 inch wheel should be a bit lighter than a 9 or 9.5 inch and the 255 tire still fits. My issue is like I said I want to go flush with the fender without rubbing and I cannot find much info on 8.5inch rear wheels and offsets used. The RS Cup car is a 9” with a 55et. If you want to leave the outside lip in the same position on an 8.5 inch wheel it would be approx equivilant to an 8.5 49et. I’m pretty sure those are not flush with the fender. I saw that people are running 9.5 et68 with 21mm spacers which is equal to 9.5 et47. If I use an approx equivilant to keep the same position of the outter lip that would be an 8.5 et35. Can anyone confirm if I am correct and if that would work. My other concern is maybe the wheel won’t fit over the calipers. Maybe that’s why the high offset wheel with the 21mm spacer is used on the 9.5 wide wheel. Is there anything else that I should be concerned about?
#2
i am currently running cup 1rims with 225 in front and 255 in the rear. I am considering getting some custom wheels made for me. I would like the rims to be as flush with the fenders as possible without having any rubbing issues. Also I want them to weigh way less than the aluminum cup 1s. My fenders are rolled from a previous owner. For the fronts I was going to go 17x8 52et. That seems like a fairly safe size from the reading I have done on the forums.
Here is is my actual question. For the rears I was thinking of having a 17x8.5 wheel. The 8.5 inch wheel should be a bit lighter than a 9 or 9.5 inch and the 255 tire still fits. My issue is like I said I want to go flush with the fender without rubbing and I cannot find much info on 8.5inch rear wheels and offsets used. The RS Cup car is a 9” with a 55et. If you want to leave the outside lip in the same position on an 8.5 inch wheel it would be approx equivilant to an 8.5 49et. I’m pretty sure those are not flush with the fender. I saw that people are running 9.5 et68 with 21mm spacers which is equal to 9.5 et47. If I use an approx equivilant to keep the same position of the outter lip that would be an 8.5 et35. Can anyone confirm if I am correct and if that would work. My other concern is maybe the wheel won’t fit over the calipers. Maybe that’s why the high offset wheel with the 21mm spacer is used on the 9.5 wide wheel. Is there anything else that I should be concerned about?
Here is is my actual question. For the rears I was thinking of having a 17x8.5 wheel. The 8.5 inch wheel should be a bit lighter than a 9 or 9.5 inch and the 255 tire still fits. My issue is like I said I want to go flush with the fender without rubbing and I cannot find much info on 8.5inch rear wheels and offsets used. The RS Cup car is a 9” with a 55et. If you want to leave the outside lip in the same position on an 8.5 inch wheel it would be approx equivilant to an 8.5 49et. I’m pretty sure those are not flush with the fender. I saw that people are running 9.5 et68 with 21mm spacers which is equal to 9.5 et47. If I use an approx equivilant to keep the same position of the outter lip that would be an 8.5 et35. Can anyone confirm if I am correct and if that would work. My other concern is maybe the wheel won’t fit over the calipers. Maybe that’s why the high offset wheel with the 21mm spacer is used on the 9.5 wide wheel. Is there anything else that I should be concerned about?
The big issue that you are missing is that the narrower wheel will not support the tire anywhere near as well as the wider 9 or 9.5. The difference in weight is negligible so that is a false economy, The loss of performance potential is not minor.
#3
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Ok thank you. I thought that the tire should be the maximum size for the rim to get max performance. I thought as the tire was stretched the performance went down. I guess I had it backwards. What is the best width with a 255. A 9” or 9.5”?
#4
964 can use 9.5ET47+/ or 10 ET~51+/- w/ 255/40
#5
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Thread Starter
Why is the tire better on a wider rim? Is it because there is less flex in the sidewall? I assume the contact patch of the tire always remains the same regardless of the size of the rim. Or does that change marginally as well depending on rim width when the tire is stretched?
#6
Why is the tire better on a wider rim? Is it because there is less flex in the sidewall? I assume the contact patch of the tire always remains the same regardless of the size of the rim. Or does that change marginally as well depending on rim width when the tire is stretched?
The tread width is pretty much the same no matter the wheel width
On a narrow wheel the side wall is more vertical giving a slightly taller OD, The side wall will also be more flexible, in a curve the tire distorts w/ the tread moving out of alignment w/ the wheel, like this, from this (__) to this /__/
when mounted though the tread remains constant the section width varies by ~.2" per extra .5" of wheel width
for example a 255/40 x17 on an 8.5 will have a section width of ~10" on a 10 it will be more like 10.6" the OD will also be shorter by a small amount
#7
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Perfect thank you for the info. Last question. When you say a 255 can use a 10” et~51 +/- . Is 51 the ideal offset in that width? The +/- is that beacause different manufacturer tires fit slightly different so you may be able to have slightly higher or lower offset on a 10” rim.?
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#8
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Here is my rear BBS E26 wheel 17x10 +47 with a 5mm spacer making the effective offset +42. Tires are 255/40/17 re71r. -2 deg of rear camber. Rear fender lips are ground down for extra clearance.
#9
Perfect thank you for the info. Last question. When you say a 255 can use a 10” et~51 +/- . Is 51 the ideal offset in that width? The +/- is that beacause different manufacturer tires fit slightly different so you may be able to have slightly higher or lower offset on a 10” rim.?
As to the ideal ET, as you get to more extreme fitments there is less and less leeway, most folks that fit 10s also use 275/xx tires, 255/40 being narrower provide a little more leeway.
some background as ET goes from 42 to 50 the whole wheel/tire assembly moves inboard.
Too far out means lip interference, too far in means oil line interference.
Shorter tires are easier to fit than taller tires.
I've seen 10" fitments from
ET44 w/ 265/35 or 275/35 x18 to ET51 w/ 275/35 x18
I've seen lip interference w/ ET 43 & 275/35 x18
from this I'd say that w/ a 255/40 x17 10ET45 to 50 would be safe 45 is closer to the lip and 50 is closer to the oil line. So ET47 +/-
#10
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#12
#13
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See my thread:
https://rennlist.com/forums/964-foru...-are-here.html
8.5" front and 10" rear, both with +45... pretty perfect, zero rub (but on very stiff suspension. I plan to camber it and lower a touch more
https://rennlist.com/forums/964-foru...-are-here.html
8.5" front and 10" rear, both with +45... pretty perfect, zero rub (but on very stiff suspension. I plan to camber it and lower a touch more
#14
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Thread Starter
See my thread:
https://rennlist.com/forums/964-foru...-are-here.html
8.5" front and 10" rear, both with +45... pretty perfect, zero rub (but on very stiff suspension. I plan to camber it and lower a touch more
https://rennlist.com/forums/964-foru...-are-here.html
8.5" front and 10" rear, both with +45... pretty perfect, zero rub (but on very stiff suspension. I plan to camber it and lower a touch more
what are are you running for caster?
I am not really concerned about it rubbing the upper fenders. I’m more concerned it will rub the inner fender liner. I have the corner of the bumper/fender liner trimmed to allow max clearance. But I don’t want to have to back the caster off below rs spec to accommodate a wheel.
#15
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No camber right now as far as I know. They are pretty square. There's still room on the inside and the tires I run fit wider than most. Shown with a 225 front and a 265 rear