Another backspacing question

[Grand_87_national]

Hello, all. I have an addition to the tired question regarding appropriate backspacing for rear wheels. 18 x 9.5 Boss 338 has a -4 mm offset, giving it a 5.03" backspacing. Searching for answers, I've found that some swear by a magic number of 4.5" on backspacing, while others say a larger wheel will allow for more backspacing room. My current rears are a 15 x 8 with 4.5" backspacing and the tires are less than a half inch from the frame. However, it is my tires, not the wheels that are near the frame, which leads me to believe that a lower profile tire will have more room. Any insight would be great, and I've seen these wheels on gn's, but I'm not having any luck finding out if they had to use a spacer. Thank you.

 

[1986 Buick GX1]

I have 17 x 9 wheels [4 5/8" backspace] w/ 285/40's on the rear with SSBI rear discs and only 1/2" clearance or so and did rub the frame on hard corners once in a while (Autocross) with the stock rubber bushings in the stock upper control arms. This is with the stock rear end and no rolling of the fenders.

The rotor hat is about an 1/8" thicker than the steel drums so the backspace is essentially 4 1/2".

Your 1/2" wider wheel will pull the inside of the tire away from the frame approx. 1/4" as compared to mine.
If you run a narrower tire (275) there will be more clearance and if you run a wider tire (295) there will be less clearance.

I would say a 1/2" spacer would work well to keep the tire flush with the fender and away from the frame.

 

[SICK_SIX]

It just depends how wide the section width of the tire you purchase is. That's the middle part that sticks out. If you go with the 4.5" backspace with the 9.5" wheels, then the tire runs the risk of making contact with your wheel lip which could cut the tire. I'd take the chance and go with the 5" backspace, and worse comes to worse a 1/8", or no larger than a 1/4" universal spacer might be required and the installation of longer lug studs. Then again, maybe no spacer at all. I wouldn't use a half inch spacer myself.

 

[Grand_87_national]

Thanks, fellas. I have a set of d.r.'s on my 15" wheels, so these will never see track time. I'm thinking just a 275/35/18 tire will do, and that shouldn't fit too bad.

 

[lhbarrett]

Boss rims fit my car fine. The front wheels were more of a challenge as as I had 12" Camaro brakes that added track width.

Sent from my LG-H820 using Tapatalk

 

[Paul87gn]

Did you end up pulling the trigger on the Boss wheels? If so how did they work out for you? Pictures/details would be a welcome addition to the official wheel/tire thread.

 

[Grand_87_national]

Actually, I didn't. Ended up going with a set of Billet Specialties instead. 16x7 front and 16x9 rear.