Manntis

peejay - you're right, but when I called around about rims before buying mine the tire monkeys would ask "What size rim? What bolt pattern? What's the offset?" and that's it. Anything else, they didn't know or care to know about 'em.

peejay

Depends on who you talk to!

All 1st-gens have the wheel offset inboard i.e. if you looked at a cross-section of the wheel, the hub surface is outboard of the centerline.

The trick is, some people call that a "positive" offset and other people call that a "negative" offset. There isn't really any standardization! For what it's worth, SAE would call it POSITiVE offset, just like most/all FWDs. But when buying wheels, ake sure that is in fact what you are getting!

12A cars have a factory 25mm offset. GSL-SEs have a 40mm offset that is cancelled out by the fact that their rear axles are 15mm longer, and the hub surface of the rotors are 15mm more outboard.

However, I'd prefer to go by BACKSPACING. Why? For one thing, it avoids all confusion - a wheel with 4" of backspacing is a wheel with 4" of backspacing. However, the most important points of contact that you should be worried about are the struts and other points of the suspension which cannot be moved. Fenders can be moved.

Let's take an example... stock 12A has 25mm offset and 5.5" wheels. They come very close to the strut, and if you have 205's on them then the tires will actually rub on the strut housing if you corner hard, since the whole tire will move sideways on the rim. If you go any wider on the rim AND KEEP THE SAME OFFSET, you would have problems.

Now let's do the math to figure out backspacing. It's easy, take half of the rim width - 2.75", and add the offset, which is positive 1", and we get 3.75". That is the backspacing you want. in this case.

The race wheels I bought are 7" wide with zero offset, which is 3.5" backspacing. (As I understand it this is the spec'd wheel for ITA? I don't have the rules handy) This gives a good amount of strut clearance, not just from the additional 1/4" at the rim but also because the rim is thinner (steel, not aluminum) and the tire (205/60) isn't pooched out like a bagel, so the tire itself is further away. It sticks out a little but it only hit the fenders once, when driving 60-70mph across a deep rut in the grass.

Like I said, backspacing is so much easier to live with because it tells you immediately if you're going to have problems with your suspension, which cannot be moved.

peejay

Part 2.

So you want to go with 1" wider wheels. To keep the same backspacing, this is actually a little easy... just take half of the width change - 1" - and subtract it from your offset - 40mm - and you get 28mm offset. Good luck finding that. I'm not familiar with going with larger diameter rims, that may help you some in that the strut slopes away from the rim and you'd get more clearance with larger wheels, so you may find that a 35 or even a 38mm would work.

rx7gslse

Thanks for the explaination Peejay, and I agree entirely.

I ordered my diamond racing wheels 13x8 w/ 4" backspacing. Backspacing is easy, but unfortunately, none of the wheel companies seem to care about that.....

hence all my confusion

rx7gslse

new thread, go vote
https://www.rx7club.com/forum/showth...hreadid=105836

(and I'll stop post whoring..)