Cheers!

How is backspacing of a wheel measured? How does the offset affect the backspacing? What is the backspacing availble for a stock TurboII with stock 16x7 wheels with 40 mm offset?

does backspacing change as a function of vechicle height?

sorry for the questions

maxcooper

Here's a picture of me measuring the backspacing on one of my wheels:



Backspacing and offset are related, but it can be harder to convert between the two than it may at first seem. The thing that makes it hard is that a 10" wide wheel is really 11.125" wide or so. The 10" nominal specification just refers to the bead width for the tire. The real wheel is wider than the bead with. Backspacing is measured relative to the real wheel width, but you have to measure the real width, too, if you want to figure out the offset from the backspacing measurement. It wouldn't be that hard if all wheels were 1.125" wider than the bead width, but they aren't. Some are just 1" wider. Some are 1.25" wider. Some might be less than 1" wider.

I don't know the backspacing for those wheels. But you might be able to make an educated guess if you assume the wheel is really 8.125" wide. (8.125 / 2) + (40 / 25.4) = 5.637" of backspacing. But it's just a guess.

Backspacing does not change with vehicle height -- it is a physical dimension of the wheel.

-Max

Cheers!

ok so a company says i need 4.5 inches of backspacing to run their suspension setup and assuming i have 5.6 inches this will not fit?

but the 2nd Gen TII does nto have any tie rods or other linkages so it shoud be able to clear?

SleepR1

That means your current wheels go 1.1 inches more inboard, which may end up rubbing those suspension components. The 4.5-inches of back space clearance will drive your choice of wheel offset selection.

For example if you wanted to run 8 wides up front and 9 wides in back on an FC. For the rears you'd need 13 mm offset. Front space will be 5.5 inches. Depending on rear fender lip clearance, you might need to roll fender lips or add wheel arches. For the 8 wides up front you'd need an offset of zero, and front space is 4.5 inches (makes sense since zero offset is the exact mid point of the wheel width )

Note that I'm adding 1 inch to the nominal wheel widths which are often quoted in wheel spec tables; this is b/c overall width is measured from rim lip to rim lip, not tire bead to bead--what wheel makers to quote wheel widths. Lip to lip is usually ~1 inch wider than bead to bead.

Cheers!

alright enough of this... bilstein coilovers for 2nd gen rx7s are only a dream for me... ground control it is.

SleepR1

Ah with those, your 5.6 inch back space should work, since the GCs are 2.5 inch outer diameter, and are NOT barrel-shaped like stock FC springs--assuming no other FC suspension components are going to rub the wheel from the inboard side.

Carl Byck

I do not know the exact maximum backspacing for every DIAMETER of TIRE, but it is a lot more than what you might think. Stock is 5-1/2, but I can fit around 7" in the back, and almost 6" in the front. remember that as you increase, or decrease wheel and/or tire diameter, the interference issues will change. You can fit a much wider tire in a smaller diameter in the rear, due to the tapered trailing arm, in the front the smaller diameter leads to less clearance due to interferance with the strut. Unfortunately, it is not cut, and dried, that said, I will post the maximum backspacing possible on a TII shortly. This does not even consider caliper height, wich also must be considered. Brad Apold is close with 9.5" rims on the rear of his TII, but he is not maxed, 25BP is past max, and is having to regroup, I hope to get it right, since my wheels are custom 3 piece from Japan, and not returnable. I expect that I will be able to fit 18x12s with a -7mm offset, and a 18x10.5 with a -13mm offset, this is assuming wilwood calipers, and a max tire diameter of 25.5". This will allow a 345-28-18 rear, and a 320-30-18 front. I will post pics of the problem areas, but depending on suspension, each car will be different. I will be running Advanced Design coilovers. Carl