rx7eaven

I am thinking of going with 17's. I know that I am going with 8.5 in the rear but should I go with 8.5 in the front or just 7.5? Furthermore, I plan on running 225 or 235 in front but what can I fit on the back with 8.5?

also what offsets do I need? I hate to sound stupid but what do the offsets mean and how do I know which offsets I need?

Thanks in advance.

SleepR1

If you have an FD Rx7, try to go 8.5 x 17 all around to maintain the FD Rx7's handling balance. 7.5 x 17 up front is too narrow.

You can go 245/40-17 all around with 8.5 x 17s.

8.5 x 17 wheel offsets can range from 40 to 45 mm. Any offset <40 mm will push the wheel out too far and cause front end clearance problems with the fender lips, especially with the left front position.

Offset is simply the distance away from the wheel centerline--if you slice the wheel lengthwise, right in the middle. Positive offsets move the wheels inward toward the suspension parts. Negative offsets move the wheels outward away from the suspension parts. +40 to +45 mm offset means you're moving the wheels 1.6 to 1.8 inches toward the suspension, FROM the wheel's centerline.

Offsets are chosen based on tire section width, wheel overall width, tire overall diameter, front space clearance, and back space clearance

3rdGenLuvr

SleepR1: Ive been reading many of ur responses on this subject for the past few weeks now (very impressed w/ ur apparant knowledge ).

I'd like to hear ur opinion on goin wider than stock for the front end?
I have two friends heavily into cars. One has been working in a tire & wheel business for over 15 yrs now and the other is a part owner in a well known Porsche perf. shop. They both gave me a similiar response to the same question being asked about goin wider at all four corners.
They didnt think it was a good idea goin wider out front cuz it wld slow down responsiveness. I had pretty much come to the conclusion at the time I was gonna go w/ 17x9's at all four corners and run 255's, they thought I wld be better off w/ stock or 9" max out front w/ 235/245max and 9.5/10" w/ 265max out back.

Your thoughts?

TIA!

SleepR1

Depends on the tire size selection and aspect ratio. Anything in a 17-inch diameter with a 40-series aspect ratio will have a much shorter sidewall height than the stock 225/50-16 tire sidewall height. It stands to reason that the lower the aspect ratio, the shorter the sidewall height and the more responsive the steering feel will be, ie. 225/45-17 would be more sensitive to steering inputs than 225/50-16, eventhough the tires are the same width.

The main reason for going with wider tires when you "plus-size" is to maintain a similar amount of air inside the tires, as in the stock size--ie maintain the weight load index. Generally, when you squash the tire's aspect ratio, you SHOULD increase the tire width to maintain the same amount of air to support the car's stated load index. The stock load index for the FD Rx7 is 92 or 1389 lbs at each corner.

If you'll notice 235/45-17 and 255/40-17 tire sizes have a load index of >92, whilst 235/40- or 245/40- have load indexes of <92. The 245/40 is probably ok with an index of 91 (1356 lbs), but the 235/40 or 225/45-17s would be a bit too light for the car's load index spec.

Load indexes are important because those numbers are what the tire manufacturer says will support the car's weight under hard cornering and braking WITHOUT any fear of tire failure!

Going wider all around will increase stick around the turns, and maintain the Rx7's nearly neutral handling balance. Going wider in back than in front will increase stick in back, but not in front, and thus will dial in understeer. This is good for high speed turns and sweepers, but not good if you want to turn the car through multiple switch backs like during a slalom race or an autocross.

The reason we maintain same size tires all around is because of the Rx7's 50 fr/50 rr weight distribution. This same reasoning is used for BMW M3s. Their hot setup is 9 x 17 all around with 245/40-17 Hoosiers. OTOH, for Porsche 911s, their weight distribution is ~35 fr/~65 rr. It should be obvious that the 911's rear weight bias will tend toward oversteer at the limit. This is why 911s come from the factory with much wider tires in the rear positions.

The consensus among FD Rx7 track enthusiasts is to increase contact patch at all four corners for better stick through the turns, while NOT screwing up the FD's intrinsic balanced handling.

rx7eaven

disregaurd my post...I am now looking into doing what I said I would not do, buy some 18" volk le37t's. I am looking at 8.5 in front and 9 in back. The reason I did not want to go this route is the money but I like them so much

3rdGenLuvr

I appreciate the response.......I think

Who wld of ever thought picking a set of wheel dimensions wld be so tough!


Any sites out there where it says what width rim is needed for a specific size tire?

SleepR1

Actually, the tire size specs should give you a rim width range to work with. Typically you want a rim width that's at least ~90% of the tire's section width. This means if you're looking to mount 255/40-17s all around, you'll need 9-inch wide rims at a minimum. Some tire makers specify 8.5 inches as a minimum rim width for a 255 mm (10.04 inches), but that's only 85%, and would allow too much sidewall flexion to allow good handling feel through the corners!