Check my Math: Wheel Widening Help
 

Okay here's the deal, I need help figuring out how to fit x width rim under x car. I have all information, I just don't know how to do the math.

Here's what it is.

I have a 1996 Mazda RX-7, I bought the stock JDM wheels from a 99 mazda rx-7. I want to retain the stock look, but the rx-7 is over 400hp, and I need to widen the stock rear wheels to maintain good traction. Here's the plan.

JDM Stock wheels
Front: 17 x 8 Offset: 50+
Rear: 17 x 8.5 Offset: 50+

Now, because the offsets are the same, I'm going to put the stock rear wheels in the front, instead of widening them, and just widen the stock fronts, and put them in the rear. This means:

Setup I want:
Front: 17 x 8.5 Offset: 50+ (originally my rear wheels, just put them upfront)
Rear: 17 x 10 Offset: 44+ (Originally my front 8" wheels, but widened them 2")

Now, I know the following:
17 x 8.5 Off: 50+ in the front of my car gives me .71" clearance on the outside of the fender, and .75" of space between tire and suspension. (running 245 tires)

17 x 10" Off 44+ in the rear of my car gives me 0.23" clearance of space from the outside of the tire to the inside of the fender lip, and 0.27" of space from the inside of the tire, to the suspension (running 275 tires). This is why I chose a 44+ offset, as the origanil 50+ offset with 10" wide rims would give me .04" clearance from inside the tire to the suspension and .046" of clearance to the inside lip of the fender.

I'm getting all this information with the attached FD (RX-7) Wheel sizing chart. It allows me to insert various offsets on the front or rear of my car in excel spreadsheet, and gives me the clearances I would have.

My question is this -

How do I widen a 17 x 8.0" wheel, with a 50+ (mm) offset, in such a way that I add 2 inches, and make the offset into 44+ mm.

For example: if I add 2" of material to the outside lip of the rim, I would make a 10" wide wheel, but shrink the offset down from 50+ mm, to 25+ mm. Width(10) / 2 = 5(center),

Or, if I add 2" of material to the back of the rim, I would make a 10" wide wheel, but grow the offset from 50+ mm to 75+ mm.

If I add 1" to the inside of the wheel, and 1" to the outside lip, I make a 10" wheel with a 50+ offset. 6mm off what I need (44+)

Check this math: If I add .75" to the inside of the wheel, and 1.25" to the outside lip, I make a 10" wheel with a 44+ offset. Turns out, when I explain things to people... I figure crap out. Can anyone verify that this last example would create a 44+ offset wheel?

Remember: Offset is the measurement of where the mounting face of the rim is, in relation to the center line of the rim. Ie, if a rim is 10" wide, the center is 5", and a 50+ offset means, the mount face is towards the outside of the car 55mm from the center of the rim. ie. from mount face to the inside lip of the rim would be half the rim width, plus 50mm, ie, 7" in this case. (25mm = 1")

Oh. Here's something else to consider. You lose 1/16 of an inch due to the material being cut. If you're adding to the outside and to the inside, you'll lose 1/8th of an inch from the 2 inches gained.

I'm still checking the math right now. Sorry it's taking a while. :(

Never mind. Actually. I just checked. I don't think you can add material to the outside of the wheel. Most people add material to the inside. You definitely won't be able to do both from my understanding. You can always add material to the inside and then space them out. Not sure if you want to go that route though.

yeah thinking of just adding 2" to the inside, you can add either to the outside or inside depending if yoru tire is front or reverse mount - I believe these wheels are front, ie, can only cut the backside of the rim - and yeah, 1/16" is what I lose on the backside. What would be idea then? I add 2" to the backside, and I end up with a offset of 75+, or 73.4+ after the 1/16". Add a 29.4mm spacer to get 44+, or 1" and .16". So... put a 1 and 1/8, or 1 and 1/4" spacer on the back. Not sure if that is too much for a 14 year wheel bearing or not. Might look damn good though