Wheel sizing - 2nd gen Firebird

[carbuff]

Yes, I know... Another one of *these* threads. Actually, I'm just looking for a little confirmation on the calculations that I've made.

I'm basing some of my numbers off of the 70 Camaro that I had previously and what I remember of my wheel/tire combination on that car. For it, I had a 17" x 11" rear wheel with a 5.75" backspacing. This was a stock width rear axle and no mini-tubs. Those tires would rub on the outer wheel well lip and occasionally (rarely) on the inner well.

For this car, I'm going to mini tub it and an 18" wheel. I took a bunch of measurements the other night with the stock axle in place, and my JRS rear housing/axle is stock width as well. Here is what I found.

From the drum flange to the outer wheel well lip, the minimum clearance that I have anywhere is 4.25".

From the drum flange to the rear frame rails, the minimum clearance is 11.25".

Those numbers were not on the same side, but I'm combining them to get a minimum space to work with.

Adding those, I have 15.5" to work with, plenty for what I want. I'll assume that I don't get the complete inner space after the mini-tub, so taking 1" off of that I'll work with 14.5".

I'm leaning towards a 305 or 315 width rear tire. I looked at several different manufacturers, and the average section width of a 315 on an 11" wheel is 12.5" (315/30/18 or 315/35/18).

I would prefer to have as much outer wheel lip as I can get, not necessarily a deep-dish look, but I don't like the center section to be as far out as the edge of the rim.

I'm probably leaning towards an 11.5" rim. So with that and the above numbers, my math tells me that I'm looking at an 8.5" backspace with 3" from the flange to the outer lip. I arrived at that by assuming the tire bulge is 1/2" on the inside and out, and using my wheel well lip as my critical point, the 3" + 1/2" bulge gives me about 3/4" clearance. That may be more than I need, but it's what I calculate.

I could be a touch more aggressive and move the wheel out another 1/2" and use 8" of bs which would reduce my worst case clearance to about 1/4".

I don't plan to drop this car in the weeds, so it's unlikely that the widest part of the tire will be close to the lip anyway.

Does all of that math make sense? Anyone else with a tubbed 2nd gen and wide rear tires have any comment on the backspacing that they chose with a given wheel width?

Thanx!

[carbuff]

Perhaps to make my numbers a little more clear, I drew a simple picture of the wheel and tire and mounting flange...

[DriverzInc]

I wrote a response on pro-t for you bud, but you can't forget than an 11.5" wide wheel, will measure 12.5" overall in width. You can't forget about the .50" bead on both sides. Also, backspace measurements are taken from the very edge of the wheel, so that .50" bead is included.

I was a little hard core over at PT, because I'm sick and tired of so called self proclaimed "experts" out there giving ill advice, so don't take it personally....

[carbuff]

Thanx Jon, headed over to PT in a sec...

I thought that BS was measured to the edge, but I saw conflicting instructions on how to measure it on a few different sites.

But if the section width of the tire is 12.5" as shown on a few manufacturer sites, the edge 1/2" of the wheel edge won't really matter for me, right? The tire width will really determine where I need to place the wheel...

As I've thought more about this, given that I'm not slamming the car low, and the Firebird wheel well is so rounded, it's hard to see how I'll have a real issue as long as I'm within the 1/4 - 1/2" that I'm working towards...

Thanx!

[DriverzInc]

Originally Posted by carbuff

Thanx Jon, headed over to PT in a sec...

I thought that BS was measured to the edge, but I saw conflicting instructions on how to measure it on a few different sites.

But if the section width of the tire is 12.5" as shown on a few manufacturer sites, the edge 1/2" of the wheel edge won't really matter for me, right? The tire width will really determine where I need to place the wheel...

As I've thought more about this, given that I'm not slamming the car low, and the Firebird wheel well is so rounded, it's hard to see how I'll have a real issue as long as I'm within the 1/4 - 1/2" that I'm working towards...

Thanx!

A 1/4 to 1/2" can be the difference between a tire rubbing on an outer fender, or not... I think it's a big deal. No matter what you may read, industry standard is that backspace is measured from the very back edge of a rim. If anyone tells you otherwise they have no idea what they are talking about. Ask any manufacturer, and they'll tell you the same.

So on that 12.5" wide tire (315mm = 12.5") and on an 11.5" wide rim, which will measure 12.5" overall in width (which again is industry standard) your sidewall will be exactly parallel to your wheel. Now if that is the look you are after, then go for it, but if you are like most guys, and want to have a little tire bulge pass the rim, go with an 11" wheel. At 12" overall, that 315 tire will look right at home. Call me anytime to talk more about wheel, tire, and the right backspace. We have recommended and proven custom figments for most cars, Which include brake, tub, and rearend variations.