Tire "Bulge"


Turbo'd since '95
I want to buy some aluminum rims like Draglites or maybe the Summit/Jegs knock-offs. I've got 275/60*15 MT ET's on stock rims right now and I've got less than 1/2" clearance to the frame rails. So I was planning on 15 x 8 rims with 4" BS. My question is what will happen with the tire's "bulge" when I go to the 1 inch wider 15 x 8's from the stock 15 x 7's with the same BS? I'm thinking with the wider rim there should be less bulge, and therefore more clearance? I also now see that there are not many 15 x 8's with 4" BS. But the way it is if I lose 1/2" they should hit for sure - unless the bulge thing helps? I also thought about aluminum drums? I've used spacers and don't really want to use them permanantly. Any suggestions would be appreciated.
The amount the tire bulges depends on tire section width, tread width, profile (50, 60, 70) and wheel width. It also varies with tire brand and construction.

There are two ways to determine how wide your tire will be when you mount it.

1. Actually mount and measure.

2. Get the info from someone with the same exact tire make, model, size and the identical rim width. Then you will be able to get the overall width without buying anything.
There is a rule of thumb

When you look at tirerack for instance, it will give you a tread width and a section width for a given size tire. The tread width will not change with wheel width (your contact patch, however, could change due to rounding say fat tire on a skinny rim).

The section width (what you are talking about) will change. Here is an example, without looking up exact numbers...

If the spec says section width = 7.5" on a 7.0" rim, but the rim you want is 8.0", the section width will grow to 7.9". The rule-of-thumb is +0.4" for 1.0" of rim above or below the number they give you in the spec.

If you vary the rim by 1/2", then use 0.2"

My old neighbor has been working as an engineer in the tire manufacturing business for 35+ years. He is my go to guy for info.
Thanks zr2tim, rule of thumbs good enough for the girls I go out with. :D So if I go to a 1" wider rim, the section should increase by .4" or .2" per side right?
I have a set of 15''x8'' widened GN rims with some Good Year Eagles 275-60-15's on them and the wheels have a rear spacing of 3.5''. More r.s. than that will have the tire's sidewall bulge to rub against the frame!...:rolleyes:

Claude. :smile: