Well, I was all ready to order some new 15x7 Weld draglite wheels for the front of my 'bird and called Summit. Weld said to order 4.5 in backspacing, but the guy from Summit said that 3.5 would be better as it won't set the wheel as far in.
Unfortunately, the front wheels I now have on the car are these tiny Centerlines made for drag racing and the rear wheels are on a Ford rearend (I know, heresy, but that's the way the car is), so I don't have something to measure for sure.
Jerry...Go with what Weld suggested (4.5") as I believe this backspacing is what GM used back then...I believe the 3.5" backspacing would make the wheel stick out too far.
I think OE used 3.5" BS for the 6" wide rims, and maybe as much as 4"BS for the 7" wide rims. This was with the factory skinnys (215 wide maybe?).
I ran an 8" wide 4.33 BS on the front for a while, and it put the 235 tire even with the fender so had to go down to 225. With a 7" wide rim, it might've been about perfect I think. Too much BS and you can add a shim or bolt-on spacer if you have to. Too little BS and tough luck.
What is the difference between Rally II wheels of different codes other than the year or application? Will all 15-7 Rally II's have the same backspace?
I can't say that I've ever heard of a 3.5" backspace on a first generation Firebird or Camaro. You would have to run some mighty skinny tires or else they would stick out of the wheelwells and look silly! I know for a fact that the first gen Camaros used a stock 4.5" backspaced wheel, and the Firebird and Camaro are the same body. As a matter of fact, all A and F bodied GM cars used a 4.5" backspacing. A 3.5" backspaced wheel would seem like it would be for todays cars.
The weld wheels I am sure are made differently than rally's and may need 3.5" backspacing. The original rally's needed 4.5" backspacing because of the way they were designed. Each wheel is different and needs a different backspacing.
Okay, so here's what was finally suggested to me and it makes the most sense. Measure the space behind the wheel from the hub then measure space in front of the wheel to the fender. Tell the measurements to the tech guy at summit along with the tire size and they will figure out what backspacing I need.
Thanks for all the comments, seems like this is the best way to make sure I don't get stuck with wheels that look weird.
Factory wheels on these older cars generally needed to run close to a zero offset, +/- as much as 1/2". Excessive offset could cause wheel bearings to wear prematurely.
The drum and disc brakes' hubs sat about 1/2" or more different relative to the spindle. Drum cars generally only ran a 6" wide rim.
Wheel face design determines caliper fitment, which has nothing to do with backspace or offset. If you determine what backspace you need by measuring your car, that is the backspace you want, regardless of which wheel design you need. Not all RallyIIs will fit the early 4piston brakes because of the shape of the wheel face, not because of their backspace.
I currently am running two different width 15" rally IIs. I think front is 7" and the back is 7.5", but not sure. I'll check at the next balance. I believe their backspacing is very similar, and may be between 4-4.5"
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Tell the measurements to the tech guy at summit along with the tire size and they will figure out what backspacing I need. </font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Don't let some clerk call it.. you are on the right track researching this first..
That's a pretty detailed diagram, but that would suggest that you have a rim that is right in the first place to measure. My problem is that the car came with a Ford rear end so I can't measure those wheels and the front wheels are tiny Centerline drag wheels. Now, would the centerlines have the correct backspace? I would think not, but if I'm wrong, please correct me :-)
Measure the backspace of your current front rims by putting a straightedge across the back of the rim, and measuring to the hub flange. Install the rims on the car and turn the steering to left lock, measure the distance between left tire and frame. Turn to right lock, and do the same. measure both sides, both positions. Take the smallest distance, and add to your current backspace measurement. Subtract 1/2" for clearence/sidewall flex, and that should be the maximum backspace you can fit.
For fender clearence, you need to know how wide your centerline is, and subtract its backspace. That is how far it sticks out past the hub. Determine how much further, if any, another rim could stick out without touching the fender at any time. This will allow you to determine how wide you can go, and your offset, with respect to backspacing.
With all of these measurements, you should be able to determine maximum backspace and width of rim and tire that you can fit. Always go a little thinner to compensate for sidewall flex.
The backspace measurement can be the same for any width rim and clear the frame, but you have to consider how far you want the rim to stick in or out (offset), so you use the maximum backspace measurement as a guideline/safety.
Like I said, a 3 1/2" backspaced wheel would look silly and incorrect!...Get the 4 1/2" backspaced wheel in whatever diameter and width you want and you'll be happy you did! :p
![[Linked Image]](http://www.volvoadventures.com/wheels/wheel_offset.gif)
