Machining Wheel Backspace?

fastsc92

Registered User
After a month or so of searching for the right wheel(s) to fit on the front of my project car, I've come up empty handed in finding a wheel with the correct backspace to sit right in the fender.

Background info:

I need a 17 x 4.5" rim to clear 13" front brakes. My measurements and trial fitting indicate that a 1.75" backspace will cause the wheel to stick out too much. There are only a few manufacturers that offer a 2"-2.25" backspace for this size wheel, and those that do are around $700ea. (Billet Specialties & Weld). My latest measurements look like a 2.25" backspace will fit nearly perfect, but a 2.375" would look even better (on a 4.5" wheel).

Here's my question...I have a set of 17x4.5 wheels with a 1.75"BS that have an extremely thick center hub section. It's so thick that the studs don't even pass thru the lug holes. These wheel use a mag shank style lugnut. The depth of the lughole is 1.5", with the hub itself being well over 3" thick of solid aluminum. I've heard of wheel shops that machine down the backside of wheels to achieve custom backspacing, but I don't have any experience in the matter. My thought is to take off 1/2" from the backside to achieve a 2.25" backspace. This would still give 1" of thickness in the lugholes and nearly 3" for the hub itself. I'd be able to use .75" long mag shanks without issue. Weld and Billet Specialties indicate that their wheels use a 1/2" mag shank, so their lug holes are even shallower than what I'm starting out with and ultimately what I'd be left with.

I have access to the equipment to make the modifications, but again I don't have any experience with this practice of modifying wheels in this manner. Has anyone done this in their past lives or seen this done before? I can post pics if it helps.
 
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Update to this:

I ended up calling the manf. to see what they had to say about it, even though I decided to cut them anyway. They informed me that this style wheel is designed to have 2" of nominal caliper clearance and that they recommend a maximum of 3/4" taken off the backside of the mounting pad. This is mainly due to the depth of the counterbored holes as they come off the production line. My initial fitment with a 1.75" backspace had some decent, but not unlivable wheel stickout from the fender. 2.25" would make it just flush with the fender and anything beyond that is gravy. I settled on .625" taken off the backside with a resulting backspace of 2.375". This allowed me to use a .75" mag shank lugnut and still have 7/8" of underhead thickness where the lugnut sits. If my numbers are correct, it'll leave me around 1/2" caliper clearance. I'd love to tuck these in a little more, but I don't want to push the envelope of what they recommend.

For the rears I may go 15 x 8 with a 6" BS. I tried to measure for a 10", but I don't think it's feasible unless the backspace is 6.75" and even then I'd expect to be trimming the upper control arm and having issues with the e-brake cable entrance. I'll cross this bridge in a few weeks time....
 
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What kind of wheels are these?

In the rear, I expect an E-brake is not an option with that deep of a 15" barrel. If it were me I would not compromise on wheel fitment and I'd trim the control arm.
 
What kind of wheels are these?

In the rear, I expect an E-brake is not an option with that deep of a 15" barrel. If it were me I would not compromise on wheel fitment and I'd trim the control arm.

The wheels are RaceStar, 92 Style. They are a simple 5-spoke design that mimics a lot of the looks of an AlumaStar from Weld or a StreetLite from Billet Specialties. As far as construction and quality, they are representative of a cast aluminum wheel manufactured overseas, and on par with their price point. The front wheels weigh around 16.5lbs ea and the rears would be somewhere around 15lbs ea. Certainly not as light as welds or billet specialties but those front wheels are around 3x the cost of these, and I just couldn't justify spending that coin for the goals of this car.

Back to the rear issue...I bought a wheel fitment tool to aid in my measurements and overall, a 10" wheel will be tough to fit without looking goofy (I'm sure you're much more familiar with this than I). Even with an 8" wheel and a more common 5.5" BS, the tire will stick out beyond the fender. A 6" backspace on an 8" wheel seems to be the best. Setting the tool up for a 10" wheel with a reasonable backspace looks to be a challenge with the control arm, shock body and actually some of the lower control arm itself. I'd need something in the neighborhood of a 7" backspace to pull off anything reasonable. Even still, it would stick out past the fender around 1". More measurements will tell the final story but I certainly welcome feedback from those that have more experience than I and have traveled down this path before. A 15x9 with a 6.75 backspace would be the best compromise.

As far as the e-brake goes....it would be a nice to have and I'd like to keep it if possible, or perhaps make some steel tubing bends to guide the cable in the correct location. I really wanted to run cobra rear brakes, but initial fitment just doesn't seem like this would happen, at least not with the 15x8 draglites (5.5" backspace) that I used for a trial fit.

The good news is that these wheels have a lot of clearance built into the mounting pad and spokes so I'm confident that you could bury the caliper in the wheel to maximize (or optimize) backspace to your advantage.

Again, I welcome any feedback.
 
Figured I'd update this issue.

Ended up machining my own backspace. Went from a 1.75" BS to a 2.375" BS. Again, this is on a 17 x 4.5 wheel matched up with a 27 x 4.5 x 17 front runner. There is a finger's thickness of clearance to the caliper. I could prob. go another 1/8", but the effort involved in the machining setup prob. isn't worth it. Maybe I'll save that activity for a rainy day in the future. If I had to do it again, I'd make it an even 2.5" BS which would still allow me to run a .75" shank stud and have 3/16" clearance to the caliper.

Overall it think it looks pretty good and fits the car well. Now on to the rears..hoping I can squeeze a 15 x 10 out back with another custom backspace.

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Looks good to me.

I used some heavy zip ties to hold the ebrake cable to the sway bar link. And i dont crank up on the handle. Only like 4 clicks.
 
The pics i posted in your front wheel thread show how they stick out. I'll have to measure the BS, but i think its 6.5". The subframe is cheated a little to the right in the pics. I've noticed a few cars with that problem. I can get pics of the cable tied to the link.
 
Here is pics of my zip tied e brake cables. This is with a '99 17" cobra wheel widened to 10" with a 3/8" wheel spacer.

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I run a 15x9 in the rear, I highly doubt a 15" 10 would be possible

Anything is possible. It'll be a tight squeeze but I do think a 15 x 10 will fit. Keep in mind that i'm not planning on it being a direct bolt-on. This will require new upper control arms. Not a huge deal because I will be running tubular upper and lower arms. It'll require some decent backspace (6.875-7") to be able to tuck it and not stick out. Wheels are on order...I'll post pics when I get around to putting them in there.
 
The control arm is not an issue. The upper shook mount and sway bar will be what you contact first. The wheel I believe you can fit, but add the section width of a drag tire and that's where I think you'll have problems
 
The control arm is not an issue. The upper shook mount and sway bar will be what you contact first. The wheel I believe you can fit, but add the section width of a drag tire and that's where I think you'll have problems

Just for reference purposes, what are the specs on the 15x9 you're running?
 
6.5bs with a 38mm offset. If everything is perfect and section width of the tire is in check. I think it can work. But too close for comfort IMO. if I knew I was running a radial with a straight sidewall like a street tire I'd worry less. But I'm not sure the added width is worth rubbing something or cutting a tire.
 
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