Help Measuring Backspacing

[AU Doc]

I'm trying to make sure there's nothing odd going on with my car before I order wheels. I got out a straight edge, marked it at 13" to estimate the radius of a 26" tire up front. From what I've read on the inter webs, a 245/40/18 is a common front tire for an 18x8 with 4.75" BS.

However, when I measure at full lock, it looks like there's going to be some serious interference with the frame (sorry for the rotated pic). It looks like there's somewhere in the neighborhood of 2-3/4" of clearance from the mounting surface back to the frame. Is that normal for these cars? Obviously the existing 15" rallye's have been rubbing, and they're a 7" wide wheel (I think).

Am I measuring correctly? Is there something in particular everyone usually checks for?

[ccmc]

I'm running a Billet Specialties 17 inch with 5" backspace and a 245-40/17 and i rub the frame just like in your picture. The rub is only at full turn. An 8 inch rim with 4.75 backspace would be perfect for a 1969 Camaro

[AU Doc]

Thanks for the reply!!!!


I'm running a Billet Specialties 17 inch with 5" backspace and a 245-40/17 and i rub the frame just like in your picture. The rub is only at full turn. An 8 inch rim with 4.75 backspace would be perfect for a 1969 Camaro

That's what has me worried. The wheels I'm running now are 15x7, and they're already rubbing (and are a little shorter than the wheels I'm planning).

Are you saying this rubbing is normal, and I need to look for other areas of interference?

[ccmc]

Measure the backspace of your current rim. Take it off and lay it face down and measure from the mounting surface, straight up to the edge of the rim, but not the bead. Put a straight edge across the rim to get a precise measure. Your current rim may have more than 5" of backspace.

The rubbing is ok, but if my 8" rim came in 4.75 I'd run that instead of the 5". If I can find a hub centric 1/4" spacer I may use that to stop the rub.

Rick

[USAZR1]

15" wheels create problems all their own, especially when it comes to clearance issues. 17" and larger alleviate many of those issues.

[AU Doc]

Backspacing on the current rims looks to be about 4". So going to 4.75 is going to make the rubbing on frame worse. The bright side I suppose is the existing tires are 235/60/15, which is just a shade over 26" according to the online calculator. So the tire size I'm looking at is a little shorter which will help with the frame rub at full lock.

[pittpens24]

See if you borrow or rent a wheel fit tool. I bought one to make sure my custom ordered wheels would fit perfectly. I bought one and then in turn sold it to a restoration shop. Don't get the one with the crappy wire thing. The wheel fit was like $375 and actually simulates a wheel(only thing is you need a tire to bolt it in. Best thing is you can actually turn the wheel with the tire on the car as it will be and mess with the backspacing until you dial it in perfectly.

[ccmc]

Wow, 4" backspacing shouldn't be touching at all. What year is the car? 1969 takes a 4.75 backspace on an 8" rim. 67-68 is different.

Rick

[AU Doc]

Yep. It's a 1969. It's rubbing on both sides. Maybe a little worse on the driver's side, but as far as I know that may be due to the.... um..... heavy driver :)

[232_ray]

Your rubbing is coming from the tire bulge I'm running the same size tires on a stock YH wheels with 4-1/4 BS and it rubs exactly where yours is rubbing also. I'm trying to fit 9inch wides on a mostly stock car with C6 Z06 brakes with 18's also . Anyways 4-1/4 BS on a 8inch side wheels will give u the widest looking stance possible. Barely clearing the inner part of the outer fender. You should be fine using anything from a 4-1/4 BS to 4-3/4 BS.

[AU Doc]

I measured again tonight. I looks like at full lock I'm going to be rubbing on the frame in front of and behind the front wheel. That back is going to be the worst. Moving back to a 25.5-ish tire, there's still only about 4" from my straight edge to the frame. It's just tire rub, so I'm guessing everyone else just lives with the increased turning radius. It looks like everything else will clear with an 18x8 with 4.75" backspacing.


Out back, it looks like the exhaust is going to be the problem.

[ccmc]

I had to remove the stock dual exhaust hanger to fit 275s. I had the exhaust shop weld new hangers to the floor pans inside of the frame rail instead of outside of the frame.

I also ended up rolling the fender lip but it only very lightly rubbed when the trunk was full and i hit a bump in the road. And then only one one side.

[AU Doc]

What size wheel and what backspacing are you running?

[ccmc]

Front 245/45-17 on 8 in rim with 5" backspace
Rear 275/40-17 on 9.5" rim with 5.5" backspace

The front would be better with 4.75 backspace but the Billet Specialties Street Lites came in 4.5 or 5 so I went with 5 and just have a touch of rubbing at full turn only.

[130fe]

AU Doc, looking at your setup, you are running a stock subframe with stock style single piston brakes. Anytime you run a wider than factory tire F70-14 (aprox 215/70-14) you have the potential for rub at full lock. I bet there is a high percentage of us that have rub at full lock. I do, but honestly how often (and long) do you drive at full lock? A bigger problem people run into is not enough backspacing, this gives you rub issues on the outer fender. Most people are concerned about rubbing the outer fender. I just upgraded to a 9" wheel up front.

I ran a 17x8 with 4.75bs wheel and 245/45-17 on the front of my car for the past 13 years without any issues. That front backspacing is the perfect fit for a first gen on the front with a 8" wheel. 245/45-17 and 245/40-18 tires are the exact same size tire. On a 69, you can run a 9.5" rear wheel with 5.5bs like CCMC talked about. 67/8s have a little less clearance due to the shape of the wheel well but I ran a 9.5 wheel on my 68 with only minor issues.

Here are a couple of tools that could help you out when trying to compare one wheel to each other (old vs new)-

https://tiresize.com/wheel-offset-calculator/

[AU Doc]

That's interesting that you're running a 5" backspacing AND the 45 series tire. I was planning to run the 40 series with the 4.75" BS because it looks like there's still going to be rubbing.

[Billet-Specialties]

Visit our website here http://bit.ly/2wOFXJx for wheel measurement guide.

[AU Doc]

Apparently I never hit submit on my last reply.

Thanks for the info, Chris! I was beginning to suspect exactly what you said. I'm guessing most people just don't notice it. I only mentioned it because it seems so many people have posted about this exact issue (how many ways can you ask, "will this fit," right? :banghead: ), that I was surprised not to see anyone else asking about the rub on the frame. It made me second guess myself.