Back spacing

[RivNut]

Alright guys, I need some one to either tell me where my thinking went wrong or to confirm that I'm not making this up.

I recently bought a set of tires and wheels (1 year old, 1,500 miles) to go on my '64 Riviera. ( I won't tell you what I paid for them, you'd just cry.) The tire / wheel combo is 245/40R20 Falken tires on Boss Series 304 Chrome rims - 20" x 8.5". I checked the back spacing on the rims before I bought them and it was a little over 4.75". I thought "too much" but the guy at the tire store said they came off a '63 Riviera. The PO pulled the tires and wheels before he sold the car so his dad could install them on a '94 Impala SS. Opps, wrong bolt pattern.

I was still concerned about the back spacing until I put the new tire / wheel combination up against what I've been running seems like forever on my '63 and '64 - 225/70R15 on the OE 6" rim. I used this calculator.

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

My revelation was/is that backspacing on the wheel has nothing to do with how the tire/wheel combo fits on your car. You have to measure the back spacing from the inner sidewall of the tire. If you run a wider rim, you're going to have a deeper backspacing. Stands to reason. The wheel is not the problem when it comes to rubbing, its the inner sidewall of the tire.

When I compared the two combos side by side, the calculator told me that there would be an additional .4" of tire toward the chassis and the same .4" of tire closer to the wheel well.

Play with the calculator and see if you see what I'm thinking.

If you have a set of custom wheels made that are 8.5" or 9" wide and specify a 3.5" backspacing, aren't you going to have a lot of tire and wheel close to the fender?

Someone either confirm what my thoughts are or let me know where my thinking went south.

Thanks,

Ed

Edited April 18, 2011 by RivNut (see edit history)

[CTX-SLPR]

Ed,

The stock backspacing on a '64 is 3 3/8in (3.375in) and with the Metric "stock" 225/75R15's I get rubbing on the frame behind the kingpin axis at full lock. There are 2 critical dimensions you're going to have to measure to see if they fit, first is the wheel backspacing (known at 4.75in) the second is the width of the tire when mounted on that width wheel. Most manufacturers of tires will give you the section (max) width of the tire on "recommended" wheel size then you need can calculate using 0.2in change in width for every 0.5in change in rim width. Divided by two this will give you the "bulge" past the rim width so you can account for it.

Tirerack.com doesn't list your particular tire but by using a Firestone Firehawk Wide Oval Indy 500, they list at 9.8in of section width on an 8.5in wheel. That give you only 0.85in bulge past the wheel and some of that is actually taken up by the wheel bead surface as well. Nominally that's a 5.4in of tire behind the wheel mounting flange if your 4.75in backspacing was for the rim, not the tire as well.

A stockish Kuhmo Solus KR21 225/75R15 has a width of 8.8in on a 6in wheel which means it has 1.4in of bulge which added to a backspacing of 3.375 gives you an effective 4.775in total backspacing. Mine rub at that amount of backspacing and it's not a small rub either from the witness marks. I'm not surprised yours rub, even if the total backspacing including tire is 4.75in. I'd suggest running a small spacer, say 0.25in, to push the tires out away from the frame. Order them from wheeladapter.com and they will custom machine them with hub centric locating rings to include ones for aftermarket wheels so you lessen the load on your wheel studs.

On a technical note... a wheel that fits a '63 or '64 Riviera bolt pattern will fit a 91-96 B-body (including Impala SS) but the backspacing might be wrong as those cars used a lot more back spacing. The back spacing for a particular car is relatively fixed for any wheel width, its that sidewall bulge that will allow you to run more back spacing as your wheels bet bigger and the side walls get shorter and stiffer. Your thinking is basically correct but like I did in the calculations above, you need a wheel and tire combo with a proper effective backspacing to avoid rubbing.

Your mileage may vary, all cars are not assembled equal, manufacturer tollerances are for refference only, and if you can break a crowbar in a sandbox or singe your eyebrows off with a bic lighter leave modification work to someone else.

[mt65riv]

...

On a technical note... a wheel that fits a '63 or '64 Riviera bolt pattern will fit a 91-96 B-body (including Impala SS) but the backspacing might be wrong as those cars used a lot more back spacing....

Your mileage may vary, all cars are not assembled equal, manufacturer tollerances are for refference only, and if you can break a crowbar in a sandbox or singe your eyebrows off with a bic lighter leave modification work to someone else.

You are correct, sir! 5 on 5". The main thing on the Riv's is the drum fins on some 15" wheels.

My '94 Impala SS has stock 17x8.5 with 5" bs. The steel wheels on Caprices were 15x7 w 4 1/2" bs.

I have 20x8.5 w/5" bs Centerlines on my '95 RM Estate which required longer studs and 1/4" spacers and they work fine. They would fit on the '65 if I put 1/4" spacers which would require longer studs, too. I set them up on it and didn't care for the look.

I love your tag... I've seen it around. Is it from something?

[CTX-SLPR]

You are correct, sir! 5 on 5". The main thing on the Riv's is the drum fins on some 15" wheels.

My '94 Impala SS has stock 17x8.5 with 5" bs. The steel wheels on Caprices were 15x7 w 4 1/2" bs.

I have 20x8.5 w/5" bs Centerlines on my '95 RM Estate which required longer studs and 1/4" spacers and they work fine. They would fit on the '65 if I put 1/4" spacers which would require longer studs, too. I set them up on it and didn't care for the look.

I love your tag... I've seen it around. Is it from something?

My "Your mileage may vary, all cars are not assembled equal, manufacturer tollerances are for refference only, and if you can break a crowbar in a sandbox or singe your eyebrows off with a bic lighter leave modification work to someone else."

Can't say it's totally original but I made it up to illistrate that while math is great, not everything lives up to it's published dimensions and if your skills seem to tend towards distruction (nothing aimed at anyone in particular), leave modification work to someone else.

[RivNut]

Not unlike some of the guys I've worked with:

"Measure with a micrometer, mark with chalk, and cut with an axe."

[1965rivgs]

Backspacing has been discussed and is critical because the wrong backspace will forces the wheel to contact the fins on the brake drum before seating on the axle. This situation creates the false impression the wheel is properly seated and can create a condition where the lugs will loosen.

Tom Mooney

[CTX-SLPR]

Backspacing has been discussed and is critical because the wrong backspace will forces the wheel to contact the fins on the brake drum before seating on the axle. This situation creates the false impression the wheel is properly seated and can create a condition where the lugs will loosen.

Tom Mooney

For a 15in or maybe even some 16in wheels I'd agree but for anything 17in and up... I don't think there is even the chance of that happening.

[1965rivgs]

My reference was in large part aimed at those folks who want to fit the later model Buick chrome wheels onto the earlier cars or any of the aftermarket wheels which are close in diameter...but the caution is relevant no matter what wheel one is considering.

By the way....

"I'd suggest running a small spacer, say 0.25in, to push the tires out away from the frame."

Wont a smaller,thinner spacer bring the tires CLOSER to the frame???

Tom Mooney

[CTX-SLPR]

My reference was in large part aimed at those folks who want to fit the later model Buick chrome wheels onto the earlier cars or any of the aftermarket wheels which are close in diameter...but the caution is relevant no matter what wheel one is considering.

By the way....

"I'd suggest running a small spacer, say 0.25in, to push the tires out away from the frame."

Wont a smaller,thinner spacer bring the tires CLOSER to the frame???

Tom Mooney

Ah... excellent point considering how popular that look is and beyond the Buick roadwheels, the popularity of wheels designed for a 73-87 C10 which also would have questionable backspacing.

If he's not running any spacer, the wheels would get farther from the frame. I missed it if he was running one right now. Beyond 0.25in I think a spacer is dangerous and an adapter would be needed which does have a minimum thickness though which I think is around 1in but I haven't looked for less than that honestly. I'm playing this game with trying to fit a 245/50R18 up front and a 275/45R18 out back on various different wheels.

[Turbinator]

On 4/19/2011 at 9:03 AM, RivNut said:

Not unlike some of the guys I've worked with:

"Measure with a micrometer, mark with chalk, and cut with an axe."

I had a customer sign a contract with a yellow lumber crayon. I told him to press hard he was making multiple copies. Deal went through.

small world