The Stovebolt Site Forums The Shop Area General Truck Talk Backspacing

How in the heck do you measure backspacing/ One sorce says from the inside mounting to the wheel edge, the 1010tire.com that is a link in here in the bolt says from the inside mount to the wheel centerline.

I've always heard it is measured from the inside mount to the wheel centerline as well. The other way wouldn't make sense, that would mean every different size tire would have a different backspace.

I've always gone by wheel centerline like shown here.

To the wheel centreline measures offset, I think. To that figure you would add 1/2 of the width of the wheel to get backspacing.

I agree with all here I was misinformed by
http://rallyamerica.com/measure/index.html
check out their diagram. makes you wonder
how good they are huh?

Gazim:
Your "source" is correct, the distance from the mounting surface to the center of the wheel is referred to as "offset". Backspacing and offset are related but seldom the same. When you are measuring to see if something is gonna rub, backspacing is usually the number to use. Don't forget there is about 1" of tire bulge also.

BigChevy3600:
From the mounting surface to the centerline of wheel is referred to as "offset". Tires do not have backspace, "wheels" have backspace. ;-)

Ghostrider:
The link you provided refers to "offset", not backspacing. :-)
Gazim wanted to know how to measure "backspacing". :-)

hotshoe36:
You are correct! Remember that "offset" could be a negetive number. It usually is not, but could be. "Offset sometimes is 0 (zero) which means the mounting surface is exactly in the middle of the wheel.

Finally Gazim,
Sorry I am being so long winded. To measure backspacing, lay the rim (without tire) on the floor or table and lay a straight stick across the backside of the rim and measure down to the mounting surface. Wallah, backspacing. This number plus 1" for tire bulge plus another 1" is what will be needed to keep the tire from rubbing the frame. Ask me how I know. Remember If the WHEEL is too wide the front side could rub on the outer fenders, not usually a problem but somethin to consider if the wheels are much wider than about 8 inches. Also, I am speaking in generalites here, not all vehicles were created equal.

Short story: Fella brings me 2 wheels, one ugly steel and another pretty aftermarket one and asked me what the difference was. I measured the rim diameter, width, backspacing, bolt pattern and told him there was no difference except that one was prettier than the other.

Hope I helped out and didn't confuse anybody. Just speaking from experience.

To correctly determine backspacing, lay the rim with the surface that butts against the hub/brake drum up. (On a whitewall tire the whitewall would be on the ground). Lay a straight edge across the rim (not the tire)...outer rim edge to outer rim edge. Measure the space vertically from the mounting surface to the bottom of the straight edge. That's your BS.

Determining your exact tire fit is a bit more complicated. I found that bolting a 14" piece of 1/8 flat stock to the mounting surface (rear wheel removed) and then clamping a piece perpendicular (like a "T") can be used to measure the space needed on the inside (bed wall side) and to the outside(fender). The tire sites give you the dimension of the cross-section of various tire sizes. Let's say you were using a 7" wide rim....and your differential is 62" from mount surface to mount surface. A rim with 3-1/2" BS would center the rim in the AD wheel well. If you divided the cross section in half you could make your perpendicular arm the same measurement and rotate the drum to make sure the mounted tire would clear.


I've found that with an AD 62" mount to mount diff...that an 8" rim with 4" BS, up to and including a 255/70/15 tire will fit. This is assuming your frame is straight, the diff is centered, and your box is properly centered. On the front a 7" rim with 4" backspacing and a 235/70/15 will give you a full wheel-well, between 3/4-1" tire clearance from both the stock and upgraded tie rod ends and clears the 5 lug disc brake conversion. This is the combo I run on my truck.

If I had it to do over again, I'd go with 7" rims with 3-1/2" BS and 235/70/15's all the way around. Hope this helps.

Dave

JJ thanks, I will print this out and hang it on the wall for future reference.

Dave I,m getting confused again but thanks. I will fugure it out when I go to buy wheels I think I will use8x18 wheels with
245/50 tires in the rear 225 in the front The more I read I better go to a tire store and not order online. They sure have to fit.

Gazim:

Rallyamerica did not go into detail very well.
If they would have not used the word "offset" then they would have been correct. Good Luck

Most of the good tire shops have a "store-bought" device like I rigged up, to verify rim and tire fit. With 18X8's you would be wise to check everything out locally. Another factor to figure in is how much you've dropped the suspension.

I had hoped to get some rims that I liked with a 3-1/2 BS but I just couldn't get what I wanted in that configuration. Isn"t that the way it goes? Good luck with your hunt!!!

Dave

Dave you lucky son of a gun, thats what I am taking off and I will be in Fl. next week. Sorry I wont be driving the truck, but you can check out the pics on the Gallery. Just pokin fun.
Zim

Nice truck....too bad you'll miss the Daytona Turkey Run! Should have 6000 registered cars/trucks/rods and 2000 more in the corral "for sale."

Should be a great gathering! BTW.....when you "come on down" be sure to bring plenty of green stuff.....we can use all you care to bring!!!! Safe journey,

Dave

Dave, I might not have been at the turkey run but a Mi. buddy took his74 cutlas there. I'll get a full report.

I go down with a lot of plastic, not gree n.

Gazim:

Take a look at the latest issue of Classic Trucks (Jan 2008). There was an article in it titled "Wheels 101" which describes the different measurements and shows some good graphics.

If you would like I could always photocopy and send to you.

Kent Butler
'56 Chevy 3100
Houston, TX

Thanks JJ. I'm going to print your info as well. You made it very clear.

Big Chev, what size rims are you using on the rear?

27Capitol,

You are welcome. Last set of wheels I bought they were using the term "offset" and I didn't know what to buy until I could understand what "offset" meant. A few days later and lots of research I had it figured out. It is kinda like Metric and SAE. Who needs the stinkin metric ??, or offset. Some wheels don't come with a backspacing number and it has to be figured from the "offset" number. I try not to buy those wheels.

The wheel manufaturers are really pushing the whole "offset" thing on rims these days. They're going for keeping correct scrub radius by keeping the wheel centerline in the same spot. But when dealing with wider wheels, sometimes you have to sacrifice that for correct backspaceing to prevent rubbing. The issue nowadays, is that most newer cars center the wheel directly over the spindle centerling, whereas older cars weren't on center. New cars and trucks are sensitive to changes in offset.
When you look at wheels, insist that they give you the backspace dimension. If they can't/won't provide it, look elswhere.