Backspacing on wheels

[fast7878]

17x10.5" wheels with a 6.8" Backspace, 315 tire, 1.25" notch, 59.5" rear end, leaves a finger left in clearance...

 

[Kwik_Cutty88]

I have 15x8" with 4" backspacing. I also run a 26x9" slick on these wheels (which is actually a 10.5" section width). There is just enough room on both sides, but it does not rub. My car is stock ride height in the rear, stock width axle, no frame notch.

 

[rogue_ryder]

Not all wheel backspacing is equal:

A wheel with 4" of backspace and Zero Offset will fit differently than a wheel with 4" of backspace and positive or negative offset. A wheel that may fit without hitting the frame with zero offset but if a wheel with 4" of backspacing and a positive offset may not fit!

Backspacing

Backspacing means the space behind the mounting plate measured to the inside lip of your rim. If you’ve got that same 18 x 8 inch rim and it’s got a 3 inch backspace, that means there are 3 inches between the mounting plate and the back lip and there are 5 inches left on the front side.That makes these rims have a deeper dish than the same rim with zero offset and 4” backspacing would have.

Offset

Offset is either called negative or positive and it’s measured in millimeters with either a + or – symbol, meaning positive or negative offset. And you thought you’d never use the metric system in your life! Since the offset of a rim is always measured in millimeters it’s important to know this conversion = 25.4 millimeters is equal to 1 inch. Even if you need to use a calculator when checking out a set of rims, at least you’ll know what you’re getting into.

If you have a negative offset, that means the mounting plate is closer to the suspension side and further away from the outside of the rim, leaving you deeper dish rims,which typically makes them stick farther out from under the truck than your stock rims. A positive offset means the mounting plate is closer to the outside of the rim, leaving you a smaller lip (or almost none at all) with a larger space behind the rim which keeps more of the wheel under the truck

Here’s an example: With our same 18 x 8” wheel, if it has a 0 (zero) offset, then the back of the mounting plate is dead center in wheel. And dead center of an 8” wide wheel with 0 offset would mean the backspace is 4”.

If the offset of this wheel is +25, that means the mounting plate is 25 mm closer to the curb (about 1”), giving you a wheel that sticks under the truck more than one with

 

[lilbowtie]

A wheel with 4" of backspace and Zero Offset will fit differently than a wheel with 4" of backspace and positive or negative offset. A wheel that may fit without hitting the frame with zero offset but if a wheel with 4" of backspacing and a positive offset may not fit!

A wheel with 4" backspace and zero offset is an 8" rim. A wheel with 4" backspace and negative offset is going to be a rim less than 8". If you are going to use an 8"rim and know the tire your going to use you can determine your backspace. Using a straight edge across the face of your brake drum measure the distance to the frame. You need to look up the specs of your tire and find the cross section width (max width) and take half of the distance between cross section minus rim width and add it to your backspace which needs to be less that the measurement you took from the drum to the frame with clearance. Using Kwik Cutty as an example he has an 8" rim with 4" backspace and 26x9 slicks that have a cross section of 10.5 minus 8 which is 2.5" half of which is 1.25" added to 4" is 5.125 and he has clearance. I run an 8 "rim with 29.5x9 slicks that have a cross section of 11.4 minus 8 which is 3.4 half of which is 1.7" added to 4 is 5.7" which didn't give clearance with a 4" backspace so I had to go to 3.5 backspace

 

[DrFrankenstein]

is frame notching the only way, or can wheel spacers be added?
I'm trying to run 295/50/15's on the back, but I'm touching frame. 1/2" would clear it and I dont want to cut the frame.

 

[Wraith]

Not all wheel backspacing is equal:

A wheel with 4" of backspace and Zero Offset will fit differently than a wheel with 4" of backspace and positive or negative offset. A wheel that may fit without hitting the frame with zero offset but if a wheel with 4" of backspacing and a positive offset may not fit!

Do not confuse backspacing and offset, they are speaking about the same thing but in different terms. Backspacing is backspacing, offset is just another way to measure it. A 15x8 wheel with 4" of backspacing is the same as a 15x8 ET0 wheel, both measurements state that the mounting surface is in the middle of the wheel. A positive offset wheel would be what most front wheel drive cars use, you wouldn't have any problems with 4" backspacing and positive offset because the 15x8 wheel would only be 7" or thinner.

Also when trying to stuff as much tire as possible it helps to square the tire on the wheel, when a huge tire mounts on a wheel with not enough width it pulls it in and baloons the sidewall pushing the tire wider than the wheel. For a drag car this isn't the same but for street cars not needing the sidewall action you would have a better tire contact patch to the road. 8" is the minimum recommended for a 295 with 10.5" being the maximum recommended width, 8" will roll the sidewall, 10.5" would stretch the sidewall.

 

[Qdub24]

Backspacing does not change with the width of the wheel as offset does. 4.5" backspacing is known to clear the rear frame rails.

 

[Qdub24]

Backspacing does not change with the width of the wheel as offset does. 4.5" backspacing is known to clear the rear frame rails.

 

[lilbowtie]

We are over thinking this. Backspacing is nothing more than a tool to get a wheel/tire combo under you car, offset will fall where it may depending on the rim width. Your drum to frame measurement is about 5.5" obviously you need to be less than that w/clearance

Here is a pic of a stock Olds rally rim 7" wide w/ 4" backspace and a P235/60/15 tire. It has just enough clearance, a tire w/ a bigger cross section wouldn't fit

Here is a pic of a 10" rim with P235/50/18 / 5" backspace, the cross section of the tire is about the same size as the rim

Here is a pic of my 8" rim with a 29.5/9/15 slick w/ 3.5 backspace

As you can see it's the wheel tire combo which is going to determine backspace needed and the tire cross section plays a big part

Dr Frank you can use a spacer to make your wheel/tire fit BUT make sure you have a long enough wheel stud to do so