Wheel Backspace Makes My Head Hurt

[MAGGEE]

I’ve got a 1972 K10 with 2 inches of suspension lift. I’m ordering a set of wheels from Detroit Steel Wheel. 20x8 or 20x9 (more likely). Backspacing is custom. Up to 4.75 inches on the 8s and 6.00 on the 9s.

This is similar to the stance I want. Front and rear. Even with or just a bit outside the fenders.

Initially, I’ll be running a 34 inch tire with 11 inch tread width (275/65/20).

Calculating the proper backspacing is making my head hurt. Any suggestions? Should it be the same front and rear? Most of the backspacing info I’ve found for our trucks relates to dropped trucks seeking to keep the tires inside the fenders.

I need help.

[harpo231]

depends on tire clearance

[Mike C]

I am quite happy with the 4.5 bs on the 9.75" wheels on my Jimmy. I am running the outers from an 8 lug Suburban on the front and the SF 14 bolt from same truck in rear though.

[MAGGEE]

What does “depends on tire clearance” mean?

[davischevy]

I think I know what your asking.

A 9" wheel is 10" to the outside of the rim.

9" wheel with 0 offset has 5" backspace and is 5" from the wheel mounting surface to the very outside front of rim.

Negative offset moves the wheel rim out, positive moves rim in.

With a lifted K10, clearance shouldn't be a problem unless you go to taller tire.

Put a plumb bob or level on the face of the fender, and measure back to the wheel Mounting Surface (WMS). For a 9" wheel, If it measures 5", you need 0 offset/5" backspace. If it measures 6", -1" offset/4" backspace.

To make it visual, put a plumb bob on the body and the WMS, then measure back 10". The distance behind the WMS is your backspace.

Confused worse yet?

Good luck, I have a few brand new wheels laying around from this exercise.

[beadblaster]

Backspacing is actually critical on a k10. The tierod end is the limiting factor. I am measuring up rims on my k10 and the distance from disc rotor mounting flange to tierod end is 4”. Any distance greater than this will hit the tierod. Right now I have a set of 16 x8 with 4 “ back space and it is a 1/16” away from tierod end.

[davischevy]

Originally Posted by beadblaster

Backspacing is actually critical on a k10. The tierod end is the limiting factor. I am measuring up rims on my k10 and the distance from disc rotor mounting flange to tierod end is 4”. Any distance greater than this will hit the tierod. Right now I have a set of 16 x8 with 4 “ back space and it is a 1/16” away from tierod end.

Will the 20" wheels make a difference.

I'm not near my truck right now, so I can't look.

[davischevy]

Originally Posted by beadblaster

Backspacing is actually critical on a k10. The tierod end is the limiting factor. I am measuring up rims on my k10 and the distance from disc rotor mounting flange to tierod end is 4”. Any distance greater than this will hit the tierod. Right now I have a set of 16 x8 with 4 “ back space and it is a 1/16” away from tierod end.

I had always wondered why the factory 8" wheels had 4" backspace.

[davischevy]

Originally Posted by Mike C

I am quite happy with the 4.5 bs on the 9.75" wheels on my Jimmy. I am running the outers from an 8 lug Suburban on the front and the SF 14 bolt from same truck in rear though.

What diameter are your wheels?

[beadblaster]

20” wheel should clear the tierod so you could then go with any backspacing. I know that. 17” rim will still require 4” backspace

[MAGGEE]

Thanks other Larry. I’ll try that method and see what I come up with. After looking, I think I agree with beadblaster. The 20 inch wheel should clear the tierod end.

[geezer#99]

Maybe this will help.
Or not!
At least you'll learn the metric system!! LOL!!
http://www.rimsntires.com/specspro.jsp

[beadblaster]

Another thing to check is the center hole in the wheel. The hole needs to be 4.25” to slide over the hub.

[MAGGEE]

Thanks beadblaster. The wheel center hole's are good. According to Detroit Steel Wheel, they will clear the hubs. And, Geezer, that site is great! I'll still have to do some measuring to ensure the proper stance, but it helps to see the results of manipulating the backspacing.
Posted via Mobile Device

[Mike C]

Originally Posted by davischevy

What diameter are your wheels?

Mine are chrome Centerline 16.5 x 9.75. I would NOT do zero offset on a 4x4 truck. I think it will rub on the inner wheelwells and the leaf springs when turning. Once again, depending on the tire. 235 75 15 would be fine LOL.

I had 33 14.5 MT's on my truck originally with 3.5" backspace 8" rims and while that is an exceptionally wide tire, they rubbed in those places.

I had 15x10 Centerlines and they needed more backspacking than the 3 5/16 that Centerline typically used but I ran them for years.

On a 10" rim I would do 4.5" backspacing and on a 9" rim I would do 4" backspace. On an 8" I would do 3.5" but tire plays a role in the whole thing.

BUT, make sure you put tape measure to YOUR truck and figure out exactly what you need/want and not just what the Inter-webs says...