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Backspacing Deep Thoughts

MIDEVIL214

Jr. Member
Joined
Feb 7, 2005
Messages
151
Current wheels are 17x9.5 with 3.50” backspace (aprox. -44 offset). Will a 9” wide wheel with 4.5” backspace (-12 mm offset) be inset .75” further? I’m removing a ¼” since the new wheel is .5” narrower overall.

I have duff disk knuckles which don’t flare out like 76-77 broncos and f150. The WH Clydesdale tire rod is where I may have some interference. I’m at work at the moment but seemed to recall having about 1” between the current 9.5” wheels and the rod end. Only losing .75" would technically still clear but be tight.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
44,848
Backspacing is a hard number, so whether a wheel is 9" or 9.5" wide won't change the relationship of the inner bead to the rod end. But the backspace does.
Now, offset is a whole different story! So for me, I like to stick with using backspacing for clarity and consistency.
Meaning that to my mind at least, going from 3.5" backspace to 4.5" backspace gets you a full inch further toward the rod ends with the new wheels.

The narrower wheel width can also effect tire sidewall location, but it's effect is extremely minimal, so that half inch change in width will probably not skew the tire clearance calculations if you need to do any.
But inner wheel bead location should be pretty constant depending on casting/stamping/forging profile differences.

I think...

Paul
 

DirtDonk

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Bronco Guru
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Messages
44,848
Oh, but being 17" wheels should minimize some of that effect for being further outward away from things too. Seems to me you should be good even with the Clydesdale.
Thanks for using our stuff too! It's a cool product who's size luckily does not always get in the way.

Your Duff knuckles supposedly mimic, or are at least similar to, the F150 SuperCab knuckles. Higher arm location, but without the outward curve of the Bronco disc knuckles.
The standard F150 knuckles are not a problem for most due to the arm location being lower and straighter than the '76/'77 Bronco knuckles.
It's the Bronco stuff that always caused the headaches.

Paul
 
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MIDEVIL214

MIDEVIL214

Jr. Member
Joined
Feb 7, 2005
Messages
151
Makes sense to me. I was definitely over thinking the width vs backspace element. Look at me over here trying to sneak in an extra 1/4".
 

DirtDonk

Contributor
Bronco Guru
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Nov 3, 2003
Messages
44,848
It might be tight. Hopefully the 17 inch factor will come to your rescue.
Are you gonna mount the tire rod over or under?
 

rjrobin2002

Contributor
Bronco Guru
Joined
Oct 13, 2007
Messages
2,592
I ran 4.5" back spacing with 17" wheels no problem.
 

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MIDEVIL214

MIDEVIL214

Jr. Member
Joined
Feb 7, 2005
Messages
151
I have the tierod mounted over. Just measured. I’ve only got about .60” between the Clydesdale tie rod edge and the 17” wheel. 4.5” won’t work for me unfortunately. 3.5” backspace in the Bronco bolt pattern can be elusive. My current wheels are atx slab beadlocks . Looking for a lighter more street friendly setup.
 

sprdv1

Contributor
REBEL
Joined
Mar 8, 2007
Messages
80,890
Backspacing is a hard number, so whether a wheel is 9" or 9.5" wide won't change the relationship of the inner bead to the rod end. But the backspace does.
Now, offset is a whole different story! So for me, I like to stick with using backspacing for clarity and consistency.
Meaning that to my mind at least, going from 3.5" backspace to 4.5" backspace gets you a full inch further toward the rod ends with the new wheels.

The narrower wheel width can also effect tire sidewall location, but it's effect is extremely minimal, so that half inch change in width will probably not skew the tire clearance calculations if you need to do any.
But inner wheel bead location should be pretty constant depending on casting/stamping/forging profile differences.

I think...
good points
 

ngsd

Contributor
Bronco Guru
Joined
Mar 2, 2019
Messages
2,355
You are not alone. That is a tough one that few can answer. I have a 68 with 77 front axle so stock disc. But it has the Duff steering (under) and t rex arms with Hellwig sway bar. So much going on I may just need to mount a tire to see but buying the wheels leaves you with having to return them if not.
 

Yeller

Contributor
Bronco Guru
Joined
Mar 27, 2012
Messages
5,069
Loc.
Rogers County Oklahoma
Due to the fitment issues and limited selection I’m on my last 5x5.5 truck. If I hadn’t just built the rear axle I’d swap it it 6x5.5 which has much better support
 
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