Optimal wheelbase / track width for different tire sizes?

4speedhandler

TJ Enthusiast
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Apr 19, 2016
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So in a whole lot of reading i see mention of wider axles and more wheel base for larger tires. I have also heard about getting proper backspacing to minimize scrub radius. does anyone have a definitive chart of optimal wheelbase/track width compared to optimal axle width and backspacing for a given tire width/diameter? like for a stock wheelbase tj to have zero scrub radius what wheel/tire would that be? or say a set of 33's or 35's what would be the optimal wheelbase/track width and the right wheel/tire? are there some good references i have missed?
 
I looked for quite a while and could not find anything definitive, just anecdotal. I was really looking for what the factory kingpin axis inclination was, but all I could find was references to caster.

I'm sure people would be happy to give anecdotal evidence of what works well. (Mine is that 35" tires seem to work fine on 3.75" net backspacing. No squirm or divergent effects that would likely come from zero or positive scrub radius. )
 
I have read from a few different sources that the centerline of the tire should be approximately 1" outside of the kingpin angle where it intersects with the ground. So that means as tire diameter increases the tire should be moved further outward. Same with tire width increases should have the tire move outward also. Again, tire centerline aprox 1" outward of the point where kingpin (or balljoint) angle meets the ground. That should optimize scrub angle and (some fancy word I can't remember right now) deflection.
 
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I have read from a few different sources that the centerline of the tire should be approximately 1" outside of the kingpin angle where it intersects with the ground.

like the OP, I've had no luck finding the kingpin angle for a TJ. Is there another name for it?
 
I looked for quite a while and could not find anything definitive, just anecdotal. I was really looking for what the factory kingpin axis inclination was, but all I could find was references to caster.

I'm sure people would be happy to give anecdotal evidence of what works well. (Mine is that 35" tires seem to work fine on 3.75" net backspacing. No squirm or divergent effects that would likely come from zero or positive scrub radius. )
It isn't hard to figure it out. A digital angle finder set on the top of the inner C with the ball joint removed will give you the KAI, and that is about 10 degrees inward at the top. Just check the long side tube for level and that will give you the angle. If you want it more exact than that, I can have my casting engineer run a degree line through the center of both ball joint holes and get that angle to the face of the knuckle to the closest 100th of a degree. He has my knuckle CAD files stored in case the casting company screws up a mold plate and we need to make another master.
 
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I have read from a few different sources that the centerline of the tire should be approximately 1" outside of the kingpin angle where it intersects with the ground. So that means as tire diameter increases the tire should be moved further outward. Same with tire width increases should have the tire move outward also. Again, tire centerline aprox 1" outward of the point where kingpin (or balljoint) angle meets the ground. That should optimize scrub angle and (some fancy word I can't remember right now) deflection.

in addition to the KPA, one would also need to know the distance from the hub mounting flange to the vertical line that intersects at least one of the ball joints, as well as the radial distance of that ball joint from the axis of the hub.

Not difficult math, just not too terribly convenient to measure if your Jeep is sitting on it's tires and you don't have a knuckle lying around.
 
thank you all for the info. Hopefully some more knowledge hits this thread to keep the ball rolling. Now that i can more easily figure out proper axle width and backspacing for a given track width does anyone know the optimal aspect ratio for track width to wheelbase? or is that just something that i should extrapolate from factory dimensions?
 
Mr Blaine has said 37s are too big for a TJ without bumping wheelbase. That is the extent of my current knowledge on WB vs. Tire dia
 
MrBlaine I thought you were basically in the most extreme location. Johnson Valley area. I wasn't calling you out, just pointing out that with all of your knowledge and experience that a 37 was your max recommendation.