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I'm licensed to operate the most powerful boiling water nuclear reactor in the world and that was quite confusing to read...
 

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That's just poorly written. Engineering concepts don't have to be hard to understand. Wikipedia has a much simpler description of Scrub Radius.

Scrub radius - Wikipedia, the free encyclopedia

The question is a good one though. As far as I know, only AEV has specified a rim to use with their lift kits, and they use a 5.2 inch backspacing. Think they know something the others don't?

Mike
 

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That's just poorly written. Engineering concepts don't have to be hard to understand. Wikipedia has a much simpler description of Scrub Radius.

Scrub radius - Wikipedia, the free encyclopedia

The question is a good one though. As far as I know, only AEV has specified a rim to use with their lift kits, and they use a 5.2 inch backspacing. Think they know something the others don't?

Mike
they work closer with Jeep then any of the other outfits and several of their engineers came directly from jeep. Hard to beat 5.2" backspacing if it gives you enough clearance for your tires.
 

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Discussion Starter #5
I'm licensed to operate the most powerful boiling water nuclear reactor in the world and that was quite confusing to read...
No crap. I need to read through it a few more times. I googled scrub radius and still seeing a bunch of opinions.
Some say tire diameter as a affect, others don't.
Positive/negative good bad, wtf ? Confusing as hell.
 

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My only experience with this was changing from using spacers to Pintlers. The Pintlers moved my wheel back in about an inch but it was very noticeable. I really like the way it drives now compared to the spacers. Thanks for the posts as I am still trying to understand all this.
 

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Good find...very interesting read.
 

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Tire diameter certainly affects scrub radius as it changes the point where the two axis intersect. This is probably why AEV moves from 6 inches to 5.2 inches of backspacing with their rims, to account for the increase in diameter from 32 to 35 inch tires with just enough space for the larger size.

Mike
 

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It's a definitely good article but it would have been simplified considerably if the author used his own consistent sketches instead of a patchwork of videos and third party diagrams. Either way he does a good job of explaining the impact of rim offset and tire diameter something common among Jeep owners.

This is evidence why it is not a good practice to change offset just to get a wider stance to increase "stability", when it can have just the opposite effect. Figure 3 also shows why having E rated sidewalls is not necessarily counterproductive.
 

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This was very educational and led me to research further. I found this video which pretty easy to understand and did a good job illustrating it. Essentially anything that moves the intersection of the tire centerline and the king pin centerline away from the surface plane (road) will cause a positive or negative scrub. This is undesirable. The goal is to have it intersect at the road surface.

http://www.dailymotion.com/video/xyr12q_scrub-radius_auto

This is why changing suspension with lifts or changing tire size or back spacing has effects on handling. Also why AEV made the geometry correction brackets. :)
 

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Discussion Starter #12 (Edited)
This was very educational and led me to research further. I found this video which pretty easy to understand and did a good job illustrating it. Essentially anything that moves the intersection of the tire centerline and the king pin centerline away from the surface plane (road) will cause a positive or negative scrub. This is undesirable. The goal is to have it intersect at the road surface. http://www.dailymotion.com/video/xyr12q_scrub-radius_auto This is why changing suspension with lifts or changing tire size or back spacing has effects on handling. Also why AEV made the geometry correction brackets. :)
Little late to the party but the last comment is not correct. Geometry brackets have no effect on scrub radius. The brackets lesson CA angles and keep the axle forward centered in the well. I'm still trying to grasp this whole thing on scrub radius. One thing I noticed in your vid .. The relation of toe to scrub radius. How the tires toe out with positive SR and in with a negative SR. Now we mostly push our SR out With less back spaced wheels or spacer. (Tires would toe out)
So this is proving why toe (in)even more important. Maybe running a bit more in would counteract the changes ... No ?
 

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I just pulled my 1.5" spidertrax spacers and ran with my stock rims (14 rubicon that had just been aligned ie toe in set) the alignment was done prior to pulling spacers and I was actually surprised how much better it drove without the spacers which leads me to believe that scrub radius makes a big difference. Unfortunately my tires toyo 285/75-17 are slightly big to run on stock rims with no spacers. I just put on AEV Pintlers which is supposed to be closer to ideal scrub radius but I would put the feel of how they feel somewhere between the stock and spacers and the stock without spacers. They do however look better to me then stock without spacers as the 1" less backspacing and 1" wider rim give a nicer look.
 

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No crap. I need to read through it a few more times. I googled scrub radius and still seeing a bunch of opinions.
Some say tire diameter as a affect, others don't.
Positive/negative good bad, wtf ? Confusing as hell.

This is a really cool and easily understood video for explaining Steering Axis Inclination (which is tied in to scrub radius) and it's effect on steering/stability. Give this a look...

https://www.youtube.com/watch?v=IZLiP_37Oso
 

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This thread has got me thinking that maybe that's why most 2 1/2 lifts require the 4.5 inch backspace on wheels. The axles shift and the one side shifts in closer to the frame by about 3/4 inch so leaving a margin of safety of 3/4 inch.

I'm wondering if a person had adjustable track bars and had their axles centered then one might not need that much backspace which would be a positive thing relating to scrub radius?
 

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This is a really cool and easily understood video for explaining Steering Axis Inclination (which is tied in to scrub radius) and it's effect on steering/stability. Give this a look...

http://www.youtube.com/watch?v=IZLiP_37Oso

Great video, thanks for sharing!
IMO: Understanding suspension and steering geometry is essential before, and when modifying. Unfortunately, I think many don't understand the importance, and get in a hurry to bolt stuff on, more worried about appearance.
 

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What I could do is to make a model mockup of the stock suspension geometry with stock wheel offset and tire heights in CAD then add in a 35" tire on a 9" wide rim with a 4.5" backspacing and see what it does to the stock geometry. My thought is the lifts effect on caster is more of an issue than the wheel offset and tire diameter effect on SAI. But that being said anything no matter how small that adds to the instability of the steering is going to make your driving experiance more exciting in a negative way.
 
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