Scrub Radius Question

[bniknar]

I've been searching the forums on the impacts of positive scrub radius, don't see much data on the impact piece. And, I suspect that may people are running tire/wheel combos without considering scrub radius. So, appreciate any feedback on this: on a non-sas Big Bend: 18x8.5 wheels with +18 offset, and 265 /70 R18 tires. According to wheel-size.com this results in a positive scrub radius (about 1 ¹³⁄₃₂").

I use my bronco for road trips and gravel road driving.

Will that positive scrub radius add wear/tear to the steering components? Reduce traction? i.e. is this a safety issue? I know what scrub radius is, I just need to understand the real-world impact.

Thank you.

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[BigMeatsBronco]

I am highly aware of scrub radius and its issues...the farther out you go the more stress it puts on Tie Rods, UCAs, and bushings, and wheel bearings...It also contributes to bump steer and clearance issues with big tires... the best design suspensions always have the pivot point of the steering as near the center line of the tire as possible.

 

[87-Z28]

Estimated scrub radius and king pin axis geometry for OEM design envelope.

The axis about which the tie rods turn the wheel (king pin axis) is three dimensional. Line between pivot points of control arms UCA and LCA Considering only the wheel camber alignment plane, the angle of this axis can be calculated as tan ^-1 (8/20) or about 20 degrees. Measured to the nearest inch for stock oem UCA/LCA combo. This ignores wheel caster plane angle, which is very small anyway (a few degrees).

As the vehicle static ride height is lifted, the king pin angle decreases (due to pivoting UCA and LCA) which pulls ground surface pivot point Inward. This also affects wheel camber. If you have lifted your rig a few inches you can easily see the disturbing new camber angles.

LCAs can be pulled outward to readjust camber, and the king pin axis should also then readjust. When LCAs are at their outward limit of adjustment, UCA ball joint may need to be pulled inward (with new UCA) to adjust camber and king pin axis for additional lift.

the king pin axis at ride height for a lifted vehicle can be set back to stock if proper alignment is achieved. Scrub radius is then only a function of wheel centerline on the ground surface; or wheel offset and wheel radius.

a 20 degree king pin axis angle will move the location of the ground pivot point (8/20) or 0.4” or 10 mm for every inch of wheel height achieved (radius). Plus or minus a couple mm??

can’t accurately measure where the king pin axis pivot point is on the ground surface. But likely there is some very small positive scrub radius in the oem design. Does anyone have this number??? The base 30” tires are on +55 offset. Moving to SAS 34.4” tires would be an increased wheel radius of 2.2” and thus require 22 mm of decreased offset or 33mm in order to maintain oem scrub radius. SAS wheels are +30 mm offset so numbers make sense.

so going from SAS 34.4” to a 36.5” (37s) tire would require about 10mm of decreased offset for a 20mm offset wheel to maintain SAS scrub radius. Using a 0 offset wheel on SAS 35s results in significant additional positive scrub radius that can add increased force couple to wheel when braking, accelerating, turning,… and thereby increasing wear.

very rough numbers but I think they are reasonable. Perhaps a bit over simplified.

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