Scrub radius
 

I just put a set of Enkei Fujin wheels on my BRZ 17x7.5 40mm offset. I wanted to set my scrub back to factory so I am using a program from racing aspirations to determine the scrub radius of the BRZ. These are my results.


Factory scrub radius with 48mm offset +20mm or .75 of an inch positive scrub.


With 40mm wheels +28mm or 1.10 inch.


With 225/45/17 1/2 inch taller tire +26mm or 1.00 inch.


http://www.racingaspirations.com/mods/tg7n7uwl

This is one way to increase your scrub radius:

Attachment 139984

Currently on the same site and doing calcs as well

Is SAI, camber adjustment at the wheel, and offset the only determining factors of scrub radius?

The upright/hub geometry and location of the ball joint are pretty important. Once that is known, then yes sai and offset are the biggest factors that change it.

Whether or not the 20mm positive scrub OP posted is correct, I assume increasing the scrub by 11mm is a non-issue since many people run on 17x9 +35 wheels. However, what if the wheels I'm looking at will increase scrub by 31mm?

If we assume factory has 20mm scrub, this means I will end up with 51mm positive scrub. Will this be so high that it's detrimental to components or the dynamics of the car?

Also, what if I plan increase negative camber at the knuckle with camber bolts? Will that increase or decrease scrub?

Are you changing rim with? If so you will need to calculate that into the equation. Tire width will not change scrub radius since it will add or subtract equally on inside and out.

Increasing negative camber with camber bolts will increase the scrub radius because it moves the centre of the contact patch outboard.

Changing wheel width doesn't affect scrub radius - only wheel offset, because it moves the centre of the contact patch, which is the critical measurement.


OP - The only way you will get back to the same scrub radius as stock is to use a wheel with stock offset and run stock camber, or increase (positive) camber with a wheel with lower than stock offset.

The effect of scrub radius on handling in a RWD car is much less pronounced than in a FWD car. In FWD, increasing scrub radius increases the lever for drive forces at the wheel to act on the steering, increasing torque steer, etc. Because the steering wheels in a RWD car are not driven, the effects are much less pronounced. You may still get a bit more tramlining and the steering will feel a bit heavier, but it's nowhere near as noticeable or potentially problematic as on a FWD car.

Increased scrub radius *can* change how the wheel bearings and other components are loaded and affect longevity, but the difference is likely minor enough as to be a non-issue within the bounds of what's reasonably possible for changes on our platform, and are most definitely outweighed by things like the improved handling from increasing negative camber, and for some, the visual / aesthetic difference of running a lower offset wheel.

Basically, what I'm saying is, in my opinion, you're overthinking things here...LOL.

Pick lightweight wheels that you like with a reasonable offset (+35mm up to stock +48mm), go with only moderate lowering and keep camber within a reasonable range, and enjoy. ;)

Yes, was not thinking correct above.