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Old 11-30-2015, 11:00 AM   #1
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Steering- positive scrub radius

How does positive steering scrub radius affect handling or cornering as it increases? I'm talking about over and above simple track change considerations and steering feel and kick back.

Right now I have 15 mm up front (245) and 23mm rear (265) on 18s and just wondering how to make things better for a more stable front end. One would think adding 5 mm spacer up front would do it.....buuuuuut...

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Old 11-30-2015, 11:39 AM   #2
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There is an article in the new Panorama about scrub radius. I read it but did not understand it... but if you haven't checked it out yet, maybe it has some info for you?
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Old 11-30-2015, 02:40 PM   #3
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Steve,

Have not read it yet but.....will do! Basically scrub radius is determined by contact patch offset outboard rom the kingpin axis .....I am sure I will be corrected shortly if I am off.

I never really liked the dancing shopping cart feel of the stock boxster steering which I believe would be something like zero scrub. I preferred the heavier feel of spacers in there even on the street. I was thinking of go wider up front than the rear!!
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Old 11-30-2015, 05:39 PM   #4
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I may have numb hands, but I have never been able to feel any difference in steering feel between various wheel offsets / spacers. I have tried just about every combination between stock and as much wheel offset as will fit in the wheel well, with no real difference, other than moving the contact patch out as far as possible definitely results in better grip due to less weight transfer.

In my view, the scrub radius has the most effect in feeling under very heavy threshold braking. The scrub radius distance between the contact patch center point and the "kingpin" inclination projection point on the ground creates a small moment arm under braking that is balanced between both sides of the car as long as the braking force is equal on both sides. The moment that braking traction is lost on one side, the moments are instantly unbalanced, which the driver experiences as a jerking in the steering wheel toward the side that still has braking force. This is good as the driver automatically makes a correction in the steering that he/she would not know to do if there weren't some scrub radius distance causing this effect.

Obviously, much of the need for this steering feel under braking is negated by the overly active ABS systems that we have.

I believe that steering feel (other than under heavy braking) is most greatly effected by toe-in. Caster angle can also effect steering feel, but these cars come with ridiculous amounts of caster angle matched by the ridiculous amount of power steering.

OP, you may want to have your toe and caster checked. If you have any toe-out, it could cause your steering to feel unstable. If you have greatly lowered your car, an unstable effect could also be caused by "bump steer", which is really a misnomer for toe change under suspension compression.

Good luck


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Originally Posted by jaykay View Post
Steve,

Have not read it yet but.....will do! Basically scrub radius is determined by contact patch offset outboard rom the kingpin axis .....I am sure I will be corrected shortly if I am off.

I never really liked the dancing shopping cart feel of the stock boxster steering which I believe would be something like zero scrub. I preferred the heavier feel of spacers in there even on the street. I was thinking of go wider up front than the rear!!

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