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Old 12-05-2012, 01:57 AM   #1
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Let's get technical about offsets and suspension geometry!

This morning started with a simple question for me. "I want better handling. Also, my wheels could be a bit more flush with the body as well. Should I buy spacers?" My first intuition was that increasing the track width would make the car more agile (short wheelbase cars are generally more agile, ergo, increasing track width improves cornering behavior). But I know this was a very rough analysis. A quick google search later I learned about "scrub radii," a suspension dimension affected by offsets (as altered by spacers) and have several unanswered questions about it. I was hoping all the motorsports fanatics could help me out.

First, here is the information that I picked up. It is a post on the Rennlist Porsche Forums.

To understand scrub radius you irst have to understand SAI -steering axis inclination. The steering axis(SA) is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. The angle between the SA and vertical is SAI, add static camber to that and you have the included angle.

The scrub radius(SR) is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.

older 911s have a small (+) SR, from 964 up they have a small(-) SR

the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability.

The bigger the scrub radius the more kickback there is at the steering wheel. If you change to wheels w/ larger o/s you will feel the increased kickback at the steering wheel

If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly(as used on 911 front) typically performs well with a lot of SAI and caster(sound familiar?), a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

abs equipped cars want a little (-) SR to allow for uneven braking forces between the left and right front wheels
As I understand (correct me if I'm wrong) McPherson Strut suspensions, like on the E46, have intrinsic negative scrub radii. This means the SAI intersects the ground further to the outside than the centerline of the tire contract patch. Adding spacers would therefore reduce the scrub radius, and bring the tire closer toward the "squirm" condition, i.e. closer to "zero scrub." From this, I conclude that adding spacers to an E46's front suspension should reduce steering effort and kickback, and make the steering less predictable (because of "squirm"). This result is contrary to what I have already read on E46F.

My questions are as follows:
- Do E46 McPherson strut suspensions have intrinsic positive or negative scrub radii?
- How does steering effort (as he says, "kickback") differ as a function of scrub radius, assuming scrub radius is the independent variable and has a domain from arbitrarily large positive scrub to arbitrarily large negative scrub? I ask because the post contradicts itself when it claims that larger offsets increase the kickback - what if the increased offset reduces the total scrub radius (As I understand it would in a car with an originally negative scrub radius - E46 or some Porsches)?
- Is my original intuition - that widening the wheel base will make an E46 more responsive and agile - supported or denied by all these other conclusions about SAI, scrub radii, and squirm? If not, in what ways will handling be affected?

The way the above Porsche enthusiast described it is somewhat inconsistent. Maybe we can do a better job and come to a better conclusion about scrub radii and how they affect steering feel, stability, and overall car responsiveness. In b4, "if you want better handling, get coilovers and sways." Partly because I'm in college and broke, mostly because I want to better understand suspension geometry. Let's expand the E46F knowledge base!

#BattleBimmer #323RS

Last edited by KrasivataSabine; 12-05-2012 at 04:16 AM.
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