Negative scrub radius(wha?), track width, and rim offset

[Venom]

Hey guys,
While reading through the VSI-8 (vehicle mod guide for Vic) the other day, i noticed that it specifically mentions that track width increases are strictly prohibited for vehicles with a combination of front-wheel drive, Macpherson strut suspension and negative scrub radius geometry. I knew L-series/Leone fit the first two, but i had no idea what the third requirement was. This had me intrigued. A bit more Google searching and i came across this excellent site describing all the aspects of steering geometry and wheel alignment which i highly recommend reading. Very enlightening:
http://www.familycar.com/alignment.htm#Scrub%20Radius

The term SAI they use is basically a straight line through the middle of your strut to its equivalent point on the ground. On scrub radius they say..

Scrub radius is the distance between where the SAI intersects the ground and the center of the tire. This distance must be exactly the same from side to side or the vehicle will pull strongly at all speeds. While included angle problems will affect the scrub radius, it is not the only thing that will affect it. Different wheels or tires from side to side will cause differences in scrub radius as well as a tire that is low on air. Positive scrub radius is when the tire contact patch is outside of the SAI pivot, while negative scrub radius is when the contact patch is inboard of the SAI pivot (front wheel drive vehicles usually have negative scrub radius).

If the brake on one front wheel is not working, with positive scrub radius, stepping on the brake will cause the steering wheel to try to rip out of your hand. Negative scrub radius will minimize that effect.

Scrub radius is designed at the factory and is not adjustable. If you have a vehicle that is pulling even though the alignment is correct, look for something that will affect scrub radius.

Based on this and the fact the the L series has a positive camber (which always totally bemused me in the past), i figure that the reason for the positive camber is to induced negative scrub geometry to improve vehicle handling (e.g reducing torque steer). Now this has me thinking that the desert rat rims I'm using which substantially increase offset is creating positive scrub radius, negatively effecting steering performance, and is technically illegal in Vic. More than likely it contributed to the huge (scary) increase torque steer i experienced after my EJ25 conversion, beyond just the power increase of course.

Now I'm also wondering whether adjustable strut tops, which i was considering for the purpose of inducing neutral camber and additional positive caster, will also negatively effect scrub radius. I'm thinking though that adjustment to the strut angle produces a lesser change in the camber angle, so if anything it may create additional negative scrub radius? Not sure at that one.

It is also interesting that they specifically mention vehicles pulling left or right and scrub radius geometry. It reminded me of problems with Liberties pulling to the left that AP and some others have had, despite having excellent wheel alignment specifications on their vehicles.

All food for thought. Feel free to comment/discuss/dispute. Personally I'm now seriously considering a set of standard offset (or closer to) set of 14 or 15 inch rims for my L series. I'm curious if this knowledge changes peoples thoughts on their wider off-set rims if they have them/thinking about getting them. Makes it hard considering they're the easiest to source and cheapest 14" rims around.

Cheers,
Rhys

[wrxer]

certainally food for thought

[phillatdarwin]

i just fitted my to my car on friday and my off set is about "1" more out

[phillatdarwin]

this i can not get to work can u re post it .
http://www.familycar.com/alignment.htm#Scrub%20Radius

[GOD]

The anchor is doing something funny. Try http://www.familycar.com/alignment.htm and scroll down

[discopotato03]

I thank the all mighty that others here are interested in learning about suspension/steering geometry and how it affects their car .

The MY and L Series Subaru's (in fact all of them really) have some form of front wheel drive geometry because Subaru's started out as front drivers and grew selectable RWD or AWD . Its the front wheel drive geometry that makes them handle like a front wheel drive car and AWD DOESN'T get around this .

Yes scrub radius is the point at which the center line of the Mac' strut would hit the ground if it extended that low . More specifically it is the distance from the middle of the tyres contact patch to the struts extended axis .

I look at scrub as zero positive or negative , zero meaning they are in the same spot .

You can't change the scrub radius by altering the location of it in the top of the strut tower - adjustable strut tops . What happens is that the tops of the struts are moved inwards to make the camber go more negative and the relationship between the wheel and strut is unaltered , you've changed the strut and wheel angle in relation to the car but not each other . Dito for moving the top of the struts rearwards to increase positive caster . If you altered the struts angle in the knuckle casting to gain negative camber then yes the scrub radius changes , better to move the top of the strut if possible .

If for some reason you wanted to change the scrub radius its as simple as altering the wheels offset positively or negatively .

Something you'll notice about L/MY front geometry is that they have positive camber , sfa positive caster and very much positive (inwards) offset wheels std . Personally I think this was aimed at minimising bump steer and torque steer and making non power steered cars not overly heavy in the steering dept .
Also manufacturers of the era were into designing understeer into cars because there is more crumple protection in these cars than there is in the side or rear . An oversteering car loses directional stability in a panic stop situation because the average driver firmly believes that brake pedal will save his/her life 99% of the time . An L or MY is designed to take collision impact through the engine bay and the engine and gearbox is intended to drop down and under the tunnel and floor pan to stay out of the passenger cabin .
Ultimate handling was not a priority in these cars standard .

There is a couple of ways to change camber and caster but adjustable strut tops give the least number of side effects geometry wise . Also the steering will get heavier so power steer is a good idea if not already fitted .

The other thing you don't want to forget is that making the front grip better makes these cars less understeery and potentially more likely to snap oversteer so you may want to alter the rear geometry as well to suit .

Out of time , cheers A .

[Matt]

Also check out K-Mac as they do a control arm adjustment which will give you more adjustment then the strut top adjustor ever can, however that is Camber only not Castor like strut top adjustor's can. I believe the strut top towers can only give around 1-1.5 degrees of change in camber however the Control arm adjustors give around 2-2.5 degrees.

Also the other thing to watch with both of these is that they will play with your CV Angles and lengths very slightly causing them to wear out a few slightly quicker. As the inner DOJ tends to run closer to the tyre side.