|
Heath,
Errol
and I have both written about scrub radius, usually when the subject of using
R31 struts on a 1600 comes up. The R31 steering geometry is unusual for a rear
wheel drive vehicle as negative to zero scrub radius is common in front wheel
drive vehicles - my 1600 is typical of modified reasonably handling 1600's and
has somewhere around 3" positive scrub radius, it could even be more than that
as it's been an age since I measured it.. Part of the theory behind the use of 0
scrub is that it lightens the steering, hence the use on FWD vehicles, but one
thing for certain is -0 scrub doesn't improve a 1600 handling one iota. Exactly
how the R31 overcomes this as well as it does is not immediately apparent to me
- Errol may have a view as he owns a R31 too. I know that my 1600 leaves my
son's Skyline for dead through the tight stuff - the Gibson RB30
(150kw) holds it's own in a straight line though. Driving a R31 in the wet is
also something akin to a 1600/FJET experience, as in nigh impossible, although
the rear of the Skyline in the wet also wants to turn you
around.
Here's
a layman's definition and also a few words about "squirm" which is
related.
Scrub radius is the distance at the road surface
between the tyre line (which is an imaginary line vertically drawn through
the centre of the tyre) and the steering axis inclination line extended
downward through the steering axis. The line through the steering axis creates a
pivot point around which the tire turns. If these lines intersect at the road
surface, a zero scrub radius would be present. When the intersection is below
the surface of the road, this is positive scrub radius. Conversely, when the
lines intersect above the road, negative scrub radius is present. The point
where the steering axis line contacts the road is the fulcrum pivot point
on which the tyre is turned. Squirm occurs when the scrub radius is at zero.
When the pivot point is in the exact centre of the tyre footprint, this causes scrubbing action in
opposite directions when the wheels are turned. Tyre wear and some instability in corners is the
result.
Here's a picture
that I just found on a wheel alignment site on the web with a few more words -
pics are worth 1000 words and all that stuff. This example shows positive scrub
radius i.e. the intersection of the lines is below the road.
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.
--membersozdat------------------------------------------------------- OZDAT Mailing List Please Note:- Send (un)subscribe requests to [EMAIL PROTECTED] Send submissions to [email protected] No unauthorised redistribution of this email http://www.ozdat.com/ozdatonline/index.htm http://www.ozdat.com/ozdatonline/listindex.html http://www.mail-archive.com/[email protected]/ --------------------------------------------------------------------- |
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). Internal Virus Database is out-of-date. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.12.4/146 - Release Date: 21/10/2005
