Bob:
Racing is so much different than cruising, I do not know if you
can generalize too much. In addition, I have not raced, all I know is
what I have read in my books, and what I have gleaned from my very
limited experimentation over the years (Fiero, Z28 Camaro, and a Mini).
Under steering is the most benign form of mis-handling, and in
fact it hides a multitude of errors on the drivers part. If a car
understeers, it is virtually impossible to get the car to spin, which
may be why your racer friend liked it.
Optimum handling is achieved when all four tires are
perpendicular to the pavement and equally loaded. When cornering
however, the tires move away from the vertical position. So adjusting
the suspension so that the tires are not vertical at rest, but become
MORE vertical while cornering will yield higher cornering forces. So
tweaking the rear camber may be beneficial if the camber changes
significantly when cornering hard. I would expect that this would be a
secondary effect, however.
Simply stated, you want minimal weight transfer (caused by body
roll), a 50/50 weight distribution, and tire tread planted squarely on
the pavement during cornering. Seems simple, but the dynamic weight
transfer and suspension geometry changes during cornering make it
complex. OK, so much for background.
When the car corners, the weight transfers to the front outside
corner of the car. The tread is distorted on that corner and the tire
wants to tuck up under the car. So the tire needing to do the most work
is least capable of doing so because of its distortion - the car starts
to slide on that tire. This results in the car pushing in a straight
line rather than turning when the tire reaches its traction limit.
The basic strategy to deal with this is pretty simple - minimize
weight transfer to the outside front tire, and extract as much work as
possible from the other three tires on the car so that the outside front
tire will not overload as quickly. How to do this?
1. Minimize body roll. This is done by stiffening the springs,
and stiffening the sway bar(s). For a car with no sway bars, the first
step is to install a front roll bar. Unfortunately, there are limits to
this approach. The sway bar does two things - reduces roll, and
transfers more of the work to the end of the car with the sway bar. So
while lateral weight transfer is reduced, longitudinal weight transfer
(from rear to front) is INCREASED. The lateral weight transfer reduction
approaches diminishing returns while the longitudinal weight transfer is
increasing - and at some point increasing the size of the roll bar is
simply increasing understeer. This happens because the outside front
tire is increasingly overloaded, and starts to slide. Make no mistake,
this is better than NO sway bar at all, but the point is making the bar
too big actually starts to degrade handling from its new peak.
2. Extract more work from the rear tires. This is where the rear
sway bar comes in. Once the front bar is stiff enough (reduced lateral
transfer) we add a rear bar to reduce longitudinal weight transfer. This
further reduces body roll and lateral weight transfer as a stiffer front
bar would do, but it ALSO reduces the weight transfer to the front, and
so keeps the rear tires loaded so they can do more work. This delays the
onset of slippage for the front outside tire.
All of this is counter intuitive. If your front tires slip while
cornering, your first response is to install a (larger) roll bar for the
front. After all, that is where the problem is occurring. My handling
books agree on a couple of basic points:
A. 1/2 of the roll resistance needs to come from the springs,
1/2 needs to come from the sway bars. This precludes too large a sway
bar without upgrading the springs as well.
B. A BALANCE between front and rear roll stiffness. This
precludes a very large front bar with no (or too small) rear sway bar.
Likewise you do not want too large a rear bar, as it will make the rear
end want to swing out when the rear tires get overloaded.
I believe that the best strategy at this point would be a stock
sized front bar (sell your current bar to a racer) and a modestly sized
rear bar (5/8" - 3/4"). This will get you a better ride, higher ultimate
cornering forces, and neutral handling. WARNING - a neutral car can spin
since all four tires will break loose at the same time. The limit of
adhesion is MUCH higher, but since the tires all slip at the same time,
there is little warning when the limit is reached. Understeer on the
other hand, can be clearly felt through the car and steering, giving you
ample warning to "slow down".
Why don't car manufacturers do this to begin with? Simple,
lawsuits. By deliberately designing in understeer, Billy-bob Boyracer
can't spin the car and end up plastered on the face of a bridge abutment
- while his grieving relatives sue for a gazillion dollars.
Cheers,
Vance
-----Original Message-----
From: Bob Danielson [mailto:75TR6@tr6.danielsonfamily.org]
Sent: December 09, 2007 11:07 AM
To: Navarrette, Vance
Subject: RE: [6pack] Sway Bar Custom Mounts
Vance,
Just curious about the understeer comment you made. When I got my car 10
years ago, I had a restoration shop rebuild the suspension for
me.........for a number of reasons. The shop was run by an ex-SCCA
Triumph
racer and, I seem to recall him saying he'd "set the car up" in one of
his
favorite handling configurations: Blue Comp Springs, Koni/Spax shocks,
7/8"
front sway bar and more negative rear camber than normal. I thought too
much
negative rear camber promotes oversteer........ If so, does his "set up"
off
set each other i.e. no rear sway bar promotes understeer yet negative
rear
camber off sets it? I have no idea how these things work together or
against
each other.
Thanks
Bob
Bob Danielson
1975 TR6 CF38503U
Running w/ Throttle Body Injection
http://tr6.danielsonfamily.org
_______________________________________________
Support Team.Net http://www.team.net/donate.html
6pack@autox.team.net
http://autox.team.net/mailman/listinfo/6pack
http://www.team.net/archive
|