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Making your car go faster, Part 3.2: Tuning the rig

Remember (I'm dating myself, I know it) Carly Simon's song,
"Anticipation"? That's what keeps F1 drivers (and more normal ones as
well) on the track. Generally in a truly fast car, you have to crank in
inputs before the car does something evil. By the time the car is doing
something bad, it's too late to correct. 

Ok, are those guys psychic, or what? How do they know the car is about
to do something evil? Here's where we get back to suspension setup.
Generally the answer is that the driver needs a car which provides a lot
of feedback, so he/she can feel what's going on. Knowing what to watch
for as a signal that something is about to happen let's them anticipate
their way out of trouble. 

Here's a frinstance: as tire lateral loading increases, positive caster
generates a ever-stronger self-righting force. That is, you crank the
wheel to make a turn and the wheel wants to return to straight ahead.
This is one of the advantages to having a lot of postive caster in your
front end (there's also a drawback, natch). If you have power steering
(here's where the 02 guys have an advantage) you can't feel that
righting force clearly, because the boost overwhelms it. Why do we care
about the righting force? 

Because just before a tire starts to slide the righting force drops
considerably, and a good driver will notice the wheel get "numb" and
take corrective action. With power steering, it's really hard to feel
that numbing in the wheel, if at all. However, the suspension has to
have most of the micro-compliance (I just made up that term, don't go
quoting it) removed for this to happen. Otherwise everything just feels
kind of dead. Micro-compliance is the rubber in the bushings all through
your suspension that keep a lot of road harshness and noise from
intruding into your trip to Grandma's. Race cars use rock-hard bushings
in place of rubber to get two improvements:

1) instant response to even tiny steering inputs, since you're not
wasting time/energy compressing rubber bushings
2) total feedback from the road.

So stiff springs and swaybars are the order of the day. 

But not always.

What about when it rains? A wet track means far less traction available
to the tires, so less lateral loading will be possible. In this case you
want to be even smoother than before, and so lots of race teams will go
with a "soft" setup, replacing springs, shocks, and swaybars with softer
rates all around. 

Sometimes what seems right just isn't. When BMW first started racing
IMSA back in the 70s with those beautiful CSLs, they found in testing
that high roll stiffness wasn't the fast way around the track. They went
with softer and softer sway bars, with the cars rolling more and more,
and kept shaving seconds off their lap times. Granted, these cars
weren't exactly NYC taxis when they were done, but they _were_ a lot
softer. Lap times never lie. 

What about camber? Why is it important? 

Take this test. Take a new pencil, hold the eraser to a smooth surface
perfectly perpendicular, and attempt to slide the eraser. Now tilt the
pencil slightly away from the direction of travel and repeat. Two things
should become apparent: The harder you push down the harder it is to
slide the eraser (duh!). A tilted eraser is harder to slide than a
perpendicular. This is called the camber effect, and race cars typically
want lots of negative camber (tires tilted in at the top) because of it.
On older, bias ply racer tires, when the tire was loaded laterally with
lots of negative camber it would actually lay flat on the road. Negative
camber was added until tire temperatures, taken right after some hard
cornering, showed even tire temperatures across the face of the tire.
Modern radial race tires (like BFG R1s) distort differently due to the
different construction method and thus won't show even tire temps across
the face. All this tire temperature stuff is vitally important to race
guys: you see crew workers show temperature slips to drivers during
practice etc all the time. They learn a lot about their setup from temps
and so can you, if you have a pyrometer. 

Sadly, too much negative camber ("Marge, I smell a tradeoff coming...")
eats up the inside tread of your tires from just diving around on the
street. Lowering your car will induce negative camber, particularly at
the rear, less so at the front. So before you buy those super-lowering
springs you might want to think about what they will do to your camber.

What the really big guys do is to set up for a given track on a given
day. Typically you'll optimize for a given section of track, since a
setup that makes you fast through one turn may suck through another.
Some turns are more important than others (see Alan Johnson's book,
mentioned in part 1) so it makes sense to optimize for them over less
important turns. This is why teams keep careful records about every time
the car is on the track, track conditions, setup, lap times, tire temps,
etc. Data acquisition for an Indy or F1 team is a serious business. As
rank amateurs we have to do the best we can with a notebook and pencil.
You also need to develop a feel for what the car is doing, and enough
understanding of the components and their interrelations that you can
figure out what changes to try. 

Rule 1: never change more than one thing at a time. Change your tire
pressures, or change your swaybars, but don't change them both and
expect to learn anything useful. 

As you get better and better, you'll want a car that allows you more
control over its attitude (no, I'm not talking about "Yo momma") so you
can rotate the car easily and point it where you want it. Until you get
to Formula Atlantic and beyond, this is the hot ticket for getting a car
through the tight stuff. It takes skill and lots of practice to master.

And the right suspension setup.

John Browne
BMW CCA
BMW ACA Puget Sound Region
M3 LTW (PeeKay)
Suburban 2500 (Godzilla)
326 iX (Spunky the brave little car)

copyright (c) John Browne; all rights reserved