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Making your car go faster, Part 2.2: Suspensions (long)

(Continued from part 2.1)

However, the car may be dramatically unbalanced front to rear. Ideally a
car should have a 50/50 weight distribution front to rear, and the M3 is
darn close stock. Changing springs won't change the static weight of the
car, so even lowering it will retain the stock 50/50 dist. But as soon
as you M3 starts to corner, the weight distribution changes. The car is
engineered for understeer, remember? BMW engineers did (and you can)
take advantage of linear (front/rear) weight transfer, which is a
function of relative roll stiffness. The end of the car stiffer will
transfer more weight to the other end. Thus if the front is stiffer than
the rear the rear gets more weight transferred during cornering and thus
"sticks" better. This is called understeer. (Sticks is a lousy word,
because what actually is going on has to do with slip angles and force
vectors. In reality understeer is when the ratio of the slip angle to
the steering angle of the front tires is greater than the same ratio for
the rear tires. But you get the idea.) 

Roll stiffness can come from many sources: spring rates, shock damping,
and anti-sway bar stiffness are the big three. Stiffening the chassis at
one end or the other can also contribute (strut braces, etc). As you
stiffen the rear relative to the front (or soften the front relative to
the rear) the car will understeer less. The "ideal" theoretical setup is
to have the car neutral: it neither understeers nor oversteers through
our hypothetical sweeper. In reality drivers like to have their cars
setup to do one or the other: I think I'll use part 3 to discuss why.

With so many variables (spring rates, ride height, shock damping (both
rebound and compression), roll-bar stiffness... did I mention tires?)
how can you possibly know what the "magic" combination is? The answer is
there isn't one (stay tuned for part 3) but you can do some things...

Different schools of thought abound. One calls for stiff springs and
soft roll bars. Others call for soft springs and fat bars. In general, I
like the springs to be no stiffer than they have to be--one test is they
should almost but not quite bottom out on the worst bump on a given
track. Springs are critical in providing tire compliance: the ability of
the tire to conform to irregularities in the road surface, rather than
just bouncing over them. When your tire is in the air, it has no
traction at all. Once you've settled on a minimum spring rate for each
end, you adjust relative spring stiffness front to rear. Then you adjust
balance with roll bars. However, that's just for racing. For driving on
the street, you need springs no harsher than you can live with on the
roads you have to drive on. Remember, too, that a swaybar is a spring,
too: a torsion spring. The stiffer your swaybars, the more coupled the
two wheels will be: ie when one is deflected the other wants to move as
well.

One consideration people make (I went for it) is ride-height adjustable
suspensions. Usually these are coil-overs with threaded spring perches.
Coilovers are useful for two reasons: adjustable spring perches make it
simple to change ride height, and the springs are sort of a standard
diameter and size, and can as a result be found in a variety of rates.
That makes it possible for a race team to tailor the spring rates to
different tracks (more about that in part 3). Some coilover kits only
allow height adjustment on the front struts; others allow adjustment at
all four coners. The latter is preferable, since it allows you lower
both ends of the car independently, and it also allows you to corner
weight your car. This is the process of balancing the car from right
front to left rear, and from right rear to left front. A car that is out
of balance corner to corner will misbehave at times when you'd rather it
didn't. It also lets you stagger the weights for a particular setup on a
certain track. The principle is simple: raising the right rear increases
the weight of the left front. Ditto. Vice versa. Etc.

Which brings us to the conclusion of Part 2: how the hell can you ever
decide? You've got two choices. You can go with a "package" or you can
roll your own. A package is a full suspension that you buy from someone,
like Dinan or whoever. They've put together a combination of springs,
shocks, and swaybars for a particular car that they believe work well
together. As long as your goal for handling exactly coincides with the
goals they established for a given package, it will suit you to a T. The
alternative is to roll your own. You either start with a package and
change stuff, or you just start from scratch and spec your own springs,
shocks, and swaybars. This is expensive, time-consuming, and potentially
dangerous when you try out your new setup on the track. The best way to
roll your own is to do like the Big Boys: get as much adjustability as
possible. If you have adjustable sway bars, you can try different
stiffness without having to buy new bars all the time. That's where Koni
shocks are better than Bilsteins: adjustability. Nobody's invented
adjustable springs yet, but coilovers are as close as you can come.
Typically you can go up or down in 50 Lb/in increments, which is pretty
fine-grain.  

We've covered a lot of stuff here, and I hope some of it's been useful.
I guess in part I was prompted by ads I saw in magazines, and subsequent
posts on the net, about different springs. These are generally
advertised in terms of how much they lower your car (achieve the look
you want!) which is a fairly meaningless metric. If you lower it a lot
and get the CG way down there, that's good, but if you wind up with the
balance all screwed up, or springs that are too compliant or too stiff,
that's bad. 

We haven't talked yet about suspension alignment or tires. We also
haven't talked about why a car that handles great today may be a beast
tomorrow. And why do people care about tire temps? Tomorrow (or
whenever) I'll try to cover that stuff as well.

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


John Browne
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