Tuning stock class cars can be a pain in the neck. It's hard to relate
to guys who drive in the SP and higher classes, who can actually solve
their problems the right way without constraints from the rules (except
for basic suspension design/mounting points). In stock, we have front
bar, shocks, and whatever alignment flexibility the car happens to have
from the factory, that's it.
For stock class, you often need to come up with creative solutions that
don't go by the book. It's a difficult thing to do, because we run on
different surfaces, tires, and types of courses and everything is always
changing. We don't get a whole lot of testing time either.
For your 330, the biggest issue is always going to be the open
differential. The car has a very sweet, torquey powerband but you
really can't use that to your advantage unless you can put that power
down early. The most effective way to cure inside wheelspin on a RWD
car in stock is to run a bigger front bar. This reduces body roll and
keeps more weight on the inside rear tire, reducing wheelspin.
What that does to the balance is the tricky part. Most stock cars are
camber limited and don't have enough adjustment from the factory. Your
330i has front struts which don't have a great camber curve and I
haven't seem more than -1 degree or so on one yet. The rear multilink
has a much better dynamic camber curve, so the net result is that you
are limited by front camber. Even though a bigger front bar should
increase understeer, as James mentioned it also reduces body roll and
the associated gain in positive camber. What you end up with is 2
curves going in opposite directions, and the trick is to find the
optimum point in between. Unfortunately there's no great way to do this
other than by trying it. To further complicate things the amount of bar
you need changes with the amount of grip from the surface and/or tires,
as James also pointed out.
A note about shocks- in theory yes, they only affect transitions and not
steady state. But watch anyone run one of our local courses from the
outside sometime. What you'll see is that the car is almost *always*
transitioning in some way- it's the nature of our sport. And I have to
say that with the exception of Saturday, most local courses do not have
as many sweepers as you get at Nationals. So what ends up happening is
that the shock does end up affecting the overall balance of the car,
because so much of the course is transitional. If you make a shock
that's stiff enough, it may never settle to steady state, even in a
fairly long (by autox standards) sweeper. When I ran my '99 Z3 2.8
Coupe, I bought a used set of custom valved Konis from Mark Sipe. I can
guarantee you these were effectively acting as springs, they were so
stiff. This isn't the best way to solve problems, but sometimes it is
the only way. Similarly swaybars are not commonly used to tune
transitional response, but they actually do help a little.
Jeffrey Macko was kind enough to loan me his STS 330i for a fun run
once. He has pretty much stock suspension except for swaybars and was
running the rear full stiff to reduce understeer. Unfortunately that
also acted to pick the inside rear wheel up on corner exit, causing
massive inside wheelspin. I said to him look, you've got all this
torque and RWD, but if the FWD guys can get on the power earlier than
you what good is it? I think sometimes you need to evaluate which
issues are costing you the most time, set the car up around those, and
then try to drive around the other ones.
If I had your car, I would make sure I had all the camber I could get
from the front. Then I'd try an adjustable front swaybar with maybe 2
or 3 positions stiffer than stock. Try them all out, and see if it
makes the balance better or worse, and also pay attention to wheelspin.
If a stiffer bar reduces wheelspin but makes the car push, you might
want to try playing with toe or shocks.
Or you can do what I did and buy an ES MR2:) Seriously, with -2.5
degrees or more of stock legal front camber, the car always responds to
setup changes as I would draw them out on a napkin. The car just
happens to fit the stock class rules perfectly- you take a loose car
with an open diff, make it even looser with alignment, and add a huge
front bar and end up with a perfectly balanced car with no wheelspin.
Peter
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