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Baselines!
by Patrick Budd, ProCar
Performance
While in the tower at the Cecil County NSCA race, I was talking
with Ned Erkman from Rolling Thunderz. He suggested I write an article about baselines, or starting points beginner
and intermediate racers could refer to if they wander too far off course and need a map back. This is by no means a
definitive article about how every car should be run; that would be impossible. It's made up of ideas the staff at
ProCar had; your experiences may yield a different solution, so feel free to experiment.
Being that ProCar is in the chassis business first and the engine
business second, it stands to reason we'd have plenty to say about shock/spring/ladder bar and four-link tuning.
Racers have a fascination with their engines; numerous test sessions are spent dialing in carburetor jetting, timing
curves and shift points. Equal time spent on the chassis can often yield even greater results. Ladder bar/four link
angle, spring rate and shock tweaking can pay off in a faster and more consistent race car- making the platform stable
enough to do its job and react to tuning input as you move back in front of the firewall and dial in for power.
When a racer chooses a ladder bar suspension, whether by rule or by
choice, they receive a predictable platform that doesn't require much ‘black magic' to get down the racetrack.
Unlike the literally hundreds of tuning choices available to the four-link owner, they have a straightforward package
that should perform well within a fairly narrow window of adjustments. When working with a set of ladder bars, it is
important to remember that the suspension will deliver a bunch of anti-squat (what you see when the rim heads into the
sidewall at the launch) and not a ton of load as the car drives out. Lowering the front pick-up point of the ladder
bar will increase the load applied down track while reducing the amount of ‘hit' (anti squat) sent to the tire.
When measured on a flat surface, the ladder bars usually run well with about 2.5 degrees of rake on the bottom bar
when compared to the ground, low front to high rear. Often, we'll see bars that angle up or not at all. One needs to
remember that the amount of hit goes up as the front of the bar rises, and load carried goes down. Balance is
critical, as we need to keep the tire hooked as the car moves off the line. If we lower the front of the ladder bars
too much, the car will spin at the release of the trans brake, as the power delivered overcomes the anti squat
available.
Four links require a bit more finesse to get the ET's down.
Because the top and bottom bar are independent of each other, there are many ways to shape the theoretical intersect
point or ‘instant center' (IC). The shape the bars take is as important as the IC location itself, as you can
juggle them several different ways and plot similar points- angularity and height contribute greatly to hook and
driveability. Our experience has taught us that most SS, TS, PO or even n/a EZ street cars want a lower bar that runs
fairly parallel to the ground, as low as 6.5” for the lower powered car (actually lower power/weight ratio) to
as high as 7.5” for the Pro Outlaw beast. Power and center of gravity make the final determination, but most of
the faster entries want a maximum of 3/8” rake in the bottom bar. Otherwise, the housing will push the quarters
down at the launch and wad the tire. (Lots of rake causes too much ‘bite', or down track loading.) We usually
stick with 3/16” and go from there- some cars run flatter bars, some more angular. IC length is fairly
straightforward; and controlled with the top bar. Starting at a height of 4” (from axle c/l) or less at the rear
(housing side) of the top bar, we take into account the wheelbase and weight percentage when choosing the front or
upper chassis hole. This is how we arrive at a workable IC length. If setting up an 8 second or quicker car for the
first time, start with a roughly flat bottom bar, top bar at 4” or less away from the housing, then angle the
front as required to reach optimum IC length. The slower the car, the more angular bottom bar the car will tolerate.
On cars going 9's or slower, the top bar can be spread as wide as 4 5/8 from axle c/l, and the bottom bar can rake as
much as a half inch. The reasoning is simple- the more torque the car has; the more it can load the tire without the
suspension's help. As power decreases, the car needs to apply the tire using mechanical leverage, using longer IC's
with more spread between the bars.
Springs and
shocks are easily the most critical tuning aid we have at our disposal. Koni's generally start at 4 sweeps from tight
on rebound (extension) and 5 clicks from loose on bump (compression). Santuffs start about 4 sweeps from tight and on
rebound and six clicks from tight on the bump. Strange shocks respond to a similar setting as the Koni's; ¾
tight on rebound and about half tight on bump, depending on the model you have and the number of adjustments
available. These are obviously baselines, and only trial and error produces the best results. No shock should be
completely locked up or dead loose- if that is the case call the manufacturer to discuss a custom valving package. The
tighter the rebound is set, the shorter an IC the car will tolerate. Longer IC's often require a softer rebound and
stiffer bump as the increased leverage on the chassis leads to more squat. Shorter IC's with a flatter bottom bar will
try to separate the chassis at launch, requiring greater clamping from the shocks to control the housing's
movement.
There are several schools of thought
pertaining to spring selection. The easiest and least scientific way we've found to choose spring rate is to
select the heaviest spring that doesn't come loose when the rear is dangling on jack stands. Most 3000 lb
backhalf cars work well with a 130 lb rear spring. Our 2500 lb Vette uses a 100. The goal is to purchase a high
quality shock that will control the housing at launch, without resorting to a heavy spring to stabilize the rear.
A slightly more pliant spring will go over bumps and irregularities better, with the shock doing the work to keep
the assembly stiff at the hit.As racers begin to play with power adders, (especially nitrous) they are often at a
loss where to start. In the case of nitrous, it is critical to establish where the motor wants to be naturally
aspirated. Knowing your engine's best timing and carburetion curves before you engage the spray will make for a
quicker learning curve than just setting timing at an arbitrary number and guessing at the amount of retard
required. We start off with the most efficient timing curve that yields the highest mph on motor, subtracting 3
degrees for every hundred horsepower of nitrous added (plus two for safety). If you're patient, ‘learning'
the system's idiosyncrasies at lower power levels can prepare you for the bigger jets. Lighter nitrous loads are
also more tolerant of richer mixtures; tune-ups in excess of 300 hp (advertised) are not. As the area of the
nitrous pills grows, it will burn an increasingly larger amount of fuel- even if you are too rich. Think of an
oxy-acetylene torch with the acetylene turned up too high- it still burns, but the flame is uncontrollable. The
oxygen (or nitrous in this example) trims the flame kernel, tightening it up. With a leaner mixture and a little
extra timing retard you may not make as much power, but you reduce the possibility of leaving un-burnt fuel in
the ring lands and distorting the piston. Still, nothing replaces experimenting with the smaller pills in the kit
and trying different mixtures. This gives you a feel for how the kit ‘works', and the smaller shot will
protect you from yourself as you learn the kit.
Hopefully, these tips will help somebody out there. If you're
confused, or see something that might help you here, feel free to call 585-247-8770 and discuss your needs. We can
provide some detail and do our best to help. Happy racing!
Copyright © Patrick Budd, ProCar Performance
