Date: Sun, 14 May 2000 21:37:34 -0700
From: "david borden"
<>
I will try to relate what I have learned, but the experts are at Ground
Control, they helped me set up my car, and it worked great right out of the
box.
<< The car is 50/50 wieght distrib, and is approx 2200lbs.(cobra replica). I
did a quick calculation and I believe it (the spring rate at the ball joint)
to be in the area of 150 lbs in the front. 190 in the rear. Seems that the
front is a bit soft. >>
That sounds seriously soft. My car is 2600 pounds and 52% front so not too
far off. I comprimised on the soft side since I do drive it to and from
events. I went with 275 lb/inch at the front wheels. I have strut front
suspension, so the spring is very close to wheel rate. You need to calculate
in motion rates for most suspensions though. My rear wheel rate calculates
out to 250 lb/inch, and I would actually like it a bit stiffer. Due to an odd
motion rate, my spring is only 180 lb/in ,n back, the spring moves more than
the wheel, usually it is the other way around.
The motion rate is squared, in other words, if your spring moves 1/2 the
wheel travel, then you need 4 times the spring rate as the dwsired wheel
rate. This can make for some seriously stiff springs, even on light cars.
<< Anyone have some input on a good place to start. What are common #'s for
succesfull autocross cars. >>
I have seen great drivers run wildly different setups and all go fast.
Balance is far more important that stiffness. A soft car with proper balance
will outhandle any stiff car with bad balance. And a soft car will handle
better if there are any bumps. My softish 275 lb/in rate paid off on a few of
our bumpier parking lots around here, but on smooth surfaces, my poor
suspension geometry let the car lean into bad camber angles and it was not as
good. We have a very fast Fiat X-19 running around here with 500 lb/in
springs on struts and it only weighs 1800 pounds or so. But the car has NO
sway bars. The great thing about the 2.5 inch coil overs is they are pretty
cheap. Set your front rate as stiff as you want for a good ride and roll
control, and you can try a few different rear rates to dial in tha balance. A
front wheel rate around 250 - 400 lb/in should be very reasonable for a 2000
lb car. Since you have a 50/50 balance, start very close front to back, and
tune from there. Softer in back will make less oversteer/more push. Stiffer
in back will make less understeer/more loose.
<< Also, I have found formulas for measuring the spring rate at the ball
joint.
Is this the proper place to measure spring rate? >>
That should be very close to true wheel rate, and close enough fo tuning it
in, especially if your sway bars are adjustable.
<>
Don't go too big here. I tried huge bars, and it was great on smooth
surfaces, but any bumps and I was screwed because the shocks could not damp
the bar motions. Stiffening the shocks then over damped the springs in normal
up down motions. Running softer bars with stiffer springs worked much better
for me. The bars are a great fine tuning tool, don't depend on them for all
of your roll control and you should be fine.
Sway bars can get very tricky, since thee are so many factors that effect the
wheel rate they provide. If you are starting from scratch, you may want to
measure what the bars provide to give you an idea. I did this when I put my
HUGE bars on. I bolted the bar mounts to a bench with a scale under one arm,
and used a big lever to twist it so it was one inch different from end link
to end link. I don't remember the numbers now, but it gave me a pounds per
inch of difference from wheel to wheel. Then you need to calculate the wheel
motion rate AGAIN for the bar. This will get you the effective amount of
spring rate the bar wil provide in body roll. My stock bars which are back on
the car provide a modest 100 lb/in, and that is a 7/8 inch bar, but with low
motion ratios. My super stiff bar was over 250 lb/in, it was a 1-3/8 in bar,
but had different motion ratios.
The sway bar roll motion force adds to the spring in roll only, and has
little effect in bump for both wheels. This is where I ran into the shock
tuning trouble. The bar was as stiff as my springs.
Good luck getting it dialed in. I would love to see a great handling Cobra
kit flying around an autocross course. We have had a few around here, but
most don't sem to have the balance dialed in too well. They also run on
street tires, and tire boiling power makes them a serious handful.
Gary M.
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