Spring Rate VS Suspension Travel

INFORMATION G2IC Archives
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My dilemma, I am not quit sure how to properly determine maximum amount of roll to maintain efficient suspension geometry VS over springing a car resulting in lateral skipping across an autocross course surface irregularities and bumps.

What is the most efficient way to measure camber changes during changes in suspension compression and rebound? With changes in wheel direction? Flipping my upper control arms is an option to increase my camber.

Strictly autocross, I presently run 450lbs springs in front and 500lbs in the rear on a 2400lbs front wheel drive vehicle approaching the dampening limit of stock Koni’s. Wheel rates, 225 in front and 275 in back.

I get occasional bottoming on the front bump stops on R tires, in high load corners with surface irregularities. I devised a method of dropping the lower mounting position of my Koni damper bodies, which provides approximately 2.5 cm of additional compression travel on top.

Should I consider sending the dampers for shortening to reduce bump stop contact? Or re valveing along with increasing my over all spring rates maintaining the roll balance and eliminating bump stop contact? I do not covet an over sprung vehicle with reduced lateral adhesion. What are some of the preeminent indicators a vehicle is over sprung?

A stiffer front anti sway bar will upset the balance, reduce overall lateral grip and induce additional under steer, not a consideration.

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ARCHIVETHIS

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You seem like you have a few ideas what to do. Honestly, in racing there is no one best way to do something. My suggestion to you is to experiment with various setups like the ones you mentioned, then test, test, and test some more. Your setup will obviously change with the location, weather, temperature etc… If you only want to go with one setup, you will need to find a compromise. If you plan on changing it, you need to gather enough test data from various conditions that will allow you to change your setup accordingly. If you have access to corner balancing scales, it would help you set your spring and wheel rates accordingly. As for watching your suspension geometry, I can’t think of much other than mounting a camera down there and doing some more testing.

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Hello Stephen,

My corner rates are 877LF, 806RF, 475LR and 436RR. I have removed around 50lbs since last weigh in and gone up 1 turn on the left rear, 8 turns per inch.

You wrote,
"If you have access to corner balancing scales, it would help you set your spring and wheel rates accordingly. "

I am not sure what you mean here, the car has been balanced with in its front to rear limits, as crossweight are with in 20 lbs.

Thanks for the comments.

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Here is some additional information that has come back to me from other source.

I was asked how I determined my Wheel rates?

Motion Ratio
Take the spring off the damper and put the damper back on. With the wheel/tire removed, move the wheel hub from full droop to full compression in 1" increments. Measure and record corresponding strut displacements. With the data, correct the wheel hub displacement numbers by the wheel off-set to get the theorectical wheel displacement.

Wheel VS Spring Rate

Integra G2 motion ratios:
Front: 1.5:1 (for one inch of shaft travel you get 1.5 inches of wheel travel.
Rear: 1.35:1

Wheel rate = (spring rate / (motion ratio squared))

Front wheel rate = spring rate / 2.25

So 400 lb front spring gives a wheel rate of 180,
and a 600 lb spring gives a wheel rate of 270.

Rear wheel rate = spring rate / 1.82

From this information Joe Chung a friend, engineer, car builder, and SCCA multi national champions driver of the worlds fastest autocross car A/MOD wrote.

"With your current set up, your wheel rate is only 0.25x corner weight for the front. For the rear it’s 0.58x corner weight. No wonder you are bottoming out in the front. Most competition cars’ suspension frequencies are around mid to high 2 cycles per second. That equals to wheel rate around 0.6 - 0.8X the corner weight. Currently, there is a lot of discussion among the Formula Ford guys as well as the D/E Mod guys from SCCA about going to 1-1.5X corner weight. I think for autox, 1X is about the upper limit and 1.5X is overkill. For a street driven car, I would say 0.5x the corner weight is about the maximum one would want to go. This would mean 430# front wheel rate. With the motion ratio you provided, spring rate up to 1000# could be used. To prevent understeer, you might need to reduce the front sway bar size somewhat. This would reduce the front roll rate which would balance out some of the increase from the stiffer springs. The net result is that at corner entry, under brake (when sway bar has no effect), the stiffer springs would reduce chassis movement, hence less chance of bottoming, also less suspension geometry changes. You will need better quality 2-way adjustable shocks to go with the set up. They will provide an increase in rebounce damping (needed for the stiffer springs) without increasing the bump rate (which might be the current cause for your car skipping over rough pavement). Don’t forget, overly stiff rebounce will also feel like overly stiff spring.

As far as shorter shocks are concerned, I think all cars with reduced ride height should use them. With shorter shocks, proper length bump rubbers could be used and that would provide a gradual increasing in spring rate rather than a sharp increase. You can alway use hard plastic shims to shim the bump rubber down but you can’t do it the other way around with a longer shock.

Joe"