RacePro Engineering

Hi Tom,

Your short answer, (which you obviously already know) is “Yup”. Higher rate springs certainly “unsettle” the car more over bumps, especially where different size bumps are affecting different wheels simultaneously. But, like most of the isolated components in a racecar, it is only a small part of the bigger picture. The softer spring will absorb more of a bump before feeding its force into the chassis, and that feels good to the driver. But, because it is compressing farther, it will also tend to rebound farther, and for a longer period of time, and that feels wrong to the driver.

Really, it all comes down to what is the best compromise as reported by lap times. Our feeling is that we spend most of our time on smooth tracks, and even the bumpy ones (Pocono) have MOSTLY smooth pavement. Higher rate springs work best MOST of the time (except when it rains.) If that means that we “take less curb”, well, then that is what it means. If it means we go “off line” for braking, well then . . . . .

Further, as a driver’s capability increases, so does his preference for a less compliant setup. To the novice, having the car sliding and darting all over the track seems chaotic. He will not be able to drive such a setup quickly. In fact, his likelihood of crashing is probably high. However, over time, he will demand that the suspension react more decisively, and that his inputs, and outputs, take effect immediately.

Most pro teams carry a variety of springs so that they can arrive at the best compromise at any given track. For we mortals, the higher rate spring, over bumps, “unsettles” the car more, AND it also “recovers” faster. This plays into ASP’s spring “frequency” in his post after yours. Adjustments in the dampers also minimize the undesirable effects of the higher rates, and help maximize their benefits.

And, of course, you are absolutely right with your “too much spring” being not good. But, in most cases, that point of having “too much spring” is so far from what most of us are running, that we could double our present spring rates, and it would still not be enough. LoPRESTI’S RULE-OF-THUMB #7: Stiffer is faster.

Ed

RacePro Engineering

Travis,

Without giving this exhaustive thought, I would think of it “the other way” - that the shocks are angled to that degree in order to effectively damp the oscillation of the transverse leaf spring. That may have inspired GM to spec higher rates for the rear, but again, FOR THE STREET. Once we move to the track, all that reasoning becomes moot.

In any event, I think that we can trust that when a 700 pound spring compresses (or bends) a complete inch, the corresponding wheel has “seen” a 700 pound bump or droop.

Ed

RacePro Engineering

Ben,

While we are DRIFTING away from Bob’s original questions, it is important to consider how the other suspension components enter into this equation.

To my knowledge, the only difference between GM’s spring rating and VBP’s is that GM uses the more scientific Newtons per millimeter nomenclature, while VBP utilizes the universal racing pounds per inch. Both measure the same thing - how much force is required to bend the spring a certain distance. If one multiplies Newtons/mm times 5.71, one arrives at pounds/inch.

Trying to limit here the discussion to springs and anti-roll bars on the track, springs limit the efffect of weight that is transferred under braking, acceleration, and cornering. The stiffer the spring, the less the weight transfer is felt. Of course, anti-roll bars come into play only in cornering. Here they serve to determine WHERE that weight is transferred, and HOW RAPIDLY it shifts.

With this in mind, most of us can envision maybe three extreme scenarios:
[1] Soft springs with high-rate anti-roll bars. Lots of dive under braking, lots of squat under acceleration, lots of weight roll to the outside in turns, very quickly, but with great control of where the weight shifts.

[2] High-rate springs, with soft anti-roll bars. Limiting of dive under braking, limiting of squat under acceleration, limiting of weighted rolling to the outside in turns, weight shift happens slowly, with limited control of weight locations.

[3] High-rate springs with hi-rate anti-roll bars. Everything is limited, and in a sense “controlled”. The only “downside” is that everything happens more quickly.

I realize that most venues for autocrossing are far (FAR) from ideal. But one must plan suspension to optimize the car’s grip MOST OF THE TIME. That is what makes someone like ASP win.

Incidentally, anti-roll bars have absolutely no bearing on wheel rate.

Good questions!
Ed

froggy47

Leaving shocks/bars/tire pressure/etc out of the picture for the moment,
how does switching the spring rates to VBP style higher front and lower rear impact "getting power down" both at launch and (more important) at corner exit?

JVetthead

And lets not forget good aero requires more spring..
1300/850 leafs

RacePro Engineering

In a vaccum, switching to higher rate springs at the rear will minimize squat, and stabilize “slewing” from side-to-side under hard acceleration. (Remember that drag racers embrace these qualities.) Because of its limiting reaction, you MIGHT have to feed the throttle more gradually, because the result will be more direct.

Although you are not asking this, it will also help limit dive under braking, limit weight transfer roll in cornering, and add control to power-on oversteer. It MIGHT delay turn-in, depending upon other factors, and may add oversteer characteristics to the chassis.

Ed

63Corvette

I like all of Ed Lopresti's valid engineering explanations.....BUT, I really wanta hear what Danny Popp is running for his track suspension (shocks/springs/bars)

PushinTheLimit

froggy47

Good point, I assume you are talking about cars with splitters & wings.

I don't have those, but still good point.

geerookie

I would bet the front springs are in the 1600 -1800 range and the rears are 1000 - 1200, probably Penske shocks and with spring rates like that do you really need bars?

RacePro Engineering

Bob,

I just realized I answered your question "backwards". I am very sorry. I should have written:

In a vaccum, switching to higher rate springs at the front will minimize dive under braking, and add turn-in control. A relatively softer rear spring will allow more forceful application of throttle without immediate oversteer. Standing starts will probably not be affected unless you go "dragster-style" soft in the rear, where huge amounts of weight get thrown rearward.

Depending upon other factors, you may notice a tendancy toward more understeer.
That is what I should have written!
Ed

froggy47

Do you mean like having a bumps at corner in?

froggy47

This thread is helping me a lot in understanding & making decisions on car set up.

The reason I am trying to isolate talking just spring rates is twofold.

We all know suspension tuning is a system & everything has to work together, but like when you are tuning the handling, you change one thing at a time, not two or three.

So that's what I am trying to grasp, one thing at a time, springs. Also it's what I haven't yet changed on the car.

I can't do coil over in the SCCA classes I run, so springs it is.

The other personal problem I have is the track I run at is pretty nice, new, smooth but the lot I run autox on is bumpy.

Rock & hard place.

Still & all, excellent very high level comments from everyone & thanks to the old pros who know all the stuff from years ago for sharing with us who are near the beginning of the learning curve.

RacePro Engineering

Yup, that would be one other factor. Other factors weighing-in would be adjustments, like to tire pressures, ride height (rake), dampers (if available), anti-roll bars (if available). But if the ONLY CHANGE were higher rate front springs, the car would tend more toward understeer.

TRACKMAN2

ed you da man!! love this info ill be picking your brain for sure as soon as i can get a dam motor to run!!!!

RacePro Engineering

Bob and George,

I truly appreciate the kind words. As I recall, George, only a short time ago a certain Forum member helped me, with great detailed suggestions, in replacing a steering column on our racecar.

It is very rewarding to contribute when folks are so receptive!

Best,
Ed

PushinTheLimit

Those are crazy stiff springs! I was thinking more along the lines of 1200 fronts and 900 rears.

Just for my applications, if I went to 1200 front and 900 rear springs, revalved Bilstein shocks... should I still run the T1 swaybars or something less? Or is it more dependent on how you like your car to handle?

I know with my basic T1 setup, the car does have some oversteer in it but its controllable and I like it so far. I would much rather have oversteer than understeer in it.

sperkins

Definitely smaller bars since you'll be getting your roll stiffness from the springs instead of the bars.


Understeer is a lap time killer.

PushinTheLimit

That's what I was figured. And those changes are in lieu of me going to a wing/splitter setup eventually. I'll just run my T1 setup until I can do the entire swap.

travisnd

I know Wally runs Callaway/Eibach coil-overs with stiffer springs (don't know the rates) and stock Z06 bars.