Help With Coil-over Spring Rates

[DLinson]

I’m getting ready to purchase my shocks and springs and I need to pick out my spring rates. Would any of you have a suggested wheel rate I should shoot for?

I am shooting for a curb weight of 3000lbs for the Nova with a 55% front bias. That’s about 825lbs per front wheel and about 675lbs per rear wheel. I’m also going to be using the QA1 single adjustable shock DR8855B on all four corners.

For the front suspension geometry, the distance from the lower inner pivot to the coil over shock lower mount is 12.38 inches. The distance from the lower inner pivot to center of the wheel will be about 19 inches. The front shocks lean in at about a 15 degree angle.

The rear suspension is just a solid axle with a custom designed 4-link. The rear coil-overs are only angled in about 2 degrees.

I’m looking for a suggestion for both the front and rear spring rates. I will be driving the car quite a bit even on some longer road trips. I’m not sure if I’ll be hitting many track events, the wife might have issues with that. Thanks for any advise you guys may have.


Thanks.
Dennis

[wally8]

I'll take a wild stab at this. Let me qualify this by saying I have zero experience with your type of car and just took a stab at some calculations. I'm sure someone will get on here that knows what they're talking about on this particular car and then I can see how my numbers compare. I used some weight transfer formulas and some spring calc formulas that I have used in that past that have worked for my applications.

I'm going to say 725 for the front and somewhere around 225 or so for the rear although I don't know your motion ratio and I'm used to working with coils, not leaf springs. Also keep in mind that this would be without a sway bar so they might be a little high. Just for comparison I figured a front wheel rate of about 309 lbs. I'm interested in seeing how my dirt track numbers compare to asphalt stuff.

Like I said, don't order anything based on my guess but it will be interesting to see other opinions.


Wally

[Salt Racer]

309 lb wheel rate may be bit too stiff for a street car especially if it doesn't see a track. But I guess my guestimate would come out about the same if I figure it w/ no bar. I've done a '55 Chevy street car before - it did 0.94G on 200' circle on BFG KDs. 3500lb running weight with 51% on nose. Its wheel rates are 250 lb/in front and 216 lb/in rear, with bars on both ends.

I can't really estimate front spring rate w/o knowing motion ratio, but I'd also go with 225 lb/in-ish rear springs if there's no rear bar. Or 175-ish with a small bar.

[DLinson]

I guess I forgot to add that I have a 1.25 dia. x .095 wall front swaybar that is about 34.5" long with 11.5" long lever arms. It is a Speedway Engineering sway bar. As of right now, I don't have a rear swaybar but can add one if required.

I included the distance from the lower control arm pivot to the shock mount and to the center of the wheel, 12.38" and 19" respectively, in the above thread if anyone wanted to calculate the motion ratios. The distance from the pivot to the balljoint center is 14.88".

Thanks for the advice.

Dennis Linson

[David Pozzi]

Dennis,
You can get a simple motion ratio that way, but a more accurate way is to also use Instant center (of FVSA) which requires the distance from tire contact patch to Instant Center.
I'd have to look up how it's done but I think you generate a "simple" MR, then multiply it by the second MR which is based on the Instant ctr to tire contact patch. This takes into account the true wheel movement, and added leverage of the wheel over the balljoint.
One method is to pick a spring frequency at the wheel and back figure the spring for that.
Here is some info on spring freq but the chart is in CPM not CPS so be careful when comparing numbers from different sources. CPS (cycles per second) is what I've seen more of.
http://www.smithees-racetech.com.au/performancefactors.html

My 67 Camaro with Guldstrand ft springs:
COIL RATE: 720 (close guess)
WHEEL RATE of 180
SPRING FREQ 1.46 CPS

I think the rear is around 1.3 CPS but I don't have a high degree of confidence in that number. These were calculated by my Perf Trends program but it isn't explained in the doccumentation how those numbers are calculated, doing it on a calculator yielded some slightly different front numbers.

Perf Trends shows a 2001 vette:
COIL RATE 390
WHEEL RATE 168
F 1.55, R 1.64 CPS.

I looked up an old file on II Much,
COIL RATE 400 lb
WHEEL RATE 189 lbs/in
FT SPRING FREQ 1.57

It looks like ft wheel rates in the 180 range would be pretty good.

[Salt Racer]

II Much is a bit different animal. The front machined face of LS1 block is almost right at the front spindle centerline.

David is right about effect of IC location on motion ratio. LCA length is needed as well. When I have someone figure it out, I cheat is a little and have him measure shock displacement at 2" of wheel centerline travel (assuming he has coil overs). That's usually close enough.

That said, assuming 16" LCA and 150" FVSA, the motion ratio comes out to about 0.714. I made further assumptions - 54" track width, 20" CG height, 3200-lb running weight, 2" FRCH, 10" RRCH, 1500 lb/in tire rate, Front bar motion ratio = spring motion ratio, etc.

I'm getting weight transfer number of 620 lbs front and 474 lbs rear at 0.92G lateral acceleration, setup for slight understeer. Based on other numbers, I get spring rates of about 475 lb/in front and 180 lb/in rear, with the said front bar and no rear bar. So my previous guess was a bit off for street tires. Ride frequencies would be 1.64 Hz and 1.47 Hz - that's more than tolerable for my butt.