rx7racerca

iTrader: (1)

I'm looking to start a discussion specifically on spring rates. I searched 2nd gen and the archive, and found some sporadic references, but little in the way of comparative discussion.
To frame my question, I'm looking to add Ground Control coilovers to my '91 na, which already has KYB AGX adjustable struts, and need to decide on what springs to order with the coilover kit. The car is autocrossed heavily, not winter driven, and I do want it to still be livable on the street. I expect to also add the RB front and rear sways, and rear toe eliminator bushings. Essentially the car will be prepped to CAC Solo 2 Super Stock allowances, roughly comparable to SCCA ST but allowing r-compound tires (I run V710's).

What I have found thus far:
apparently stock front and rear spring rates are ~110-112 lb/in. It wasn't specified but I am assuming this is for TII/GXL/sport package models.
A friend who has heavily modded an RX-7 for racing (and by heavily, I mean full cage, significant lightening, and a 400hp/400ft/lb aluminum Chevy v8 under the hood, although thanks to the lightening measures, the car is only 70lb heavier (mostly on the front) than a TII. He runs 375lb/in front springs, 275 in/lb rears, and suggests this is not a very streetable stiffness, although from the passenger seat, I can say it works damn fine for Solo 1 or 2.

So the question is, where in between is a happy medium, keeping in mind the RB sways will contribute their bit to resisting body roll. And what is the reasoning behind different front and rear rates when the stock rates are the same?

Icemark

Well the stock rates are not the same for front and back.

The rates you have listed somewhat close to what Mazdatrix found on their spring tester:

Front :
Stock 95
Racing Beat 130
Suspension Techniques 120
Eibach 70/185

Rear:
Stock 90
Racing Beat 80
Suspension Techniques 120
Eibach (non-Turbo) 85/165
Eibach (Turbo) 90/165

and as a side bar, I personally suspect the fact that your buddies V8 has front springs 400%+ heavier than stock suggests he is lieing about the vehicles true weight difference, as if there really was only a 70 lbs differnce there would be no reason to run such heavy weights even on a pure race only car. Even if it was pure race, running 400% heavier springs in the front, but only 300% heavier in the rear says something about the balance because of the weight differences. I would suggest that setting your car up off of anything based on his set up may not be the best approach

You may wish to post in the racing/auto cross section and suspension sections instead of here as there will be a more race oriented group there.

lovingthefc3s

iTrader: (2)

Anybody Know what the rates are for Intrax 1.6'' lowering springs for a turbo?

-Thabks, Dustin.

drago86

Most japanese coilovers are 8/6 kg front and rear, which works out to 440~ front and 340~ rear in lb/in, so your friend might not be to far off.

Needa13b

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My book is autographed. Half the book is on Tires, which everyone in the world should read, and the other half is about static and dynamic chassis movement. Calculating weight transfer laterally/longitudinally, spring rates, ARB's, suspension movement and design. But don't take my word for it...

"Tires have always been a big mystery and a black art. Paul has figured out some things about tires and racecar handling and explained them so people can understand." -Mario Andretti

LI FC Greg

Greg,

I have a very similar setup on my GXL to what you are describing. I chose to go with GC's recommended base spring rates and with those springs I run full stiffness on the front AGX and 4/8 on the rear AGX. With stock height tires and the car corner balanced, I have to run maximum ride height or the wheels hit the inner fenders on large dips. It is still lower than stock. I'm switching to a shorter tire.

I have the RB front sway bar and have completely removed the rear bar. The car was a virtually undriveable oversteer monster with the RB rear sway bar. Remember, with coil overs and a large sway bar you are making your semi-trailing arm rear suspension essentially a live axle, but one that lifts its inside wheel in every corner. My LSD could do nothing to help me get off the corners. Removing the rear bar made the car more neutral and grip like crazy at the rear. More negative camber (-2.75) in the front prevented understeer.

I also have the rear steer eliminator bushings and poly bushings all around.

I woud suggest doing the coil over setup with camber plates but skipping the sway bars until you try the set up with out it. Truthfully, I think the stock 24mm front bar and no rear bar at all is best. A SPEED World Challenger driver who used to run FCs agrees with this set up.

Hope I have helped. Any questions, let me know.

Greg

rx7racerca

iTrader: (1)

I did come across some references to Teins with the rates you quote - the posters did say it was extremely stiff and not really suitable for daily driving (maybe in California or Arizona where roads don't crack and heave from frost). As it sits the car currently is very comfortable on our frost-heaved roadways, and I am willing to sacrifice some of that for better track behavior. And thanks for the suggestion about the racing/autocross forum - I didn't actually realize there was one (I mostly browse the Canadian regional subforum).
Interesting the RB lowering springs are that low a rate - I've actually heard good things about that setup too. Other references I found were for 350lb/in fr 275/rear, and a 300/200 setup, who said he had toned it down to that from 375/300 as he found that too punishing.
As far as camber plates, they'd be great, but bump me to CAC Street Prepared (roughly the equivalent of SCCA Prepared, except with fairly lax restrictions on turbo/supercharging, meaning it tends to be a class of big horsepower cars up here. Pretty much would need to do a TII conversion with a better turbo and IC to play there).