FD suspension motion ratios
 

Maybe I should rather post this in the Suspension section...anyhow.

Today, I jacked the suspensions and measured directly spring and wheel motion from unloaded (wheel hub hanging down) to fully loaded. I calculated a motion ratio 0.60 Front and 0.68 Rear.

Has anyone else done this I could compare notes with or, are such figures available anywhere?

I am doing this because I am moving from stock class and upgrading my suspension. I wanted to calculate spring rates at different frequencies, and get some reference points before starting playing with real springs.

Thanks,

Sandro

I haven't done much suspension work, but thanks for posting your info.

If you're interested in corner weights, I had access to some racing scales a couple of months ago, and posted the info in this thread:
https://www.rx7club.com/race-car-tech-103/had-my-fd-corner-weighed-advise-comments-suggestions-430565/

-s-

Hi Sandro,

Those look like they are in the ballpark, though I'd need to check my suspension model. If you lower the car significantly, you'll need wheel rates in at least the 300's to keep roll reasonable for an ASP car (very low cars with stock suspension probably have the front roll center under ground).

Best thing to do is use your calc's as a starting point, then real world experience to fine tune.

-Andy M.

Thanks Andy.

Yes, that's what I coming to. Like 12 kg/mm in the F and 10 in the R. That would give an equal frequency of about 140 cpm. I will start with that, then try going up. Next year project at this point...

- Sandro

Hi Sandro,

IMHO, for a serious ASP car those rates are too soft unless you plan on using the bumpstops as added spring rate. I think you need to be 30% or more stiffer than that.

-Andy M.

I value your opinion Andy and will therefore follow your suggestion. 30% more over 12/10 kg/mm would make something like 900/750 lb/in F/R I will start with those rates then.

Thanks again,

Sandro

spring rates
 

Those sound like good rates... Good job Andy...

Jeff

Sandro,
Those rates sound very good, as we are running 950/750 on Topeka ashalt on our 3-rotor which does have some extra weight on the nose. Make sure you get a good shock to control these springs.

That sounds more like it, though you will need to balance with your swaybar choices. You're going to want something sizable up front like the Tripoint bar, but shouldn't need alot of bar in the rear. If you can get the car to work with no rear bar that would be best for putting power down. I run a near stock sized rear bar and one of these days I'll try and get rid of it altogether.

-Andy

We have the 0.188" wall thickness Tripoint bar, and it is available in 4 wall thickness from 0.093" to 0.250". Also as Andy and I have discovered, you must get a KAAZ 1.5 limited slip (set to 67% locking) for autocross, as you will be spinning the inside wheel with the stock differential. This will help the most on getting power to the ground.

Andy, Eric,

Thanks a lot for your valuable advices.

As for the shocks, I have been using the Koni 8211 (matched to the stock springs so far) for a number of years now and found them very reliable and consistent. I asked Paul at Koni NA (who has been tuning them to perfection) about using them with higher spring rates (1,000 lb/in or so). He indicated that after proper revalving the 8211 should be still adequate for handling those rates. Having said that, I know you Eric have been very happy with the 2812, which you recommended to me in the past. I will give a try to my 8211 first, but if they won't work I will definetly go with the 2812.

It's good to know about the Kaaz. With the car stock and a super beefy front sway bar I had never experienced wheel spinning. With one exception, when one of the aluminum arms of a tubular Racing Beat front sway broke off in the middle of a run. The rear wheels were spinning so badly that I though the clutch was gone, as the rpm needle kept spiking up while the car would not accelerate...

Best,

Sandro

When you put enough rubber on the car in ASP/SM2 trim, you can unload the inside rear enough for the stock Torsen to spin. This tendency is worse if you run a larger rear swaybar. On lower traction surfaces, the effect won't be as pronounced.

-Andy

Is there any advantage in keeping the rear sway bar? It certainly helps with the stock springs, but why would you need it when you can use stiffer springs?

-Sandro

If you can get the car to transition well without one, then I would definitely do it. I played around without one for awhile, but never got the spring rates right and ended up keeping a small rear bar.

-Andy

hi erik, my kaaz is set to 33% but i have never really used it with good sticky tires...just used up kumhos victoracers. next season i'll have fresh v710's. what will i experience with the 33% that the 67% would fix?

i can't believe the springs rates you guys run with. i have old mazdaspeed coilovers (kyb) with like 5kg/mm front and 4 rear. i thought those high rates would guess skipping in a bumpy lot.

i race a little with sandro so i think we'll both be learning a lot this coming season (i don't even know what sandro is talking about with motion ratio but i'll save that for his email haha).

Since you will be able to put more power down, you may create some extra push. I really didn't see much difference from the factory Torsen other than I didn't spin the tires all the time. It did help to be able to drive the car more with the rear-end, as the locking stays consistent.

I have tried rates all the way up to where we are at now, and with good shock valving, the ride is suprisingly "soft".

awesome thanks.
do you run with a rollbar or cage? if so, curious where they mount to.

i actually just learned of your success and andy's from sandro, and posted this lookin for your setup:

https://www.rx7club.com/race-car-tech-103/rotaries-make-great-showing-solo-nationals-582888/

No rollbar or cage. Extra weight not needed in autocrossing.
As for Andy and my setups, Jeff Kiesel will be opening back up his website at www.7parts.com in a week or so, and both of our cars will be in there, complete with pictures and stats. I hope to get up to Dallas within a few weeks to have Steve Kan tune on the car again, as when we dyno'ed it earlier, it had a log manifold, trailing spark out on rotor 1 , and leading spark out on rotor 3, and it was 107 degrees. I will say that the accidental boost spike to 26psi on the first pull had about 500 ft/lbs of torque at around 4500 rpms. Exhaust guy flipped the hoses to the wastegate. Thank god for Steve's quick reflexes/ignition settings/and 112 Octane.

The consenous in this 5 year old thread is that 900#/inch front and 750#/inch rear are desirable spring rates for a FD prepared to SCCA A Street Prepared rules.

Are there any new opinions gleened over the past 5 years? The application would be on 275/15 A6's with the Street Prepared aero, i.e., small splitter and spoiler, not the SSM extended splitter and big wing.

Thanks, Philip

I believe those rates still stand.

My FD is currently 700/550 with 285/30/18 A6's, and the springs are way to soft.

Once I get my shocks rebuilt this winter, I will be going with about 900/750

i roadrace with slicks and run 1400 front / 1150 rear! car does not feel stiff.

Thanks guys for those two data points and your comments about them.

I just recalled that I did the motion ratio exercises a few years ago and put them in an excel spreadsheet.

Just set the spring rates in Sheet1 in the green boxes to see the wheel rates.

My measurements for the motion ratios are pretty close to those that Sandro calculated.