What tires?

 

Kooks. I'd say most of use are running kooks so well go with that.

 

No one will be able to answer this question, btw. There are too many other variables.

 

[QUOTE=josh7owens,Dec 16 2010, 09:00 AM] I'd be interested in seeing how that turns out. People that have tried disconnecting the front bar have been amazed at how "rolly" the car is. Isn't the solid Gendron FSB on full stiff around 1000 lb/deg? how much spring would it take to compensate for this? has anyone ever done the math?


ps- So I'm totally lost on on this calculations but I read on here a min ago somewhere a 850 spring/ .7 mr = 600 lb/deg wheel . Im guessing You all took 850/.7 and got 1214 and then divided by 2 to get the "600". From guessing would you just add the 1000 lb/deg of spring divided by 2 and now have a "wheel weight" of 1100 per wheel?

If that above is correct would a

 

Here's a reference for the calculations: http://performance-suspension.eibach.com/c...nsion_worksheet

And a calculator (note it takes the tire stiffness into account which none of us are doing here - you could put 9999 into the tire stiffnesses to be more like what we're doing): http://farnorthracing.com/autocross_secrets16.html

For natural frequency and wheel rates you just have to fill out the top row of stuff and the motion ratios.

Edit: Actually the tire stiffness is pretty significant even when it's 9999. The calculations will be in the ballpark but not exact (for example a 900 lb spring with 9999 lb tire stiffness becomes a 825 lb spring). I'm sure there's another calculator somewhere.

BTW, if you just want to get wheel rate, just multiply the spring rate by the motion ratio squared. So 900 lb/in spring * 0.7^2 = 441 lb/in wheel rate.

 

<---- too simple minded... Maybe I should go back to Stock class.

 

I was under the impression the AP1 and AP2 motion ratios (or installation ratios if you're referencing Milliken's "Race car vehicle dynamics") are very similar, if not the same. Guess it's time to take some measurements...

My wheel rates, using .7 installation ratios, are 441 lb/in front and 392 lb/in rear. This neglects the tire spring rate.

If I assume a linear (I know it's not) tire rate of ~2000 lb/in, the rates are 361 lb/in front and 328 lb/in rear.

EDIT: coil rates are 900/800 lb/in, for reference

 

There's this thread, which references 0.7 and 0.67 for an AP1:
https://www.s2ki.com/forums/index.ph...c=380006&st=0&

I believe that one personally. I was going to double check it myself on my AP2 just to double check as well as to make sure it's the same on an AP2, but I haven't had a chance yet.

There is an earlier thread that lists .82 and .77 but this was done in kind of a dubious way and the guy obviously meant to check it but never did: https://www.s2ki.com/forums/index.php?showtopic=169926

Edit: There's also this thread with a drawing that people have measured and come up with the same .7 and .67 as the first link: https://www.s2ki.com/forums/index.php?showt...0&#entry7439630

 

Don't worry Dave, most of this is bench racing/mental masturbation.

I prescribe to Nick's "drive your car, try stuff and see what works" philosophy, it's significantly more useful than a good portion of this discussion. The math is helpful to get you started, but doesn't replace real world testing.

 

Unless we know what the Ideal wheel rate is, these numbers mean nothing, correct? Or what the idea bias is front to rear?

For example if we knew the idea wheel rate was 600lbs/deg then we could just plug that in and come up with the correct spring rate to use. Although it seems we haven't yet come up with the idea rate therfore these numbers mean what?

anyone know the dia of the miata bar and thickness?



dave your car is fast, period. Just put it on a diet!

edit- I did just see this...
With a good setup, wheel rate for each axle should reflect the corresponding weight on that axle except for about 15-20% bias towards the non-driving axle (i.e. 15% towards rear for FWD and 15% towards front for RWD). Then, you choose sways to fine-tune front/rear balance. This was how my miata and BMW suspension were done.

The rule of thumb does not suggest particular spring rates that are ideal. Instead, it suggests suspension frequency for certain application. This frequency is mostly determined by the ratio of wheel rate to corner weight. For a street/autox/track car, high 1.X to low 2.X Hz are good start. For more performance-oriented streetable car, start at low 2-ish Hz. High 2-ish becomes bone-jarring and 3Hz is a bit too much for the streets IMO. Sways do affect roll-stiffness and in fact, they contribute more than springs in most if not all cases. The fact is it's the wheel rate with respect to corner weight that determines the natural frequency of a suspension which should be the starting point when setting up suspension. Sways are then used to fine-tune roll-stiffness. Shocks are then used to fine-tune damping for the chosen spring rates. I've ridden in many FWD cars that they run so much rear spring rates for the light rearend that it basically keeps bouncing off the pavement. That's not the way to set up suspension.

Take some easy number to start with. Say for the S with 2800lb with driver and 50/50 weight distribution. Corner weight is ~700lb. For low 2-ish suspension frequency, that works out to about 260lb wheel rate. Since you don't want exactly the same frequency front to back to avoid "resonance", you want to lower the wheel rate for one end by at least 15%. It's always favorite to lower the wheel rate on the drive axle for better drive traction. So with our RWD nature, you want 15% less wheel rate for the rear, another rule of thumb. With my measured motion ratio, that works out to 530lb/in front and 490lb/in rear. Don't these numbers sound familiar to us?