Hey GRM!
I'm looking for some advice on how to calculate ideal spring rates to optimize motion ratio and suspension frequency on a fwd hpde vehicle.
I recently upgraded my 2012 Mazdaspeed3 suspension to Feal 441+ coilovers and now that I have the actual corner weight values, I think there's room for improvement.
Current rates are F7k/R7k. 3044 lb curb.
LF 983 RF 945 (63.34%)
LR 532 RR 584 (36.66%)
L 49.77% R 50.23%
Cross (LF/RR) 51.48%
Cross (RF/LR) 48.52%
Any input is greatly appreciated.
Here's the article that got me thinking.
https://grassrootsmotorsports.com/project-cars/2019-nd-mx-5/how-fast-can-custom-built-coil-overs-at-off-the-sh/
dps214
SuperDork
10/31/24 9:20 a.m.
Are those weights with driver? I'm any case you really should get those cross weights better. And by cross weight, what I really mean is you want the ratio of lf/lr to match rf/rr. Right now there's an almost 13% difference between the two.
As far as motion ratios you need to do some fairly accurate (like better than a tape measure) measuring of suspension geometry points, at static ride height. IMO this is one of those places where no data is better than inaccurate data, or at least you need to be very aware of your accuracy level.
Similarly suspension frequency requires fairly accurate measurement of unsprung weight. This is a place where no data is *really* better than inaccurate data. I'd use it as a check to make sure you're in a reasonable range but not be making tuning choices based on frequency alone.
Previous numbers were without driver, new numbers below.
3226 lb with 180 lb ballast.
LF 1028 RF 993 (62.65%)
LR 662 RR 583 (37.35%)
L 51.15% R 48.85%
Cross (LF/RR) 49.94%
Cross (RF/LR) 50.06%
I haven't had a chance to fully dig into the article yet but it seems there's a ton of helpful info to point me in some right directions.
Motion ratio is easy to understand until the angles of shocks and springs are calculated as well.
in simplistic form if you have a 12" arm that pivots at a fixed point on one end and place a spring,or load out at the end you have 100% motion ratio,if you put it at 6" you have 50%.
As a practicle example a 68 Mustang has very close to 55% motion ratio,I moved the spring mount out 1" improving the motion ratio to about 65%. It's been years I don't remember exact numbers now. I was running a 550inlb spring mounting that spring 1" out made it near uncompressable with my 200lbs pushing on the fender. Also raised the ride height over an inch. IIRC a 400inlb spring would create the same wheel rate as the 550 spring. Again the 1" move did increase the angle of the spring and would have to be included in the calculations if accuracy of the math was most important.
Typically, someone has already done the work to calculate the motion ratios for your platform, so unless you've modified the suspension in some way that significantly changes the geometry it's fairly safe to copy their homework. Corksport lists them here as .98 front and .71 rear
dps214 said:
Similarly suspension frequency requires fairly accurate measurement of unsprung weight. This is a place where no data is *really* better than inaccurate data. I'd use it as a check to make sure you're in a reasonable range but not be making tuning choices based on frequency alone.
That's a little bit dramatic, an estimate of unsprung weight that is off by 30lbs for a given corner is still within .05Hz. That's way off if you're setting up a LMP2 or something, but in the ballpark enough for a DE car.
Driving4fun said:
Previous numbers were without driver, new numbers below.
3226 lb with 180 lb ballast.
LF 1028 RF 993 (62.65%)
LR 662 RR 583 (37.35%)
L 51.15% R 48.85%
Cross (LF/RR) 49.94%
Cross (RF/LR) 50.06%
Something is recorded incorrectly with those numbers. The corner weights add up to 3266lbs not 3226, the front % is 61.9 based on the corner weights, and the cross weight works out to 50.7%.
Anyhow, here's how that works out:
Wow thanks red_stapler! So you think the 180+ lb weight reduction would have little influence on spring rates? The factory claimed curb is 3,281 for the .98F and .71R and I'm at 3266 with ballast and a half tank. Also, I found the missing 40 lbs. It was a typo for LR weight. 622, not 662 doh.
It might be worth mentioning that the vehicle still has full interior, stereo, A/C, etc (lowered like -1.5" f/r, good rake) and the majority of the weight reduction came from the lightweight wheels, front bbk, coilovers and race seats (with min 4" lower seating position). I'm running a reverse stagger front 17x10 255/40 ad09 (41.8 lb) and rear 17x9 245/40 ad09 (42.6 lb). Wouldn't it now be ideal to recalculate the new reduced mass / lower unsprung values specifically for my vehicle?
To my limited understanding, any given vehicle (FWD, RWD, AWD) is sprung according to the weight bias front to rear and the shock valving. The idea is that a FWD is sprung stiffer in front vs rear because there is more weight in the front (and the car is being driven up their as well so you have a HP and TQ aspect), plus the rear end is usually much lighter and just being dragged around.
Ime regarding FWD, the softer rate in front vs rear is what I've seen in what I consider to be well sorted cars and I think there's most likely room for improvement with my setup.
Also just found this old article. Although I think the OP took a few liberties with some of the numbers.
https://grassrootsmotorsports.com/forum/grm/mazdaspeed-3-suspension-set-up/106548/page1/
