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Originally Posted by RandallNeighbour
Um, I have matched my spring rates to yours. Should I use the ones that came with the set instead or what?
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did you get the 400/500 setup, or the 300/400 setup? i thought you traded out your rear springs for 7kg.
i'll try to explain the harmonic issue i'm talking about. when we design suspension for a car, one of the first things we do is to choose the natural frequency of the suspension. this is the rate that, if undamped, the car would bounce up and down at. higher natural frequencies make the car respond faster while lower natural frequencies make the car respond slower (and they make it more comforable).
the natural frequencies for road cars tend to be in the .5 - 1.3Hz range. street / track cars and some formula cars tend to be in the 1.3 - 2.0 range. high downforce formula cars are over 2.0. these frequencies are determined by the spring rate.
we actually want to choose slightly DIFFERENT frequencies for the front and rear axles. the reason for this has to do with how the car reacts over bumps. if the frequencies are the same, the rear of the car is at maximum height when the front of the car is at minimum height. this excessive pitch / dive is felt as bounciness at lower speeds. ideally, we want the rear frequency to be about 10% - 20% higher than the front.
the frequency is based on the motion ratio of the suspension (this is the distance the wheel travels compared with the distance the shock compresses in the y direction), the SPRUNG mass, and the spring rate. for our cars, the motion ratio is about 1.43. if i assume a 3000lb car with 100lb unsprung mass per front corner and 115lb unsprung mass per rear corner, then the sprung mass at each front corner is about 600lb and each rear corner is 680lb. the springs i received are 7.0mm/kg (392 lb / in) up front and 8.7 mm/kg (486 lb/in) in back.
this works out to a frequency of 1.76 up front and 1.85 in back. this is a difference of only 5%. the car would ride much better on the street if this were greater than 10%.
FYI, the stock KSport frequencies are 1.53 and 1.88; this is a 23% difference and is too great.
the softer spring rates i recommended (5.3 / 7.0) would work out to 1.53 and 1.66, an 8% difference.
usually this problem isn't so tough to overcome; the rear biased nature of our car means we have to compromise: the best spring rates for balance (under/oversteer) aren't necessarily the best for ride quality
if one uses the 5.3kg stock KSport front springs, they'd want a rear spring rate between 403 lb (7.2kg/mm) and 440 lb (7.9kg/mm) for a 'flat' ride.
if one were to use the 9.0kg rear springs (mine showed up as 8.7 rather than 9.0), they'd want a front spring rate between 322 lb (5.8 kg/mm) and 360 lb (6.4 kg/mm).
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