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From a qualitative evaluation of suspension stiffness, here is some information on perceived suspension stiffness:
The natural frequency of road car suspension is usually between 1 Hz and 2.5 Hz, with the following ride qualities:
"Pillow soft" - 1.0Hz is a soft traditional Cadillac ride
"Normal but a bit stodgy" - 1.5Hz is typical of a modern SUV or family sedan
"Quite hard and sometimes uncomfortable" - 2.0Hz is typical of many sports cars
"Teeth rattling harsh" - 2.5Hz is stiff and somewhat harsh, typical of the stiffest OEM suspensions.
With a sprung corner mass of 350kg (close to that of an M240i with driver and tank of fuel, minus unsprung mass), the effective spring rate when hitting a speed bump squarely and not invoking the anti-roll (sway) bar for each of these frequencies will be expressed as f = √(k / m) ÷ 2π, or k = (2πf ^ 2)m.
k = spring rate in N/m (1 N/m = 0.00571 lb/in)
m = sprung mass in kg
f = natural frequency in Hz
For
1.0Hz - 13,821N/m (79 lb/in)
1.5Hz - 31,089N/m (178 lb/in)
2.0Hz - 55,270N/m (316 lb/in)
2.5Hz - 86,359N/m (493 lb/in)
Track cars can go as high as 5.0Hz if bump absorption is not a significant consideration, leading to spring rates as high as 345,000 N/m or 1,970 lb/in.
From the perspective of tyre impact absorption, a typical tyre at 2 bar / 30psi pressure will deflect about 10mm under a total mass of 400kg (sprung + unsprung), leading to a spring rate of about 400,000N/m (2,284 lb/in), so will not affect the perceived ride quality when hitting large bumps to any significant degree, instead being responsible for high frequency / small deflection bump absorption.
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