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Old 05-07-2013, 01:06 PM   #97
u/Josh
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Quote:
Originally Posted by Shankenstein View Post
To contribute:
m_1 (unsprung mass) = 37.6 kg
m_2 (sprung mass) = 280.4 kg (front) 244.5 kg (rear)
k_1 (stiffness of tire) = 350 N/mm (at 30 psi)
k_2 (stiffness of suspension) = motion ratio * spring rate = 2.11 N/mm (front) 2.8453 N/mm (rear)

We can construct a piece-wise interpolation that's easily mapped using a "spline" fit. How to Spline Like a Boss

note: Sign convention usually has compression as positive displacement/velocity and rebound as negative displacement/velocity.

damping coeffficient = Force/velocity

damping ratio = actual damping coefficient / critical damping coefficient

We want a damping ratio of nearly critical (1.0) for roll and pitch modes, but ride can be 0.5 - 0.8 for some driveability.

If anyone has not read these, please do! OptimumG Technical Papers

WRT ride frequencies:
spring rate = 4 * pi^2 * ride frequency^2 * sprung mass * motion ratio^2
2294.2 = 4 * pi^2 * f_r^2 * 280.4 * (1/0.92)^2
f_r = 0.42 Hz

That's a very soft ride. Sway bars stiffen it up in roll though. That's another calculation for another day.
I'm still getting 21.11 N/mm. Perhaps your are still in units of Kg/mm?

The good news is this works out to be 1.6 hz ride frequency. Anyone know why we see the highest gain in the bode plot at around 5 hz not 1.6 hz?
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