Total Lateral Load Transfer Distribution

[Sleeper68]

Are those measured ride frequencies, or calculated?

Calculated, using measured values - namely corner weight, motion ratio, and spring rate (normalized) using the std. equation for natural frequency ω=√(k*MR/m) , where ω is the natural frequency of the sprung mass, k is the spring rate, MR is the motion ratio, and m is the sprung mass.

When I actually measured my ride frequencies, I was surprised to find they were quite a bit higher than calculated, especially in the rear. My car (1968 Firebird, 3410 lb, 58% front) has a rear ride frequency of 1.75 Hz with stock 5 leaf rear springs, which have a published rate of 110 lb/in.

To measure the ride frequencies, I took the shocks off one end of the car, put the other end on full stiff (to minimize coupling effects), and bounced it. It'll be kind of like pushing a swing - it's fairly easy to feel the natural frequency. The quick and dirty way is to count bounces in like a 30 sec timespan, but I got fancy and used the accelerometer on my phone and then analyzed the data in Matlab. I think it's worth measuring (if you haven't already) to give you a baseline.

That is very interesting. Your measured value and theoretical value for natural frequency vary quite drastically in the rear. My calcs show you should have been in the 1.56Hz range for rear ride frequency (assuming a motion ratio of 1.0), however with a motion ratio of ~0.7 (the actual leaf spring simplified motion ratio in roll) you get 1.1Hz. This brings up an interesting point - the wheel:spring motion ratio for a solid axle changes depending if the sprung mass is moving in heave or roll, or some combination of the two. This may be why you got a higher measured value than "expected", you were moving the sprung mass in heave. Additionally, as you probably already know, leaf springs have a progressive rate when installed with shackles like G1 and G2 F bodies have. This may have contributed to the higher measured ride rate/frequency as well.

To be clear, my estimate of my rear ride frequency (1.3Hz) actually uses the measured "pure" roll motion ratio of ~0.7:1. Using a heave motion ratio of 1.0:1 I get 1.85Hz rear ride freq for my car.



I will check the rear ride frequency using your method on my car when I have a chance and report back.