An Experiment without Shocks

[davidbrady]

I removed my steer axle Koni shocks and drove around the block. Actually, I had a pair of worn out Koni 90-2497SP1's which I drained of the damping oil and I bolted them on. I didn't want to drive without shocks because it's the shock that limits rebound travel.

What I found is that the bus body wildly oscillated at it's fundamental (resonant) frequency at even the slightest pavement input. Speeds had to be kept down to keep the bus drive-able, less than 40 mph. I counted the number of oscillations in a 10s period and found my bus has a resonant frequency of approximately 1.3 Hz.

Any abrupt, small amplitude road imperfection like a manhole cover or a slight change in road elevation caused the axle to tramp. This motion was around 10 Hz which is the resonant frequency we'd expect from a tire.

From the bus body frequency of 1.3Hz I can calculate the spring rate of my air springs: Res Freq = 1/2pi * sqrt (K/m)

(K)'s the spring rate and (m) is the sprung mass on the steer axle. This formula calculates to 2180 lbs/in, but there are four springs so each spring is providing 545 lbs/in.

I think a resonant frequency of 1.3 Hz is on the high side, and a spring rate of 2180 lbs/in is a smidge stiff. Most luxury passenger cars are around 1 Hz and most performance cars are around 2 Hz. I have some Prevost air spring documentation which shows that they're achieving 1 to 1.1 Hz, or a 4 spring equivalent of 322 lb/in per spring, almost half of my LXi.

The shock can make the suspension overly stiff by not allowing sufficient time for rebound. This forces the suspension to be slightly compressed when it hits the next bump. Compressed means it's operating in higher range on the spring rate curve. (Air springs are very progressive).

Driving without shocks taught me just how necessary they are on our buses. The bus floated over bridge abutments and manhole covers which was good, but the body oscillated wildly and the axle tramped badly. I realized just how good the Koni's are even on their softest setting. I set mine to the softest setting and drove 135 miles from Charlotte to Asheville NC. The softest setting is actually quite a bit of damping compared to no shocks and it allows the suspension time to rebound for the next bump. I have a whole new appreciation for how hard the shocks work and how important they are!

For a long time I thought the optimal setting was 1 turn from full soft; I like full soft better!