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+--- Thread: An Experiment without Shocks (/showthread.php?tid=10018)
An Experiment without Shocks - davidbrady - 10-02-2013 10:02
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!
RE: An Experiment without Shocks - timetravelers - 10-02-2013 13:53
Great Info, I'll reset my front Koni steers per your test. Do you think the Tags need the Koni FSD, or if there a less expensive solution?
RE: An Experiment without Shocks - davidbrady - 10-02-2013 14:30
Morey,
What I'd really like to do is find a way to lower the resonant frequency. Prevost did it using auxiliary reservoir tanks, but they also added conical bump stops to the inside of the air springs. This gives them a soft initial spring rate when the suspension moves in short strokes but a very aggressive spring rate when the suspension moves in long strokes. (The spring rate curve has a pronounced knee). We don't have this option. Clearly Prevost added conical bump stops because they needed to. If we added reservoir tanks we may find that the suspension quickly bottoms without conical bump stops! The other alternative is spec-ing a different air spring - one with larger internal volume. We have to go wider since we can't go taller but I don't think there's room for any addition in air spring diameter. Alas, it is what it is! And, truthfully, it's actually quite good when set up right. Slightly on the stiffer side but it still handles and rides well. Admittedly I'm looking for that last 10%.
If anyone wants to know what true Blue Bird Bounce is, then take a spin around the block with no shocks. Wow! This little exercise completely reset my notion of what a misbehaving bus is really like. Koni deserves a gold medal for providing a heavy duty shock able to tame the beast without terribly compromising ride. This experience taught me that the 90-2947SP1 Koni on full soft is nothing short of fantastic. Now, folks may say it still bounces more then they'd like, but be aware that every little bit of bounce control that is dialed in results in a harsher ride all around.
Morey, someday I'll do back to back tests of the adjustable Koni's versus the FSD's on the Tag and Drive axle. I'm running FSD's and I like them but it's hard to say just how different they are without testing them back to back.
RE: An Experiment without Shocks - Kev - 04-30-2019 19:31
(10-02-2013 14:30)davidbrady Wrote: Morey, someday I'll do back to back tests of the adjustable Koni's versus the FSD's on the Tag and Drive axle. I'm running FSD's and I like them but it's hard to say just how different they are without testing them back to back.
Hey David. The search bar didn't give me the answers I was looking for so I'm having to resurrect an old post.
Did you ever get around to testing the shocks on the Tag and Drive axle?
I'll be replacing mine soon and am unsure if I should go with the adjustable Koni's or the FSD's.
RE: An Experiment without Shocks - davidbrady - 04-30-2019 20:12
Hi Kevin,
I never did get around to doing a back to back test, but I would definitely run FSD's at the back. Here's a video that describes frequency sensitive shocks. We definitely need good damping at the natural frequency, around 1.3 Hz to prevent porpoising, but we don't want it at the expense of high speed bumps like patchy pavement. FSD's give us this capability.
RE: An Experiment without Shocks - Kev - 04-30-2019 21:04
Thanks, David. It looks like the auto-adjusting FSDs are the way to go. After having watched that video I now feel motivated to go tune the suspension on my mountain bike.
