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Why cheap shocks might be better
09-26-2013, 13:21 (This post was last modified: 09-26-2013 14:27 by davidbrady.)
Post: #1
Why cheap shocks might be better
There's an interesting graph in "The Shock Absorber Handbook", by Jason Dixon. I uploaded a copy to our Library at Document Library -> Theory.

Here's a snippet of the graph: ─░mage

Transmissibility is the degree to which road imperfections are transmitted thru the suspension to the body of the bus, the seats, and the occupants. It's a measure of how the suspension system reacts or overreacts to road input. If you bounce the suspension at it's natural frequency then the bus will theoretically bounce out of control (frequency ratio 1 on the chart). For our buses the natural frequency is around 1 Hz.

Looking at the graph you can see that there's a range of frequencies where shock absorbers actually make the ride worse. We'd be better off without them. The transition is where the graph transmissibility passes thru 1. This occurs at sqrt(2) times the resonant frequency of our suspension. For all road inputs above this frequency we'd be better off tossing our shocks!

This means for any road imperfection that comes at us faster than seven tenths of a second (1/(sqrt2*resFreq)) we'd have a better ride without dampers! So for every pothole, bridge abutment, railroad track out there, your ride would improve if you removed the shock.

So how do cheap shocks make the ride better? The road imperfections listed above are all low amplitude motions and there's a correlation between low amplitude road disturbances and frequency - low amplitude equals high frequency and we've already said that high frequency ride is better w/o shocks. Cheap shocks are only speed sensitive; they're not frequency sensitive, so you'd expect them to be worse on these types of road inputs. But wait - cheap shocks also come with cheap bushings. They're overly soft and overly compliant. This is the key to providing a good ride over choppy surfaces. The soft and compliant bushings deflect on the choppy surfaces and act closer to no shock at all! The cheap shock acts as a poor man's frequency sensitive damper.

Incidentally, the Koni Frequency Sensitive Damper (FSD) does just this. It has special valving that provides maximum damping for large amplitude road inputs and minimum damping for small amplitude road surfaces. This way slow resonant frequency road inputs won't bounce us out of control, while sharp and fast disturbances are minimally damped.

Koni offers FSD's for our wanderlodges but only for the drive and tag axle, so if you want a good ride on rough road, take your adjustable Koni steer axle shocks and back the stiffness off or put in some soft bushings. Air springs by nature have built in damping so there's little to lose and much to gain.

david brady,
'02 Wanderlodge LXi 'Smokey' (Sold),
'04 Prevost H3 Vantare 'SpongeBob'

"I don't like being wrong, but I really hate being right"
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Why cheap shocks might be better - davidbrady - 09-26-2013 13:21

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