Monday, Oct 13, 2003 at 08:40
Re Simplicity suspension, BTs, semi trailers etc and the need or otherwise for shock absobers.
One could write a number of books on this. Years back I almost did but turned into a series of
articles (in Modern Motor) instead.
Start by considering what happens when a wheel travels over a bump on a vehicle with leaf springs and no shock absorbers.
Wheel is thrust upward as it encounters bump compressing the spring. The kinetic energy of that rapidly rising wheel and part of the axle assembly is now held within that compressed spring (just like a bow that has been pulled taught by an archer).
Once the wheel is over the bump it is now hurled down again by the energy held within that compressed spring (i.e as if the archer has released the arrow).
The downward thrusting wheel impacts the road surface with one hell of a thump. This shock load is transmitted back through the vehicle structure where it does the damage that one reads about on sites like this.
The impact of the wheel onto the road compresses the tyre to the extent that the wheel rebound and thrust the wheel back up. The cycle repeats at a lower level of energy - and if you see a car with coild spring suspension and a broken or crook shock absorber the wheel literally bounces up and down like a demented yo yo.
What's needed therefore is a way of dissipating the energy imparted to the springs. This can readily by done by converting it into heat. That's what a shock absorber does and how it works. It's just a heat pump.
With leaf springs there is some resistance as one leaf slides over another but this occurs mainly on the upward cycle, little on rebound. It is fairly effective on multi-leaf springs, but as the stresses on such springs are huge, they must either be very short or have limited travel.
For a soft ride however, the requirement is for the opposite, supple springs with substantial movement. This can only be achieved by using adequate damping via the somewhat miscalled shock absorbers.
Simplicity suspension is a special case. It has very little spring travel, relying instead of the walking beam action to allow a wheel to rise over a bump, with the imparted intertia partially counteracted by the inertia of the other wheel of the pair.
Whilst this action does reduce the workload on a damper, l am far from convinced that it would not work a great deal better with adequate damping - but off-raod it's a lot better than most that's around.
Most semi-trailer suspension is of a similar walking beam nature - but that on the 6 X 6 ACCO for example has massive shock absorbers as well. The move to air suspension on trucks has made shock absorbers absolutely vital, as of course they are also with coil springs (as they have virtually zero effective damping).
Curiously, and with rare exceptions, caravan builders believe that none of the above applies to them.
Collyn Rivers
AnswerID:
559836
