Originally posted by JimAttrill:
You may be interested in the following:
A while back Annetjie and Andy Grafton were having a discussion regarding shocks. I took the liberty of sending the posts to someone who has been invloved in shock design for over 30 years. The response is below which you may find interesting.
Several points are raised in these two letters.
First. Resistance values (softness or hardness) of shocks is almost entirely dependent upon the valving used. Springs, orifice plates, leak plates, flexible valve plates and other things are used to create the desired resistance at different operating velocities. Velocity refers to the speed of the collapse or exrension of the shock itself. Typically on 4X4 work this will be between about 0,1m/s and 1,5m/s. This has no direct connection with the speed of the vehicle. Therefore the resistance depends on the way the engineer designed it, i.e. what valving he used.
Secondly. Since oil (and most liquids) are almost incompressible it is necessary to have a gas space of some sort in the shock operating system. This is normally a gas space inside the shock. This is because when you compress a shock you put the rod inside the tube and there has to be room for it. This room is created by allowing the gas space to be compressed. In "twin tube" shocks the basic difference between conventional units and gas units is that this gas space is charged at about a thousand kilopascals in "gas units" and left at atmopheric pressure in "conventional". (Other changes are made but these are not significant in terms of effect on the vehicle.)
Note. There are also "mono-tube" gas shocks. e.g. Bilstein and a few others. I will say more of these at the end. They are slightly different animals.
The advantages of "gas" units derive from the fact that the ambient internal pressure is higher with the result that foaming, vapourisation, etc., occur at a higher temperature.
Also the replenishment of oil during the upstroke is better as it is dependent upon the pressure differential between the top surface and bottom surface of the replenishment valve in the base assembly. This differential is enhanced by the high ambient pressure. (The low pressure on the so called suction side will not be significantly changed on rapid movement to detract from this.) However, to set against this, gas units are doing a little bit of the work of the spring as they do support some weight by virtue of their tendency to extend. (They normally try to extend with a force of about twenty kilogrammes if my memory serves me correctly.) This means that the shock is doing additional 'work' in terms of energy. This additional energy tends to translate into heat which causes the gas unit to run a little hotter than the conventional unit.
All shocks are merely devices to change unwanted movement energy into (equally unwanted) heat energy. The heat can then be dissipated into the surrounding atmosphere.
This brings us to the remark that "bush bashing" doesn't normally cause fade in shocks. Probably substantially true. The worst condition for shocks is fairly slow forward speed combined with rapid wheel/suspension movements. The sort of thing you would experience when driving aross a series of ruts about 70 to 100mm high and a forward vehicle speed of around 20 to 25kmh. The shocks are working hard and the cooling air around them is quite slow. Very slow severe terrain is not putting movement energy in fast enough to be serious and rough but quicker road keeps a good air flow around the shocks to cool them.
Gas and non-gas "normal" shocks have a seal failure temperature at around 150 degrees C although prolonged use at around 135 degrees C may well cause failure. Some units which are made for high energy usage utilise "viton" seals which will take 200 dgrees C for short bursts and run for prolonged periods at around 150. All (I think) mono-tube shocks of the Bilstein type use viton seals. This very high temperature problem is rarely serious in off road work unless it is being done competitively.
Gas or non-gas? If your priority is rapid response of damping to changing conditions then go for gas. If your priority is keeping temperatures down go for non-gas. Or, go for one of the mono tube types a la Bilstein with high temperature capability.
Mono-tubes. A quick word on these. They are different.
There is a single shock tube which carries two pistons. One is attached to the rod and has the valving on it. The other piston is not attached to anything but is free to move inside the single tube. This floating piston has the gas chamber between it and the end of the tube and has the oil chamber with the normal piston on the other side. The volume change of rod inside or rod outside during normal shock movement is compensated by the floating piston moving and causing the gas chamber to get smaller or larger.
All the valving is on the piston so there is no eplenishment from an external chamber as in the two tube variety. Mono tubes are expensive as they have to be built to tighter tolerances and with more finicky materials. Consequently they are much more expensive. Mono-tubes are very good shocks but not necessarily any better for off road applications. There are many good arguments for not going this route for off road work, even for off road racing. Failures on mono-tubes tend to sudden and total when they happen.
The comments about Monroe being softer etc. This depends on the way the engineer designed them. Any good shock manufacturer can achieve hard or soft shocks so the choice is theirs.
Mixing shocks? Never mix from left to right - keep the same brand side to side. Front and back is a different story, there is no real reason against putting one make or type on the front and a different one on the back. No need to stick with gas or non-gas for the whole vehicle either. Again keep the same side to side. (Gas front and non-gas rear may tend to make the vehicle more prone to oversteer but on a 4X4 even that may not be noticeable.)
Cheers
Michael Ings, 98 Disco Tdi
Cape Town, South Africa
mike@infocare.co.za
