To the layman, seeing 2 shock absorbers on one wheel may give the comfort of double capacity or “bullet proof” suspension. This concept is not taken up by 4WD manufacturers; and for good reason. There is a much better outcome to increase the size, quality and design of a single shock absorber with better suspension design than just bolting on an additional $120 shock absorber.
The science of Shock absorbers
In simple terms, a shock absorber’s purpose is to dampen the suspension as it goes up and down. Without them, your caravan or camper trailer would continue to bounce up and down until all the (kinetic) energy is finally dissipated by friction in the springs.
Energy can’t be “destroyed”, so the shock absorber converts this (kinetic) energy into heat as it dampens the “bouncing” of the springs. So in difficult terrain, shock absorbers have to get hot or they aren’t doing much.
How does a shock convert to heat energy and then dissipate it?
By forcing a piston through oil, shocks develop hydraulic “friction”. There are tiny holes (Orifices) in the piston head and these orifices let only a small amount of fluid through the piston, which in turn slows down spring and suspension movement.
The heat is then dissipated through the body of the shock absorber. If the shock absorber can’t dissipate the energy fast enough, the seals will overheat and fail. Then the shock absorber can no longer function. Then it is a pain-full “limp” to the repair centre.
So surely 2 shock absorbers would dissipate twice the heat?
Well yes they do but this can still be less than one shock absorber of a different design and capacity. And it is the design of the shock absorber that is critical over corrugations.
There are broadly 2 types of shock design and generally dual shocks are 2 sets of the cheaper “twin tube” design. The alternative is the single “Monotube” design. These have been around for many years. The technology trickled down from Formula 1 racing through NASCAR to dirt track racing.
Monotube shocks have an advantage, since their larger pistons displace more oil for a given stroke. This is especially true for the rebound stroke. The greater the amount of oil moving through a shock’s piston, the easier it becomes to regulate the flow and thus control the dampening forces of the shock.
Monotube shocks are also less prone to fade and failure due to heat. They stay cooler because the piston and oil are in direct contact with the outside wall of the cylinder allowing for better dissipation of heat. Most twin tube shocks use a rupture-prone gas bag or “foam seal” to hold the nitrogen. When this seal or bag breaks the nitrogen mixes with the oil making the shock skip as the gas moves through the piston orifices. A bubbling noise can be heard when the shock is actuated by hand.
Corrugations are the killer and should be the determining factor in shock absorber choice!
When travelling over corrugations, the “up and down” cycle, whilst small, is fast. The faster the speed of the cycle, the more resistance the shock absorbers provide. The more resistance, the higher the heat energy.
Because at higher speeds the shocks do not move very much (as low as 5-10mm across corrugations), the build up of heat is concentrated around a very small area adjacent to where the valving and seals are located. The heat build up is in the oil, which is in contact with the shock body, which then dissipates the heat. The critical success factor becomes how quickly the shock absorber can dissipate that heat before the seals “blow”.
The energy dissipation calculation for a typical 41mm twin tube shock in the corrugation example is less than half that of a 52mm Monotube shock. This is because of several design factors:
- the increased surface area
- The increased oil volume (and the quality of the synthetic oil in a monotube)
- the difference in the heat transfer: In a twin tube, heat has to pass from the inner tube, through a steel case and air to an outer tube and then to the ambient air. Whereas, in a mono-tube, the heat passes from the oil to the Aluminium tube to the ambient air. Aluminium is also a far great conductor of heat than steel. And it is during the corrugations that the rate of heat build up is dramatic.
Kimberley originally used OME Twin tube shocks. the failures were more frequent than we liked and the "bushings" of polyurethane often melted. When you see melted bushings it is a case of high speed over corrugations. We now only use stainless steel bushes.
Desert “buggy” racing is completely different. Generally the shock absorbers operate with bigger strokes and this moves the oil up and down the cylinder and disperses the heat far more effectively. Remote oil cylinders are used with great effect. However, our tests showed no improvement in dissipating the heat over corrugations with such a system as the oil hardly moved around.
Shock design is the first factor and geometry is the second factor with shock absorber performance.
Often, dual twin tube shock absorbers are used because the suspension design does not allow a 90 degree orientation of the shock to the wheel travel and it is a way to gain more capacity. Twin tubes are also typically shorter. So you see 2 short shocks at an off set angle in the suspension frame.
If the suspension design requires the shock absorber(s) to be “laid back” from the vertical, the effectiveness of the shock decreases significantly.
In the ideal world, each shock should be mounted as close to the wheel as possible, be exactly perpendicular to the angle of motion at full compression. This will provide a progressively stiffer suspension as you come closer to full compression.
The third factor with shocks is in fact not the shocks but the "unsprung weight" of the suspension. The "unsprung weight" includes the weight of the independent suspension arm, the hub, the brakes, the wheels, and the tyres. It is called "unsprung" because it sits below the springs.
The best suspension design reduces this weight to a minimum. It is why alloy wheels are so popular with performance cars. By reducing this unsprung weight to a minimum increases the resonant frequency so at any speed the effects on your caravan are similar to what you would get if you SLOWED DOWN towing a caravan with a higher unsprung weight. The biggest culprit with weight are the large 12inch electric drum brakes. These are twice the weight of hubs with disc brakes.
So the final conclusion is that an off road caravan fitted with lighter hubs and disc brakes, long 2inch Monotube design shocks properly set at 90degrees will beat 2 x twin tube shocks across corrugations all day long! And not by a whisker, but by a mile!
Travelling corrugations (for more than 20 minutes say):
We are assuming that everyone wants to travel as fast as they can.
If you pick up speed to say 100 klm/hr over corrugations then your tyres only hit the road at the tops of each bump and the car is in the air the rest of the time.
- The benefit at this speed is the suspension should not resonate and the forces on you and your suspension are less. The shock absorbers will experience less heat buildup.
- The disadvantage is you will have poor steering and braking. You are in the air so the tyres cannot help you if you need to change direction or speed. (Please don't try this as it is really deadly).
As you slow down, the reverse occurs. You get better road control for safety but the heat build up in the shock absorbers is increasing.
A better approach is to lessen the tyre pressures. Set these pressures to below 30PSI for corrugated roads. Then travel at the safest (high) speed for the road and conditions (visibility, unseen corners etc) and for these lower tyre pressures.
Reducing tyre pressures reduces the frequency of oscillation of the tyres and makes the tyres absorb more of each bump. When the tyres are hard they act as a solid and the whole wheel and suspension must bounce upwards on each bump they hit. But if the tyre is softer, it flexes and the suspension receives less of the energy of the bump thus reducing the stresses on the suspension while still maintaining tyre to road contact.
At Kimberley, we try to guide customers on where they are with speed and tyre pressures. We do this by fitting heat sensors to the back of the shock absorbers. For a given road, you can see the temperature impact of speed and pressure. You dont need to keep examining these, it is a guide for you to get a feel and education on what is working best. Once you establish this, you may not look at the guide for weeks.
This customer guide has been a huge success with very, very few shock failures in a travel season. We estimate that 50% of the 6,000+ Kimberley’s built are travelling every year so the facts speak for themselves!