You may read of some off-road camper trailers and off-road caravans with a single axle and yet fitting dual shock absorbers per axle for a total of 4. This is not an overkill, it shows inadequate suspension design. Fitting single high quality shock absorbers with the right suspension design is far superior. This blog explains why.
Firstly good design dictates the shock absorbers must be Mono-tube for high temperature dissipation in off road conditions. Monotube shocks are able to get more of the temperature buildup dissipated through the body more efficiently than twin tube shocks. The heat buildup occurs rapidly going over corrugations. The shock absorber piston barely moves more than 10-20mm for long periods of time as you ride over the top of the corrugations. This builds up temperature around the seals. The size of the oil reservoir doesnt have a big impact, as this oil is not circulating with large piston strokes. It is sitting in the body getting hotter and hotter around the piston edge where the seals are. To dissipate the heat buildup, a large diameter alloy body mono-tube shock absorber is needed.
Lets look at the actual construction of the Fox monotube shock absorbers accross a range of popular brands including Bilstein, Rancho and Pro-comp. This is shown in the picture with actual cut-aways of specific models for this comparison at similar price points.
The Fox shocks have a larger one piece alloy body from a single machined billet. They are larger by 4mm.
The Fox Piston Shaft diameter is larger by 2mm. This dissipates more heat.
The Fox Shock has 3 redundant shaft seals versus 1.
The Fox shock has integral bump stops.
Secondly, the weakest link in shock absorber design can be the upper or lower bushing. If a polyurethane or rubber bushing is used, this can fail ahead of the shock absorber.
Up until 2005, Kimberley used exclusively Old Man Emu (OME) twin tube shock absorbers. These failed when the polyurethane bushing melted. They deformed and melted when travelling over corrugations. Once the bushing deformed or melted away, the shock absorber failed soon after. This happened all too frequently and we wanted a better solution for the Kimberley Karavan. It was with this background that we started the engineering journey of eliminating the polyurethane bushings and moving to mono-tube shock absorbers.
Fox were using steel bushings in their Dakar off-road shock absorbers after they had a similar experience to Kimberley. Kimberley adopted these special Dakar steel bushes into the shock absorbers and still use the same ones today. It keeps all the pressure on the shock absorber piston as it should be.
Finally, the choice of oil in a shock absorber is critical. The oil has to perform at high temperatures. If a poor quality oil is chosen, then multiple shock absorbers may be needed. The highest quality commercially available synthetic oil is used in the Fox shocks for Kimberley.
In summary, the shock absorbers used by Kimberley are the highest quality single piece per side to match the suspension, whether air or spring. They are:
|Shock Absorbers||Kimberley Use Fox for Single Axle Suspension||Comment|
|Type||Mono-tube||Better heat dissipation than twin tube designs|
|Size||50mm from single alloy billet||Large Size for heat dissipation than twin tube or other mono-tube shocks|
|Sealing||Tripple viton seals||Others use a single seal|
|Bump stops||Integrated||Others use plastic or no stop|
|Bushings||Steel||Other use polyurethane or rubber|
|Rebuildable?||Yes||Others are disposable|
|Protection||Rubber boots over shaft|
The Fox Mono-tube shocks we use in our camper trailers and single axle off road caravans are especially designed and built for our application. They are valved to match the unsprung weight of the Kimberley insependent suspension.
The overall design means only one unit per side is needed and is supperior to dual sets of smaller twin tube off the shelf units!
If you havent seen the air suspension now fitted to Kimberley Karavans, have a look at this page. http://www.kimberleykaravans.com/off-road-caravan-air-suspension
For suspension design of an off road caravan, we recommend this eBook.