Sunday, 1 December 2019

Problem Identified

Readers may recall during our outback trip in August last year there was a problem with the loan trailer suspension.  The four shock absorbers shattered and the head of the suspension arm bolts sheared off.  Fortunately we were able to make temporary “bush” repairs and continue on to civilization. 


The camper trailer we have purchased has the same suspension system and I want to ensure the problem from last year doesn’t repeat itself.  therefore I have been carefully examining the Chinese designed suspension and I believe I’ve identified the problem.

The trailer has independent coil suspension with two shock absorbers attached each side.


Side view

B - Red coil spring which expands and compresses

Grey suspension trailing arm

A - Yellow shock absorber which expands and contracts

The wheel fits at point C

The red coil spring expands and compresses as the trail travels over bumps and humps.  The shock absorbers also extend and compress as the trailing arm goes up and down.  However the shock absorbers to this at a slower rate than the coil spring damping the movement.  Without the shock absorbers the trailer would bounce like you were playing on a trampoline.

What is the issue?  There are actually two issues.

  • The shock absorbers are very poor quality and not extending or compressing properly
  • They are also too long.

When the trailer is fully laden the shock absorbers should be at the halfway point (ie, half compressed and half extended)  However these shock absorbers only have 35mm of compression left even though the trailer isn’t fully loaded.  To compound the problem the spring has another 150mm of compression before being fully compressed.  This means the shock absorbers will be fully compressed before the spring.   The problem is reversed when the coil spring is fully extended.  At this point the shock absorber has another 90mm of extension left which is unused. 

When removed from the trailer and tested the shock absorbers fully extend by 180mm.  Therefore they should be at 90mm when fitted to the laden trailer.  But the actual measurement is closer to 30mm.  The consequence of this is the shocks a ‘bottoming out’ before the spring and shattering under the impact.  Moreover I suspect the force of this impact was being transferred up the trailing arm and shearing the head off the retaining bolts.

To resolve this the shock absorbers need to be replaced with a shorter and better quality model.  The current 415mm long shocks need to be replaced with a 320mm version. 

Time to start searching online.


Paul (from Waterway Routes) said...

Under British Law you've identified a good reason why the trailer supplied was not fit for purpose and the supplier who sold you the trailer should be tasked with fixing it.

Dave said...

Hi Tom

I'm not an engineer, but even to me that shock mounting is just wrong and inefficient if its that far away from the spring and the arc that the swing arm makes. Surely the shock should be mounted in the spring or to the right of the spring, more like your truck suspension is..