TOWING SAFETY
Which suspension produces more body roll – beam axle or trailing arm?
WORDS BY JOSEPH VAN WOERKOM PHOTOGRAPHY BY CARAVAN & MOTORHOME
Have you ever wondered why independent coil sprung trailing arm suspension has become virtually standard on new caravans, especially on the larger, more expensive models?
The main reason cited for this is that coil suspensions fitted to modern cars gives a much softer ride than leaf springs and can also provide longer suspension travel, especially in 4WDs. Both of these characteristics imply less shock and stress to the caravan frame and its contents when travelling on rough outback roads (which is apparently where most of us want to travel).
Without entering into the argument as to whether the above assertions are correct or not, are there any drawbacks using independent trailing arm suspension in lieu of beam axle suspension? Well, after doing some research on the subject I was surprised to find that there is at least one problem with the ‘better’ suspension which has a significant effect on the dynamic stability of the caravan.
The problem is to do with the location of the caravan’s roll centre and centre of gravity.
Now the roll centre is the point about which the caravan will roll when pushed from the side and the centre of gravity is the imaginary point where the total weight of the caravan may be thought to be concentrated. The moment arm is the distance between these two points. The longer the moment arm, the more the caravan will lean over for any given cornering force.
Now while the centre of gravity is fixed for any given caravan design, (typically around half to 1m above the chassis) its roll centre is determined by the type of suspension fitted to it.
Unfortunately trailing arm suspensions produce much lower roll centres than beam axles. When measured at the middle of the axle group, the roll centre of a trailing arm suspension will typically be at ground level while for a beam axle it will typically be at chassis level. This longer moment arm (especially on high clearance off-road caravans) causes the caravan to lean over more than the same caravan with a beam axle for the same cornering speed. Since leaning over also moves the centre of gravity outwards, the caravan is more prone to sway and rollover.
To make matters worse, the roll centre typically moves downwards towards the back of the caravan. This is another reason why heavy objects like extra spare tyres or toolboxes should not be mounted on the back of the caravan as they will produce a much greater rollover force here than if they were located closer to the centre of the caravan.
While it’s true that shock absorbers can slow down the rate at which the caravan leans, they can’t actually stop it from leaning and the longer the corner the more it will lean. What surprises me, given the significant cost of trailing arm suspensions, is why aren’t they fitted with anti-roll bars (like cars and trucks) to keep them more level when cornering.
Ultimately, trailing arm suspension is a very good and safe option for your suspension system, however vanners need to consider weight distribution and understand how their van handles when cornering.
