In this third article of our Beginners’ Guide to Suspension series, Robert Pepper discusses the differences between caravan and towcar suspension and the types of suspension found in caravans and trailers.
Your caravan needs suspension just as much as your vehicle if you’re to have a safe and efficient tow. Just like the towcar, the caravan has a body and wheels. The wheels go over bumps, the caravan goes around corners and it slows down. All of this means you need suspension.
The fundamentals of caravan suspension are the same as the towcar; a spring absorbs the bump, and a damper (shock absorber) stops the resulting bouncing. The springs and dampers are very similar too. Caravans typically run coil, leaf or air springs. The dampers do the same job too, and are of the same design; twin-tube, monotube, foam etc.
Differences between caravan and towcar suspension
There are, however, significant differences between caravan and towcar suspension. A driver will feel poor suspension much more in a towcar than a trailer, and generally, the suspension design in trailers is nowhere near as sophisticated as that in the towcar. You can often see this when you follow a rig over rough bitumen; the trailer will bounce, but the car won’t.
That’s the difference in suspension design and quality. The reason for this is that the towcar will have had the benefit of a lot of engineering around suspension geometry, alignment, spring and damper tuning. The aftermarket will begin with that carefully engineered base and then tune for a given purpose.
The trailer, on the other hand, will probably be an off-the-the-shelf chassis with off-the-shelf suspension, and on top is built whatever the vanmaker likes. Despite asking, I am yet to see much evidence of engineering dynamic design on caravan suspension – most seem to just bolt on shiny independent suspension and call it good, with just a couple of notable exceptions who design their own chassis and suspension unique to their vans.
It’s not a question of quality, there are some very robust and well-designed suspensions out there, it’s matching the suspension to the van itself.
Types of suspension in caravans
Caravan suspension will either be a beam axle, where both wheels are connected to one axle, or independent, where each wheel has its own, shorter axle. Independent suspension offers handling and ride benefits in cars, but in trailers, the advantage is less clear-cut. Again, taking advantage of the tuneability of independent suspension requires significant engineering and testing.
The independent suspension does offer greater offroad ground clearance (pictures above and below), but even the beam axle is likely to offer more ground clearance than the towcar as its axle doesn’t have a differential or a driveshaft. Overall, independent suspension should, in theory, tow better, but only if it’s designed to be so for that specific van.
Many caravans run twin shocks …. why?
Many caravans run twin shocks, not singles. Why? Heavy trucks run single shocks, so it’s not the load, even if leaf springs offer some damping there are many heavy-duty coil-sprung trucks and 4x4s with single shocks. There is also little argument for redundancy; well-designed shocks rarely fail, and if they do, you can still tow but the trailer will bounce – and each shock has to be tuned to do half the work, so if one fails the other will quickly follow as it is then significantly overworked.
It is also not true the caravan suspension does twice the work of the towcar. There isn’t likely to be twice the load, and for a single axle at least as it’s a tripod, both wheels are more in contact with the ground than the four wheels of the towcar.
This is not to say twin-shock suspensions never work; I own a van with that design and it tows very well. I have asked around the industry and am told that a big factor is cost. Two small shocks can be procured for less than a large one, even if it were made, and also, two shocks looks tougher.
From the buyer’s perspective, do not consider two shocks to be better than one; the overall engineering design and quality of the components are more important. However, humans tend to fixate on “more = better = tougher”. This is also what has driven larger and larger shock tube sizes and remote-reservoir shocks, all of which can be sold at a premium, and 12,000lb winches where 9500lb would be adequate. Anyway, back to caravan suspension.
Caravans springs
Caravan suspension springs can be coil, leaf or air. Air has the advantage of self-levelling when you’re at the campsite, can also help with trailer hookup, and can be raised for offroad use and then lowered onroad. It is more expensive and not as reliable as plain steel spring, but reliable enough for almost everyone.
Leaf springs are derided for their poor handling in cars, but in trailers, it’s much less of a problem. I don’t think there’s anything wrong with leaf-sprung, live-axle trailers and I own one which tows great. However, marketing says independent suspension for caravans with shiny components is what sells. Consequently, you’re unlikely to find much leaf sprung, but if you do, then don’t assume it’s sub-standard or a poor tow. It all depends on the quality of components and engineering.
Loadsharing suspension
Some caravans have four wheels, not two, and these are known as tandems. The dynamics are quite different between single axles and tandem axles as the singles operate as a tripod, so the van wheels are always in contact with the ground. A tandem might have diagonal wheels in the air or unloaded. The tandem’s wheels also need to operate in pairs on a side; when the front left wheel goes up over a bump you don’t want the rear left wheel lifted off the ground as that means less grip and more bouncing. This is why loadsharing suspension exists, and it looks like this:
If you intend to tow a caravan or trailer offroad, then you should be using loadshare suspension for better stability and traction – braking and lateral grip. If you tow onroad, it’s less important, but do make sure you tow such that the trailer is level.
Modifications to caravan suspension are quite rare compared to towcars, as typically, the van suspension will be designed for the job at hand. Probably a bigger factor in ‘towability’ is weight and weight distribution. Heavy weights up top and at either end will ruin the towability of even the best-designed trailer, whereas weight central over the axles and low can make even a poorly designed trailer seem like a towing dream.
In summary …
So in summary, your caravan has, and needs suspension but it’s likely to be less well designed than that of your towcar. Shiny independent double-shock suspension isn’t the amazing differentiator it seems, you’re unlikely to need to modify your van’s suspension, and if you experience towing instability weight, distribution is probably the first place to look unless your van suspension is worn, or the van is much heavier than the makers envisaged.
