Caravan chassis and suspension design and ratings

ByPhilip LordJuly 1, 2020
Caravan chassis and suspension design and ratings

In part three of our series, How well engineered is your caravan? it’s the turn of caravan chassis and suspension design and ratings to go under the microscope. Not to mention the tyres!

It’s all well and good if the caravan chassis and body are well designed and engineered, but all this falls down if the suspension is not set up correctly or if the chassis is not correctly rated for the load it is intended to carry. There are some questions you should raise with the dealer to clarify how well the van’s suspension is tuned to suit the specific caravan design.

If the suspension is not designed and built by the caravan manufacturer, is it just a generic suspension tune or were spring rates calculated (and where applicable, damper rates) to match the van’s characteristics?

For a non-load-sharing tandem-axle suspension, has a safety factor been incorporated in the load rating of the axles? If so, how is it determined? Is the Axle-Group Rating at least 1.2 times the GTM Rating, as required under VSB1?

If the caravan has a non-load-sharing suspension system, is the front wheel/tyre set strong enough to withstand hard impacts?

Apollo says it ensures suspension is engineered with the appropriate load ratings, “as are all critical design elements, including drawbar, chassis, axles, wheels and brakes.”

Zone RV says that it takes a lot of time and effort to make sure that the chassis design is fit for purpose and work with suspension suppliers that utilise similar software with the ability to share models to make this integration seamless.

Caravan engineering

Ezytrail warrants its new chassis for a lifetime (for original owner)

Ezytrail sources suspension components from Pedders, and says that in co-operation with the Aussie suspension company ”we can now tailor spring rates as well as shock absorber compression and rebound dampening forces to specifically match the rated load of a particular product and provide improved handling characteristics across all driving scenarios.”

Ezytrail suspension stress test

Interesting to see the static stress areas on the Ezytrail arm

As for the chassis, if it was built by a supplier rather than the manufacturer itself, does it have a placard noting its maximum (GTM and axle group) ratings? These ratings should meet or exceed the caravan’s GTM rating.

What’s the risk of not having a well-engineered caravan?

Damage and wear can be accelerated as a result of poor or insufficient design and engineering. With the impacts from the road surface, body panels and cabinetry can crack or loosen on a van where those impacts have not been accounted for in the CAD/CAE design and engineering stage. Leaks and accelerated wear are inevitable, as is the potential for even more series issues, such as chassis or A-frame cracks.

Caravan design and engineering

And finally, the tyres!

The combined load ratings of the tyres on any potential new van you buy must meet or exceed the caravan’s GTM to be safe and legal. You can check the load and speed ratings by finding the load index on each tyre’s sidewall, translating it to kg and adding them up (see examples in table below). The kilogram total should exceed the caravans GTM rating.

To be on the safe side, the tyre’s maximum speed rating should be a minimum of ‘L’ (120km/h) for a caravan, although most are N (140km/h) or higher (see speed rating table below).


Caravan Engineering Tyre Speed Rating Table

The recommended tyre pressures on the van’s placard should correctly calculated – so the question to ask the manufacturer or dealer is, were the tyre pressures calculated according to the Tyre & Rim Association of Australia Handbook?

Tyre load index can be found adjacent to tyre size found on the tyre sidewall. For example, in 225/70R16 100T, ‘100’ is the load index. Examples of load indices and the correlating maximum load in kilograms.

Guide Caravan Engineering Tyre Load Index Table

So now you’ve read this part about caravan chassis, you can read parts one and two so you should know exactly how well engineered your caravan is!