Dangerous Trailer Dynamics

With summer time approaching numbers of caravans and trailers on the roads skyrockets with people heading off for their holidays. Invariably this means, up and down the country, there are incidents involving trailers and caravans, predominantly on fast paced roads.

So what makes a safe car and trailer combination, versus a dangerous one? In the UK, the law is predominantly based on size and weight. A trailer must not exceed the specified towing capacity of the tow vehicle, and must be less than 3,500 kg. To be able to tow this amount, a new driver requires a B+E license (correct at publishing date). But how dangerous can a trailer be while still adhering to these laws?

The intention of this blog post is to investigate how to best avoid dangerous situations. The situations that I wanted to investigate were:

  • High lateral wind gust, such as going across a tall bridge
  • Sudden half lane change at highway speeds, such as avoiding a dropped obstacle
  • Highway speed emergency stop, such as stopping for an accident ahead

To test these situations I used 2 cars, a medium sized family car and a large 4×4; and 2 trailer types, a laden car transporter and a caravan. The differences between the two vehicles demonstrate the difference in response of different vehicle types (mass, distribution etc); and the two trailers demonstrate different number of axles, weights, weight distributions and aerodynamics.

Throughout this investigation the VeSyMA – Suspensions library is used, and models are extended from it. The family car is taken from the library, with the 4×4 model extended and adapted from it. The trailers are extended from and slightly modified versions of ones that exist in the library.

High Cross Winds

For the high lateral wind experiment the family car and caravan are exposed to a high 15m/s wind (rated 7 on the Beaufort Scale) with 2 extra gusts of 15m/s, 10 seconds apart. The road is also considered to be wet which decreases the coefficient of friction significantly. This experiment investigates the difference in dynamics between travelling at 100km/h (roughly the UK legal limit of 60mph) and 50km/h, highlighting the impact of speed on the stability of the train.

The video above shows that at the lower speed, the vehicle deviates very little overall whilst the caravan yaws a large amount; at the higher speed later in the clip, the vehicle is a lot less recoverable. Once the vehicle starts to yaw, it quickly becomes excessive to the point of lane deviation. The second gust totally unsettles the car and causes both to yaw off the road.

Obstacle Avoidance

The next test was the object avoidance test with the car transporter; this was performed with a larger 4×4 vehicle with much more than required towing capacity. This test was to investigate the difference in how loading the car can impact the dynamics of the vehicle. In this case a car with 45/55 (front/rear) weight distribution was loaded forwards and then backwards to compare the stability between the two.

As can be seen above, with the car loaded facing backwards the train spins out of control after the rear starts to swing. Whereas with the car facing forwards the vehicle regains stability soon after the avoiding event.

Emergency Brake

The last experiment conducted was the sudden braking event, which was conducted with a slight cross wind of 10m/s (5 on the Beaufort scale) and poor weight distribution. This induces the caravan to be slightly offset from the rear of the car. This experiment is conducted with the 2 different vehicles to demonstrate the difference on the effect of weight in the tow vehicle on the stability of the train as a whole.

The video above shows that the heavier 4×4 achieves a more controlled decrease of speed with the caravan returning to straight ahead, whereas the lighter family car is pushed off course. This demonstrates the benefit of having a heavier tow vehicle for added stability.

These tests demonstrate the importance of good forethought when loading the trailer and selecting a tow vehicle. Loading the trailer with a higher tongue weight (centre of mass closer to the front of the trailer) and a weightier tow vehicle would yield the best stability. It also highlights the impact that speed has on the stability of a vehicle.

Written by: David Briant – Project Engineer

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