Doctor Diesel

Getting the wind up about towing

Dear Doc,


Your article ìSide Wind Assist and Crosswind Stabilisationî in issue 381 was very interesting. The only thing that wasnít mentioned was whether this feature works when towing. Iím thinking in particular of towing on the motorway in a strong side wind when there are two things to watch out for. The first is when there is a steady strong side wind and a large vehicle overtakes and cuts off the side wind and the caravan tends to swing towards the overtaking vehicle. Or in relatively still air when a large vehicleís bow wave upsets the caravan before it affects the car. For instance, a bow wave would initially push the rear of the caravan into a clockwise yaw which would pull the tow-hitch to the right making the car yaw in an anticlockwise direction. This then changes as the large vehicle bow wave reaches the front of the caravan reversing the effect on the car. The bow wave finally reaches the car and things continue to be unsettled. Would Side Wind Assist and Crosswind Stabilisation iron all this out? When the above occurs on the motorway, I automatically input the corrections, having the benefit of 40 years and thousands of miles of towing, but I can well imagine that it would be a bit scary to someone new to caravanning. 


What really frightens me is the number of ìnew caravannersî that you see on the motorway. Usually itís a brand-new top-of-the-range 22-foot twin-axle caravan and a new 4×4 towing it at 70+mph, and probably feeling totally bomb-proof. I wish they could see the picture from the Caravan Club magazine of a Range Rover that was flipped by a caravan, and maybe they would slow down. 


Might I just add that my still current Mazda CX-5 is the best tow car that I have ever had! 


Mike Arnold, Barnstaple


Hello Mike. Interesting to have your input on this, as a towing man. I’ll fire some questions at Ford, Mercedes-Benz and Volkswagen. I have done no significant towing in my motoring life, save for when, in my very modestly powered 1970s 1200 Beetle, I towed a friend’s broken-down Ford Cortina GT all the way from near Sleaford back to Elland near Bradford in Yorkshire. It had put a rod through the crankcase, and I remember it all only too well! But I digress! Inescapably though, these systems rely on the yaw sensors in the towing vehicle. If maintenance of a straight-ahead course, and avoidance of any deviation from the lane is what the towing vehicle should be doing, one would think that they can only be of assistance. But I suppose that there is a possibility of over-correction though, with the sideways momentum of the towed item significantly influencing things, possibly not for the better! In researching this, though, I came across the following web posting that may be of interest to you ñ and to DC readers ñ if they are towers.


“There is an old saying that ìPrevention is better than a cureî and following this mantra is still true. The whole concept is based upon control of Inertia… and it is easier to control ìinertiaî by preventing it starting to take effect, rather than allowing it to start and then trying to stop it. This is what some electronic trailer stability systems fail to address ñ they wait until a number of swings occur before applying a braking force to the trailer ñ by which time a sway can be out of control. To try and apply the brakes on the first swing would mean that the whole system becomes a speed governor and the brakes would need adjusting quite often. To overcome this, some providers are installing self-adjusting brakes, but this means the brakes will need more maintenance. In ìDifferential Stabilityî the system is pro-active, since very small natural sways are controlled by the hydraulic damper up to the extent that ìSimple Harmonic Motionî is acting, over that the individual brake action just kisses the appropriate brake to control the acceleration of that wheel and keep the trailer in line. Since this is not trying to slow the whole combination, the wear rate on the brakes is minimal. In practice, it means the driver will only feel a sway following a severe push, but this will immediately be stopped as it returns to the centre of equilibrium. It works without any electronics.”


In my short, but arguably almost heroic, towing career, I picked up the fundamental law that the towed vehicle should best initiate any braking ñ well, certainly when towing with a rope, not a rigid tow bar, anyway! So, the use of caravan/trailer braking to stabilise any side wind generated situation seems quite logical to me. I should be somewhat cautious if I had side wind assist and might well be tempted to disarm it when towing. But it will be interesting to hear what feedback I get from the vehicle manufacturers, and I’ll let you know what responses I get! 


Best regards, 


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