In the first part of Understanding the dynamics of towing, we looked primarily at the effect towing a caravan or trailer has on the overall vehicle-trailer combination, now we will have a more detailed look at what affects the tow vehicle.
One of the important things to consider in a tow vehicle is wheelbase and tow bar lengths. In general, a longer wheelbase (axle to axle distance) and shorter tow bar length (rear axle to tow ball distance) will make for a more stable tow vehicle. Lets look why that is.
NOTE: The tow bar lengths quoted are to demonstrate the principle and are not related to any vehicles.
If we first look at the tow bar length…
In the above drawing we can see the two vehicles, one with a 500mm axle to tow ball distance, the second with a 750mm axle to tow ball distance. Now using leverage calculations, we can calculate what the same side load or lateral load (red arrow) on the tow ball will have (blue arrow) on the rear wheels with the 750mm tow bar length compared to vehicle with a 500mm tow bar length.
Lets assume that the caravan or trailer doesn’t impart a sideways load on the front axle. The front axle then effectively becomes the pivot point.
Oh no… here comes the maths!
Now for the maths….
So it can bee seen that the resultant force of a 100Kg acting on a 500mm tow bar is 120Kg’s while the same force acting on a 750mm tow bar is 10Kg more.
We learned in Pt 1 that there is an effect called “dutch roll”, most often felt when on motorways where the inside lane has been worn down to two grooves by heavy goods vehicles and the caravan or trailer is ‘hunting’ between these grooves. This will set up a rolling motion in the caravan, and will have the effect of trying to move the hitch from side to side. As the vehicle and caravan travel forward, the undulations will also set up a rocking backwards and forwards motion in the caravan, which has the effect of first lightening the load (nose weight) on the hitch, then increasing it. The combined effect of this is the caravan hitch will move in a circular motion first sideways then down, then sideways in the opposite direction and finally upwards again, repeating continuously. This one source of lateral load on the tow ball.
The rear tyres only have so much grip. The grip the tyre have is a function of the friction of the road surface, the friction of the tyre compound, the tyre pressure and the axle load…. (and if you are into Formula 1 it should include the temperature of the road surface and the tyre temperature as well) This grip is not unlimited…. this is easily displayed when trying to pull away rapidly and the tyres spin, likewise when trying to stop quickly the tyres ‘lock up’ and start to slide (untill ABS takes over!) If we were to put a rope through rim of the rear wheel, it might only take a ‘pull’ of 300Kg to drag the rear of vehicle sideways on the road. As you start to apply the force, the tyre wall will deflect and distort the tyre reducing the grip available (more about this later). A static wheel on a vehicle has the most grip, as soon as it starts to rotate, this grip reduces and will continue to reduce the faster the wheel rotates. This is one reason a car can go round a sharp corner at say 30 Mph but at 50 Mph, the dynamic side load has increased, but also the grip (friction) the tyre has on the road surface has also reduced and the rear end will slide outwards resulting in a skid.
Does the length of wheelbase matter?
Lets revisit our earlier calculations to see what happens:
Now we have two vehicles, one with the same 2500mm wheelbase and a 500mm tow bar length. The second has a wheelbase of 2000mm and the same 500mm tow bar length.
Using the same equation as before….
We now calculate for differing wheel bases:
Looking at the table above, we can see that the same 100Kg sideways force on the tow bar has a greater effect by 5Kg on the short wheel base vehicle when compared to a longer wheelbase vehicle.
There is also something else that happens when you compare a short and long wheelbase vehicle. If you apply a sideways force on the rear of the vehicle it has the effect of ‘steering’ the back-end – i.e. pushing it to one side. If we compare the two vehicles we can see what happens:
Given the same 100mm of sideways movement of the tow ball by the forces imparted on it, basic trigonometry demonstrates that a short wheelbase (2000mm) vehicle is deflected 2.3 degrees off path, while the same 100mm of sideways movement only deflects the longer wheelbase (2500mm) vehicle by 1.9 degrees. While this does not seem much, remember, you can usually feel though the steering wheel if the tracking is out by more than half a degree, so the lateral push/pull felt in a Short Wheel Base (SWB) vehicle will be more noticeable than a Long Wheel Base (LWB) vehicle. While it will be easier to ‘feel’ the deflection earlier in a short wheel base vehicle, the long wheelbase vehicle will be more stable.
Why are articulated lorries so short then?
With an articulated tractor unit, the pivot point (tow ball in effect) is actually directly in line with or slightly in front of the rear axle, therefore any lateral load imparted on the pivot point is actually less at the rear axle as some of the force is also transmitted to the front axle. It’s a bit like having your tow ball just behind the front seats.
Earlier we were talking about the grip of the tyres on the road surface. There is something else to consider though. We know that sidewall flexing has an effect on grip and as we have just seen, towing a caravan or trailer will put more lateral force on the rear tyre side walls. So what’s the solution? “Put low profile tyres on” OK who said that?
That is one solution and on the face of it, low profile tyres would reduce the amount of sidewall flexing therefore be more resistant to the lateral forces imparted on it by the caravan or trailer. But before you rush out to change the tyres, lets have a closer look at this. With a “tall” tyre (some 4 x 4’s have a taller side wall) There is quite a bit of ‘feel’ in the tyres, i.e. you can easily feel when something is pulling you from side to side well before the tyres loose grip. With a low profile tyres, the side walls still deflect but usually with little warning to the driver. The ‘feel’ is almost non existent. The lateral load will increase without much “feel’ and the first you know of it… well it’s usually too late as you have passed the limits of grip and the rear of the vehicle is being pulled side to side in the dreaded “snake”. Low profile tyres are great, if you are a racing driver and have super sharp reflexes and know how much you can push the tyres to the limit holding back just before they break loose. If you don’t believe low profile tyres deform…. just watch the super slow motion clips of Formula 1 cars clipping a kerb.
So what is the solution? Well normal profile tyres actually. It’s the best of both scenarios, enough ‘feel’ to give you some clues all is not well and enough rigidity to help resist the lateral loads imparted by the caravan or trailer. However, you do need to check the tyre pressure guide in the owner’s manual for heavy loads and towing. Almost all vehicles, the manufacturer will recommend a higher running pressure in the rear tyres. Please check you own owners handbook for details, as there are exceptions for certain vehicles and wheel/tyre combinations.
So what does this all mean?
You are travelling along the road at 50Mph towing your caravan or trailer that weighs 1500Kg. The caravan or trailer starts to sway as a lorry passes you and a tiny amount of the mass of the caravan or trailer….just over 6.5% or 100Kg of the weight now starts acting sideways on the tow bar of the tow vehicle, trying to pull the rear axle sideways. We know there is only so much grip available from the rear tyres and now the force acting on the rear tow ball is taking some of that grip. We feel the caravan or trailer starting to pull us from side to side and the natural reaction is to brake. WRONG!
We now know there is only so much grip available at the rear tyres and some of that grip is being used to resist the sway of the caravan or trailer. If we brake, we also use some of this grip…. possibly more than is now available which we don’t want to do. We also know (from Pt 1) that braking reduces the load on the rear axle as the centre of mass of the towing vehicle moves forward therefore reducing grip available still further. If we accelerate we also need some of this grip. Accelerating with 1500Kg’s swinging about behind us will use a lot of the available grip and we might just run out again and we really don’t want to go faster at this point do we?
What else have we learned?
Well we have proved that a longer wheelbase is preferable to a shorter wheelbase given the same towing conditions and trailer, and we have also proved with a bit of school maths that a short tow bar to rear axle length is better than a long tow bar as it imparts less sideways (lateral) force on the rear axle and tyres. We also now know why it is important to keep an eye on the tyre pressures and condition of the tyres on the tow vehicle, as they are now doing so much more work.
Is there a downside to having a short tow bar?
Yes there is. With a short tow bar, the caravan or trailer will “cut the corner” more than a long tow bar vehicle. So it requires a wider ‘sweep’ when going round corners. On the upside, when reversing the reduced length of the lever effect makes it slightly easier to manoeuvre as the steering effect on the trailer is slowed down, so it becomes less sensitive and gives you more time to react to a deviation off course, although you will have to put slightly more steering input to correct.
Further Reading: A friend and noted fellow author of caravan books – Collyn Rivers has recently updated his article on “Caravan Dynamics“. While primarily based on caravans for the Australian market it is an excellent article and well worth taking some time out to read it along with a companion article: “Making Caravans Stable“.
Happy, safe and knowledgeable towing.