How much do you actually know about what’s going on behind you when towing? And we’re not discussing the view from your towing mirrors; we are talking about understanding the dynamics of towing
This series of articles is designed to give you an insight into what’s going on when you are towing a caravan. It’s a UK-sourced story, so while there are European references as to the use of over-run brake systems, etc. the article is dealing with physics, and that doesn’t change from country to country! No matter if it is your first time or you are seasoned veterans, we hope that there will be something here that expands your knowledge and helps you understand what’s happening when towing.
Under acceleration or braking, if the caravan is in line with the vehicle, the forces acting will always be in a straight line, however, if the caravan and towing vehicle are not aligned the forces under braking conditions will have a different effect.
Caravans have two main types of braking systems – over-run and electronic. Electronic brakes are the most popular system in Australia, whereas the most common system in the UK is the over-run type brakes. While there is also a hybrid of this, AL-KO offers a stability system to electronically operate the caravan over-run brakes in the event of the caravan becoming unstable while being towed. Importantly, the effects we are discussing here are almost the same no matter what braking system you have.
Caravan overrun brakes are operated by compressing the coupling against a tension device, once the tension device is compressed, the brakes are activated by a system of levers or cables applying force to the brake shoes in the hubs on the axle, slowing the caravan down. As the caravan slows, the force on the tension device is reduced, releasing the braking force.
In a perfectly maintained system, the manufacturer calculates the maximum force required on the braking system and creates a tension device to give the correct forces to enable the most efficient braking. This action will also reduce the pushing effect on the towing vehicle to only the force required to activate the braking system, helping to prevent it from becoming unstable.
However, there are limits. If a caravan is overloaded, for example, the tension device may not be able to resist the mass of the caravan pushing into the rear of the towing vehicle and the force acting on the brakes may be more than the designer allowed for and lead to the caravan wheels locking up completely.
Tyres also play a part: If they are badly maintained (under or over-inflated, worn tread, etc.), the grip on the road surface will differ to the manufacturer’s original calculations and will affect the efficiency of the braking system.
If the caravan is off to one side, under braking, the force transmitted to the tension device will not be in a direct line and therefore have a slightly reduced effect. Further, the force acting on the car will have the effect of trying to push the rear of the vehicle in the opposite direction, compounded by a slight reduction in the loss of efficiency of the caravan braking system.
Another effect on braking efficiency is the forces acting in a vertical axis. If the caravan and towing vehicle are correctly matched, the hitch height will be in line on both units. Therefore, any forces transmitted during braking will be in line, but we will see shortly why this might not be the best position.
If the caravan is nose-high this can have an effect under braking and reduce the stability of the towing vehicle and caravan. The normal cause of this is the hitch height on the towing vehicle being too high, even with the correct nose weight, this could cause instability problems.
As can be seen we have a similar situation to braking with the caravan not being aligned with the towing vehicle. In this case, the forces acting as the caravan ‘pushes’ into the rear of the braking vehicle are not in line and have the slight effect of trying to ‘lift’ the vehicle’s rear end. This is also compounded by the fact that under braking, the vehicle’s centre of gravity moves forward, transferring weight to the front wheels and off the rear wheels so the rear of the vehicle is already lighter than in a normal towing condition.
However, it is very unlikely that the hitch height will be high enough to cause significant problems. Like braking, while the tow vehicle and caravan are not aligned (above), it will reduce the efficiency of the over-run brake hitch slightly too.
In practice, it is unlikely that the hitch height will be high enough for this to be an issue, but it is worth mentioning so you can appreciate that there are numerous things, however small, that can affect the overall stability of the tow vehicle and trailer combination.
Centre of Gravity
The centre of gravity of a caravan is designed to be as low as possible to make it tow as safely as possible. Good advice is always to load heavy items on the floor and keep the overhead lockers for the light stuff. Due to the way the caravan is constructed, and if correctly loaded, the centre of gravity is always in front of the axle and above it.
However, because of the distance between the centre of gravity and the hitch, the effect of braking (or accelerating) means, in practice, there is a limited change in the nose weight.
This can be demonstrated quite easily, if you put a 20kg weight directly over the axle and measure the nose weight, then move the 20Kg forward by 500mm the actual increase in nose weight is only a few kilos. Under braking, the whole mass of the caravan moved forward slightly, but because of the lever arm distance from the pivot point (the axle) to the hitch, the actual weight transferred to the hitch is not excessive. The height of the centre of gravity above the axle will affect the transfer of weight; the higher the centre of gravity, the more weight will be transferred. The height of the centre of gravity above the axle is also important for lateral (side to side) stability.
It is a good idea then, that we tow with the caravan slightly nose down and here’s why. Under braking, the direction of force acting on the towing vehicle from the caravan will be in a slightly downward direction, increasing the load on the rear wheels of the towing vehicle.
We know from above, under heavy or emergency braking that the centre of gravity of the towing vehicle moves forward and pitches the nose down increasing the load on the front wheels and reducing the load on the rear wheels. Similarly, the forward shift of the caravan’s centre of gravity will increase the nose weight.
By towing nose down, when braking the caravan’s acting line of force resists this and imparts a further downward-acting force on to the rear wheels and effectively increasing the grip on the road surface. There are two advantages to this: firstly, it will resist any tendency for the rear wheels to lock up as the increased grip will resist the force of the brakes and keep the wheels turning, improving overall brake efficiency of the vehicle.
Secondly, if the caravan is out of alignment with the towing vehicle, the increased grip will help resist the lateral force imparted on the rear of the vehicle that wants to push it sideways, therefore reducing the chances of jack-knifing.
However, we also have to consider something else. Vehicles always have larger brakes at the front than at the rear, and there is a good reason for it. The designers take into account the ‘weight shift’ under heavy or emergency braking and know that they can put much more braking effort into the front axle than the rear. So if the caravan imparts too much nose weight under braking, although the rear brakes on the vehicle are assisted by extra load pushing down, the braking ability of the front axle is reduced as the centre of gravity change (weight shift) in the braking action is reduced.
This is one of the reasons there are nose weight limits on the tow hitch of the towing vehicle. It’s not the strength of the towbar or towball, but the manufacturer has determined what the maximum load can be for all sorts of reasons including what effect it will have on the braking efficiency and performance of the vehicle.
It’s all a balancing act, so don’t exceed the vehicle or caravan nose weight limits or load the caravan above its GTM and keep all your heavy items as low as possible.
Click here to read part two of Understanding the Dynamics of Towing, where we look at twin-axle trailers.
This story was written by and reproduced with kind permission of Simon Barlow at Caravan Chronicles
Images RV Daily.