The simple answer is the wheel bolts come loose, that’s why the caravan wheels fall off. So the question now becomes, why do the wheel bolts come loose?
The normal scenario is this, the van has just been for a service or back to the dealership, and the next time the van goes out, the caravan wheels fall off. It’s normally the near side wheel as well because this wheel rotates in the same direction as the bolts do when loosening, so the rolling action facilitates the loosening.
So, it’s easy to jump to the conclusion that it’s the engineers fault, he obviously didn’t tighten the bolts up properly. Well…. you’d be wrong with that conclusion. A caravan engineer will go to great lengths to ensure the wheel bolts are tightened to the correct torque, he will check the manufacturers spec and will use two separate torque wrenches to ensure the correct tightness.
Caravan wheels are held on by four or five bolts which clamp the wheel to the hub, so in reality it’s camping force that holds a wheel in place. With the correct torque (tightness) on the bolt the wheel has been tested by the manufacturer to be stable and secure for traveling. The bolt is made to a tolerance that allows the bolt to expand (not stretch) a minuscule amount and in doing so creates the clamping force necessary to hold the wheel in place. When the bolt is removed it ‘relaxes’ back to it’s normal state ready to be tightened when the wheel is put back on. Over time however, after the bolts have been undone and retightened a number of times, the bolt has a tendency to forget and ends up remaining in the expanded state and it’s at this point that the bolt is then deemed to be ‘stretched’. This expansion can not be measured by a service engineer and they have no idea how many times those bolts have been on, off or even if they have been tightened to much, so they have no idea of what state the bolt is in before they get to work on them.
Some people say that you should tighten the bolt until you can’t tighten it any more, then give it an extra tweak for good measure. Doing this will stretch the bolt for sure and will cause nothing but trouble down the line. A bolt that has been over tightened will fail because they are being used in away that is against their designed and approved usage. We have all seen in other areas of life where accidents have occurred because someone was using something in a manner other than what it was intended for.
So why is over tight bad?
Over tightening a wheel bolt will stretch the bolt beyond its tolerance. That means it is then not able to create the clamping force that it was deigned to do. Clamping force is vital for keeping the caravan wheel on, as soon as you loose clamping force there is only one outcome and that is wheel detachment. A good example of clamping force and how it can be so easily ‘lost’ is when playing with Meccano. If you have ever put a Meccano set together you will know that the bits that you have already added and tightened up using the the nuts and bolts supplied, inexplicable work loose as you work on another area of the model. How can that be? Bolts that have been tightened are now loose, did you over tighten them (very much doubt it) so what caused it? Flexing of the structure of the model could cause the problem, bumps and knocks could cause it, temperature changes can cause it and movement in the components will definitely cause the problem. So you can see on a model that uses bolts to connect things together clamping force is very easily lost. Scale this up to a caravan and ask yourself, does a caravan flex? When traveling does it encounter bumps and knocks? Does the temperature of my wheels change as I travel? And can components move against each other? The answer is of course yes in all cases so you can see it’s almost inevitable then that caravan wheels fall off.
Many people assume ‘…if I tighten the bolts up mega tight and I always tighten them like this then I will be safe and my wheel won’t fall off…‘ The answer to this is of course, no. Once the bolt is stretched it is damaged and won’t do the job it was designed for. However the person clearly asks ‘…if I always keep them this tight, then I’ll be alright…‘ but again the answer is no because very often the person doesn’t check. When a service engineer, dealership or a tyre mechanic next works on the wheels they are duty bound to tighten the bolt to the manufacturers approved torque setting. They won’t tighten them up ‘…mega tight…‘ so unless the customer checks afterwards then the bolts won’t be ‘…mega tight…‘. If the bolts are stretched because they are too old or have been overtightened then potentially the clamping force will fail the next time the van starts to travel. This is why it appears that the engineer is at fault for a wheel detachment, customers think that because the engineer was the last person to touch the wheels then they obviously missed something or they didn’t do their job properly.
What can I do to prevent my caravan wheels falling off?
First and foremost, always use an approved engineer. Most caravan engineers are MCEA members and many go on and join the AWS (approved workshop scheme). These caravan engineers are instructed to use calibrated torque wrenches, they know to tighten the bolts to the manufacturers approved specification and to use a second wrench in front of a witness to prove that the bolts have been tightened correctly.
Second, never (ever) over tighten caravan wheel bolts and if you suspect that they are or have been, then they must be replaced. Wheel bolts can be changed for as little as £50 per wheel and will pay you back far more than that in peace of mind and potential repair bills. (One word of caution, don’t be tempted to buy cheap bolts off ebay, these could be inferior replicas – talk to your service engineer about getting the correct replacement bolts).
Third, buy yourself a torque wrench and learn how to use it. Find out the torque settings for your caravan wheels and check the tightness before every trip. Ensure your wrench stays in calibration too – it’s no use checking with a tool that is out of calibration.
Fourth, once you are traveling pull over and recheck the torque. Don’t think ‘I checked them before traveling, it’ll be OK‘ Pull over and check. There is no harm in checking, there is no shame in checking and don’t think that because you have been caravanning for years that it won’t happen to you. We are talking about factors beyond your control so don’t leave it to doubt. Check them every time!
The potential for a serious accident when a wheel detaches can not be understated. A caravan is a large metal box traveling at speed behind a car and a wheel detachment can cause serious damage not to mention injury to the people in the car and other road users.
These simple checks therefore can not be ignored.