At Bristol Caravans we have heard a lot about caravan wheel loss over the years. And from time to time we are asked for more details on the subject when we recommend WSL wheel bolts.
The following may be seen by some in the caravan industry as controversial, so I preface the following by saying this is the view of Bristol Caravans (ACORNCO LTD), and is shared in the interest of improving safety within the caravan community – Without Prejudice.
We hope caravan owners adopt the recommendations at the end of this article to improve road safety. We hope caravan manufactures investigate Lost Wheel Syndrome, publish their findings, and change the standard brake drum, wheel, wheel bolt set-up.
We are not professors of mechanical engineering. But having researched what we can find on the subject of caravan wheel loss, including speaking to a professor involved, here are our thoughts.
From a layman’s perspective (we are not mechanical engineers), we believe there is a design issue in the wheel, brake drum and standard wheel bolt set-up. These issues converge to influence/reduce the clamping force (also called Pre-Load) on N/S caravan wheels.
1. Direction of Rotation
Incident reports indicate it’s the N/S wheel that usually detaches.
It is unusual for the O/S to detach, the thinking is that the O/S wheel rotation, works together with the wheel bolt thread direction to keep the joint tight.
The opposite occurs on the N/S where they work against each other.
An alternative theory is that the N/S wheel hits more pot holes than the O/S in the UK. That said the short distance between wheel change and wheel detachment suggests it is not a pot hole issue.
Perhaps N/S wheel bolts should have a Left Hand thread?
2. Friction on Torque Reduces Clamping Force
A wheel is held in place by a clamping force applied by the wheel bolt.
Unfortunately no tool is available to test the actual clamping force (between the wheel and brake drum).
Therefore the industry uses a torque wrench to test the tightness of a wheel bolt (which is different to clamping force).
WSL tested “standard” caravan wheel bolts, and found that when a “new” bolt is fitted, 91% of the torque was consumed by friction, and only 9% was applied to clamping force.
The WSL bolt design reduces friction.
3. Uneven Surfaces
Whilst the mating surfaces of brake drum and wheel look smooth, under a microscope both surfaces look more akin to mountain ranges, with peaks and troughs. Where the two surfaces meet, you have peaks touching peaks. As the wheel bolts are tighten the peaks grind together, the torque wrench “clicks” and the wheel is deemed fitted. However, as the vehicle is used, the peaks further mash together, as the wheel surface and brake drum surface bed in. As they bed in, the clamping force is reduced. Hence the industry recommendation to re-torque wheel bolts after 25 miles.
However we are aware of incidents where the bolts became loose at circa. 15 miles.
Perhaps the recommendation should be re-torque wheel bolts at 10 miles?
4. Bolt Stretch
To apply clamping force, a wheel bolt needs to stretch. The part of the bolt that stretches is the shaft between the end of the thread (which sits in the brake drum), and the start of the cone (which sits against the wheel). If this distance is small, little stretch is available. The standard wheel bolt fitted to most caravans has a very short stretch zone.
A WSL bolt has approx. 3x the stretch zone (bolt elongation) of a standard bolt.
Wheel Bolt Research
WSL shared some of their research with Bristol Caravans.
An M12 zinc coated bolt (standard wheel bolt fitted to most new caravans), produced the following results:
1 x use: 27 KN clamping force at 130Nm torque setting.
5 x use: 19 KN clamping force at 130Nm torque setting.
As a bolt is used, the clamping force gets worse!
There is no “minimum” clamping force published in the caravan sector, but the outcome we find shocking is the 30% reduction in clamping force after 5 wheel off / on’s.
This could mean, that a new caravan in 2010, that has original tyres fitted, and only has the wheels off at the yearly service, could have a 30% reduced clamping force by 2015. We think this means a caravan going in for it’s 5th service, could loose a wheel shortly after.
WSL advise their bolt design achieves:
1 x use: 70 KN clamping force at 130Nm torque setting.
5 x use: 70 KN clamping force at 130Nm torque setting.
20 x use: 65 KN clamping force at 130Nm torque setting.
Bailey Caravans now fit WSL to new caravans as standard. Well done Bailey.
Without trying to “sell” WSL bolts, their wheel bolts have a different design. They have increased the stretch zone, and reduced the influence of rotational friction. In short, a higher clamping force combined with a greater on/off frequency.
We recommend:
1. You fit WSL bolts to your caravan. If in doubt contact your local MCEA mobile engineer, AWS approved workshop or WSL direct to ensure the WSL bolt is compatible for your caravan.
2. Check your wheel bolt torque 10 miles after any wheel removal/refit.
We publish this information as our view, following our search for an answer to Lost Wheel Syndrome. We recommend you also investigate documents published on this subject, and if you find additional information, especially scientifically tested research or data, we would welcome your feedback.
We believe this is a real issue that impacts caravans, owners and repairers every year, the sad thing is, there seems to be a solution on the shelf – but as a community we are not promoting it.
We publish the above in the spirit of positive change, and hope it aids and educates everyone involved. We hope it inspires you to investigate Lost Wheel Syndrome in caravans.
We respectfully ask the leaders in our industry to take action and remove this risk from caravan ownership.
How will we help with this issue?
We offer all Bristol Caravans service customers the opportunity to change to WSL bolts, see our offer page or ask for more info when booking your next service.
Schedule your service right now by calling 01454-612893 or fill in the online form on the contact page.
