Michelin Helps Farmers to Stop Throwing Their Weight Around

Michelin made a worthy contribution to agricultural vehicle safety during the summer through a group of experts in the field. The Michelin Agricultural Contractor Panel, made up of five agricultural contractors – all of whom are past winners of the Farmer’s Weekly Contactor of the Year Competition and benchmark industry specialists – met on a regular basis and used the Michelin brand to help communicate technical issues facing the industry. The five members, Jonathan Cole, Troy Stuart, Roger Dickinson, Mike Simpson and Daniel James, made their first panel appearance at Stoneleigh’s Agricultural Showground.

The objective of the initial event at Stoneleigh was to raise awareness within the agricultural industry of shortfalls in trailer braking efficiency. This is a topic that has gained additional relevance of late due to safety concerns over rigs, often over the 24,390kg gross train weight legal limit, travelling in excess of 20mph on public roads.

The trend towards ever-greater tractor horsepower, speed and weight means that larger tractors with trailers are being used on our roads in increasing numbers. Operators may now be unknowingly breaking the law and relying on inadequate trailer braking systems. Changes to EU regulations on trailer braking performance have already been proposed and are expected to lead to improvements in this area, including better trailer maintenance.

Research funded by the Health and Safety Executive, The Department for Transport and the UK Industry (coordinated by the AEA), carried out by Dr. Andy Scarlett of Scarlett Research Ltd, was used by the panel to highlight the braking problems experienced by tractor-trailer combinations. On a representative selection of ten trailers and trailer appliances it was found, for example, that 90 per cent failed to meet the UK legal requirement of 25 per cent braking efficiency for 20 mph maximum operation. Even after corrective maintenance, 60 per cent still failed to meet the minimum legal requirement.

The demonstrations hosted by the Michelin panel at Stoneleigh, and run under the guidance of Dr. Scarlett, used two tractors fitted with equipment capable of accurately measuring trailer braking efficiency. These machines were used to tow two laden trailers, one fitted with standard agricultural spec brakes and the other with commercial spec ‘high speed’ brakes. The braking tests were carried out from 20mph just using the trailer’s brakes and the results were as follows:

Agricultural spec

A 1997-build 14-tonne tandem-axle trailer with rocking-beam under-gear incorporating Ø355 x 90 mm ‘flat cam’-type ag-spec brakes operated by four Ø20 mm hydraulic rams.  This build spec is typical of trailers of this size built during the mid-1990s, yet whilst the test-trailer’s braking system was fully-operational and adequately adjusted, it only delivered a braking efficiency of 13 per cent; – only around half of the minimum required by UK law and about a quarter of that required for operation at speeds above 20 mph. The Michelin Agricultural Contractor Panel’s verdict was that braking performance may improve following a thorough service, but the system is undersized for use behind modern 25 mph / 40 km/h tractors and will overload the tractor’s braking system causing rapid and expensive wear.  Some scope exists to upgrade the trailer’s braking system (fit larger brake rams), but the small brake size will remain a limitation.

Commercial spec

A 2008-build 16-tonne tandem-axle trailer fitted with sprung axles and Ø420 x 180 mm ‘S’ cam brakes, operated either by four Ø25 mm hydraulic rams or air chambers via a pneumatic load sensing system.  Only the performance of the hydraulic braking system was assessed, this demonstrating an acceptable 25 per cent efficiency. The Michelin Agricultural Contractor Panel gave the verdict that this was a much more suitable braking system, adequately matched to the trailer size and more appropriate for the demands of modern tractor-trailer operation.  However, the panel cautioned that, whilst fitted with ‘commercial spec’ brakes, the performance of the hydraulic braking system in this instance was only just suitable for operation at speeds up to 20 mph.  For legal (and economic) use at higher speeds (25 or 30 mph) higher braking performance is required.  The brakes are large enough to deliver this, but only if applied with sufficient force, which may be available from the pneumatic system fitted to the vehicle.  In any case, higher braking performance (when laden) needs to be moderated when unladen to avoid over-braking and excessive tyre wear: a load sensing system is consequently a vital necessity.

The Trailer Braking Workshop at Stoneleigh, comments Michelin, confirmed what many farmers have already found to their cost: their tractors are doing most of the braking during trailer work and the trailer brakes simply are not “stopping their weight”.  Whilst historically this has not proved too great a problem at lower travel speeds, the increasing use of faster ‘conventional’ tractors has highlighted the inadequacies of many on-farm trailer braking systems – the main symptom being premature tractor brake failure.  Selecting the right trailer braking system spec, Michelin adds, is essential if this expensive problem is to be avoided.  Upgrading the brakes of existing on-farm trailer may prove a viable option, however Michelin recommends the seeking of guidance before doing so. A good place to start, the tyre major recommends, is the HSE / DfT / industry Agricultural Trailer Braking Study report. This can be downloaded at: http//www.hse.gov.uk/research/rrhtm/rr697/htm.