Heavy vehicle wheel detachment report: Executive summary

Heavy vehicle wheel detachment: Frequency of occurrence, current best practice, and potential solutions

Executive summary

When a wheel becomes detached from a heavy vehicle it may simply come to rest without causing any further damage or harm. However, in the wrong circumstances, it can collide with other vehicles or road users and cause an accident and, in some cases, this has resulted in fatalities. Typically, it is only the more serious accidents that occur as a result of wheel detachment that are identified by standard reporting mechanisms. Such detachment of wheels from vehicles, particularly heavy commercial vehicles, has been a cause of concern for many years and there has been a considerable amount of investigative work and comment on the subject. In 1997, the Department for Transport (DfT) published an advice leaflet ("Careless Torque Costs Lives") with the intention of reducing the scale of the problem. Despite this previous research, comment and advice, there is evidence to suggest that wheel detachment still occurs. There is, therefore, a need to more accurately quantify the current frequency of wheel fixing problems and to assess current practices within the industry in order to assess whether the previous advice has had a beneficial effect and to identify new action that could be taken to reduce the scale of the problem. The research was funded by the DfT and this report describes all of the work undertaken and the results and conclusions drawn from it.

The work involved:

• A survey run by VOSA and ACPO to investigate the current frequency of wheel fixing problems in the UK
• Studying existing sources of data on wheel fixing problems.
• Obtaining information from other countries to assess the scale of the problem in the EU and elsewhere
• Carrying out a survey of heavy vehicle drivers, operators and manufacturers in order to gain their views and perceptions
• Review current standards related to wheel fixings and procedures for wheel nut tightening.

The main findings were as follows:

• The various studies carried out produced quite variable estimates of the frequency of wheel fixing problems. However, with the exception of the TRL driver survey, there was general agreement that the frequency of loose or missing wheel nuts is in the low thousands each year, the frequency of wheel detachment is in the low hundreds each year and that annual fatalities resulting from wheel detachment are likely to be in single figures.
• Based on the data and a range of assumptions about the data, TRL has estimated that the typical annual frequency of wheel fixing problems is between 7,500 and 11,000 wheel fixing defects resulting in between 150 and 400 wheel detachments. Of the wheel detachments, it was estimated that between 50 and 134 would result in damage only accidents, 10 to 27 in injury accidents and 3 to 7 in fatal accidents.
• It was not possible to reliably determine whether the frequency of occurrence has changed since 1997 and the publication of "Careless Torque". There was some evidence to suggest that the frequency had decreased but there was also some conflicting evidence that it had stayed constant or increased.
• Requests for data on wheel fixing problems were sent to a wide variety of countries. There is sufficient evidence of wheel fixing problems to state confidently that this problem is not unique to the UK. There was insufficient detailed information to enable an accurate comparison with the estimated frequency of occurrence in the UK but there was sufficient evidence to suggest that the levels are "broadly comparable".
• There is a bias toward problems occurring on the nearside of vehicles (i.e. left in the UK). However, this bias is small for loose wheel nuts but large for wheel detachments suggesting it may be of small influence in the root cause of nut loosening but may have a larger influence on the rate of progress between the first nut loosening and full wheel detachment.
• Findings in Finland showed that there is a strong bias toward wheels becoming detached from the left side of vehicles, the same as in the UK, despite the fact that they travel on the opposite side of the road. This lends support to the theory that the direction of wheel rotation in relation to thread direction could have an influence on how quickly a loose wheel nut turns into a full wheel detachment.
• There has been a considerable amount of research into the mechanics of wheel nut loosening and the causes are now well understood. Clamp load is considered the main parameter and the clamp load must be sufficiently high to withstand all applied forces despite any effects of joint relaxation, variation in the torque to clamp ratio, relaxation due to temperature. However, this must be achieved without the clamp force being so high that the yield point of the stud is exceeded.
• Current designs of wheel fixing are capable of achieving this providing joint relaxation is accounted for with re-torquing and all components are in very good condition. However, the factor of safety allowed means that considerable maintenance of the joint is required to maintain its effectiveness over several years' service, particularly where vehicles are exposed to harsh operating environments.
• A wide range of best practice guidelines are available that recommend practice and procedures for fitting and maintaining wheels. In general the research into wheel fixing problems has meant that many of the requirements are now common to all of them but there are still areas such as actual torque levels and the issue of lubrication where there is no standard approach. Further development towards common standards may well be beneficial.
• Although most operators (86%) reported that they do have formal policies and procedures in place regarding wheel fixings, the survey showed evidence to suggest that these were not always in accordance with the guidelines discussed above. For example 12% of operators reported "never" re-torquing a wheel after it has been refitted.
• Similarly, there was strong evidence that a large proportion of drivers did not understand and/or adhere to the best practice requirements. For example, despite all operators stating that drivers should conduct a daily visual inspection, 26% of drivers admitted not always doing this and 32% admitted not always looking for signs of loose wheel nuts. Two percent admitted to "never" carrying out daily visual checks.
• The surveys also showed that there was evidence to suggest that those drivers or operators who better understood and adhered to the requirements reported a lower incidence of wheel fixing problems.
• It can, therefore, be seen that maintenance is a key issue for the current design of wheel fixing. The theory shows why it is important in physical terms, there are a range of guidelines in existence, there is evidence to show that these guidelines are not always followed and evidence to show those that do follow them report lower levels of wheel fixing problems.
• A range of potential measures capable of reducing the frequency of occurrence or mitigating the consequences were identified. These range from indicators of wheel nut movement to a complete redesign of wheel fixing methods. The physical effectiveness of these potential solutions was not evaluated but the survey showed that they were generally perceived to be beneficial by the industry. However, 2% of the defects found in the VOSA survey were found on vehicles equipped with wheel nut retention devices and 15% were found on wheels fitted with movement indicators. This shows that adding these devices alone will not eliminate the problem, although it may well reduce it, and good maintenance practice will still be required.
• A substantial difference was found between the perceived importance of wheel fixing problems in relation to other heavy vehicle safety issues and the relative importance in terms of fatal accident statistics. The industry respondents considered wheel fixing problems as second only to collisions with other vehicles. However, fatal accident statistics showed that wheel detachment contributed to 0.57% of fatal accidents whereas about three-quarters of fatalities in accidents involving HGVs involved collisions between vehicles, 17% involved collisions with pedestrians and approximately 3% involved contributory brake defects.

The report recommended that consideration was given to routinely monitoring the frequency of wheel fixing problems, possibly through the use of the VOSA prohibitions database combined with detailed exposure data. It also recommended that consideration be given to further work to investigate improvements such as:

• Further standardisation of best practice wheel tightening procedures, in particular, simplifying the identification of recommended torque settings and standardising on the use of lubrication.
• Further education and or enforcement to encourage universal adoption of rigorous wheel maintenance procedures.
• The possibility of a fundamental re-design of the method of wheel attachment for heavy vehicles to produce a much lower maintenance solution.
• The potential mandatory use of wheel nut retention devices or movement indicators, if independently shown to be effective.

Summary Report

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Reference

Report by I Knight, M Dodd, C Grover, R S Bartlett & T Brightman

PPR086

Clients Project Reference Number: SO535/V6