Assessment of the effect of automatic slack adjusters on brake adjustment

Prepared by:
Centre for Surface Transportation Technology
National Research Council of Canada

Prepared for:
Motor Vehicle Safety
Transport Canada

TP 14214 E
April 2002

How to get the full report


All provinces and states across North America inspect heavy vehicles following standard procedures agreed under the auspices of the Commercial Vehicle Safety Alliance (CVSA). A vehicle may be put out-of-service if it has any of many specified defects defined by objective inspection criteria. Airbrake defects have been the principal reason for vehicles to be put out-of-service, and airbrake pushrod stroke in excess of the prescribed limit has been the principal airbrake defect. CVSA inspections conducted at random over the last ten years or so have consistently resulted in 25 to 45% of all vehicles inspected being put out-of-service. A vehicle with some airbrakes out of adjustment may not have a consistent and reliable capability to stop, so a requirement for automatic slack adjusters was introduced to improve the reliability of airbrake stroke. U.S. Federal Motor Vehicle Safety Standard 121 was amended to require automatic slack adjusters, and a simple visual means of checking brake stroke, on vehicles built from 20 October 1994. Ontario introduced this requirement for certain vehicles with the same effective date, and extended it to all vehicles from 30 April 1995. Transport Canada amended Canadian Motor Vehicle Safety Standard 121 to introduce this requirement for vehicles built from 31 May 1996. Five years have now passed, and Transport Canada wished to assess the effect of the regulatory change on the state of brake adjustment on vehicles with airbrakes. The objectives of the work were to determine:

  1. The fitment rate of automatic slack adjusters on vehicles with airbrakes;
  2. The extent to which original equipment automatic slack adjusters may have been replaced by manual slack adjusters; and
  3. The effectiveness of automatic slack adjusters in maintaining airbrake adjustment.

Transport Canada engaged the Centre for Surface Transportation Technology of the National Research Council of Canada (NRC/CSTT) to conduct this work. NRC/CSTT identified that Ontario Ministry of Transportation (MTO) had detailed roadside inspection reports on large numbers of heavy trucks, which included vehicle type, number of axles, model year, license plate jurisdiction, brake chamber type, slack adjuster type and brake stroke. MTO graciously provided access to about 4,500 records collected from CVSA brake inspections during Operation Airbrake in 1999, 2000 and 2001. Operation Airbrake consists of three days of inspections in all provinces and some states, with the objective of focusing drivers and carriers on airbrake issues and gathering data to assess the state of airbrake adjustment. Vehicles were selected at random in all parts of Ontario.

The sample of vehicles for each of the three years appeared to be representative of traffic in Ontario. Fitment of automatic slack adjusters on power units jumped to over 90% when the U.S. mandated them in 1994. Automatic slack adjusters were found on about 93% of all tractors in 2001. This number will rise gradually as older vehicles equipped with manual slack adjusters disappear from service, and may reach about 98% by 2010. Automatic slack adjusters were found on about 85% of all straight trucks in 2001. Fitment of automatic slack adjusters on Canadian trailers did not increase significantly until they were mandated in Canada in 1996. They were found on about 75% of all trailers in 2001. The fitment rate of automatic slack adjusters on straight trucks and trailers is lower than for tractors, because straight trucks and trailers tend to be kept in service longer than tractors. Thus, a larger proportion of the fleets of both straight trucks and trailers that pre-date the requirement for automatic slack adjusters are still on the highway than tractors. These older vehicles predominantly have manual slack adjusters. About 30% of older straight trucks, tractors and trailers that pre-date the requirement for automatic slack adjusters are nevertheless fitted with them. Some may have been fitted as original equipment, because these devices have been available for at least 25 years. It is also possible that some carriers believe that the value of retrofit outweighs its cost. Overall, in 2001, almost 90% of all vehicle units (tractors, straight trucks, trailers and converter dollies) on the highway in Ontario were fitted with automatic slack adjusters. Small numbers of older vehicles fitted with manual slack adjusters may still be operating well beyond 2020, though most of these will be in local use for limited travel.

The survey showed that about 4.2% of power units and U.S. registered trailers built since 1994, which would have been expected to have been built with automatic slack adjusters, were reported as fitted with manual slack adjusters. It is not possible to identify from the data whether these vehicles were built with manual slack adjusters, or the automatic slack adjusters fitted in the factory had been replaced with manual slack adjusters, or whether there were errors in identification of the slack adjuster, or in recording the inspection data. If it is assumed that all the vehicles were built in compliance with regulations, then this establishes an upper bound for an inspection and reporting error rate. However, 9.4% of Canadian registered trailers built since 1996, which would have been expected to have been built with automatic slack adjusters, were reported as fitted with manual slack adjusters. There is no reason to expect a different inspection and reporting error rate between power units and trailers, so this suggests that at least 5.2% of Canadian registered trailers were either not built with automatic slack adjusters, or have been retrofitted with manual slack adjusters. If U.S. built trailers are assumed in compliance and are removed from the sample, it is possible that as many as 10% of Canadian trailers that should have automatic slack adjusters do not have them. The level of non-compliance for all vehicles will be higher if the inspection and recording error rate is less than 4.2%. Transport Canada could address this issue, by working with the provinces in future editions of Operation Airbrake, to conduct a careful assessment of each vehicle unit that should have been fitted with automatic slack adjusters but is reported as fitted with manual slack adjusters. It could either be done on-site, or by following up from the paper inspection reports.

The distribution of stroke in the normal operating range appears very similar for both manual and automatic slack adjusters for a brake chamber of a particular size, regardless of whether it is standard stroke or long stroke. However, a higher number of manual slack adjusters end up with a stroke outside the prescribed adjustment range. While manual and automatic slack adjusters each manage to maintain about the same mean stroke, the standard deviation of stroke is slightly larger for manual slack adjusters. Estimates of the probability that brakes would be out of adjustment using these statistics were reasonably consistent with the results from the survey.

Vehicles with manual slack adjusters were put out-of-service at a rate 150% higher than their population, simply because there is a higher probability that a brake with a manual slack adjuster would be out of adjustment. Ontario impounds vehicles with defined critical defects significantly beyond the threshold for putting a vehicle out-of-service. The impounded vehicles also match the provincial fleet profile reasonably well. About 97% of all vehicles had been impounded for brake system defects, and about 90% of impounded vehicles have manual slack adjusters.

Long stroke brake chambers began to appear about 1997, and the rate of fitment is increasing rapidly. It was about 13% for Canadian registered tractors, and 21% for U.S. registered tractors, in the 2001 model year, with lesser rates for trailers. A long stroke brake chamber with a typical automatic slack adjuster as seen in this study should virtually never be out of adjustment, as long as the slack adjuster is functioning.

The process of integration of automatic slack adjusters into Canada's truck fleet is about 90% complete, though the last manual slack adjuster may not disappear for another 35 years or so. There is no doubt that current models of automatic slack adjuster are able to maintain brake stroke more reliably than manual slack adjusters, though it must be recognized that automatic slack adjusters are predominantly on relatively new vehicles and manual slack adjusters are on older vehicles. Trucks are put out-of-service in Ontario mostly for brake system defects, and the principal defect has always been brakes out of adjustment. The out-of-service rate has diminished by about half over the last ten years. The introduction of the automatic slack adjuster has undoubtedly played a significant role in this, but so also have other initiatives, such as MTO's focus on the responsibility of the carrier to inspect and maintain vehicles, allowing drivers to adjust brakes, and providing brake adjustment training to drivers and mechanics. 

The work described here has developed an approach and data processing methodology that could be applied to similar data that may be available from other provinces. 

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