OTTAWA, Ont. – The Council of Ministers Responsible for Transportation and Highway Safety has approved several amendments to the MOU on Vehicle Weights and Dimensions, including increased weight limits for new generation wide-base single tires.

The changes are expected to come into effect by July 1, 2008, according to John Pearson, secretary, Task Force on VWD Policy.

The Council states in a release that while research shows the new wide-base single tire designs offer improvements in fuel-efficiency, vehicle roll stability and reduced tare weight as well as reduced impacts on pavement compared to the older wide single tires, they still appear to have “slightly higher impacts than the dual tire configuration they replace if used at dual tire loads.”

So to improve the consistency of weight limits applicable to use of these tires across Canada, while also protecting highway infrastructure, the national standards for weight limits on single and tandem axle groups are being increased to the federal limits currently applicable in the US.

“According to research, this change should not have detrimental impacts on pavements in Canada, as the impacts associated with the wide single tires at the new weight limits would be no more than the impacts associated with conventional dual tire axles at their current weight limits,” the Council states.

The MOU has been amended to include the following clause that applies to axles except steering axles: Its axle loads, when an axle is fitted with two single tires, each of which has a width of 445 mm or greater, do not exceed 7,700 kg for single axles and 15,400 kg for tandem axle groups.

Three jurisdictions have registered exceptions in the MOU to the higher weight limit for single tires:

• The Northwest Territories will retain a weight limit of 3,000 kg for any single tire except on steering axles;

• In New Brunswick the weight on a single tire (except on steering axles) having a minimum width of 445 mm cannot exceed 3,080 kg on highways rated as Class 3 – Gross Vehicle Weights up to 50,000 kg and Class 4 – Gross Vehicle Weights up to 43,500 kg. The weight on a single tire (except on steering axles) having a width of less than 445 mm cannot exceed 3,000 kg on all New Brunswick highways;

• In Newfoundland and Labrador the weight on a single tire (except on steering axles) cannot exceed 3,000 kg on secondary roads within the designated route network.

Replacing dual tires with the new generation single tires on existing trailers can result in a violation of the minimum track width requirement stipulated in the MOU (2.5 m), according to the Council.

It adds that industry representatives indicate that use of axle spacers or rims with large offsets can compensate for the dimensional deficiency, but can create other problems due to higher stresses in axle and wheel bearing. (This situation can be avoided with the purchase of new trailers, as longer axles can be fitted on the trailer when single tires are to be used).

“To avoid creating potentially unsafe situations, the minimum track width requirement has been relaxed for carriers who wish to retrofit existing trailers with new wide-base single tires,” the Council states.

The MOU has been amended to include the following clauses:

• It is understood that the minimum track width for trailer axles fitted with single tires must be no less than 2.3 m for trailers built in 2007 or earlier;

• It is further understood that the minimum track width for all axles on trailers built in 2008 or later must be no less than 2.5 m.

Other changes included in the MOU include accommodation of aerodynamic devices installed on the rear of trucks and trailers and standardization of width limit allowances for auxiliary equipment.