The Trick With Tridem-Drives

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Tridem-drive tractors, and maybe straight trucks with tri-drives, in Canada’s national agreement on truck weights and dimensions? Maybe. But not any time soon. There are too many technical issues and differences among the provinces for quick action on this.

The trucking industry is trying to have the national agreement amended to allow tridem-drive tractors. The agreement, which any province is free to exceed for trucks operating within its boundaries, only recognizes tractors with single or tandem-drive axles. Some provinces, though, do allow tractors with more axles within their borders. The Task Force on Vehicle Weights and Dimensions Policy passed a motion at its last meeting in Montral on December 3, 2003 to develop specifications for tridem-drive tractors that would be acceptable across Canada. The Task Force reports to the Council of Deputy Ministers Responsible for Transportation and Highway Safety and it was responding to a request from Manitoulin Transport from Gore Bay, Ont. to develop these specifications so that it could extend its use of tridem-drive tractors. Manitoulin operates four tridem-drive tractors in Ontario.

But provincial representatives on the Task Force were silent when this motion was passed. There are too many differences of opinion to allow such a proposal to be greeted with enthusiasm.

Tridem-drive tractors are useful in some situations and the extra axle only adds about $20,000 to the cost. A heavy piece of machinery on a lowbed trailer is one such situation. If only tandem axles are used, they are overloaded. On bush roads, the tridem drive has more mobility because it has better traction than the alternatives. In general freight operations, a tridem-drive tractor pulling a trailer with a tridem axle (7 axles in all) comes close to B-train payloads without the operational disadvantages of a double-trailer configuration.

So, what is stopping the introduction of tridem-drive tractors to the national agreement?

One thing is the confusing mixture of current rules. Because tridem-drive tractors are not in the national agreement, each province and territory has its own rules. Some allow them only under permit. In Ontario, a tridem-drive tractor or, for that matter, a tandem-plus-tag axle is legal (non permit) under the old regulations – the ones that predate the national agreement. But, because these are the old rules, the semitrailer is limited to 14.65 metres (48′); the national agreement allows 16.2 metres (53′). Or , if operating doubles, the overall length is restricted to 23 metres (75′); the national agreement allows 25 metres (82′). Alberta and British Columbia have had permits for several years allowing tridem-drive tractors, most notably within the forest industry. In fact British Columbia decided several years ago to dispense with permits and simply wrote the specifications for tridem-drives into its regulations. So anyone can now use a tridem drive in BC, as long as it meets the legal specs. Alberta continues with permits (perhaps approaching a thousand) and there are now a wide number of situations, besides logging trucks, where they are common. Saskatchewan allows tridem-drive tractors under permit to haul non-divisible loads. In addition, there are a few truckers in the oil-field service using them under Saskatchewan’s Trucking Partnership Program.

The second thing holding back the national introduction of tri-drives is a concern over technical standards. When the national agreement was first developed in the mid 1980s, experts agreed to performance standards for trucks. Most configurations in the national agreement meet these under most operating circumstances. For example, there is a standard for low-speed off-tracking that controls how far out the last axle of a vehicle tracks in a turn compared to the inside steering axle. Or, as another example, trucks are supposed to have a rollover threshold higher than 0.4 (some say 0.35). Trucks with a threshold lower than 0.4 (or 0.35) have a nasty habit of rolling over in tight turns.

When the forest industry in BC and Alberta wanted a better truck to haul heavy payloads it went to the Forest Engineering Research Institute (FERIC) in Vancouver. Eric Amlin of FERIC explains: “Back in the 1980s, the industry was looking for a truck that went beyond the standard 5-axle tractor-semitrailer (or pole trailer) logging truck. They did this by adding trailing chassis units. These increased payloads but there were problems.” For one thing, the increased number of articulation points and sometimes the increasing loads on the tandem drives made it more difficult to manoeuvre these configurations, particularly on bush roads. Another problem was that by adding trailing axle units (“jeeps” and “boosters”), the height of the log bunks increased. The result was that centre of gravity was higher and rollover thresholds were less than 0.35.

These logging trucks, with various trailers, pole trailers, jeeps and boosters, have two or three articulation points. This generally means that they have poorer dynamic performance characteristics than the standards used in the original national agreement. For example, the agreement is based on a maximum of 0.6 for the dynamic load transfer ratio – the fractional change in load between left and right-hand side tires as a driver makes a rapid steering manoeuvre. Some logging trucks – a tandem-axle tractor+single-axle jeep+tandem-axle pole trailer+single-axle booster (“jeep-dog logger”) – were found by FERIC to have transfer ratios as high as 0.666. In an evasive manoeuvre at highway speeds, this rig comes close to having all tires on one side lift off the pavement.

So, in the 1980s, with the help of both the British Columbia and Alberta governments, FERIC began a research program into tridem-drive tractors. This resulted in the specs now used in BC and Alberta to control tridem-drive configurations. There are slight differences between the specs used in the two provinces and that’s another reason why it might be preferable to introduce tri-drives in the national agreement: it would standardize the specs.

One would think that, with all this work (FERIC alone has published 20 technical papers on the subject) and the years of experience with tri-drives in the west, the rest of the country would readily accept tri-drives in the national agreement. Well, it’s not quite that simple.

There are problems. For example, as you increase weight on drive axles – all else equal – you increase what FERIC’s Amlin calls the “lateral friction utilization of the steering axle.” In plain English, in a tight turn there is a greater tendency for the rear end to push the front end of a tractor straight ahead. And, obviously, with tridems there is going to be more weight on the drives than with tandems. Alberta allows up to 23,000 kilograms on tridem drives and BC allows up to 24,000 kilograms versus 17,000 kilograms on tandems in both provinces. To compensate for this increased weight on drive axles, FERIC’s research suggested that steering axles should carry up to 7300 kilograms (the national agreement only allows 5500 kilograms). In fact, BC regulations require that the steering axles on tri-drive tractors carry a minimum of 25% of the drive-axle weight when the truck is loaded and Alberta permits require that the steer axles carry a minimum of 27% of the drive-axle weight at all times. All this to keep lateral friction utilization down to acceptable levels.

Two other factors that keep the lateral friction utilization down are relatively short spreads on the drive axles and relatively long wheelbases for the tractors. BC and Alberta both limit the spread on the drive tridems to 2.8 metres, and they both require wheelbases to be in the range of 6.6 to 6.8 metres. The national agreement sets the maximum tractor wheelbase at 6.2 metres (244 inches). This is to limit low speed off-tracking. By allowing tridem-drive tractors into the national agreement, some provinces are concerned that the industry will soon want long wheelbases for other configurations. The truth is that any limit is arbitrary because the importance really depends on where that tractor is going to operate. Long wheelbase tractors will always have a problem turning corners on city streets. But, then, as BC and Alberta officials note, not many logging trucks actually operate on narrow, urban roads.

Another technical issue causing problems among the provinces is the weight allowed on the tridem. With a 2.8-metre spread, the national agreement allows 21,000 kilograms on a tridem (trailers only). That’s because some bridge engineers say any more weight over this spread would cause a problem for bridge decks. Obviously not all bridge engineers agree as BC has always allowed 24,000 kilograms on a 2.8-metre spread. Alberta, which sticks to the national agreement for most trucks, has deviated in its permit conditions for drive tridems by allowing 23,000 kilograms. Until the bridge engineers can sort out these differences, this tridem-drive weight issue will remain a stumbling block to the introduction of tri-drives in the national agreement.

And there are other technical issues. For example, overall lengths for some tri-drive configurations have to exceed the current limits in the national agreement. While tri-drives may gradually become more common in some parts of Canada (typically, by permit) don’t expect their introduction to the national agreement any time soon.

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  • We want to travel a tridem drive truck through the province of Quebec. It was purchased new in Ont. and would be travelled through Quebec empty. We may have to wait up to 10 days for approval.
    Question; would it be possible to remove the tires and rims from the front drive axle and drive it without doing any damage to the rear diffs?

  • Try taking off the section of drive shaft?
    We went to California with a 53′ tridem chip trailer which put us over length for axles so we took off last tires and chained the axle up til returning to Oregon.