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By Jim Kerr

How much can your vehicle tow? Looks and vehicle size alone are no judge of towing capacity, and neither is engine size or power. Fortunately, the auto manufacturers provide maximum towing capacity information either in the owner’s manual or on placards on the vehicle.

Sometimes the information is given in the form of Gross Combination Weight Rating (GCWR), which is the combined weight of the vehicle, contents and trailer. Governments use these specifications to regulate the weight of trailer a vehicle can tow, but how do the manufacturers arrive at their numbers?

You might be surprised to find out there are no standards for determining vehicle towing capacity. Unlike engine horsepower ratings, door latch specifications or tire load ratings, there is no association such as SAE (Society of Automotive Engineers) or RMA (Rubber Manufacturer’s Association) to set towing standards. Each manufacturer sets its own criteria.

I used to think that axle bearing load ratings were used to determine towing capacity, but brake size limits the maximum capacity. While these components do affect safe towing, they are usually not what determines the limits.

In a recent discussion with Chrysler engineers about their truck towing capacity, I found out that their criteria is often based on what it takes to get a load moving. They select the transmission options and test the vehicle on a seven per cent grade. The vehicle must be capable of moving away from a stop with the maximum load. Manual transmissions are favoured by many for heavy duty towing, but using Chrysler criteria, an automatic transmission would be rated higher because the torque converter allows easier starting on a hill than a clutch would. If both manual and automatic transmissions are available on a vehicle, then the one with the lowest maximum rating sets the specification for all variations of that model.

With trucks, you usually find different towing capacities with optional axle ratios. A higher number axle ratio (low speed ratio) will have a higher load capacity, not because it provides more pulling power but because it allows easier starts on a hill.

Ford may use different criteria. Talking with their engineers, I was informed their limits are often determined by cooling system capacity. The vehicles are driven with increasing loads at high ambient temperatures (those desert testing facilities are there for a reason). If the vehicle can be driven without overheating, then that is what determines the maximum towing capacity.

There is much more to towing than just the maximum weight rating, however. A trailer that has the same frontal area as your tow vehicle will tow much easier than one with a larger frontal area. Boats are streamlined, so they may tow easier than a lighter camper trailer.

Tongue weight is another towing consideration. Approximately seven to 18 per cent of the total loaded weight of the trailer should be on the trailer hitch. If there is too much weight, it can overload the rear springs of the tow vehicle or lift the front end of the vehicle so steering is unsafe. If there is not enough tongue weight, the trailer hitch can lift the rear of the vehicle so there isn’t enough traction on the tow vehicle rear tires and cause poor stability. Loading the trailer correctly is key to safe towing.

Always consider braking capacity when you are towing. Many provinces require trailer brakes on all trailer axles once the trailer weight (with contents) is over 1,360 kg (3,000 pounds), but be sure to check your local regulations. Some older trailers were required to have brakes on only one axle. Regulations do vary from province to province, so be sure to check before towing outside your own locale.

Finally, because it is possible for trailers to disconnect and break-away, causing serious accidents, safety chains are required on trailers. Safety chains should be looped and cross under the trailer tongue and connected between the tongue and the vehicle frame or fixed part of the trailer hitch. Heavier trailers may be required to also have an automatic braking device that will activate the trailer brakes if the trailer separates from the tow vehicle.

Towing a boat to the lake, a recreational trailer on holidays, or a moving trailer across the country can be done easily if you choose the right tow vehicle with the correct equipment, load the trailer properly and keep the speeds suitable for road and traffic conditions. Remember, too, that it takes longer to stop safely when there is a trailer behind.

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