Trailer Tire Wisdom

Tech Shorts September 25, 2004 1
Trailer tires for snowmobile trailers are NOT all created equal! You can find several tires of the exact same size, yet with widely varying...

Trailer tires for snowmobile trailers are NOT all created equal! You can find several tires of the exact same size, yet with widely varying load ratings and number of plies (internal layers). For example, a popular two-place snowmobile trailer tire (18.5 x 8.5 x 8) can be found with load ratings of 590 pounds, 770 pounds and 940 pounds from a single tire supplier.

With sleds weighing in at over 600 pounds (some even top 700 pounds each full of liquids), you should quickly realize when you factor in the weight of the trailer you can easily exceed or be close to the rated load capacity of some of the “lesser” tires. Anytime you approach the load capacity of a tire, it becomes even more critical to have the tire pressurized to its full pressure rating. Any trailer tire that is under inflated will have a reduced load capacity, which translates to an increased risk of tire failure.

How often do you check the pressure of your trailer (and tow vehicle) tires? Tire inflation pressures change due to time and temperature. According to the tire experts at the Tire Rack ( tires lose about 1 psi per month due to air escaping through the rubber as it stretches. If you were to check your tires only every six months, it would not be uncommon to find them underinflated by as much as 6 psi (six pounds)!

Tire inflation pressures also fluctuate with changes in outside air temperature, a HUGE consideration when it comes to pulling a trailer in sub-zero environments. This occurs at a rate of about one psi for every 10° F (plus OR minus). So the trailer tires you set correctly with an ambient temperature of 50° F will be underinflated by seven psi at -20° F!

If you add the variations of time and temperature together, it is easy to understand why a tire’s inflation pressure should be checked frequently, especially when you consider how close you may be to the tires rated load capacity on a fully-loaded trailer.

According to the Tire Rack, tire pressures should ideally be checked once a week, in the morning before the tires have been used (heated) at the coldest time of the day. The heat of the day and/or driving more than a few miles will cause an incorrect cold pressure reading.

If the tires are warm the general logic is that the pressure will be approximately four psi higher than the cold pressure. NEVER let air out of warm tires (those that have been used for over one mile within the past three hours) to arrive at a desired cold pressure specification. However, you can ADD air to warm tires if they do not have the specified cold pressure. Again, NEVER let air out of warm tires to reach your target pressure, as this can result in a low pressure, but it is OK to add pressure to warm tires to reach your target. Recheck the pressure again when cold.

Next time you’re shopping for a spare or a new set of trailer tires, pay attention to the load capacity and number of plies, which will also increase the tires resistance to puncture and overall failure rate. And yes, they do cost more, but one flat tire out on the highway in the middle of the night at –20 will forever cure you from being a cheapskate.

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