BOAT TRAILER TIRE HOW-TO

Learn how your trailer’s tires should be in­flated.

Boating - - FRONT PAGE - — Charles Plueddeman

Boat-trailer tires re­quire a lot of air pres­sure — in most cases, be­tween 50 and 65 psi. In fact, the cor­rect pres­sure is al­most al­ways the max­i­mum­rated pres­sure for that tire, which is molded right on the side­wall. Main­tain­ing that pres­sure is crit­i­cally im­por­tant to tow­ing safety. Tire pres­sure de­ter­mines the load the tire can safely carry. The load ca­pac­ity of the trailer was de­ter­mined with that spe­cific tire pres­sure as a fac­tor. If you roll out of the drive­way tow­ing your boat with less than the max psi in the trailer tires, you have ef­fec­tively re­duced the load ca­pac­ity of the en­tire trailer. Un­der­in­flated and thus over­loaded tires are the lead­ing cause of trailer-tire fail­ure; the tires over­heat and fall apart.

Here’s an ex­am­ple. A sin­gle-axle boat trailer with a 2,490-pound GVWR (trailer plus cargo) rolls on ST175/80R13C tires with a max psi of 50 pounds. At 50 psi, each tire is rated to carry 1,360 pounds, or a com­bined 2,720 pounds for the pair of tires. If the tire pres­sure is re­duced to 40 psi, the in­di­vid­ual tire-load rat­ing drops to 1,190 pounds (2,380 pounds com­bined), or 110 pounds less than the trailer ca­pac­ity. If you are in the habit of air­ing those trailer tires to just 35 pounds (your truck tires take 35 pounds, af­ter all) the com­bined tire ca­pac­ity drops to just 2,200 pounds. This could be a prob­lem if the boat (in­clud­ing out­board, bat­ter­ies, fuel and gear) plus the trailer weighs 2,400 pounds.

But wait, you won­der. If I fill my trailer

tires to the max psi, won’t they be over­in­flated when they warm up af­ter tow­ing for 100 miles? What if they ex­plode right off the rims? Air pres­sure will rise if the tire warms, or even as am­bi­ent air tem­per­a­ture changes, but only by 2 per­cent for each 10 de­grees, and the tire is en­gi­neered to han­dle that ex­tra 2 to 4 psi. If you add air to warm tires, sim­ply re­duce the pres­sure by 2 to 4 psi.

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