Understanding Tire Pressure – Written by a woman, for women.
So…I did just learn, that just because tires read Max. 50psi on the sidewalls, that’s not necessarily the pressure they should be inflated to. Maybe everyone knows this already, but at 56 years old, this was the first I’d heard of it.
I thought my tires must be low on air, because when I checked them, they were only in the 30’s. I aired them up to 50 PSI, and instantly found I was pulling a bobber behind me down the highway. The trailer bounced around behind me as trucks went by. It’s different than sway; when the trailer fishtails back and forth because of weight distribution.
I had no idea what I’d done wrong. I thought of pulling over and letting air back out of the tires at random, but I wanted the real answer.
I called 3 men whom I knew would be knowledgeable about trailer tires, and none of them answered their phones. I called one more, whom I knew would be more likely to be knowledgeable about computers than cars, and he read off tire pressure specs from Google, which told me nothing more than I already knew. Snappy due to my panic, I got off the phone and put on my thinking cap.
Figuring the pressure was better before, when my trailer pulled along behind my vehicle as though it was a feather, I decided to stop and let a little air out of the tires, down to 40 psi. It couldn’t be worse than the previous low pressure, right? And life got a little bit better after that. I got back onto the highway, but still had to keep both hands on the wheel. I only had another hour to go, so I stayed the course and headed for my next campground.
Of course, as soon as I got back on the highway, my phone rang. The first man I’d called was on the line. We’ll just call him Hero.
Hero, who is always a hero as soon as he is free from being a hero for someone else, explained I needed to look at the weight rating on the tire to make a calculated adjustment for the PSI I needed. The sidewall on the tires marked the rating at 1360lbs at 50psi. (I’d never paid any attention to the weight rating before, always going by the psi when I aired up tires. I guess that’s part of having a Dad, or other male role model in your life to explain mechanics when you’re growing up.)
Anyway, two tires (one axle) on the camper meant my weight rating on my tires was a total of 2720 (1360 x 2 tires.) Since my trailer weight was 1800 pounds at the scale, I was only carrying about 66% of the tires weight capacity. So I needed to put the tire pressure at the same ratio of it’s capacity. 66% of 50 is 33psi. I aired the tires back down to 35psi (to accommodate extra weight I might have added to the trailer since I had it weighed,) and they feel perfect. No more bobber on the back.
Here it is as a simple math calculation:
The weight rating on the tire multiplied by the number of tires on your trailer = the weight capacity. (Mine was 1360 x 2 = 2720.) This is the maximum weight you can put on the trailer tires.
Your trailer weight. I weighed mine on a truck scale. You can have your trailer weighed at any truck scale, and also at salvage yards. (Mine was 1800 pounds.)
The psi rating on the tires = the maximum psi at the total load weight (My tires are rated at 50psi/1360pounds.) You can find these numbers for your tires printed on the sidewall.
Divide your trailer weight (mine was 1800 pounds), by your maximum tire load weight (mine was 2720 pounds). The answer is the percentage of the maximum tire pressure you should have in your tires. (Mine was 66%.)
Multiply your maximum psi rating by the percentage you came up with. (i.e. 66% of 50psi is 33 psi.)
This is the air pressure your trailer tires should be inflated to.
Hero also threw in a couple of extra tips about tires. He explained the tires need to be aired down enough to meet the pavement correctly when they are on the road. If they don’t have enough surface area, you not only get a bobber situation, but the tires wear unevenly as well. If they are underinflated, they will overheat, and blowouts become more likely. He suggests walking around the car and trailer when you stop for fuel. Quickly place your hand on the outside of each vehicle and trailer tire. If one is too hot, you likely have a problem. Trailer tires will be too hot if they are under-inflated. Same goes for the tow vehicle tires, but there are also brakes, bearings and other parts which could be overheating. This is a quick way to find potential problems before you have an issue while driving. Keep an eye on your tires and you’ll save yourself not only blow-outs, but wear on your tires, and the stress of trying to hold your trailer on the road with sweaty palms.
Thanks Hero.