Weight and Tire pressure

[BehindBars]

Originally Posted by shepardmike

Ok, so this thread has been on my mind and when I went and picked up the MH up for our next trip I took pictures of the stickers inside my overhead cabinet. They are clearly stating 110 PSI. Do I believe the generic Michelin docs or the stickers?

FWIW - when I upped the tire pressure to 110 it rode a lot better. Not sure there will be a clear answer but I wanted to share.

Heading out in the morning but was happy to see that after a month in storage I only lost 1LB PSI in all six tires. TPMS is next on the to buy list.

The stickers will usually show the tire pressure required to carry the maximum axle weight rating for that particular axle.

Tire pressure varies with ambient temperature. To determine if any air was lost in a month's storage time, you would need to check the tire pressure at the exact same temperature before and after.

[gordonsick]

The sticker on my 2015 Berk 34QS says “120 psi”. The OEM tires were 255/70R22.5 Load Range H tires. That is also the maximum allowed pressure for a Load Range H tire, and gives allowable tire load that exceeds the GAWR of the chassis. So it is either there to cover someone’s butt or to increase the fuel efficiency of the Coach to meet someone’s guideline. I tend to think it is the latter.

Your newer coach comes with 275/80R22.5 Michelins. Michelin makes tires in that size with Load Range G (max pressure of 110) and Load Range H (max pressure of 120). So, to cover the possibility that the coach was shipped with Load Range G tires, they have to lower the sticker to 110 to cover their butts with Michelin.

OTOH, when I bought some new Michelins recently, the dealer filled to a much lower pressure than was consistent with the published Michelin guidelines for the load in my coach. I didn’t see them weighing my coach. The salesman just said “you run Michelins at lower pressures”. If I had a blow-out, he wouldn’t be there to help, I’m sure.

I’m a Formula 1 racing fan and the teams always conspire to find ways of running lower tire pressure than Pirelli mandates, because they get better traction. And, they get blowouts. Usually, they can drive back to the pits after a blowout, but do a lot of damage to their body work. But, they are the world’s best drivers in well-engineered vehicles, so it is an acceptable risk.

My recommendation is to do a 4-point weighing of the coach with your desired fuel, water, passenger and payload. Then, look up the tire manufacturer recommendations for the tires in use, and set each axle pressure to the required pressure for the highest corner weight on the axle.

This is the standard recommendation from most of us on the Forum because it is the safest approach. It’s just amazing how much snake oil is out there... Even on official-looking stickers on the coaches.

The pressure recommendations are for cold tires, but the tire manufacturers are pretty silent about the definition of “cold”, except for sub-zero temperatures. I usually regard “cold” as about 15°C or 50°F, but no authority has published that for me. So, if I start out the day at 0°C or 32°F, the tire pressures fall below that. I used to add air in those circumstances. But, I notice that my tire pressure monitoring system (TPMS) soon shows pressures consistent with my standard definition of cold, after a bit of driving, so I’m now comfortable with not adding air in cooler weather. If I was a trucker driving in 0°F temperatures or lower, I would go back to the Michelin guidelines and add more air because of the difficulty of getting tire temperature up in such low temperatures. Michelin does publish adjustments for extremely low ambient temperatures.

—Gordon