Originally Posted by Top
I just wanted to give an update on these Michelin tires. So far I've had them on for two and a half years and around 18,000 miles. I'm very happy to have these tires on my trailer. I'm sure they will need replacement by date before the tread wears to the TWI.
I run them at 43 PSI. Very happy with these tires so far.
In a PM, Top asked me to comment here on the fact that Extra Load tires are rated for maximum load carrying capacity at 41 psi – in spite of what may be written on the sidewall of the tire. In particular he wanted to know if it was true that using a higher pressure serves no useful purpose – and if there were any advantage to using a higher inflation pressure.
First, I am going to generalize the question this way: Every tire has a “Rating Point” – the point where the maximum load carrying capacity is obtained. For Standard Load (SL) Passenger Car tires, it is 35 psi (36 psi if the tire is a metric system based standard – notably European and Japanese standards).
This also applies to LT tires, which use 50 psi (Load Range C), 65 psi (Load Range D), and 80 psi (Load Range E).
Please note that for Passenger Car tires, it is common for the sidewall to state a maximum inflation pressure – and that is NOT (usually) the rating pressure. LT tires do not state a maximum inflation pressure on the sidewall.
OK, now that the stage is set, onto the question.
When tires are tested for durability, they would be tested based on the rated load - the maximum load carrying capacity as written on the sidewall of the tire (or something derived from that load) – AND – the rated inflation pressure (or something derived from that). By “durability” I am talking about the integrity of the tire itself, not the treadwear. This could be a load test or a speed test or a combination.
There are many different durability tests. Each test is designed to look at a particular aspect of durability. These tests are usually company specific and trade secrets.
As a general rule, increasing the inflation pressure helps the tire achieve better results. This is partially due to the reduced operating temperature and partially due to the reduced stress cycling. But there is a law of diminishing returns. Increasing the pressure 10 psi doesn’t give twice as much improvement as 5 psi.
But there is one aspect that is important here. Because increased inflation pressure reduces the operating temperature, it reduces the aging affects. This is particularly important for trailer tires which operate fairly close to their rating points for their entire life (unlike Passenger Car tires and Light Truck tires).
Other advantages? Fuel economy, hydroplaning resistance, snow traction (it helps you get down to the road surface.
But there is a downside to using a higher inflation: Reduced dry traction, reduced wet traction (as opposed to hydroplaning resistance), reduced snow traction if you are riding on top of the snow, poorer ride quality.
For trailers, I hope no one is towing when it snows. I also think that ride quality for a trailer is almost a moot point. So in the real world, there are advantages to using a higher inflation pressure, but there are disadvantages as well.