Tire Load Rating at Lower Tire Pressures vs. Axle Rating: A Case for "E" Load

[Smileyshaun]

Just my opinion, you’re overthinking this . Just slap on whatever tires you want air them down as needed and just drive . I have been lazy and not aired up after traveling drive home and you wouldn’t know a difference. You have to drive for a really long time to actually get the tire hot enough to worry about it .

 

[rruff]

I have been lazy and not aired up after traveling drive home and you wouldn’t know a difference. You have to drive for a really long time to actually get the tire hot enough to worry about it .

What did you air down to? Do think a half hour is a long time?

 

[Smileyshaun]

What did you air down to? Do think a half hour is a long time?

Matters on the terrain but 8 is usually the lowest I go . I’ve driven around at 15 for a couple days .

 

[RoyJ]

The high speed test sets load and psi, and varies speed (increasing). Temperature is usually the limiting factor at ~200F as the rubber compounds deteriorate in performance above that.

The main thing generating heat is hysteresis in the sidewalls and tread where it meets the ground. So you should be safe on the road at low psi so long as the tire is no more squashed than it was at the max load.

When airing down offroad you're typically nowhere near the speed rating of your tire, so the heat generated (which is a little above linear with speed) will also be less. The cooling component due to airflow will also be less (varies less than linearly with speed), but the conduction to the ground (doesn't vary with speed) will be greater, due to more surface contact. Since I can't find any reliable analysis or testing of this... I dunno...

If you're concerned about it, measure your tire temperature with a IR deal.

Agree with what you said, but will add: tire deformation + thickness of the rubber determines your heat generated. i.e. a 1" thick sidewall vs 1/8" sidewall, both deformed by the same amount under (different) load, running the same speed - your thicker sidewall generates significantly more heat. Hence why E-rated tires need more psi for the same load, it's to reduce deformation.

Yes, off road it won't matter much. But as you mentioned earlier, rock crawling vs bombing down a washboard at speed running King long travels are 2 different things.

A P265/75R16 at 22psi, will retain more of its speed rating so to speak, compared to an LT265/75R16 at 22 psi. How much speed rating is retain, we won't know without temp readings.

 

[Datsun Man2]

Instead of focusing on the load range, I simply see what is available in the size I'm looking at and for particular tires. For my current vehicle I have 265/75-16 Firestone Destination XT's. They are a Load E, but the actual weight rating is just above Factory. How do they handle? Just fine. The main thing is running the correct pressure on the street and lowing pressure on the trail.

 

[Peter_n_Margaret]

I think "Load Range" is a term used only in the USA on USA made tyres.
Actual weight rating and at what pressure is what counts.
Cheers,
Peter
OKA196 motorhome

 

[Smileyshaun]

Don’t always assume a higher load range means a thicker sidewall my 255/85 E rated wildpeaks where thin compared to the 35x10.50 D rated kendas I replaced them with I would say they where half as thin 15 psi would have the wildpeaks nice and squishy but I would have to drop to 8 to even come close to the same on the kendas

 

[rruff]

Agree with what you said, but will add: tire deformation + thickness of the rubber determines your heat generated.

Hysteresis of the compound is a big factor as well.

 

[luthj]

Sidewall flex, sidewall ply orientation, rubber chemistry, all combined will determine how much heat is generated in a tire for each rotation. Simply looking at a tires load range or weight capacity won't tell the whole picture.

In moderate conditions at lower speeds, having lower than spec (for the axle weight) tire pressure isn't an issue. But running well under the load table spec pressure in 100F weather at 70mph? Yeah, that's asking for tire to overheat and fail. Remember, temperatures over a certain threshold are cumulative, and will contribute to a tire failing internally long before the tread is worn out.

In general high load index tires have stiffer carcasses, and thus produce less heat for the same load and pressure. Of course to reach a higher load they need more pressure than a lower load index tire of the same size, per the load tables.

When aired down you are driving slower, and thus producing less heat in the tire. My general rule of thumb is to weight the vehicle, and then air the tires to the load table recommended pressure for that axle weight. Measure the sidewall height from the ground to the edge of the wheel rim.

Reduce the tire pressure until the sidewall height is 25% reduced from that number. This is a good starting place for rough trails or roads. You typically need to limit your speed to about 30mph at this pressure.

 

[rruff]

Sidewall flex, sidewall ply orientation, rubber chemistry, all combined will determine how much heat is generated in a tire for each rotation.

As a side note, this heat generation is also an energy suck, and why 2 different tires with similar specs can have very different MPGs. I wish someone would test truck tires for rolling resistance, but until then I think the speed rating at the same load and pressure is the best proxy we have.

I was involved in testing bicycle tires for rolling resistance about 20 years ago when no one was doing it, and it was a real eye opener. You can't go by weight or thickness or feel, or any of the normal things people use. When the testing became more common, manufacturers needed to get honest and actually pay attention and make better tires.