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

[Greyota]

I've searched quite a bit for an answer, or discussion at least, on this aspect of lowering tire pressures while off-road but haven't found what I'm looking for, so I'm hoping to get some clarity from the experts here on the forum.

I'm in the market for some new tires, and I'd prefer to use a "C" Load tire, however, I don't see how they can be used at lower tire pressures safely as lowering tire pressures also reduces the max load for the tire. So what happens when you lower the tire pressure so low that the max load is now lower than the axle rating?

For example, let's consider the new GX550 that has a Front GAWR of 3,460 lbs and a Rear GAWR of 4,100 lbs. That's 1,730 lbs per front tire and 2,050 lbs per rear tire.

Now, using the Toyo Load Inflation Tables, on pages 28 and 29, it provides load carrying capacity at various inflation pressures but only goes down to 22 psi. Let's consider a "C" Load tire with a load index of 117 (In this case, let's assume the Toyo Open Country AT3 285-75R17, Load C, 117 / 114Q).

The inflation table would indicate a load carrying capacity of 1,885 lbs at 22psi. This is sufficient for the front axle, but is quite a bit less than the Rear axle rating. If you were to interpolate down to 18 psi, it gives you a load rating of about 1,600 psi; not enough for either axle.

Doing the same analysis on an "E" load tire with a load index at 125 or higher at 22 psi yields approximately 2,550 lbs and at 18 psi gives about 2,160 lbs; appropriate ratings for both the front and rear axles. This really shows how much lower an "E" load can go and still meet the required load ratings.

In summary - am I thinking along the correct lines? Is there something I'm missing on this topic?

 

[ExpoMike]

My first thought is, GAWR combined front and rear, tends to be higher than the actual GVWR. GVWR is overall max weight distributed across all four points. Depending on how the vehicle is loaded, you could have more weight on one axle than the other. Trucks are more prone to this than SUV/Cars. That said, I think most of the tire pressure ratings are related to driving down a highway at 60+ MPH, not <30mph on a dirt road. Heat is what kills tires and the lower the pressure, the higher the load and the faster you drive, the more heat is put into the tire. When you are driving slow on dirt, you are not putting the same heat into a tire then when driving down a blacktop road at 60mph.

As for different load range tires, (lets say C vs E), the main difference is sidewall stiffness. This limits the overall load rating of the tire but it also changes how much sidewall flex you get at lower pressures. Sidewall flex contributes to heat, so the more flexible they are (i.e. C rated), the more heat they will put into the tire is running at lower pressures. Again, this is more related to high speed, pavement driving. You need to find that sweet spot for YOUR specific rig and YOUR specific needs. There are so many factors to consider, it's not quite the black and white of just looking at rated numbers.

 

[rruff]

Is there something I'm missing on this topic?

Yes... what you are doing is irrelevant, because the rating doesn't just involve load, but also speed (the speed rating of the tire).

What it's really about is temperature... or overheating. It's not a linear relationship because aero cooling is involved, but slower speed means cooler tires.

So if you air down your tires to 15 psi and drive 100 mph on washboard, you might have a problem... but if you are driving slow, then no worries. Well, you have to worry about rim damage, but not overheating.

If you are concerned, check your tires with a IR thermometer. According to Bendix, 195F is totally fine and > 250F will degrade the rubber compounds.

BTW in case it wasn't obvious, the flexing of the tread and casing is what causes the tire to heat up.

NOTE: I just looked up these tires, and it appears that the C and E load are the same tires, just rated differently. Look at other sizes as well, and you'll see that C and E have nearly identical weights and other specs. It's possible that they have slightly different tread compounds or something.

Open Country A/T III | The All-Terrain Tires for Trucks, SUVs and CUVs

The Open Country A/T III has exceptional wet and dry braking and handling, great off-road traction, a quiet ride, great in snow and 127 sizes.

www.toyotires.com

 

[Greyota]

Good points, I wasn't considering the impact that speed has in the load rating and was only considering the static load. That makes me feel better about "C" load.

 

[dstefan]

Other related factors that haven’t been mentioned enter in here and are worth considering. The three big reasons to air down are 1) traction, 2) comfort, and 3) protection from flats. Numbers 2 and 3 are affected by wheel size to a degree. Smaller wheel size = more sidewall = more flex for the same diameter tire. More flex (softer tire due to air down) more protection from punctures generally.

A “C" rated tire will flex more and be more comfortable, but at low pressures can be more prone to pinch flats on the side wall (ask me how I know!), more so with a larger wheel size for the same tire diameter. The stiffer the sidewall the less comfortable and more we need to air down, but there’s typically more protection from pinch flats.

The style of the tire also affects things — many now have aggressive side lugs which can help protect and allow for more air down with less risk of sidewall damage and better traction in soft stuff.

 

[Gravelette]

Why buy C if the tire you want is available in E?

 

[Greyota]

Why buy C if the tire you want is available in E?

"C" load is generally considered less firm and usually weighs less.

 

[rruff]

Same weight in this case, so... ?

 

[Peter_n_Margaret]

For 20 years I have run these Michelin tyres that have a single ply all steel sidewall.
The thinner and lighter the sidewall, the less heating when running at low pressures and high flex.
Michelin also publish speed/load/pressure tables for this tyre. I use them religiously and have never overheated them.
The pressures in the pic are below the yellow line, but that is OK for very low speeds and short durations to get over a soft sand dune.
Cheers,
Peter
OKA196 motorhome

 

[charlieaarons]

This simply reiterates Peter’s post, you see the pressure-speed relationship in print, endorsed by MB engineers.
Last year I drove a 12.3 metric ton vehicle on deep sand tracks in January (summer), 5 days, maybe 200km in Central Kalahari Game Reserve. With my tires at about 45% usual highway pressure. Top speed maybe 25kph.
The only safety issue is that one must be mindful of de-beading. It’s a real concern (especially turning and other side stresses). The only 100% cure for de-beading risk is internal beadlocks, usually used with Hutchinson type wheels.
On the other side of the curve, when I calculate highway pressure (defined as cold, before starting in am), after a linear interpolation, I somewhat arbitrarily add 10% “for safety”, as long as it doesn’t exceed the max pressure printed on the sidewall.