What the heck -- tire pressure

[jensht]

Interesting thread. It has me doing this thought experiment:

We make the run between Southern New Mexico and San Antonio TX (9 or so hours) about 2 times a year. Left lane traffic (which we are in of course) averages 87 to 95 mph (80mph speed limit), ambient temp between 105 and 110 degrees F, blacktop temp has to be significantly higher then that in the west Texas sun.

Which vehicle is less likely to suffer a catastrophic tire failure, my Bronco on OEM 315/70r17 C rated LT tires at 31psi cold, or my wife's 2016 Toyota Sequoia which currently sits on 275/55r20 "extra load" p-metric tires which are V rated (149mph) that I keep at 36-38psi cold?

The Sequoia is a serene highway cruiser, quiet, but lots of sound deadening means it sits at 6700lbs with a full tank, family and gear. The Bronco would be loud and stiff, but would be "only" 5600lbs loaded with gas, family and gear.

My gut tells that even at the lower pressure the Bronco tire has the easier time of it. I could bring the Bronco up to 34 or 35psi for a more clear advantage especially given its volumetric air capacity (the 55 sidewalls of the Sequoia approaching a lowish profile).....BUT.....the Sequoia's tires have the speed rating which ostensibly gives you a tire compound able to handle the high heat of these circumstances.

In the end, I think I trust the raw lower weight loading and higher air volume (and thus heat absorbing capacity) of the Bronco's tires over the Sequoia's manufactures claims of loading and speed rating (and thus heat capacity)....or do I? Not that we would ever take the Bronco on this trip

What are Y'all's opinions?

I'll play along

My guess is that you are probably fine with either, and should have a lot more tire capacity left over in the Bronco. But for maximum peace of mind I would bump the Bronco up to 35 psi as that is the minimum "certified" pressure for LT tires, and keep the Sequoia at 38 psi.

As far as I can tell the P275/55R20 tire, in an XL rating (113 load rating), has a max load rating of 2833 lbs at max psi. But, since this is not mounted on a passenger car, we need to deduct 10% from the load carrying capacity, resulting in 2575 lbs.

Let's assume that the Sequoia has a 60/40 weight distribution, this is just a guess on my end as I couldn't find any numbers. So with 6700 lbs, that would be 4020 lbs on the front axle, approximately 2010 lbs per tire. So at max pressure there would be plenty of overhead.

I found a load inflation table from Falken that has this tire load rating in extra load (https://www.falkentire.com/load-inflation) and it looks like that at 36 psi, with the 10% SUV deduction, you are sitting at 2045 lbs load capacity, so that would be pretty close to the estimated weight on the front tires. I would probably want a bit more headroom than 35 lbs

Tire Rack has a good article on how the speed rating of tires can only be achieved when the tire pressure is increased accordingly: https://www.tirerack.com/upgrade-garage/how-do-i-adjust-tire-air-pressure-for-high-speed-driving. So having a V Rated tire, won't give you the V Rated speed capability unless pressure is increased. Based on the Tire Rack chart for V-Rated tires (https://www.tirerack.com/upgrade-ga...ent-for-high-speed-driving-vspeed-rated-tires) the full V Rated speed can only be achieved with a 5 psi increase from the standard pressure and at only 91% of the tires load rating.

In Germany, many car and tire manufacturers start recommending higher tire pressures once you start exceeding 160 km/h - 100 mph. That aligns pretty well with the Tire Rack article, and it makes sense as the article states that this is based on the recommendations of the European Tyre and Rim Technical Organization.

Taking for example my Volvo V60:
Curb weight per Volvo: 2064 kg - 4550 lbs
Tires: 235/45R18 Y Speed Rating (300 km/h - 186 mph), Extra Load rated

Below 160km/h - 100 mph
With up to 3 passengers:
Front 36 psi
Rear 36 psi

Max load:
Front 39 psi
Rear 39 psi

ECO:
Front 39 psi
Rear 39psi

Above 160km/h - 100 mph
With up to 3 passengers:
Front 45 psi
Rear 45 psi

Max load:
Front 49 psi
Rear 49 psi

ECO:
N/A

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[dgorsett]

Surprisingly it seems that both the European (ETRTO) and American (TRA) organizations that do these standardizations have the documents available only for paying members...Toyo still has an older chart on their website that should apply to other brands as well: https://www.toyotires.com/media/pxcjubjs/application_of_load_inflation_tables_20200723.pdf

You can also play around with the calculator here and change the OEM tire pressure to align with the actual weight: https://tiresize.com/pressure-calculator/

For fun I ran the last one at lowest pressure available, 26 on my OEM SLs and still had to run 38 on my Es. Rating was still overweight for the Bronco though. About 2000 lbs.

Running for C and D rated came out the same, LT rating must be what does it.

 

[Jtbob3]

Tire wear and heat are a big concern. But for vehicle manufactures vehicle stability is a bigger concern. Tire pressure affects vehicle stability in the real world, but importantly for them, standardized stability tests that they must meet. Manufacturer pressures will be specified first for stability, then wear/comfort. The tough required tests are not normal driving but accident avoidance type events. Tire sidewall stiffness (from both construction and pressure) makes a huge impact on these.

A tire dealer who does not set to manufacturer recommendations opens themselves up to liability if you have a rollover. For a tire not recommended for a vehicle (and therefore no OEM pressures) setting them high is much safer.

Someone mentioned the Explorer rollovers. Lots of finger pointing, but ultimately Ford recommended a pressure at the lower limit of stability to improve ride quality. Still safe, but no factor of safety. When people drive with under inflated tires, the stability becomes questionable and you get rollover.

I spent 15 years designing vehicle suspensions and doing stability testing. Now I work in motorsports where tires are our most influential tool for handling (get your pressures wrong and nothing else matters). The manufacturers will set pressures for stability, at the expense of ride, noise, off road performance, etc. That means you can certainly improve tire performance for your specific needs but likely at the expense of stability.

I’m not saying don’t set them lower - it’s your car, do what you want. Just understand the manufacturers had more to consider than just ride quality and wear.

 

[dgorsett]

For fun I ran the last one at lowest pressure available, 26 on my OEM SLs and still had to run 38 on my Es. Rating was still overweight for the Bronco though. About 2000 lbs.

Running for C and D rated came out the same, LT rating must be what does it.

For more fun I input my 265 LT LREs as OEM @ 35 psi (backed into it) and it spit out a load of 1890 pounds at that pressure. So I'm good to go with that.

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