My club has two pairs of planes that are similar in weight and configuration, but the POH has way different pressure specified for tire pressure for the same size tire.

For instance, the 172S and Archer both top out at 2550 lbs, but Piper says to use 24 psi while Cessna says to use 38. The Cessna is newer so the POH specifies this is for a 6 ply tire, while the Piper spec is for a 4 ply tire.

Why would the Cessna pressure spec be 50% higher? Is it just because the spec was for 6 plies?

Less pressure in the Piper tires allows for a softer landing............so the wings don't break off

The main difference between 4-ply and 6-ply tires lies in their inflation pressures and load capacities:

- **4-Ply Tires**: Typically have a lower maximum inflation pressure (around 22 psi for some models) and a lower load capacity (e.g., 1,320 lbs at 22 psi). They are lighter and more flexible but less durable under heavy loads[2][4].

- **6-Ply Tires**: Have higher maximum inflation pressures (often around 32–44 psi) and greater load capacities (e.g., 1,640 lbs at 32 psi). They are sturdier with thicker sidewalls, making them better suited for heavier loads and rougher conditions[2][6].

Higher ply ratings require higher inflation pressures to support their increased load-carrying capacity effectively. Always refer to the manufacturer’s guidelines for proper inflation based on the tire's load range[4][6].

Sources
[1] new tires 4 ply vs 6 ply thoughts? [Archive] - ATV Riders Forum https://www.atvriders.com/vbb/archive/i ... 79607.html
[2] 4-ply or 6-ply Front Tires? - TractorByNet - Tractor Forum https://www.tractorbynet.com/forums/thr ... es.126949/
[3] Trailer Tires Ply Ratings Guide: What You Need to Know https://www.tires-easy.com/blog/trailer-tires-ply/
[4] What is Ply Rating for tires? Here we explain it clearly https://www.sttc.com/ply-rating/
[5] What Is Tire Ply Rating? | Help and Advice | Tirebuyer https://www.tirebuyer.com/education/tire-ply-rating
[6] Question: Rule of thumb for tire inflation vs ply rating? https://forums.yesterdaystractors.com/t ... ng.862750/
[7] 4 Ply Vs. 6 Ply, Big Difference? | YFZ Central https://www.yfzcentral.com/threads/4-pl ... nce.57178/
[8] Are 4Ply Bias Trailer Tires as Strong as 6 Ply Trailer ... - etrailer.com https://www.etrailer.com/question-58670.html

Is there any reason you can't run the 6-ply at 24 psi for soft fields?

Tire inflation pressure is roughly equivalent to ground pressure.

That’s a really good question and don’t know the definitive answer, but I sat through a presentation by a Michelin rep years ago. My recollection is the answer would be no because the sidewalls of the tire need the inflation to dictate the amount of flex.

He showed lots of testing data, and as I recall under-inflation creates significant heat in the sidewall. YMMV, as all from my memory of 10 years ago.

It certainly does. Most auto tire blowouts are caused by underinflation followed by weakening of the sidewall and eventual failure.

I'm not convinced that a plane going to backcountry strips with short taxis can heat the tire enough to make a difference.

I started down this rabbit hole when I noticed the difference. We have a big club with a lot of pilots, and a lot of pilots moving between planes regularly. I figure that making things as consistent as possible makes it more likely that they'll get things right.

It certainly does. Most auto tire blowouts are caused by underinflation followed by weakening of the sidewall and eventual failure.

I'm not convinced that a plane going to backcountry strips with short taxis can heat the tire enough to make a difference.

Next tire change go to 4 ply, for consistency. Probably a lot cheaper too.

Definitely not going back to 4 ply. It's not a lot cheaper and the club went to 6 ply for all of them a while ago. Plus I think the 172S requires 6 ply tires, and I'm trying to increase fleet commonality, not decrease it.

Sorry.

172N with 2300 Gross is 4 ply tires. Nose 31 psi, Mains 29 PSI.
When the Air Plains STC is installed for gross weight increase to 2550 it requires 6 ply tires and also higher tire pressures. Nose 45, Mains 38 PSI. Quite a change percentage wise compared to the percent increase in gross weight.

But for the 6 ply vs 4 ply why should there be any pressure change? I would think the stiffer sidewall should need less pressure for expected tire flex in the certification drop tests but it is opposite.

I think low pressure is related to chafing of the tubes from the tube moving inside the tire and eventually getting pinholes at higher weights. Enough talc on tube installation should help prevent this.

The other issue is with low pressure and heavy braking the tire can rotate on the wheel halves shearing the valve stem off. It's a real problem with J-3 Cubs that have 8.00X 4 wheels and already operate at low pressure 15psi or less. Frequently airplanes that have wheel pants seem to never get the tire pressures checked. I've seen many with the wheel pants dragging in the grass as they taxi to the runway with almost flat tires. The tires are worn on the sidewalls from rubbing on the pant edges. Cracked tire sidewalls at annual.

There was a discussion about this a while back either on here or some other forum. IIRC, one of the tire manufacturer rep's said that the manufacturer of the tire sets the range for the tire pressure and the aircraft manufacturer then adjusts that number for each particular wheel. The published tire pressure will take into all kinds of factors from performance, strength and angle of the strut, etc.

It's been a while, but it seems like they used a PA28-181 as an example that has a different pressure between the mains and the nose with the same tire.

What's the ratio of Pipers losing wings, versus Beechcraft losing wings?