I frequently hear/read people talking about the danger of sheared valve stems when running low PSI in aircraft tires but I have yet to meet someone with real world experience If it's happened to you I'd like to hear your story. Please include the type A/C and tire as well as the psi you were running if you remember. I've also wondered if a tire like a 29" Airhawk or 26" Goodyear has a stiff enough sidewall to still provide some lift to the wheel with zero PSI as long as the bead doesn't break from the wheel (I'm sure the weight of the A/C makes a difference). If so, it seems like you would still be able to fly the A/C home if need be.

Been there, used to run 8 lbs pressure in Goodyear 26 tires on Super Cub. I sheared valve stem taxiing.

Sidewall is not stiff enough to taxi and tire collapses completely. Now I run 10 lbs minimum in Cub's current Goodyear 26 inch tires. But like to keep them at 12 for more peace of mind.

Brad

I can't comment on the sheared valve stems, but I do have some experiences with the Goodyear 26s concerning this subject. I recently upgraded the C180 from 7.00x6s to GY26 and Cleveland double puck brakes. Combined with big feet and having to work the rudders aggressively on windy Colorado days has caused me to accidentally land with the brakes locked on a asphalt runway. That has to be an incredible amount of torque. With tires at 15psi, I've never had an issue with the tires slipping on the wheels... although I think having a couple short strips of adhesive helps with that.

I've made a conscious effort to ensure my toes stay at the bottom of the pedals now though.

Yes, it happens.

As I've pointed out in other threads, ignorance of tire pressure is almost universal amongst pilots in most of this country.

Want proof? Next time you're at a fly in or airport, check out how many airplanes you see which are equipped with "hub caps" which cover the valve stems.

So, how often do you suppose THOSE pilots actually check their tire pressures?

The KEY to operating low tire pressures is maintaining a safe tire pressure, and to check pressures regularly. Inner tubes actually pass some air at the molecular level through the tube....not much, but.... This is one reason why many jets run nitrogen in their tires....nitrogen is a larger molecule than oxygen, and the high pressure in jet tires amplifies the pressure loss in these tires.

IF you want to run low pressure in your tires, figure out how low a pressure you can run safely. That does NOT mean the lowest possible pressure...it means a pressure high enough that a little pressure loss won't cause problems.

Then buy a GOOD quality air pressure gauge (I use digital pressure gauges calibrated for low pressure) and check pressures regularly. Every pre flight? Not necessarily, but figure out a schedule, and stick to it.

A lot of pilots think that they'll notice low pressure in a tire by seeing what appears to be a "flat" tire. In fact, I've seen tires on aircraft with pressure so low a standard gauge wouldn't register it (hence the logic for a digital low pressure gauge), and the tire didn't appear low on a visual inspection.

For lighter planes (cub, scout, etc) with tube type tires, I run pressures at 12 psi. If the tire sags to 10 psi, it should still be okay, but I'll catch that and re inflate to 12 psi ASAP.

For a heavier plane, like a 180/185 or 182/206 I keep pressures to 24 psi. Again, if pressure creeps down a bit, I've got a little cushion.

Slippage marks are also a great idea, whether you're running low pressure or not.

But, I've pre flighted airplanes and asked the owner/pilot what pressure he's running in tires, with a response of "oh about 35 psi. A check of tire pressures revealed pressures in the low teens.....last time pressures were checked was at the annual.....if then.

FWIW

MTV

I wouldn't compare Gar aeros to ABW first off.
29s used to slip on gar-aeros pretty easy. Too easy. Could kick them off when you let air out. Needed to stake the tires to the wheels with screws to run soft. It takes ALOT more air to put a 29 on a ABW 10x10. The bead seat is wider/thicker on the ABW wheel. Only takes one install to figure that out. Better yet- try to break the bead on one.

FWIW
I haven't ran over 10-12lbs in the skywagon with this setup. 8 while working it in fall. No slip yet. Maybe this year….

ymmv

I saw a friend's 185 with 26 GYs running 15psi, the big brakes. We were talking about this very question, and upon looking at his stems one was clocked over 45 degrees *(about *this* close to shearing).

So yes it happens.

Ps. I have read lots of other reports too. The general gist is:
If you stay above 18-20 psi you can be sure you'll be safe in the medium-term. Below that pressure it seems the weight of the plane and the braking force regularly used comes into play. It seems like it happens slowly at higher pressure, slowly creeping around, so you get a chance to see the problem and fix it.

A similar thread is taking place on maulepilots.org and here is one of my posts based on familiarity with your plane.

On a Maule, I dont use pressure, I use bulge which is dependant on load and density altitude.
Starting with 8.50s for normal use on a Maule, the outer rib is barely contacting the tarmac at about 20psi, Ive run them at 12psi but my guess is that 14 might be better. You dont want to wack the rim on a rock.
Blimp tyres are much stiffer so 12 or even less should be good but again I eyeball.
If pressure is too low, a lot of drag on the ground is induced.
These were originally built as the single castoring wheel under a blimp when tethered at the mast and thus were tso'd.

If the hubcap screw holes happened to go all the way through the rim, a sharpened longer screw could protrude into the tyre bead a quarter inch and the tyre will not slip at low pressure.

Make sure the calipers are on the front so a squeeged tyre at inside bottom cannot grab the caliper.

maules.com wrote:A similar thread is taking place on maulepilots.org and here is one of my posts based on familiarity with your plane.

On a Maule, I dont use pressure, I use bulge which is dependant on load and density altitude.
Starting with 8.50s for normal use on a Maule, the outer rib is barely contacting the tarmac at about 20psi, Ive run them at 12psi but my guess is that 14 might be better. You dont want to wack the rim on a rock.
Blimp tyres are much stiffer so 12 or even less should be good but again I eyeball.
If pressure is too low, a lot of drag on the ground is induced.
These were originally built as the single castoring wheel under a blimp when tethered at the mast and thus were tso'd.

If the hubcap screw holes happened to go all the way through the rim, a sharpened longer screw could protrude into the tyre bead a quarter inch and the tyre will not slip at low pressure.

Make sure the calipers are on the front so a squeeged tyre at inside bottom cannot grab the caliper.

Jeremy,

Those observations are pretty subjective, but if you really know what you're looking at......

A couple of observations, though:

The shape of the Goodyear 8.50s is very different than that of the MacRearys. Which one are you referencing here?

Pivoting a tailwheel airplane around 90 degrees or more to park can put additional load on the outboard tire.....so pay attention to that if using this technique.

MTV

Mike, the groove/outer rib reference can be used for either manufacturer and in fact size too.
Set a pressure with gauge, note heavy or light, note the reference distance of rib, groove, sidewall markings or any of the above and from there on you can estimate the required pressure for that flight's mission within a couple lbs when you glance at it preflight.

What's the best bush fix for a sheared valve stem? Are you guys that are running Oversize tube style tires carrying a spare tube?

ABW 10" rims have serrations milled into the rim to prevent the slippage.
Old GarAeros and Schneiders have countersunk screw holes to accept screws into the bead.
GW26" and GY or Mc can be held from slippage via extended screws in hubcap holes.
I have never had a torn stem in thousands of landings in rough conditions.
Pro-active prevention is the name of the game.
Re-active attention and the game's not the same.

I just run stock size tires on my P172D, and with wheel pants, they're pretty inconvenient to check and air up--not something I do for every flight, for certain. So yesterday I did that in anticipation of going off to OSH this Saturday, after not checking them for a couple of months, just before the La Garita fly-in. Results: 18 psi in both mains, 12 psi in the nose. Way low. So yeah, they do lose air--and yet they didn't "look" squishy.

Cary

maules.com wrote:GW26" and GY or Mc can be held from slippage via extended screws in hubcap holes.

Pro-active prevention is the name of the game.

Exactly what he said. Once you have a flat you're already saying "Oh, bother!" Better to prevent.

Here is a link to the extended screws trick:
http://bearhawkforums.com/forum/tools-technics-and-best-practices/6365-bead-lockers-for-running-low-tire-pressure-with-inner-tubes