Tire Pressure

[Robert Eilers]

I recently installed Michelin 7.00-6/6/120 tires on my C-170. The paper work that came with the tires did not include recommended tire pressures. The Desser tire chart reflects 38 lbs for a 7.00-6/6 tire - unloaded. Adding the .04% for a loaded tire the pressure I get is 39.52. Does this seem reasonable to those of you operating on 7.00-6/6 tires? Seems high to me. I have been operating on 24 lbs - previous pressure for the 6.00-6 I replaced.

[Robert Eilers]

I am still tracking down an answer to this tire pressure problem. After doing some research, I have begun to wonder whether tire pressure is set by the wheel, i.e., Cleveland, or the tire mounted on it. The Mechilins I bought do not have any tire pressures stamped on the tire. The paperwork did not contain recommended tire pressures. The old/original owners manual reflects 24 lbs., but that was also on 6.00 x 6 tires. The Desser Tire chart suggest 38 lbs for the 7.00 x6 tires I installed. The tires seem to perform fine at 24 lbs.

[N2865C]

I don't have any official answers, but I do know that Cessna Spring gear doesn't like over-inflated tires. 140's and 170's both use the same wheel and 6.00x6 tires. On the 140 tire pressure is 16 psi vs. 24 psi on a 170. It's a function of weight. In both cases correct pressure yields a tire that looks "just a tad" under-inflated. The softer tire helps absorb some of the "spring" from the spring gear. When I bought my 120 there was 50 psi in the tires . Needless to say my landings were very interesting until I figured it out.

[GAHorn]

The recommended tire pressure for a given airplane is derived by a combination of factors. One of the most important is the pressure required to give the maximum footprint for the gross weight of the airplane. This is directly affected by the aircraft weight and within reason, to a lesser extent the tire size. This not only gives the best wear for the tire, but also it gives the best coefficient of friction for braking while also offering a reasonably low rolling resistance. (Higher pressures will cause the tire to wear faster in the center of it's tread and subject it to a greater chance of cuts, bruising, (from objects) and possible scorching due to skids. It will also be more likely to cause "thumpers" when landing.
Lesser pressures will cause the tire to suffer excess wear near the outer edges of it's tread, and also subject it to possible heat and cord damage from excessive flexing of sidewalls. It will also subject the weakest portion of the tire (the sidewall) to an increased danger of object damage, and in the most extreme cases can lead to sidewall cuts and bruising imposed from the wheel itself. An even greater risk of an underinflated tire rolling off the wheel also exists, and lastly, the tire can actually slip it's position around the wheel which in-turn can lead to catastrophic failure when the tube is dragged along with the tire until it snaps/cuts off it's valve stem.)
The airframe mfr's derived pressure takes all this into consideration, and also considers the operating limitations of the tire design. (This is one reason the tires are made to meet TSO standards, so that airframe and tire mfr's are singing from the same hymnbook.) The pressures stamped on the sidewall of the tire is that at which the tire is capable of handling the most weight while still meeting it's speed rating. In other words it's not a recommendation for that tire size for all installations....it's a limitation of that particular tire in any installation. (That limitaion pressure stamped on the sidewall also considers materials, plies, and other factors.) It is NOT the pressure at which the tire should be operated regardless of the installation aircraft.
The mfr of the AIRFRAME supplies the recommended pressures based upon the type of landing gear and the weight of the aircraft, and the braking capability of the aircraft's factory supplied brakes. In fact, not following the airframe mfr's recommended tire pressures can negate the published runway field performance of the aircraft, not only for takeoff, but for landing and braking and some other performance matters including operating weights.
Several other factors can come into play, but again the primary considerations are aircraft weight, aircraft runway operating speeds, aircraft field-length performance, landing gear type, and tire footprint.
In other words: Do not use the pressures stamped on the side of the tire except as a MAXIMUM operating pressure and in accordance with the specific recommendation of the airframe mfr for which that tire is approved. Always use the Airframe Mfr's recommendation for tire pressures.
Now there's some differences if any tire size other than the standard is mounted on the aircraft, but here's the effect of a different-than-standard tire size: A larger than standard tire size (i.e. anything larger than 600-6 for our C-170's) will provide greater footprint and greater air capacity than the standard tire. Theoretically a larger tire will require slightly LESS pressure than the standard, not greater. This is because the larger tire will have a greater footprint at lesser pressures (due in turn because it will have a greater load-carrying capacity...yet will actually be carrying a lesser weight than maximum design for that tire.) If larger tires are inflated to the same pressure as smaller tires....yet no change in weight has occurred...then the tire footprint will wear excessively in the middle of it's tread because the tire will see it as an over-pressure condition.
Of course, this is again a case of measuring something with a micromenter, marking it with a grease-pencil, and cutting it with a hatchet, when it comes to installing the only-slightly-larger tire sizes already approved on the type certificate for our aircraft. I doubt most of the tire gauges used by the average pilot is capable of measuring more accurately than to the closest 2-4 psi anyway. (In fact, it's more important to use the same gauge each time the tire pressures are checked than the actual accuracy of the gauge being used. As long as the same gauge is being used consistently, then any abnormal tire wear can be easily assigned a lesser or greater recommended pressure using that particular gauge and while operating at that particular aircraft weight. See how this can get?)
Increased plies simply increase the strength of the tire carcass and allow it to carry higher air pressures, which in turn allow it to carry more weight. But if you are still installing it on a Cessna 170 operating at 170 weights, guess what pressure it should be operated at? Yep. The same as the lesser ply tires recommended in the type certificate. You can almost think of it like buying 100 octane fuel for your 170. It's more than you need, and more than you can use, and it doesn't do anything more for you. (Well, mostly that's true. It does make for a stiffer tire sidewall and can add a small amount of additional protection from punctures due to thorns, etc., but that's about all. It also makes for firmer landings and adds weight to what the airplane has to carry around. But some popular aircraft tires only come in 6 ply, or at least the ones usually on sale are.)


PS- The standard tire pressures for our aircraft are 24 psi Mains, and 34 psi for the 3200 Scott tailwheel. I recommend using those pressures for all tire sizes approved on the Type Certificate.

[Dave Clark]

Gee George, great narrative and I hope you took a breath in there somewhere

I suppose there is some difference in what the airplane likes between the early and later gear due to the stiffness difference there. I usually run my mains at 28-32 (my gauge) with Goodyear Flight Custom lll's 6:00X6 and 180 gear. At that I get pretty even wear. Over 32 and it seems to be a little squirrelly landing, under 28 and it's a bit hard for this old man to push and I know it's time to add air. The airplane is usually loaded near gross. If I flew it light I'd probably like it at 24-28. Like you said, operating weight matters. My van likes 60 lbs when light but 80lbs when loaded and towing my RV. In fact the maximum load rating is only achieved at the highest rated pressure. My tw tire is run between 35 and 45. I think my 180 gear makes the tail a little heavier plus I seem to always have a load aft.

I think you can buy a pair of Michlens for $50 on eJunk??? At least I've seen them listed at that. I wonder how well they are performing. That's a heck of a price.

[Robert Eilers]

Thanks gahorn,

Great lecture on tire pressures. I feel very good about the tire prssures I am using now.

[GAHorn]

Gee George, great narrative and I hope you took a breath in there somewhere

In a former life I was a Pearl-diver. (Hopefully no one present knew her.)

[blueldr]

Pearl? I knew her well. Great sport! Loved to go swimming. Was crazy about divers.

[blueldr]

P.S. She spoke very highly of you, George. Something about technique.

[N170CT]

As usual FWIW.....I have flown my 170 with 800 tires and pressure so low it would not register on a common/cheap stick type pressure gauge with absolutely no problem. (This was when I was in the process of buying the airplane and did not realize the owner was not checking the tires) Today i am flying with smaller tires and never less than 30 PSI in the mains and 35 PSI in the tail wheel tire also with no problem. Based on this, I wonder if some problems with aircraft handling attributed to tire pressure is not, in fact, due to something else such as wheel alignment. Or perhaps high/low tire pressure accenuates an alignment problem. Just a thought .
chuck

[mrpibb]

I'm running the same mich 700/6/6. 24 psi has been working real good. I put them on when I bought the airplane over two years ago, I have to say I have close to 1000 landings on the with 600 of them on pavement. They are not even 50 percent worn yet, been very happy with them, I would rate them up there with the GY flight customs.