Tire thickness: 4 ply vs 6 ply

tawood

tawood

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Tim
I took my wheel pants off for winter / snowy runway flying, and now that its spring, I went to put them back on. It was then that I noticed that all three have been lightly rubbing with the top of the tires...
My plane currently has 6.00 x 6, 6 ply tires...I wondering if going to 4 ply would stop the rubbing (I would assume "yes", but you know what they say when you assume). I should note that Piper says OEM is 4 ply as well.
 
Aviation tire catalogs usually list external dimensions (when inflated). You might want to check them.
 
Are your tires actual 6 ply or simply a 6 ply rating?
 
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I don't know what you're flying but if it's not an experimental I think you're required to use the tires (including number of plies) that your POH (or parts manual or whatever) specifies. Maybe for just this reason.

Someone with more knowledge in this area might chime in to confirm or dispel that statement.

Edit: A quick search revealed this on the AOPA website "The approved tire sizes and ratings are included in every airplane owner's manual or pilot's operating handbook (POH)." An AvWeb article had similar verbiage.
 
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Isosceles said:
Aviation tire catalogs usually list external dimensions (when inflated). You might want to check them.
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Tried that...they only seem to list a maximum tire diameter, not actual (which might sound the same but my research shows it isn’t...for example, all 6.00x6 tires by all brands, regardless of ply numbers, are the same: 17.5").
 
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Aircraft Spruce was able to line up several of their 6.00x6 tires for me then told me the difference. They said the 4 ply rated tires all had a significantly smaller radius (3/4") than the 6 ply rated. I just got the shipping notice for 3 brand new 4-ply rated tires.

Anyone want to buy 3 slightly used 6 ply rated tires?
 
finding 6ply on aircraft that should have 4 ply is a common thing. people, including A&P's, think that more is better and the tire is stronger. the problem is the gear is designed for the flex characteristics of the tire it calls for. a higher ply rating puts more stress on the gear than it was designed for. in the opposite case using a 4 ply where a 6 ply is called for can cause enough sidewall flex to allow the rim to contact the ground.

bob
 
unsafervguy said:
finding 6ply on aircraft that should have 4 ply is a common thing. people, including A&P's, think that more is better and the tire is stronger. the problem is the gear is designed for the flex characteristics of the tire it calls for. a higher ply rating puts more stress on the gear than it was designed for. in the opposite case using a 4 ply where a 6 ply is called for can cause enough sidewall flex to allow the rim to contact the ground.

bob
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More stress on the gear? Does the weight of the aircraft carried by the gear change? Stress is still force divided by area isn't it? I'm just not certain I can believe that stress is greater in any meaningful way. Clearance problems? Sure. Increased weight? A little bit. Stress? not so much.
 
Clark1961 said:
More stress on the gear? Does the weight of the aircraft carried by the gear change? Stress is still force divided by area isn't it? I'm just not certain I can believe that stress is greater in any meaningful way. Clearance problems? Sure. Increased weight? A little bit. Stress? not so much.
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Yeah, I'm not sure I'd buy the "more stress" argument either...Still, my plane is proof you should stick with the right ply: wheel pants don't fit otherwise. And the added benefit is that my aircraft is about to lose 4.5 lbs. (Might not seem like much, but in a PA28, every little bit helps)
 
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timwinters said:
And a rougher ride while taxiing. Enough to matter? I'm not sure.
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At my home-drome, prolly enough to matter. The asphalt cracks from clay shrinkage are a rather prominent feature. I taxi at a slow walk...but there's this guy in a Cirrus who taxis at about 20 kts - you should see the nose gear bounce around. The cantilever arm bounces like popcorn in an air popper. My A&P said that he tried to talk to him about it but made no impression.
 
The Piper PA-14 Family Cruiser had a solid landing gear with no bungees or springs. The tires were the only shock absorbing item in the system. Fly Baby homebuilts also have no shock absorbers of any type. Seems in this case, stiff tires would certainly impart more stress to the gear.
 
Clark1961 said:
More stress on the gear? Does the weight of the aircraft carried by the gear change? Stress is still force divided by area isn't it? I'm just not certain I can believe that stress is greater in any meaningful way. Clearance problems? Sure. Increased weight? A little bit. Stress? not so much.
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your looking at it in a static sense vs a dynamic sense. when the rubber meets the road, the force put on the airframe is affected by the amount of deformation of the tire. a higher ply rating deforms less than a lower ply rating, therefore the force (mass X acceleration) transmitted to the airframe will be higher. the post about the pa-14 is a very good example. also, the gear on all the 150's and 172's is a good example. the only shock absorbing thing for the gear leg is the sidewall flex of the tire. aircraft tires are designed for a higher sidewall flex just for that reason.

bob
 
unsafervguy said:
your looking at it in a static sense vs a dynamic sense. when the rubber meets the road, the force put on the airframe is affected by the amount of deformation of the tire. a higher ply rating deforms less than a lower ply rating, therefore the force (mass X acceleration) transmitted to the airframe will be higher. the post about the pa-14 is a very good example. also, the gear on all the 150's and 172's is a good example. the only shock absorbing thing for the gear leg is the sidewall flex of the tire. aircraft tires are designed for a higher sidewall flex just for that reason.

bob
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It’s an interesting story. Shall we dig into it? What exactly provides the supporting strength of a tire? If you answer anything other than air then you’ve got the wrong one. The air pressure in the tire sets the amount of load (pressure * contact area) that can be exerted on any other part of landing gear. How could it be otherwise? You suggest the sidewall carries load. Fine. Deflate the tire. How much load does the sidewall carry? If the load exceeds the tire’s ability to provide support then and only then can greater stress be passed along to the gear.

As for you example of Cessna spring gear, well the spring provides shock absorption. That’s the whole point of spring gear.
 
Clark1961 said:
It’s an interesting story. Shall we dig into it? What exactly provides the supporting strength of a tire? If you answer anything other than air then you’ve got the wrong one. The air pressure in the tire sets the amount of load (pressure * contact area) that can be exerted on any other part of landing gear. How could it be otherwise? You suggest the sidewall carries load. Fine. Deflate the tire. How much load does the sidewall carry? If the load exceeds the tire’s ability to provide support then and only then can greater stress be passed along to the gear.

As for you example of Cessna spring gear, well the spring provides shock absorption. That’s the whole point of spring gear.
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and the working pressure on a 6.00 x 6 4 ply is 29 psi a 6 ply is 42 psi and a 8 ply is 55 psi. yes that is point of spring gear, but put solid tires on a cessna and see how long the gear lasts. the deformation rates of the tires ARE taken into consideration when designing gear. stop applying static calculations to a dynamic math problem.
 
unsafervguy said:
and the working pressure on a 6.00 x 6 4 ply is 29 psi a 6 ply is 42 psi and a 8 ply is 55 psi. yes that is point of spring gear, but put solid tires on a cessna and see how long the gear lasts. the deformation rates of the tires ARE taken into consideration when designing gear. stop applying static calculations to a dynamic math problem.
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Lol. Don’t even try to tell me to what to think. That is way out of line. I’ll think about things as I have been taught and have practiced for many years.

At least you have admitted that I am right that air pressure is a major factor. Now you should admit there is no measurable stress difference with four and six ply tires when they have the same air pressure. Now the next thing you should admit is that the gear doesn’t push up (load) any harder than the aircraft pushes down. It doesn’t matter if the aircraft is at rest or rolling. The stress on the gear is the aircraft load. Tire deformation has nothing to do with it other than the tire must support the design load.

One other thing. You’re guessing about the effect of solid tires on Cessna spring gear. That spring won’t be hurt by that application. The rest of the aircraft structure and particularly the mechanical instruments would be destroyed but the spring steel won’t be bothered.

HTH and HAND
 
Six ply tires also weigh more. I'd need to look it up but I think the 4 ply 8.50x10 I use is 6-8 pounds less per tire than the 6 ply.

Obviously with smaller tires the difference will be less, but it is still there.
 
When your aircraft was designed and approved with 4 ply tires, putting 6 plies on it makes it unairworthy.

YEAH, it is that simple
 
I went through this a while back. The manual stated 4 ply, which was also lighter by a few lbs, I went with 4 ply. They get changed once in a while anyway.
 
So I changed back over to the 4 ply rated tires this weekend...I love split rims, btw, as they make tire changes a breeze. I took this pic to show the size difference, as the 4 ply rated tires are not just less diameter and less weight, but also thinner:

upload_2018-4-2_7-42-46.png

4 ply rated on left, 6 ply on right.
 
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