8.00-6 tire ply rating?

[learaviator]

I was wondering what ply rating you would order for this tire? 6 or 8? I read a lot of threads about sizes but no opinions of what ply would work the best. Probably 75 % hard surface and the rest dirt or off airport.

[blueldr]

Considering the allowable gross weight of the C-170, I can't immagine any need for more than a six ply rated tire.

[learaviator]

I can't either but I was wondering if through experience, someone had an opinion about ply ratings versus air pressures and such. Really more on the off airport stuff than hard surface. Higher ply less air, lower ply more air? Vice versa? Just curious before I press the "Buy" button.

[GAHorn]

Ply-ratings are only that.... ratings. They do not necessarily mean what they seem to imply. (pardon that silly pun)

In the "old days".... (bluEldr will remember this and perhaps have a better explanation).... tires were engineered to increased strength of their casings by adding "plys"... layers of fabric.... within their vulcanized-rubber construction. The "Plies" or "Plys" are that actual strength of the tire,.... the vulcanized rubber is merely an anti-abrasion material intended to prevent the "wearing out" of those sidewall plies. (This does not apply to the "tread" area which may have different or additonal "plies" and/or "belts" to support tread contact area.)

The sidewalls of the tires take a great deal of punishment and heat due to the constant flexing of those sidewalls during each rotation of the tire. (Think about it: As the sidewall reaches the six-o'clock position they must flex outward as the weight of the vehicle settles upon the lower portion of the tire... and then must expand upward/inward again as the air pressure pushes the tread back upward at the 12-o'clock position. If you are travelling at 50 mph (Miles will calculate this for us shortly as to how many revolutions-per-minute to which this equates ) then the tire sidewalls generate considerable heat and wearing/tearing flexure. By increasing the number of "plies" the tire mfr'r could increase the durability of that tire. This was all started back when tire-fabric was made of cotton or linen.

When synthetic fabrics were introduced to tire-mfr'g the additional strength, flexibility, and heat-capacity of those synthetic fibers allowed the tire makers to reduce the number of "plies" of fabric necessary to reach any given strength-of-design. So they resorted to a "new" system of specification called a ply-RATING. So a 4-ply nylon or rayon-fabric tire might be as strong as a 6 or 8-ply cotton or linen-fabric tire. These days "poly-ester" is the more common fabric, but also steel, kevlar, and even carbon-fiber is possible in more expensive lines.

Why would mfr's want to reduce the numbr of plies? Because it saves weight, which makes the tire easier to balance. Imagine that 6-lb tire rotating at 60 mph and how a few ounces of rubber and unnecessary fabric can "pound" vibration into the airframe. It also makes a tire fit into a smaller fairing or wheel well if it's dimensions can be reduced. Or it can make a same-sized tire stronger by using stronger fabric to thin sidewalls which generate less heat.

If you are planning to operate a "heavy" aircraft then you'll want a higher ply-rating tire. In the old days, if you operated off of gravel, rock, thorny runways... you'd want a higher-ply-count tire because that would automatically provide you with a thicker tire sidewall .... which might provide greater protection against cactus/mesquite/etc. thorns and rocks, etc.

But with today's "rating" system, the tire may achieve it's higher rating ...not with additional plies... but with higher-thread-count fabric or with fabric of a different, stronger material. So, there may be absolutely no advantage for you from the standpoint of external abrasion-resistance or puncture-protection.... the only advantage might be in weight-carrying ability. This might be important if you''re using a 800 X 6 tire on a Caravan or Cessna 206/207 hauling freight... because it might give you longer service between re-treading costs.... (re-treads benefit more from higher ply-ratings than new tires in that regard)... But it's also going to give you harsher ride/landing conditions and will subject your landing-gear/wheels/airframe to greater vibration abuse than it might receive from a tire of lesser-but-adequate/suitable rating. (Higher-ply-ratings also weigh more than necessary and require more energy to "spin-up" on touchdown, and therefore "shock" your landing-gear/gearbox, etc. more than necessary.) If you utilize rough-stirps with rocks/thorns often, a closer examination of the actual tire-tread-design is probably more beneficial. For instance, look at the "Aero Trainer" tread versus some of the more robust designs of higher-priced tires, and you will likely see that it's worthwhile to spend a little more on the better tire than to attempt economy of purchase.

The Cessna 170 type certificate specifies a 4-ply tire. Since it's a 2200 lb gross wt aircraft, a 4 ply tire is adequate. ( The original size 600 X 6, 4-ply typically is rated around 1150 lbs capacity. 6-ply is typically rated around 1750 lbs, and 8-ply are usually rated around 2350 lbs. Larger sizes are rated higher.) Anything more than that is probably a waste, in my opinion, but in some sizes/brands no other choice may exist. My own choice of tire on my B-model is a 600 X 6 Desser "Aero Classic" with a "Vintage" tread design which only comes in a 6-ply rating. With my landings it makes little difference.

Your question regarding TIRE PRESSURE: The tire pressure to use is a function of aircraft operating weight versus tire "footprint" (which is a function of size.) A common error is when an operator reads upon the tire sidewall a particular pressure-rating and assumes that's the pressure to use for his airplane. NOT CORRECT. The number emblazoned on the tire sidewall the mfr's suggested or design-pressure for that tire's maximum weight-carrying capacity. It has little to do with what a specific airplane should use. For example: A tire sidewall that indicates 40 psi at 2350 lbs.... means that the tire's maximum weight carrying capacity by design is: 2350 lbs of weight... but only if the tire is inflated to 40 psi. (Less pressure will allow the sidewalls to flex more and therefore generate more heat than the tire material is designed for, and more pressure will exceed the construction-strength design of the tire.) Incorrect tire pressures contribute to greater tire wear and to greater possibility of tire damage from rocks/abrasion etc. (Higher pressure than necessary will contribute to "bruising" of the tire carcass. Lower pressure will subject the sidewalls to damage not only from heat but also from ground-hazards. The sidewalls are the most fragile part of a tire's construction. The tread area can take MUCH more abuse than the sidewalls. IN fact, if you ever damage a tire's sidewalls... throw it away. Never attempt a repair to a sidewall, from anything including simple punctures. Sidewall punctures and damage is reason to condemn a tire.)

The specified pressure for Cessna 170 600 X 6 tires is 24 psi. Tailwheels is 34 psi. Most larger sizes are not significantly different, but if you use a significantly larger tire size you will likely discover that slightly lower tire pressures will give the tire the correct "profile". Look for only slightly bulged sidewalls at the 6-o'clock position and from the front or rear observe the tread area. You want it to be "flat" not rounded or concaved where it contacts the surface.

[PilotMikeTX]

Technically isn't the 4 ply the only one that can be used without a field approval since it's specifically listed in the TCDS?

Also, if you were too cheap to buy bushwheels and didn't really have any intention of landing on big rocks, but gravel bars or other unimproved strips weren't out of the question, wouldn't sing a 6 ply (at weights sufficient for a 4 ply) tire have a stiffer carcass and be able to withstand operating at a lower pressure for a wider footprint without deforming past it's limit?

Are the tire pressures listed in the POP (Pilot Operating Pamphlet) loaded or unloaded pressures? I think I remember reading somewhere that if unloaded pressure is specified, then add 5% when servicing a loaded tire (yeah, that's only 1.2psi on these guys...)

[GAHorn]

Generally, unless otherwise stated, specifications are for aircraft at maximum operating weight. (2200 lbs)

Tire plies can be controversial. It's my understanding the the ply-rating specified in the TCDS are minimums for the sizes indicated. (Remember, tire plies are also weight-carrying specifications.) Like using a fuel of higher-than-necessary octane,.... tire Ply-ratings are also minimums. If the TCDS specifies a 4-ply rating tire, a 6 ply may be used, however the reverse may not because the weight-carrying capacity would not be met.

Remember also ... the TCDS gives tire weights for the "ratings" of certain tires, but actual tire weights must be used when installations are made. (There also errors that are sometimes published in the TCDS. I've corrected a few over the years with FAA and they subsequently issue a new Revision. The listed tire weights in Revision 54 are typographically misleading, and they refer to BOTH tires together, not individuals.)

[Jr.CubBuilder]

I got the 8-ply goodyears they seem to be working out fine.

I picked the 8 over the 6 because I do land on dirt strips sometimes and I wanted the stronger carcass for puncture resistance. I'm sure there's pros to the six ply besides the weight, but a flat tire sucks.

[PilotMikeTX]

That's good to know. With my luck I'd get ramp checked by some smart SOB who had a 170 TCDS in his pocket. I've only found one 4-ply, and it cost a couple bucks more than the 6 ply. I think the 8.00x6 is what is used on the PA-23 Apache/Aztec, so that might explain why there are more options in the 6 and 8 ply varieties. I'd really like 8.50s but don't need the paperwork headache for some silly tires.

[blueldr]

The members of TIC170A should be eternally grateful that George is not getting paid by the word on these forums. "Keep up the good work", George.

[db99577]

Judged by George’s frequent use of large obscure words, we’re thankful it’s not per pound either.

[gparker]

Is the weight rating for each tire, or for the total weight of the plane ?. I see 4 ply tires are rated only to about 1200 lbs. Would that be 2400 for two tires,not including the tailwheel, and therefore ok on our birds? I ask because a friend has two Flightcustom II 8.00x6 4 ply tires with only a few landings and he said I could have them.

[Bruce Fenstermacher]

Greg, what you suspect would seem to be the case, that it is the accumulative load of all the tires that counts.

What is important regardless is that 8:00 x 6 4 ply tires are the specified tires by the TCDS. So put them on and forget about it.

[Ryan Smith]

Please forgive the dumb question, but assuming a 170 with Cleveland wheels (if it matters), will that wheel support both tube and tubeless tires? Any advantages/disadvantages to running either? I assume perhaps that with tubeless tires, there is a possibility of breaking the bead on a hard landing and blowing a tire, quickly ruining your day.

[Bruce Fenstermacher]

The wheels are not designed to run tubeless. All small aircraft wheels I'm aware of are split rims and won't hold air.

[Ryan Smith]

Fair enough. Not being overly familiar with the mechanics of aircraft wheels, that makes sense that they would be split rim. I saw some tubeless Goodyears on Aircraft Spruce and was curious.