The International Cessna^® 170 Association

I have a tire ply question. What is the difference between the 4, 6, and 8 ply tires? Why would you want to buy a tire with more plys? I am looking to replace the 600x6 on my 170. I am narrowing in on 700x6 - just a little confused about the ply question.

In our case the TCDS calls for 4-ply tires for all of the allowable sizes. I believe the plys refer to the number of layers of the rubber/chord make up. More plys means a possibly heavier and stiffer tire. Anyone correct me if I am wrong here.
David

David has it right more plys equal stiffer sidewall. Some would say that only 4 ply is approved for our plane. I never saw a 4 ply tire used on a 170. Mine are 6 ply.

IMHO from a practical stand point this becomes more critical where the tire has to fit in a close tolerance such as a wheel opening on a retractable gear.

FWIW, I've got 600 x 6 - 4 plys on mine.

A similar topic came up a while back. Some one said that there 8.00s were 4 ply, yet I have never seen anything but the 6.00s advertised as 4 ply. I am not sure there would be much difference.
David

N9149A wrote:David has it right more plys equal stiffer sidewall. Some would say that only 4 ply is approved for our plane. I never saw a 4 ply tire used on a 170. Mine are 6 ply.

IMHO from a practical stand point this becomes more critical where the tire has to fit in a close tolerance such as a wheel opening on a retractable gear.

I have 4 ply tires on mine. I previously had 6 ply and decided to try the 4s. I must say that the airplanes seems to land better with the more flexible sidewall, the 6 ply are just a stiffer tire which may be why Cessna recommended (required?) the 4 ply tires.

The TCDS says:
"202. (a) 2 Main wheel 4 ply-rating tires, 6.00-6, 18 lb. Type III (with reg. tubes) (b) 2 Main wheel 4 ply-rating tires, 7.00-6, +1 lb. Type III (with reg. tubes) (c) 2 Main wheel 4 ply-rating tires, 8.00-6, 10 lb. Type III (with reg. tubes)"

In other words, the tires do not have to be 4-ply construction, but they must meet 4-ply ratings. Additional ply-ratings may or may not be a stiffer tire depending upon several issues.

Tires are made up of synthetic rubber over a set of fabric "plys". The rubber protects the fabric from abrasion and makes a good friction surface for traction and braking, but it's the fabric that is the actual strength of the tire. In the old days with less sophisticated fabrics (everything from cotton, linen, and wool had been tried) the number of plys or layers of fabric denoted the relative strength and load-carrying capacity of the tire.
Modern tire construction utilizes more modern materials. Rayon, Nylon, and Polyesters are much stronger than earlier fabrics and fewer plys of modern fabrics will equal or exceed more plys of original materials. So mfr's came up with ply-ratings. Your tires may actually be constructed of only two plys, yet be 4-ply rated.
Another advantage of modern materials is how the tire behaves. Few plys can mean more flexible sidewalls. It can also mean cooler-running tires because flexible sidewalls are actually not conducive to tire longevity. The sidewalls are a weak area of any tire.
Anyway, the 4-ply rating is a requirement in order to support the weight and impact loads to be expected from our aircraft. Heavier aircraft will require higher ply-ratings. It does not hurt to use a 6-ply tire on an airplane with a 4-ply minimum rating. The additional plys do not make for a larger tire in any given size. But it's never OK to do the reverse. The TSO which tires are mfr'd under specify the maximum outside measurements, speed ratings, load capacity, etc. of aircraft tires. Still, it's a good idea to check for clearance in wheel wells, fairings, perform retract tests, etc. several hours after inflating the tire to max pressure, in order to allow the tire to reach it's max expansion and determine clearances, prior to flight.
The tire pressure should also be checked after 24 hours to adjust for natural loss due to "weep" holes which are part of the normal construction. This is especially true if tubeless tires are used. (Not approved for 170's according to the type cert.)

Out of curiosity, what brand / ply tires are the guys who are running 8.50-6 tires using? Looks like I'll be swapping sometime in the near future.

Goodyear 6-ply 8:50x6

I've read that some other brands can be all over the
place dimensionally (manufacturing tolerances).

My records show I have 617.8 hours on the 850s
and that they are 4 years / 2+ months old. To be
honest, they're basically worn out and I need a
new set....

FWIW, Desser Tire used to have free shipping (not
sure if they still do or not....?)

http://www.desser.com

Craig i should point out the 8.50s are not approve for the 170 so you will need some bases for their approval. 8.00s are approved.

Thank you all for the excellent advice and education on tires and tire plys. This is one heck of a web site. I have ordered two Michelin 700-6x6 from Desser. I also ordered the adjustable jack pad from Wag-Aero. This will be my first tire change on the 170. I did several on the Champ. I assume the tires are balanced. Is there anything in particular I should be on the look out for, i.e., shims, etc.?

Robert Eilers wrote:Thank you all for the excellent advice and education on tires and tire plys. This is one heck of a web site. I have ordered two Michelin 700-6x6 from Desser. I also ordered the adjustable jack pad from Wag-Aero. This will be my first tire change on the 170. I did several on the Champ. I assume the tires are balanced. Is there anything in particular I should be on the look out for, i.e., shims, etc.?

Always install new tubes when installing new tires. Don't buy cheap tubes, they leak air when made in China. Use a tablespoon of tire talc and roll the tire all around to distribute it (lubes the tube) but be careful not to breathe the dust. It's as bad for you as asbestos.
The tire will have a red dot on it's lightest place. Install the red dot on the tube (the tubes heaviest place) at the same location in the tire. If the tube has no red dot, then install the air valve at the tire's red dot.
Be careful when deflating/inflating split wheels such as these. Completely deflate the tire first. Inspect the bolts and if any defects, rust, deep scratches exist, then replace them. Torque them to the proper torque (150 inch-lbs) and stand clear when inflating the tire. If the bolts let loose, it can take off a hand or head.
While your airplane is jacked up (only inside a hangar, out of the wind, on a firm surface such as concrete or at the least a piece of plywood) keep a large block of wood beneath your axle, in case the jack fails. That way the axle will be less likely to be damaged. Don't jack your airplane anywhere near other airplanes. If it falls off the jack, it can damage other aircraft when a wing falls or the opposite wing lifts. Be sure to chock the other wheel before jacking.
Your old tire is likely stuck to the wheel. Avoid using sharp tools to pry the old tire off. Scratches on your wheel will weaken it and may damage the new tire bead. Inspect your wheel for cracks, scratches, corrosion, before installing a tire. Lay the deflated wheel on plywood and use a 2X4 across the tire sidewall to stand on to push the tire down off the bead, then work your way around the circumference.
It's best to chemically strip the old paint off for the inspection. I like to paint the wheel with silver lacquer after the inspection. It is cheap and can be easily found in aerosol cans. (Enamel can cover and hide a crack in a wheel. Lacquer will crack with the wheel and will be more obvious.) Don't get any paint in/on the bearings. I recommend waterproof grease for the wheelbearings. (Marine trailer wheel bearing grease is good.)
Be careful.

N9149A wrote:Craig i should point out the 8.50s are not approve for the 170 so you will need some bases for their approval. 8.00s are approved.

Don't worry, I've got a field approval in the bag.

I bought a set of 4 ply AirTrac 800's from Desser last spring. The last set was 4 ply also, either from Spruce or Chief. The catalog might not list them, but they got 'em- ya just gotta ask.

Eric

So you guys running 4 ply tires. Do you think the side walls have more give than the same size and brand tire with the same pressure but with 6 plys?