# Tire sizes

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##### Well-Known Member
Can someone with more experience than me explain what the heck the numbers in an aircraft tire specification (e.g 7.00x6) actually mean?

#### wally

##### Well-Known Member
Hi,
The first number is how wide the tire is - I think it really is some kind of formula about width to height but that may be car tires, not sure.

The second number is how big the hole is through the middle of the tire or the diameter of the metal wheel that tire fits.

Wally

##### Well-Known Member
I was sort of afraid that was the case. So:

1) How is diameter determined? Is it just convention; that is, a 6.00x6 is always 15" (or whatever)? Or is it even going to vary from manufacturer to manufacturer?

2) What's with 15x6.00x6? Is this the "three part" usage, meaning an actual 15" diameter?

3) What does "nominal" mean in this context? Should it be read as "approximately" (nominal bolt sizing) or "by convention" (the size of a 2x4)? If the latter, how big is the difference?

4) What's with tubeless tires? They seem like a good idea all around, and they don't seem to be catching on; or at least, availability still seems limited.

#### pwood66889

##### Well-Known Member
Regarding #4, Add; Aircraft tires have to withstand side forces from cross wind landings, etc. That would "brake the bead" and cause grief.
Percy in NM, USA

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Tubeless tires, like those used in automobiles, require a rim that is sealed, and the tire must be rolled over the rim with a rotating tire lever. The tire sizes used in aircraft are pretty large compared to the rims, and would be nearly impossible to roll over the rims. So, rims are split to install and remove tires. How to seal them? No good reliable ways exist, except using a tube. So that is what is done.

Tubes would be bad on highway equipment due to heat build-up, but aircraft tires run very slowly in ground operations, and then only briefly at high speed as the aircraft takes off or lands. Little heat buildup occurs.

According to my copy of Pazmany, 5.00-5 tires (C152 and many homebuilts) is 14.20-13.65 diameter, 4.95-4.65 wide. 6.00-6 (C172, PA28) is 17.50-16.80 diameter, 6.30-5.90 wide.

Billski

##### Well-Known Member
(I'll post this to a new thread if there's no response here, or people prefer; just trying to keep things compact.)

So I'm working on a design that customarily uses 8.00x6 tires, and I will be using it for mild off-airport use (less than 50% pavement use, but mostly nice grass). I'd like to strip off a few more pounds; even a 5 lb or 10 lb difference is tempting.

Would it be reasonable to consider using 8.00x4 (Piper Cub) tires/wheels for this? The total weight savings would be about ten pounds. The smaller wheels decrease static load rating, braking energy dissipation, and braking torque, but it should not be a problem for this design. The only problem I see is that tires are significantly more expensive, and that there's really only one size available (24x10x4s exist, but are unaffordable). Anything else?

#### orion

##### R.I.P.
The three number designation refers to the tire diameter at rated pressure, the maximum tire width and the wheel size, which refers to the bead seat diameter (I think).

#### Dan Thomas

##### Well-Known Member
I was sort of afraid that was the case. So:

1) How is diameter determined? Is it just convention; that is, a 6.00x6 is always 15" (or whatever)? Or is it even going to vary from manufacturer to manufacturer?

2) What's with 15x6.00x6? Is this the "three part" usage, meaning an actual 15" diameter?

3) What does "nominal" mean in this context? Should it be read as "approximately" (nominal bolt sizing) or "by convention" (the size of a 2x4)? If the latter, how big is the difference?

4) What's with tubeless tires? They seem like a good idea all around, and they don't seem to be catching on; or at least, availability still seems limited.

The first group, the 6.00, is section width. Measure the useable tread, even that bit that reaches down the side of the tire, and see the six inches. The second size, the 6" is the wheel's bead seat diameter. The hole in the tire will be smaller so that it jams on the bead seat so braking forces won't spin it on the wheel. There will be tolerances for both of these sides.

A three-part number, like the 15x6.00-6, is giving the tire's actual diameter. In the case given, that is a smaller-diameter tire used on airplanes likle our 182RG where the wheels have to slip into rather small wheel wells. And they cost twice as much as a 6.00-6.

A third factor is ply rating. 6.00-6 might be four or six ply rated, while actually having fewer fabric plies than the rating suggests. It comes from a time when the fabric was cotton or linen instead of nylon, which is stronger. More plies means a stronger and stiffer tire for more weight and pressure.

Larger aircraft use tubeless tires. They don't stretch them over the flange, like we do with car tires. The wheel splits and has seals to prevent the escape of air (or nitrogen, which they commonly use to reduce the fire risk). Off-highway earthmover wheels and tires have been made like this for a long time already, and I used to sell these in the '70s. They had a removable lock ring, a flange, and a bead seat ring that fit over an o-ring in the wheel itself. A BIG o-ring. These came in sizes as big as six feet or more.

The use of tubes in light-aircraft wheels is an anachronism, like the felt grease seals that brand-new Clevelands still come with. Automobiles and trucks abandoned felt in the '30s for leather, and leather in the '60s for nitrile. And then they abandoned grease in many wheel bearings for oil, since the synthetic seals were so good. Oil stays with the bearing instead of being squeezed out, and carries heat away better. Cleveland could do the same, with sealable two-part wheels and oil-filled bearings. But it might cost a little money, and their parts sales might drop off. Tsk.

Airplanes still use magnetos, too, when a self-contained electronic mag would be better and probably more reliable. And we use Buna (SBR) o-rings in gear struts and many other places, while for decades other industries have had Viton, silicone, fluorosilicone and a bunch of other magic compounds that would outlast the rest of the airplane if we were allowed to substitute them. "Advanced Flight Technology." Yeah, right.

Dan

#### PTAirco

##### Well-Known Member
Regarding #4, Add; Aircraft tires have to withstand side forces from cross wind landings, etc. That would "brake the bead" and cause grief.
Percy in NM, USA

Surely a car tire travelling at 70 mph in a hard turn would see far greater sideloads than an airraft tire ever would?

#### jgnunn

##### Well-Known Member
does a 15x6.00x6 tire fit on the same wheel assembly as a regular 6.00x6?

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Guys, the book with all of this and much much more is Landing Gear Design for Light Aircraft by Ladislo Pazmany. Get it through the EAA...

Billski

#### Dan Thomas

##### Well-Known Member
does a 15x6.00x6 tire fit on the same wheel assembly as a regular 6.00x6?
Yup.

Dan