# Bicycle Mechanics - Standard Tire Size?

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View Full Version : Standard Tire Size?

shipinabottle
08-17-06, 07:30 PM
I have 26 X 1.5 inch tires on my bike. However, when plugging in my wheel/ tire size for my new cycle computer, I saw that my options included 26", 26.4", 26.5", 26.6" and maybe some others. The 26.6" size was in boldprint, so I guessed that was sort of the standard size and plugged that in. Readings don't seem to be outrageous or anything, but I want to make sure. I just bought one of the tires, but its label just had 26", nothing more. I know 26.5" said tubular next to it and I don't have that. The others had something like 700 times some number CC. Nothing on my tire package had any of that. Any help?

thomson
08-17-06, 08:13 PM
The number you enter is the number of millimeters the tire moves in one revolution. What you can do is line up your valve on a line and roll your bike until the valve returns to the bottom. Measure, convert to millimeters, and enter that number.

moxfyre
08-17-06, 08:16 PM
I have no idea what 26.4" .5" etc. means but tire sizing is a very confusing business. Fortunately there are basically only two common sizes today 700C (622 mm b.s.d.) and 26" (559 b.s.d.)

Tires of different widths have different diameters. You need to know the tire width as well as diameter, then look at this page: http://www.sheldonbrown.com/cyclecomputer-calibration.html#ccc

Column F gives the circumference of the tire in millimeters, so for your 26x1.5 tire the circumference is 1985 mm. Convert that to whatever units your computer uses, and you should be good to go.

HillRider
08-17-06, 08:36 PM
I calculated the circumference of a 26" (ISO 559) rim with a true 1.5" tire (38mm) as 1995 mm or 199.5 cm. Possibly the 1985 number assumes the marked tire size is a tad optomistic (as are most bike tires).

DanPT
08-17-06, 09:40 PM
I will assume that if it has 26", 26.4", 26.5", 26.6" it must mean it need the diameter of the wheel as input. ( Do you have the cyclocomputer manual?)

The exact diameter of the wheel is dependent on the thickness of the tire over the wheel. That why there is a range for thin or thick tire.

Other (I think most of them ) cyclo-computer calculate wheel size by circonference instead of diameter.

I think it's easiyer and more precise to calculate the circonference under full weight with a tiny dot of paint, then rolling stait and mesuring the distance. You can always convert circonference to diameter if your cyclocomputer only accept 26", 26.4", 26.5", 26.6" and such.

shipinabottle
08-18-06, 12:43 PM
I have the manual.
It says use the Wheel Diameter Size Chart to find the correct wheel factor (or multipy the wheel diameter by 3.1416). I'd try and measure mine, but I don't know how exact I would be anyway. Here's what it says for diameters.
26" (650A)
26.4" (700 X 20C)
26.6" (in bold) (700 X 25C)
26.8" (700 X 28C)

DanPT
08-18-06, 03:16 PM
Ok if they ask you to multiply the diameter by 3.1416 (Since Diameter x 3.1416 = circumference) it mean you can input the circumference somehow. That must be the wheel factor.

An easy way to mesure the circumference is to use paint. put a tiny dot of paint on top of the tire in the center at a place where it will touch the gound when rolling. Get on your bike and pedal until the wheel pass twice on the dot. Use a mesuring tape to mesure between two dot on the ground. Us as little paint as possible it will cleanly rob off the tire just by rolling.

If not Thomsom method above is also a good one

What you can do is line up your valve on a line and roll your bike until the valve returns to the bottom.

foolish heart
08-18-06, 06:13 PM
Have you tried looking up the website of your cyclocomputer brand? The manuals are usually downloadable. For example, Cateye pegs 26x1.50 with a value of 201 which is supposed to be programmed in their cyclocomputers. Don't know if that's any help.

shipinabottle
08-19-06, 07:36 AM
Found something else. It also says later in the chart
ATB 26" X 1.5
Whats ATB? Is that mountain bike? I have a Specialized Rockcombo, which is supposedly a hybrid. Could I fall under this category?

DanPT
08-19-06, 03:42 PM
Yes 26 x1.5 ATB is the good size.

700c and 650a are road bike wheel size.

If you have 26" tire that a mountain bike tire.

It would be more accurate to actualy mesure the real circomference but if you don't mind use the 26x1.5 ATB.

The truth is for most people slight error in speed/distance reading don't realy matter much.

foolish heart
08-19-06, 11:09 PM
Whats ATB? Is that mountain bike?

As far as I know, that stands for all-terrain bike. I could be wrong.

Wogster
08-20-06, 07:45 AM
I have 26 X 1.5 inch tires on my bike. However, when plugging in my wheel/ tire size for my new cycle computer, I saw that my options included 26", 26.4", 26.5", 26.6" and maybe some others. The 26.6" size was in boldprint, so I guessed that was sort of the standard size and plugged that in. Readings don't seem to be outrageous or anything, but I want to make sure. I just bought one of the tires, but its label just had 26", nothing more. I know 26.5" said tubular next to it and I don't have that. The others had something like 700 times some number CC. Nothing on my tire package had any of that. Any help?

Scales do not always work well, because the actual diameter of a tire, depends on several factors, the type of tire (knobby, inverted, smooth), the make and model of tire (some have a deeper tread then others of the same type), the weight of the rider, the tire pressure. It's often not by much, but a 2mm difference in diameter varies circumference by 6.28mm. That means every time the wheel turns, your either going 6.28mm further then indicated, or 6.28mm less then indicated. Every 1,000 turns of the wheel, your off by 6.28m. One revolution of a 26" wheel is around 2m so over a distance of 100km (60miles) your wheel goes around ~50,000 times. Your off by 1km every 319km or 1 mile every 308 miles, this is with a 2mm difference in tire diameter (there are 25.4mm in one inch) so not much of a difference but it adds up:D

Best is to pump up the tires to the pressure you normally use. Now make a mark on the ground with a piece of chalk, and the side of your front tire in the same spot, extend the mark around the top of the tire, then get on the bike, and move it until you have gone exactly 5 revolutions of the wheel, make another mark on the ground. Get a tape measure and measure between the two marks. divide by 5, this gives you the exact circumference of the tire. This is easiest to have someone assist, as they can count the revolutions of the tire, and make the marks while you remain on the bike. You can also go 10 revolutions and divide by 10, but that requires a longer tape measure. Chalk is best for making marks, because it's easy to clean up.

Of course if you change tire type, pressure or gain/lose a lot of weight, you may want to recalculate.

frquenttrvlr
10-23-07, 08:47 AM
When I roll out my tires, I hardly ever come near to the standard tire settings supplied by the computer manufacturer, or found on the web. To those of you who roll out your 700 x 23, what do you usually come up with? (I usually come up with 2117-2120) For 700 x 25? (I usually come out with about 2131-2133)

tellyho
10-23-07, 08:52 AM
You have (as far as I can tell by your post) mountain bike tires. Your best option is the ATB 26x1.5. If you have bit of time and would like to know about tire sizing, read Sheldon:

http://sheldonbrown.com/tire-sizing.html

JanMM
10-23-07, 12:38 PM
As far as I know, that stands for all-terrain bike. I could be wrong.

A long time ago, (1980's) when factory-produced mountain bikes were new, there was much discussion about whether they should be called ATB's or MTB's. ATB fell by the wayside, for the most part.
26x1.5 would be a moderately skinny ATB/MTB tire.

Bill Kapaun
10-23-07, 03:54 PM
For my 26x1.50 tires (Serfas Drifters), I measured 2020 MM for the circumference.
Even if you are off by 20, that's only 1%!