Mountain Bike Wheelsize WTB and the Global Measuring System

I had to change my WTB Velociraptor 44 / 50 tyres last week due to age and wear but I didn't know what to replace them with, all knew was the inner tube that I had was marked as thin 26" x 1.5" / 2.2" so what does the 44 / 55 refer to ?

I knew that my rim was a 26" Mavic X138 quick release rim with a thin inner 16mm outer 20mm which I found was standardized by the following system

ISO System ( aka E.T.R.T.O.)
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The ISO system uses two numbers for example 55 - 622

1. The first is the rim width is the inner width measured between the flanges as shown in the diagram so this would be 55mm, this is a wide rim most WTB are 20mm and narrow to save on weight
2. The second ISO number is the critical one, it is the diameter of the bead seat of the rim, in mm ("B.S.D.") in this case 622m or a 29" wheel, note this is not a straight conversion 520mm is only20.5"" this is because its the Bead Diameter not the total Diameter in this case would be 736mm

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29" x 55/50 aka (55 622) on the wall of the a WTB Tyre this is what is means

1. The 29 is the size in inches of the wheel
2. The second set of numbers 55/50 is called the Global Measuring
System by WTB describes the tread size at 60psi
3. The third set is the Tire Size Designation is the Tire Section Width or the outside width of the tire casing in this case 55mm, not the rim size which might be as narrow as 21mm
4. The second number is the Tire Bead Diameter in this example is 662 mm, this is not the full Diameter of the Wheel

26" X 44 / 50 is my WTB Velociraptor this is all I had to go on I knew it was a mid-range tyre due to the treads

1. The 26" is the BSD size in inches of the wheel ( 599mm )
2. The second set of numbers is called the Global Measuring
System by WTB which will take up to a 1.9" inner tube with a deep tread.


GMS ------------> TRADITIONAL 26" DIAMETER x WIDTH in INCHES

-- / -- ---------------> 26" x 1.5 " --- slicks or road tyres
-- / -- ---------------> 26" x 1.75 "
44 / 50 ------------> 26" x 1.9 " ---- WTB Velociraptor
47 / 50 ------------> 26" x 1.9.5"
47 / 52 ------------> 26" x 2.1 "
49 / 52 ------------> 26" x 2.14"
52 / 54 ------------> 26" x 2.24" ---- WTB Motoraptor
50 / 55 ------------> 26" x 2.3" ---- WTB Exiwolf
55 / 52 ------------> 26" x 2.4"
55 / 62 ------------> 26" x 2.5"
58 / 55 ------------> 26" x 2.55" ---largest tread chunky knobbly or ballon tyre


NOTE:

It is also worth adding that if you own a Bike Computer you will need to input the Wheel Size (WS) this is also the circumference in mm this can be calculated using 22/7 (approximation of Pi) x the Diameter, where the diameter of a fully inflated MTB tyre is the full 26', taking 1' = 25.4mm

26' = 660.4mm

660.4mm x 22 /7 = 2075mm

Read more tips from Owen Stephen Dawson