Tire, Wheel and Inner Tube Fit Standards
This article will review compatibility issues and considerations when purchasing bicycle tires and inner tubes. Aspects of performance and the type of ride resulting from the choices will not be reviewed.
When selecting tires and inner tubes for a bicycle, it is important to get compatible components. There are many different tire and wheel sizes available. Unfortunately, the bicycle industry has not always had consistently clear nomenclature for tire standards.
Tires are made with a steel wire or fabric cord molded into each edge of the tire called the _bead_. The tire bead effectively forms a circle and the diameter of this circle determines the tire fit to the rim. The tire bead is sized to fit into the rim bead seat, which is the area below the outer rim edge.
Do not attempt to mix tires and wheels with different bead seat diameters. Although the bead seat diameter determines the tire and wheel fit, there is little consistency between manufacturers in how tires are labeled or identified. Different countries at times have used different nomenclature in marketing and labeling their tires. This causes confusion when selecting a tire for a wheel and frustration when installing a tire.
An antiquated but still common system uses _inch_ designations, such as 26-inch, 27.5-inch, 29-inch. The inch size does not refer to the bead seat diameter or measurement. In fact the number of the inch designation has no actual inch measurement associated with it. It is simply a code of the vaguely approximate outside tire diameter. For example, there are several 26-inch tires that use different bead seat diameters. A 26 x 1 3/8 inch tire, for example, will not interchange with the common MTB 26 x 1.5 inch tire. There are three even more obscure tire standards also referred to as 26-inch diameter, but none are interchangeable. As a rule, tires marked with fraction sizes, such as 1/2, 3/4, etc., do not interchange with tires marked in decimal sizing, such as 0.5, 0.75, etc. For example, a 26_ x 1-1/2_ tire does not interchange with a 26_ x 1.5_ tire. There is not a logical reason for this system, it is simply what manufacturers have offered for nomenclature when selecting a tire.
Another common yet misleading system is the older French system of sizing. The numbers are reference numbers and are not accurate measurements of anything. Road bicycles commonly use a 700c tire that has a bead diameter of 622 mm. The _700c_ does not refer to bead diameter. The _c_ is simply part of the code system. There are also 700a and 700b tires and wheels, but none interchange with the more common 700c. Additionally, the 650b tires and wheels will not interchange with the 650c tires. There is not a logical reason for this system, it is simply what manufacturers have offered for nomenclature when selecting a tire.
The ETRTO (European Tire and Rim Technical Organization) system, sometimes called the ISO, short for the International Standards Organization, is now becoming more commonly used and understood. The ETRTO system uses a two number designations for both tire and rim sizing. The larger number is always the bead seat diameter. Rims and tires with the same number are made to fit one another. For example, tires marked 622 will fit rims marked 622, because the bead seat diameter is 622 millimeters for both. Look for this sizing system on the tire.
Rims also come labelled in ISO sizing, in the form of a two number system. The smaller number is the width in millimeters inside the rim sidewalls. Generally, a wider rim will accept a wider tire. A narrow tire on a relatively wide rim will mean the tire profile shape will be less rounded. A wide tire on a narrow rim will result in less support for the tire in cornering, which can cause the tire to laterally roll or twist. Additionally, rim caliper brakes will have very little room to clear the tire with a very wide tire on a very narrow combination. As a loose rule, the ISO tire width should be between one and a half to two times the ISO rim width. A rim with a width of 25mm between the sidewalls should use an ISO tire width of about 37_50mm.
Another consideration in selecting tires is the frame and fork. Although a tire might be correctly and safely fitted to a rim, the frame may lack clearance for the size. Inspect the bike if you are changing to a larger tire profile.
There are three types of valve stems on bicycles, Schrader, Presta and Woods (_Dunlop_).
The Schrader or _American-type valve_ is common on cars and motorcycles. It is also found on many bicycles. The valve stem is approximately 8mm (5_16 inch) in diameter. The valve core consists of an internal spring plunger to assist in shutting the valve after inflation. The length of the valve can vary and should be longer for tall or deep rims.
The Presta or French-type valve is common on mid- and higher-priced road bikes and on higher-priced mountain bikes. Presta stems are nominally 6mm in diameter and thinner compared to the Schrader valves. At the top of the Presta stem is a small valve locknut, which must be unthreaded before air can enter the tube.
Some brands of Presta tubes use a valve shaft that is fully threaded and usually include a locking nut or ring. These come with an extra locking nut or ring. Loosen the ring by hand and remove it before installing the tube. Install and fully inflate the tube. Then install the lockring and snug only by hand. When deflating the tube, loosen and remove the nut first.
Some makes and models of Presta inner tubes use a removable valve core. Inspect the end of the valve for two wrench flats. Use a valve core tool such as the Park Tool VC-1 or a small adjustable wrench to secure or remove the core.
Inner tube valve stems are available in different lengths. Rims with a very tall cross section require longer valve stems (60 mm or 80 mm). There are valve extenders available that screw onto the Presta valve and allow the tube to be inflated (figure 2.38). If the inner tube uses a removable valve core, use an extension that screws into the valve_s inner threads. There are also designs that are simply a tube to lengthen the stem but do not permit the Presta valve locking nut to be secured. If the locking nut cannot be closed, the valve may leak. Extenders that do not allow the valve nut to be tightened may allow the tube to leak slowly.
The wheel rim valve hole should match the valve of the tube. If a rim has been made with the smaller valve hole for Presta valves, it can be typically drilled and enlarged safely to the 8 mm size by using an 11_32 inch (8.5 mm) hand drill. After drilling, use a small round file to remove any sharp edges. Rims that are less than 15 mm outside width should not be drilled. It is also possible to use the smaller Presta valve in a rim intended for the larger Schrader by using an adapter sleeve.
A third valve type is the _Woods_ or Dunlop valve. These are seen on less expensive bikes outside the United States. The stem looks like a combination of a Schrader and a Presta stem. The top of the stem is narrow, with a large locking ring under it, and then the main shaft.
To deflate the Dunlop valve, partially unthread the nut. Pull on the tip of the valve if air is not already escaping. Remove the nut fully after the inner tube is deflated. Remove tire and tube as any other.
To inflate the Dunlop valve, use a Presta compatible pump head. Simply engage the head and inflate. Remove the head. There is no need to unthread the locking ring.
The tube should match the tire size diameter closely. However, tires that are close in bead diameter may use the same inner tube. For example, an inner tube for an ISO 630 tire (27-inch) will also fit an ISO 622 (700c) tire.
The inner tube should also match the tire width, but, because inner tubes are elastic, one inner tube may fit a range of tire widths. If the inner tube is too narrow for the tire width, it will become very thin when inflated inside the tire body. This
will cause it to be more susceptible to punctures and failures. If the tube is too wide for the tire, it will be difficult or impossible to properly fit inside the tire casing and seat in the rim. Part of the tube may stick out of the tire and blow out when the tire is fully inflated.
Tire Sizing Table
The following table shows only some of the bicycle tires made. These are listed by ETRTO diameters (bead seat diameter).
|Tire Labels for Inch or French Standard||ETRTO (ISO) Bead Seat Diameter||Common Uses & Notes|
|16_ x 1_ to 2.2_||305||Juvenile BMX bikes|
|20_ x 1_ x 2.2_||406||
Juvenile bikes, BMX, freestyle bikes, recumbents
|20_ x 1-1/2 or 1-3/4||419||BMX, older Schwinn bikes|
|20_ x 1-1/8_ and wider||451||BMX bikes|
|24_ x 1.0_ - 2.0||507||
Junvenile MTB bikes, some small road bikes
|24_ x 1-3/8_||540||Wheelchair tires|
|24_ x 1-3/8_||547||Older Schwinn tires|
|26_ x 1.0 to 4.8_||559||MTB bikes and fat tire bikes|
|26_ x 1 1/2_. Also called 650C||571||Smaller road bikes, some specialty tri-athlete bikes|
|27.5_ or 650B||584||MTB Bikes in 27.5_ sizing|
|26_ x 1-3/8_||590||Commuter bikes|
|26_ x 1-3/8_||597||Older Schwinn bikes|
|700c||622||Common road bike, hybrid and others|
|27_ x 1-1/4||630||Older USA road bikes standard for mass market bikes|