Round Up: Everything you need to know about bicycle tyres

Cyclescheme, 08.11.2018

Round Up: Everything you need to know about bicycle tyres

Tyres come in a bewildering range of sizes and tread patterns for every conceivable usage. Here's how to choose the right ones.

Early bicycles had tyres made of iron on wooden rims; they were called boneshakers for a reason. Iron was superseded by solid rubber and then pneumatic tyres, which transformed comfort, rolling efficiency, braking security, and handling.

Tyre types

Clincher – The standard tyre. It’s U-shape in section, with a circular line of wire or kevlar (the ‘bead’) embedded at the edges. When the tyre’s innertube is inflated, air pressure forces the tyre beads up against the lipped edges of the wheel rim, holding the tyre in place.

Tubeless – A clincher designed for use without an innertube. Tubeless tyres are tighter fitting and should be used on tubeless-compatible rims. The tyre’s inner surface is less porous and special rim tape is used to cover the wheel’s spoke holes (if present). A small amount of liquid sealant is poured into the tyre when fitting. This plugs small punctures automatically.

Tubular – Circular in section like a sausage, with the innertube inaccessible inside. There are no tyre beads; a tubular is glued onto a lipless rim. Tubulars are only for racing. For everyday use, performance gains (if any) are outweighed by fitting and repair hassles.

Solid – Now usually some kind of polymeric foam rather than solid rubber, such tyres can’t puncture are less comfortable and don’t roll or handle as well.


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Tyre sizes

Tyre size names are a mess of approximate imperial measurements and anachronistic French designations. A 700C tyre has the same diameter at the bead as a 28-inch tyre and a 29-inch tyre, while a 27-inch tyre is bigger than all three!

How do you know if a tyre will fit your bike’s rim? By the ISO designation, which is stamped on the side of the tyre: two digits, a dash, three more digits. The number before the dash is the width in millimetres. The number after is the diameter at the bead. If it’s the right type of tyre (see above) and the ISO diameter matches, it will go on the rim. Here are some common ISO diameters: 622 (aka 700C, 28in, or 29er); 584 (650B or 27.5in); 559 (26in MTB), 406 (20in BMX); 349 (16in Brompton). A 700x28C tyre is 28-622. A 29x2.2in tyre is 56-622.

When it comes to width, you’re limited by the bike’s frame and fork clearance and by the room under the mudguards (if used); many road bikes won’t fit tyres wider than 25 or 28mm. You’re also limited by the width of the rim. A wider tyre needs a wider rim to support it. Tyre manufacturers such as Schwalbe (https://www.schwalbe.com/en/reifenmasse.html) have charts showing what tyres best match which rim widths. Your bike’s rim may have its size shown like this: 622x13. (Width comes second for rims.) If it’s not given, you can work it out by subtracting 6mm from the external width 

Construction

There’s more to tyres than rubber. There’s the bead, which can be steel wire or kevlar. The latter is lighter and enables the tyre to be folded up for storage.

Then there’s the casing, the fabric carcass that supports the rubber. It’s usually nylon but cotton or even silk can be used. The thickness of the casing threads is shown the Threads Per Inch number (TPI). A higher TPI, such as 120, means thinner threads, so the tyre will be more pliable and will roll better. A lower TPI, such as 60, means thicker threads; the tyre carcass will be stiffer but will resist cuts better.

Between the tyre bead and the tread there’s the sidewall. This can be ‘skinwall’ – traditionally tan coloured – where the rubber over the casing is very thin, or ‘gumwall’, where the sidewall rubber is thicker. Thinner sidewalls save weight and improve rolling performance. Thicker sidewalls resist cuts and abrasion better. Some tyres have a polymer coating or mesh in the sidewalls to improve cut resistance without requiring thicker rubber.

The tread is the part that rolls on the road/ground. Thicker is tougher and wears better. Thinner is easier rolling but more fragile. A thick tread gives decent puncture resistance by itself but many tyres have a protective layer underneath. In lighter tyres it’s a synthetic such as kevlar. In the toughest (and heaviest) it’s a thick layer of springy rubber.

Tread

Off-road tyres are knobbly because they grip by pressing into the ground. Road tyres are slick because the tarmac presses into them. Tyres that never leave tarmac don’t need tread patterns – perfectly slick is fine. For mixed use, on both tarmac and firm tracks, a good compromise is a slick centre rolling strip and an incised tread pattern on the shoulders of the tyre. 

Grip also depends on inflation pressure and on the compound of the rubber. Softer, faster-wearing rubber grips better. This is why many road tyres are dual compound: they use harder rubber in the centre of the tread and softer rubber on the shoulders for cornering traction.


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Pressure

Most casual cyclists have their tyres too soft. Many cycling enthusiasts have them too hard. The right pressure will depend on: intended usage (off-road, road, etc); tyre width (narrower tyres need pumping up harder); and your weight (lighter riders can use lower pressures and vice versa). A 60-584 tubeless mountain bike tyre might work best at 20psi; a 23-622 road bike tyre might need over 100psi.

Pressure determines a tyre’s ‘footprint’. Lower pressure means more rubber on the road/track, which means more grip. If you go too low, especially on a narrow tyre, you may get pinch-punctures and/or a buckled wheel. Too low also means more drag and vaguer steering. Too high results in less comfort and less grip. It can even slow you down, as a tyre rolls best when it can absorb bumps.

Don’t be afraid to experiment with tyre pressures. All you need is a pump with a gauge. Check the pressure rating stamped on the tyre. Try at the maximum, then in the middle, then the minimum. Which works best for you?

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