Two small patches of rubber
Your whole weight, plus every braking and cornering force, passes through two small areas of rubber touching the ground. That area is called the contact patch. Its size and shape are shaped by the tyre’s width, its construction, how much air is inside and how much load it carries. Tyre tread and grip depend on this patch, on the surface beneath it, and on the rubber itself.
The points here are general. Real tyres differ a great deal, and results depend on the rider, the bike and the surface, so treat these as principles rather than predictions.
What grip actually is
Grip is the friction between tyre and surface. Two things contribute. One is adhesion: the rubber is soft enough to cling to tiny bumps and textures in the surface. The other is interlocking: on loose or soft ground, the tread knobs dig in and push against the material.
On hard, dry tarmac, adhesion dominates. That is why a smooth or finely patterned road tyre can grip very well in dry conditions, since a smooth surface puts more rubber in touch with the road. On loose dirt or mud, interlocking dominates, and bigger, more widely spaced knobs help the tyre bite and shed mud.
What tread does on the road
On a wet road, a film of water sits between rubber and surface. Tread grooves can help channel some water away, but on smooth bicycle tyres at typical speeds, the rubber compound and the surface condition usually matter more than groove depth. Wet roads also have slick patches, such as paint lines, metal covers and fallen leaves, where grip can drop suddenly regardless of the tyre. Our article on wet road traction looks at that side of the story.
The sensible conclusion is not that tread is useless on the road, but that it is not a magic ingredient. Riding smoothly, braking earlier and leaning less on wet days does more than any pattern can.
Width: more than comfort
A wider tyre at a suitable pressure generally has a shorter, wider contact patch, while a narrower one has a longer, thinner patch. The area may be similar, but the shape is different, and each shape has benefits.
Wider tyres often:
- absorb small bumps better, which tends to be more comfortable
- allow lower pressure, which can improve grip on rough ground
- feel more stable on loose surfaces
Narrower tyres often:
- weigh less
- have less air drag on a fast, smooth road
- feel lively on smooth tarmac
Which is better depends on the surface and the ride. Many everyday riders find a moderately wide tyre a good compromise. The behaviour of a wide tire is discussed further in its own article.
Rolling resistance
Rolling resistance is the energy lost as the tyre flexes while it rolls. A tyre that flexes less, or that is built with a supple, efficient casing, generally rolls more easily. On smooth surfaces, a firmer tyre can roll well. On rough ground, a tyre that is too hard bounces over bumps rather than following them, which wastes energy and reduces grip, so a slightly softer tyre may actually roll faster in practice.
Tread plays a part as well. Large knobs on tarmac squirm and flex, which adds drag, whereas a smooth or low tread rolls more freely. This is part of the balance between grip and speed, and it relates to the wider question of drag and rolling resistance in cycling physics.
Pressure: follow the markings
Air pressure changes the contact patch, the ride feel, the grip and the risk of damage. Too low a pressure can allow a tyre to squirm in corners or the rim to be damaged by an impact; too high a pressure can make the tyre skip over bumps and feel harsh.
Each tyre has pressure markings printed on its sidewall, usually a recommended range. Rim and bike manufacturers may also give limits. Stay within those figures and follow the instructions, rather than trusting a number from an article, including this one. The right pressure inside that range depends on rider weight, load, tyre width and surface, so some adjustment is normal, and a bike shop can advise.
Tyre condition
Rubber ages and wears. Cracks, cuts, bulges, exposed casing and a worn-flat centre are reasons to look more carefully or replace the tyre. A quick tyre inspection before riding is a worthwhile habit, and knowing how to fix a flat is useful if a tyre fails on the road. If you are unsure whether a tyre is safe, a bike shop can check it.
Choosing sensibly
Match the tyre to where you actually ride. Smooth, supple tyres suit paved roads; tyres with more texture suit gravel and trails; mixed-use tyres sit in between. A change of tyre changes how the bike feels in corners and when braking, so ride carefully at first on a new set and give yourself extra distance to stop until you know how they respond.