Dr. David continues his series of useful tips on keeping your bike in tip-top shape. This article covers everything you need to know about bicycle tyres and is being published in two installments. Here is the first:
Welcome to my cycle tyre FAQ Part 1
First thing to say is, I’m not going to recommend any specific tyres. We all have our favourites!
Tyre pressure – is higher better?
Yes and no!
On a smooth surface in a lab, higher pressure can mean faster but the benefits get smaller the higher you go, especially over 130psi (pounds per square inch). Lower pressures are more comfortable and by allowing the tyre to squash more provide a larger surface area in contact with the road and hence more grip. As the rolling resistance of a tyre is mostly produced by the friction in deforming the sidewall then underinflated tyres waste energy. Some people have studied the ideal pressure for performance and grip and came up with the 15 percent rule.
How much pressure?
100psi means that the air in the tyre is exerting an outwards force of 100lbs per square inch. So if you load that wheel with 100lbs the tyre will deform to a contact area of 1 square inch to carry the load. A lower pressure of 50psi will require 2 square inches to support the 100lbs load. That’s why giant mining earth moving trucks have very large tyres which spread the many tonnes over a large area. Too low a pressure will mean pinch or “snake bite” punctures.
The weight of a cyclist is more over the rear wheel so that needs to be a higher pressure than the front. Time trial bikes move the rider forwards and may put more weight on the front wheel. If necessary weigh the front and back wheels with you in your riding position.
It seems that to go faster, you need high pressures. Track bikes may use in excess of 220psi on the smooth wooden velodrome but road tyres often have a maximum recommended pressure of 120-135 psi. On uneven roads, high pressures make the tyre roll up and down every little bump effectively going up and down tiny hills and shaking the bike. This uses energy and slows you down. Having some give in the tyre and smoothing out the rough road surface improves rolling resistance. Importantly, added to this is the effect on the power source (cyclist) that vibration drains power sometimes by hundreds of watts (the tank driver effect).
See this article for more on tyres and pressure.
23mm v 25mm which is faster?
For the same tyre pressure a 25mm tyre has less rolling resistance. Seems a bit odd, but true. The slimmer the tyre the longer the contact patch and the more of the sidewall is deforming and therefore wasting your energy. But the faster you go the more wind resistance becomes the dominant force you have to cycle against. So a narrower tyre gives less wind resistance especially on the front. Also, 23mm tyres can be run at higher pressures. In general 25mm is a better all-round tyre.
TPI – is higher better?
High threads per inch is often seen as a mark of a better tyre. The carcase is made up of woven material which is then impregnated with rubber. Cheap tyres will have fewer, thicker, fibres probably of nylon while lighter expensive tyres will have a very fine weave often made of cotton. Lightweight cotton is less resistant to sidewall punctures but is lighter and more flexible and hence faster. Thicker nylon is much more resistant to wear and punctures. Finer more flexible tyres roll faster due to less energy wasted in deforming the tyre.
Most racing tyres are smooth and some winter tyres have a tread. For riding on a road tread is not required for grip. Obviously off-road tyres need a knobbly style tread to grip earth and mud. Car tyres are wide and have a flat radial tread formed by steel belts. They need tread to allow water to escape from between the tyre and a wet road to prevent aquaplaning. Bicycle tyres are narrow and cut through the water so a tread is not required. As with many things, Sheldon Brown’s site has some useful information on this.
Smooth tyres roll better than coarse treaded ones.
— The next part of Dr. David’s tyring blog will reveal the truth about rolling resistance, the choices of compound and weights for a tyre, and inner tube choices —