Making Life Easier… tyre pressure

This article was published in 2008, in Newsletter 80.

This article was first published in Newsletter 37 back in 2001, but the good advice it contains is just as useful today!

Most of the bicycles people bring to maintenance classes or to Dr Bike sessions have poorly inflated tyres. Over 100 years since John Dunlop first fitted pneumatic tyres to a bike, you might expect that everyone would have realised that riding on soft tyres is hard work but-no! Mr Dunlop would turn in his grave to see the widespread misuse his wonderful invention is put to. Here’s a review of the facts, plus an easy way to gauge correct tyre pressure.

First a bit of theory. Pneumatic tyres are designed to cushion you from the uneven road surface. They do this by ‘squashing’ a bit where the tyre rests on the ground, so matching their shape to the bumps instead of the wheel (and rider!) bumping up and down. The bigger the area of rubber in contact with the ground, the more grip (or traction) you have to keep you upright and moving; but the more a tyre squashes onto the road the more pedalling power it uses up. Now, the trick is to get enough air into the tyre for sufficient cushioning effect and traction, while reducing rolling resistance to a minimum.

Highly-inflated tyres roll along very easily and, for most riders on the road, nice firm tyres are preferable. Riding on a relatively smooth surface, your weight squashes a small (but adequate) area of rubber onto the ground which provides enough traction. Pressure should be high but not excessive: extremely high pressure may cause tyres to blow off the rim (not recommended), and it is bad at absorbing road bumps, making the ride very harsh and tiring. Also, on rough surfaces, highly-inflated tyres will bounce on each little bump and, when a tyre is slightly airborne like this, traction is zero. Keep the tyres nice and firm (using the method below). In general, the more load a tyre carries the higher its optimal pressure will be. Rear tyres carry more of your weight, so they should always be inflated about 10% more than the front tyre.

[A] Soft tyres are liable to ‘snakebite’ punctures when pinched by a kerb or pothole
Image as described adjacent

Under-inflated tyres have many disadvantages. Low tyre pressure puts too much rubber in contact with the ground, increasing rolling resistance. In short, you work harder and go slower. When cornering, soft tyres can ‘squirm’ and deform so that they will not track properly. So, if you find riding slow, hard work, notice that your tyres are noisy or that cornering feels ‘strange’, simply raising tyre pressure is often the answer. Softer tyres do soak up minor bumps quite well, but if you hit a bigger rock or ride over a pothole, insufficient air pressure allows the tyre to ‘bottom out’ against the metal rim of the wheel. This often traps the inner tube causing ‘pinch-flats’ [A]. Another less obvious but detrimental effect caused by soft tyres is excessive flexing of inner tube and tyre side walls. A tell-tale sign of long-term under-inflation is gritty, black powder all over a punctured inner tube, where the tube has literally flexed itself to bits. Usually, numerous tiny weak spots leave you no alternative but to buy a new inner tube. You might find that the tyre side walls are also nearly worn out.

[B] Correctly inflated tyre can’t be easily squeezed in
Image as described adjacent

A quick word on tyre pressures for off-road riding. Off-road (or ‘mountain bike’) tyres are wider than road tyres, giving a bigger contact area, more traction (and increased rolling resistance). Deep knobbly tread patterns are designed to dig into soft surfaces to increase traction further. It is generally best to keep MTB tyres well inflated to minimise rolling resistance and to avoid squirm in the corners; tyre width and tread pattern still ensure that you have enough traction. If you take your road-tyres onto rough ground, slightly lowering your tyre pressure will give better traction. But don’t take this too far or you risk getting pinch flats.

Simple Squeeze Test

[C] Under-inflated tyre squeezes ineasily
Image as described adjacent

How do you judge the right pressure? Simply squeeze the sides of the tyre together with finger and thumb. A correctly inflated tyre will feel very firm and you will not be able to deform it much [B], whereas an under-inflated tyre will be easy to deform [C]. This sounds a bit simplistic but it’s a quick method which works, and you’ll always have the right tool with you! Some people prefer to check tyre pressure with a pressure gauge. Most tyres have recommended inflation pressure rating marked on the sidewall in PSI (pounds per square inch), BAR (atmospheres) or kPa (kilopascals). Providing you have an accurate gauge, you can inflate tyres to an exact pressure. Remember, though, that rated pressures are only approximate: they generally feel excessively hard to me. I tend to stop pumping when I can’t squeeze the sidewalls in at all, even if it is below rated pressure. A correctly inflated tyre, then, provides just enough ‘cushion’ to soak up the bumps without allowing too much rubber on the ground, or too much tyre flexing. Ride well-inflated tyres and you can forget pinch flats, smile knowingly as you zoom past less well-inflated cyclists, and never have to replace a tyre or tube due to premature disintegration!

David Green