Bike tyres gradually deflate as time passes, a bit like balloons. This is noticeable if you take your bike out after not using it for ages, because cycling for exercise is one of the few eligible reasons to leave your house. The tyres of a bike brought out of hibernation might be completely flat; more likely they’ll just be very soft. Even if you ride your bike regularly, however, the tyres will still slowly deflate. You’ll just become aware of it sooner.
Problems with too-soft tyres
More punctures. ‘Snake bite’ punctures are the main problem. This is where there isn’t enough pressure in the innertube to prevent the tyre bottoming out against the rim when you ride over a bump or pothole. Both edges of the rim pinch the innertube, creating twin punctures that look like those from a snake’s fangs – hence the name. Punctures from thorns or shards of grit or glass or also more likely with soft tyres, as a soft tyre picks up detritus more easily. This is then embedded as the wheel keeps turning.
Worse bike handling. Too-soft tyres will squirm on the rim when you lean the bike, which makes the steering vague. Cornering is thus unpredictable, especially at speed. Shear forces from cornering can even make an overly soft tyre come off the rim!
Worse rolling performance. Tyres that are too soft are draggy as there’s more rubber on the road creating friction. This means you have to pedal harder or cycle more slowly.
Given these problems, you might think that bike tyres should be pumped up really hard. Not so.
Problems with too-hard tyres
Less comfort. The air in your tyres is pneumatic suspension. A soft enough tyre will absorb bumps and vibration. A rock-hard tyre won’t so your body will take a battering. Riding on really hard tyres is like running in clogs rather than trainers.
Less grip. The harder the tyre, the less of it there is in contact with the road or trail. That means less grip. This is very noticeable if the surface is loose or bumpy, as it often is off-road, or if it’s wet. Lowering the pressure gives more traction.
Worse rolling performance. Yes, tyres are also slower when they’re too hard. A softer tyre will absorb small bumps and surface irregularities, the tyre deforming and ‘swallowing’ them while you and the bike continue to travel horizontally, unperturbed. A bike with rock-hard tyres bangs into these bumps and irregularities and has to climb up and over them, so you and the bike go up and down a little as well as forward. This slows you down. It’s obvious off-road on rocky singletrack but the same thing happens at a micro scale on tarmac that’s less than perfectly smooth – which is to say, most of it.
The right pressure
Like the bears’ beds that Goldilocks lay on, tyres shouldn’t be too soft or too hard but somewhere in between. Suitable pressure depends on your bike, your riding, and you.
Your bike. Narrower tyres need higher pressures than wider ones, otherwise you’ll get pinch punctures and really bad bike handling. Conversely, wide tyres can’t be inflated to the same high pressures as narrow ones. (They’d explode off the rim if you somehow managed it.)
Your riding. Higher pressures work best on smooth, hard surfaces such as good roads and velodromes. Lower pressures work best on bumpy and/or soft surfaces, like unsurfaced roads and off-road terrain.
You. The heavier you and your bike are, the more pressure you’ll need in your tyres and vice-versa. It depends what your priorities are too. For example, do you want to optimise speed or comfort?
Settling on the right pressure need not be guesswork. Tyres have a pressure rating stamped on the side in psi (pounds per square inch), bar (multiples of atmospheric pressure), and/or kPa (kilopascals). This rating is a good guide: inflate to at least the minimum (if given) and don’t exceed the maximum. But feel free to experiment. Just 5-10psi either way can make a tangible difference.
Precisely adjusting your bike’s tyre pressure is only possible if you have some way of measuring it. The simplest way is to invest in a floor pump with a gauge. Floor pumps, also known as track pumps or workshop pumps, are much quicker and easier than hand pumps to use so they make it trivial to keep your tyres well inflated. If you don’t have a pump with a gauge, squeeze the tyre by hand. It should feel firm – more like an apple than an orange.
When to pump up your tyres
Different tyres and tubes lose air at different rates, so it’s difficult to say exactly when you’ll need to top up yours. The narrower the tyre, however, the faster the pressure will drop, mostly because there’s less air in the tube to begin with so any given air loss means a bigger drop in psi. Check skinny tyres (road bikes) every few days or at least every week. Check medium width tyres (hybrids, gravel bikes, most e-bikes) every week or two, and fat tyres (mountain bikes, fat bikes) every two to four weeks.
Some types of tyre lose pressure faster. Tubeless tyres tend to be less airtight than those with tubes so need reinflating more often. Tyres with innertubes made of latex rather than butyl rubber also deflate faster because latex is more permeable.
If your tyres are going flat overnight, this isn’t just the natural seepage of air. You’ve probably got a slow puncture, a hard-to-see small hole that needs patching, or a problem with the valve. Tubeless tyres will also deflate if the seal between tyre and rim is imperfect.
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