Why Do Bike Tires Lose Air? [PRESSURE LOSS EXPLAINED]
Bike tires (or tyres) lose air more rapidly than vehicle versions due to size, shape, material, and a constant pressure forcing air out of them. The day-to-day hazards of punctures are easier to identify, so it can be frustrating to have to re-inflate a tire that felt firm the last time you rode.
What Causes a Flat Bike Tyre?
Punctures from surface debris, poorly inflated, worn or cracked tyres, plus constant pressure forcing air out of any gaps in the rubber compound and valves are the main cause of a bike tyre going flat. Day-to-day, a tyre will also lose pressure despite efforts to keep it all in.
The tyre is our only contact point with the surface so we must treat it with due care and attention. They are responsible, in no particular order, for grip, comfort, safety, fatigue, efficiency, and fun. Tyres can be directly filled with air, or even sewn together then stuck directly onto our wheels.
Most day-to-day cycling is done using clincher tyres, where an inner tube is housed between the tyre and the metal rim, then is inflated with air. The right pressure and a correctly housed tyre ensures a tight fit.
Tyres are designed in most cases to resist rapid deflation but cannot be designed to fully keep air in without being made impossible to ride comfortably. The rubber compounds of a tyre and the valves used to inflate them must be soft enough to be comfortable to ride on.
Any harder and you’d lose enjoyment (and some teeth maybe!). Before the pneumatic tyre came along, early bikes were called bone-shakers. They were made with wooden wheels and iron tyres!
Surface debris will cause more modern tyres to deflate rapidly. Small pieces of flint, glass, metal and stone are to be found on the ground and will always find a way past your tyre’s defences. You normally get that feeling in your stomach around 30 to 90 seconds after they strike, as you feel the sudden squidgyness from your rubber grinding against the surface underneath you. Your control soon starts to waver.
Sometimes you won’t notice or feel it until the next day if the hole only allows the tiniest amount of air to leave. If you have decided to ride with a tubeless tyre, the liquid sealant rotating around the inside of the tyre will hopefully do its job and close up the gap to stop any more escaping.
Poorly inflated tyres can also cause rapid deflation. This happens with clincher tyres when your inner tube is under-inflated. This is different from a puncture caused by debris. It’s known as a ‘pinch puncture’.
You’ll hit a sharp edge, like a kerbstone, pothole, or tree root, hard enough to press and pinch the inner tube against the metal rim of the wheel hard enough to go through the tube. It usually leaves a dracula-like bite – like the mark from two fangs – next to each other on the inner tube.
All punctures need to be fixed as soon as possible. You should not ride your bike on a flat tyre as the risk to permanent damage to the rim is almost unavoidable. Replacements are not cheap.
We’ll look at correct inflation later but our attention now moves to dealing with the fact that tyres simply, over time, lose air and need to be re-inflated from time to time.
Why Do Bike Tires Lose Pressure?
Bike tires are not like car tires. They are more porous, and rubber will let air escape naturally. They lose pressure because of the higher pressures they run. They have a larger surface-to-volume ratio which means air escapes more quickly. You can limit the loss of pressure but not eliminate it.
A car tire subject to average use should be checked once a month and the pressure topped up. A bike tire loses a burst of pressure almost as soon as it’s inflated then slows to a steady pulse. Between seven to ten days of use, most should be examined and pumped up.
You really should be checking before each ride. Even a quick bounce of the wheels should suffice to get a feel. Better to check at base before noticing it when you are on the move.
Can a Bike Tire Lose Air without a Hole?
Pneumatic bike tires deflate naturally through a process known as permeation. This is the process of molecular penetration of gases, vapours or fluids into, through, then out of the material membrane of a solid. In this instance the penetration of the air through the inner tube, or tire.
Why Does a Bike Tire Keep Losing Air?
Compared to a car tire, a bike tire loses air more rapidly. Permeation differs between materials and a quick cross-reference shows that car tires are much denser and firmer, which means the air passes through more slowly. A bike tire should be reviewed and topped up once a fortnight at the least.
Air is only able to escape from the outer edges of an inflated surface. Although car tires have more volume than bike tires, and a greater surface area, the surface to volume ratio is the key calculator. Without getting too scientific, the surface to volume ratio of a road or hybrid bike is more than nine times that of a standard car tire. This therefore increases the chances of air escaping.
The most common reason for losing air repeatedly and rapidly is that the flint, glass or objects which causes the flat tire in the first place is not sufficiently removed before attempting to install the replacement.
This has happened to me on a handful of occasions – usually when I’m trying a replacement on a cold and wet roadside and all I really want is to be at home. Spending 90 seconds locating and rooting out the source of your puncture before re-inflating on the road and checking alternatives will save a lot of time.
New Bicycle Tire Keeps Going Flat: Why?
Permeation occurs no matter the tire. Even a new tire with firmer rubber, with more material compared to a worn version, will still go flat. Other reasons are low tire pressure, a damaged inner tube before replacement, or a spoke has pierced the rim and is rubbing against the inner tube or tire.
You should avoid allowing a wheel with a completely flat tire to rest on a firm surface as this could cause irreparable damage to the tire and rim.
How Long Should a Bike Tire Stay Inflated?
An average bike tire should stay inflated for around a week to ten days. Riding without puncturing does not impact the rate at which the air escapes naturally. A tire inflated to its maximum recommended level will leak more air in the period just afterwards, then the rate of deflation slows.
It’s not unusual for the average commuting cyclist to see a tire lose between five and ten pounds per square inch per week. This is between 0.34Bar and 0.69Bar. The air will continue to work its way out from there at a slower pace. You’ll use more or less depending on use.
From personal experience I find that a tubeless tire (no inner tube) loses air more rapidly than a clincher tire where the inner tube runs inside. It is highly likely that this is down to less than adequate tubeless tire maintenance, which can easily cause air to escape between gaps between the tire and rim. No such worries with a clincher tire.
Different types of tire will lose air at different rates. The length of time it needs to stay inflated is also related to size and pressure. They only hold a small amount of air, but are faced with a lot of pressure.
It used to be considered normal to have road bike tires rock solid. Track bikes still have very hard tires but road bikes have softened slightly. A road bike clincher tire will still run to around 120psi / 7.58Bar and is usually the tire type which needs inflating most frequently.
Hybrids/Gravels are next in line for speed of deflation, then mountain bikes and finally fat bikes. There is no limit to the amount of times that a tire may be inflated. It has not been proven that different surfaces affect the speed of deflation.
How Do I Stop My Bike Tire Going Flat?
The best advice to avoid a bike tire going flat is to ride good quality tires at the correct pressure via regular inflation, and safely avoid debris on your ride. It’s vital to check your tire inside and out for evidence of sharp objects stuck to it. Otherwise a repeat puncture is sure to follow.
If you are planning a long ride, then inflate your tires to the correct pressure for you the night before. All tires should have a PSI ‘range’ on the sidewall. Stick to that. If you commute each day, keep a good pump at home and maybe a smaller travel one in your bag. Like most maintenance in cycling, routine work prevents failures and expensive servicing. Check tire pressures once a week at least.
Remember to get the right pump for the job. Try to stretch to a pump with a pressure gauge attached. You can buy separate pressure measuring gauges but most decent pumps have them attached these days. Track pumps have a much longer handle and push more air in for each stroke of the handle. They are less portable than standard hand-held pumps.
There are two main types of tire valve – presta (mainly found on thinner tires) and schrader (also found on vehicle tires). Schrader tires should not be inflated via a petrol station forecourt system as this releases the air far too quickly for bike tire design. The risk of a blow out is significant.
The more durable the tire, in most circumstances, the better protection from flats. There are various strips layered around the tire to prevent punctures. That is why you see some tires with nicks and cuts, but no history of punctures due to the resistance of these barriers around the circumference.
If you ride with a clincher tire (and most bikes still do), then you can opt for different types of material for your inner-tube which lets less air out over time. The regular inner tube material is known as butyl and actually holds up very well. Latex tubes are much more expensive but lighter. And thicker tubes made of the same material hold more air.
Another top tip is to shake the tube in a bag of talcum powder to reduce any rubbing on the inner tube’s surface. This reduction in friction limits the wear on the inner tube.
Paying for the extra protection of a rim strip or a tire liner is another relatively inexpensive tip. One fits onto the outer edge of the rim, where you can see the flat tips of the end of the spokes resting. The other slides onto the inside of the tire and each offers an extra layer of security against punctures.
Run your bike on good tires and replace them. Mileage tends to average out at around 3,000 but regularly check for signs of wear, damage, dry rot or thin metallic chords which might show through the compound.
A tubeless tire system has been advocated to reduce instances of a flat tire. There are two major reasons – the absence of an inner tube eradicates pinch punctures and then the liquid sealant plugs the small holes caused by surface debris.
Bike Tire Pressure Guide (PSI*)
Refer to manufacturer’s range on sidewall of tire before inflating and never go above or below recommended tire pressures
|Bike type||Road||Hybrid/ Commuter||Mountain||Gravel||Folding|
|Guide range (clincher)||70-120||65-90||30-50||35-60||65-115|
Other factors which influence tire pressure guide
- Wheels come in different sizes which changes the pressure range
- Body weight: assuming a 165lb rider is running mid range of pressure, a 130lb rider could knock 20% off and a 200lb rider could add 20% – provided each rider stays within the limits of manufacturer’s range
- Cargo / carrying weight – increase pressure if you transport heavier equipment
- Lower pressures are more comfortable and promote more grip at the expense of speed
- Outside temperature: air is transported out of the tire around 2% quicker per 10° drop in Fahrenheit
- The trend has moved away from rock hard tires for speed less than maximum pressure to improve comfort and grip
- Tire widths are widening on road / hybrid / gravel bikes for comfort – narrow tires require a higher pressure