This post answers a frequently asked question: “To what pressure should I inflate my bicycle tyres?” Related posts explain: Slick bicycle tyres, Rolling resistance – are narrower tyres faster and Bicycle winter tyres.
1. What is the role of air pressure in tyres?
2. What is the optimal tyre pressure?
…2.1. What does the optimal tyre pressure depend on?
3. To what pressure should I inflate my bicycle tyres?
…3.1. Rear tyre
…3.2. Front tyre
4. Effects of over and under-inflation
…4.1. Pressure higher than optimal
…4.2. Pressure lower than optimal
Tyres are filled with air in order to achieve several effects at the same time:
- Enough rigidity and firmness to allow good control and steering
- Lower weight
- Absorbing road irregularities (bumps, holes etc.)
Optimal tyre pressure is the one that allows the tyre to carry the weight without deforming too much, but so that the tyre is not too hard, making the ride harsh, and traction poor.
It depends on two things:
- Total volume of the tyre
The bigger the air volume (larger wheel and/or wider tyre), the lower the pressure required.
- Weight carried by the tyre
The bigger the weight carried by the tyre, the higher the pressure is required.
Typical example is a tyre with dimensions 622-23 (23 mm wide) that is inflated to pressures over 7 bars. On the other hand, a tyre with the same diameter, but 47 mm wide (622-47) is inflated to pressures below 4 bars – for the same weight carried.
The optimal pressure of the rear tyre is 15% drop. This will be explained:
First both tyres are inflated to be firm (feel by squeezing with a hand). Then, the bike is loaded with luggage that it’s supposed to carry (if any). Finally, the rider sits on the bike. Then it is measured how much the rear tyre is compressed compared to fully unloaded bicycle.
This can be measured visually – no need to use a ruler. Tyre should just slightly “bulge” at the place it contacts the ground. If it doesn’t deform at all, the pressure is too high. If it deforms too much, the pressure is too low.
The pressure at which there is 15% drop depends on load and tyre width. First the tyre is inflated/deflated to achieve 15% drop. Then the pressure is measured. This is the optimal rear tyre pressure.
After the optimal rear tyre pressure was determined, it is the time to adjust the front tyre pressure. Which factors are important here?
In case of hard braking, or hitting a kerb, front tyre takes the same load as the rear tyre, at least temporarily. That is why the front tyre shouldn’t be much softer than the rear tyre.
Still, for most riding conditions, rear tyre takes generally more load. On road bikes, when a rider is leaned all the way forward, load distribution of front – to rear wheel is about 45% – 55%. For city bikes, where the rider sits almost upright, weight distribution is about 30% – 70%. This is why the front tyre can be a bit softer than the rear tyre, but shouldn’t be harder.
Generally, the front tyre should be inflated to a pressure about 10% lower than the determined optimal pressure for the rear tyre. For aggressive riding with often loading the front tyre, the pressure can be the same for the front as for the rear. For laid back riding style, or for city bikes, the front tyre can be inflated to a pressure about 20% lower than the rear tyre pressure. Reducing pressure below this limit increases the risk of getting pinch flat (“snakebite”) when hitting a bump, or a kerb, as well as less control when braking hard (with the front brake).
What happens if a tyre is inflated to a pressure much higher than optimal? Or if it is deflated more than it should?
These things will happen:
- Ride will become significantly less comfortable – since the tyres will not dampen road irregularities (bumps, even small ones).
- Energy put into pedalling to move forward will partially be wasted on small bouncing up and down. So speed on flats and climbs will drop slightly, for the same effort. On descents it will remain the same.
- On extremely flat, good quality pavement, speed will slightly increase compared to optimal pressure. Especially if tyres are of poor quality.
Inflating rear tyre to a pressure more than 5% over the optimal hardly makes any sense. As well as inflating the front tyre even 1% higher than optimal pressure for the rear.
- For the same effort, bicycle will go slower. This goes for both climbs, flats and descents.
- On rough roads (very bad pavement, or off road, snow etc.), ride will be a bit more comfortable. If tyres are fat enough to avoid pinch flats (a.k.a. snakebites), running a bit lower pressure is a good idea for bad roads.
- Grip on rough roads (as well as climbing speed) will be a bit better (and faster).
However, even on rough roads, even for very wide tyres, there is a limit how low makes sense. Rear tyre can be inflated up to 30% of the determined optimal pressure. Front tyre up to 35% below.
Going below those pressures further increases risk of pinch flats, speed loss is higher even on rough roads, while there’s hardly any improvement in comfort or grip.