Optimising tyre pressure for mountain bikes
Why is there no calculator?
There are many different styles of mountain biking, and each has different requirements for materials and riders. The choice of bike models, tyre sizes and inner tubes is as varied as the sport itself. This makes it virtually impossible to get all the necessary information into one formula, and still produce useful results.
Steps for optimising tyre pressure for mountain bikes
To work out the correct air pressure for your mountain bike tyres, use the following approach:
- Get the average value for your tyre size by our bike tyre pressure calculator
- Use the tips on this page to make adjustments to this value, depending on your individual needs
- Hop on your bike, test the ride feel and adjust if necessary
On level tarmacked surfaces the following rule applies: Higher tyre pressure means lower rolling resistance. Off road, the exact opposite is true. Studies have shown that, even on gravel tracks, lower air pressure is associated with reduced rolling resistance.
In principle, the higher volumes of wide mountain bike tyres can sustain a lower air pressure without the tyres becoming destabilised. However, this does not mean that the air pressure can be reduced indefinitely, since there are other import factors to consider, along with rolling resistance. Specifically, these are grip, pinch-flat resistance and the stability of the tyres on the rims. It’s important to find the right balance. If in doubt, follow this rule of thumb: as low as possible - as high as necessary.
|Low air pressure
|High air pressure
✓ Lower rolling resistance off road
✓ Increased comfort
✓ More grip
✓ Lower rolling resistance on asphalt
✓ Increased pinch flat resistance
✓ Greater tyre stability on the rim
Although there are some basic rules of thumb for choosing the correct air pressure, every rider must experiment with their own set up. As a general rule, heavier riders would usually be advised to go for a slightly higher air pressure, but this also depends on the terrain and individual riding style.
When a tyre is travelling over sticks and stones, every little bump represents a step, which the tyre must get over. You can imagine a firmly pumped up tyre going over a root: As it passes over the root, it raises up, and then it falls back down again to ground level on the other side. On uneven tracks, this occurs several times per second and every time the tyres bump over an obstacle, momentum is lost.
But if the tyre pressure is lower, the root sinks into the tyre, almost as if the obstacle is being ‘absorbed’ by the tyre, and this prevents all that bumping up and down. So lower pressure gives you better suspension, and it also saves a tonne of energy and helps you to maintain forward momentum.
Lower air pressure also has a positive impact on the grip, which in turn improves the cornering and braking behaviour. It gives the tyres a larger surface area and means that a greater portion of the tread is in contact with the surface of the ground. A softer tyre will also sink more easily into uneven ground surfaces, which further increases the levels of grip. Even slight differences in air pressure can have a significant influence on the handling; the best way to find out what works for you, is to try it for yourself!
For better road holding capabilities, it’s best to go for the lowest possible air pressure. However, the air pressure cannot be lowered indefinitely, since this will have a negative effect on many factors, including the stability of the tyres on the rims. As a rule, mountain bike wheels feature a width of 20 millimetres and over. The narrower the wheel, the more ‘pear shaped’ the tyre will become under pressure. For cornering and drifting, this can be risky for the tyres, as they will become compressed over the edges of the rims. This leads to a spongy ride feel, and as the pressure sinks, the danger rises: the tyres may slip from the rims, which will cause an immediate loss of grip and control, and result in accidents and falls.
On the other hand, wider wheel rims offer a wider platform and therefore minimise the risk. A peculiar characteristic of tubeless tyres is that if the tight fit between the edges of the tyres and the rims is lost for even a short time under cornering, a leak can occur, causing air to escape, also known as burping. If this happens on downhill runs, it can cause the tyre pressure to sink steadily and this compromises the handling.
A higher pressure can be especially useful for providing a higher level of pinch flat resistance. A pinch flat occurs when the tire is compressed down hard over something like a stone or root, with the result that the tire is pinched against the rim; this can often cause double punctures known as Snake Bites. In more serious cases, the rims can also be affected. The rims may be dented and damaged, or in some cases even broken. As far as possible, it’s best to avoid these pinch flat punctures in the first place!
Fat bikes with their balloon tyres are all the rage right now. The advantages of these extremely wide tyres include their incredible grip and outstanding suspension, even on rigid bikes with no suspension springs. Because of the high volume of the tyres, you can get away with much lower air pressure, without risking pinch flats or instability. Air pressures of under one bar are possible, and some fat bikes only need 0.5 bar of air pressure!
As their name suggests Tubeless Systems have no tubes at all. Tubeless ready tyres are equipped with soft rubber lips which fit snugly into the rims, thereby forming a single airtight unit. So that the spoke holes are also completely air-tight, a special rim band is bonded into the rim base. Even the valves are air tight, and the tyres are filled with a sealant, so that the system is capable of sealing itself, and preventing deflation from small holes like punctures from thorns.
Along with the reduction in weight - which amounts to around 200 grams per tube - this puncture protection is another compelling reason for choosing this kind of system. Since: Where there is no tube, there can be no hole! Because of their obvious advantages, tubeless systems have achieved widespread popularity, and the number of tyres which support this technology is steadily rising.
Finding the right air pressure is largely a matter of 'trial and error'. However, to help give you a starting point, the following information should provide you with some general guidelines. For a system consisting of a 2.4 inch wide tyre, which is fitted with a conventional rubber tube, you would start with an air pressure of 2.0 bar. In general, you should pump up the front tyre with 0.1 - 0.2 bar less than the back tyre, because the rear of the bike takes most of the strain from the weight of the rider. On flat, even surfaces, you can use a lower pressure than on, for example, rocky terrain with lots of sharp obstacles.
When using a tubeless systems, you can get away with less pressure - as low as 1.2 bar - assuming that you have a wide rim of around 27 mm. Many riders prefer to play it safe, filling their tubeless systems with an air pressure of around 1.5 bar. But it’s not just about the technology, your riding style also affects how much pressure you’ll need: If you are more observant, and tend to notice obstacles and avoid them, you can use a lower pressure than those who ride straight over everything in their path.
A new system has recently come on the market, which combines the advantages of tubeless and tubed systems. The supplier Schwalbe has introduced a system featuring a small tube sitting directly on the wheel rim, which is pumped up to a higher pressure. A tubeless tyre is then placed over the tube, and is filled with a very low air pressure. The inner tube ensures that the edges of the tyre press firmly against the rim, giving the system stability and protecting the rim from punctures.
This set up allows the pressure of the outer tyre to be lowered even more, meaning a massive increase in grip. Unfortunately, the entire system is significantly heavier than a true tubeless structure, even though it is slimmer in comparison to a conventional tube. As an alternative to conventional tubes, the user can also try bike inner tubes made from other materials, for example latex, which are lightweight and relatively puncture resistant, but these can also be more expensive and don’t hold the air as well as traditional models.