Tyre pressure is something we get asked about a great deal and quite rightly so. The pressure that you pump in to your tyres can make a remarkable difference to the quality of your ride. It affects the rolling resistance and speed of your bike as well as the comfort and cornering grip so getting it right is of great importance if you take your riding seriously. The trouble is that it’s not that easy to find the right tyre pressure for yourself and the terrain over which you ride.
So, where do you start and what variables need to be taken in to account when deciding on your final pressure. In this little article we’ll try to give some good pointers to help you maximise your riding experience.
THE QUESTION OF TYRE PRESSURE
There are a couple of important principles to consider before we delve a little deeper into tyre pressure. These are tyre-to-road contact patch and vertical tyre compression. Both of these have a big effect on ride quality with the ideal tyre pressure optimally blending both.
When the tyre rolls along the road it is continually deforming and then “springing” back to it’s original shape. This constant, rapid movement robs energy via heat loss which in turn slows the bike down. This is rolling resistance. The amount of deformation and therefore rolling resistance is influenced greatly by the tyre-to-road contact patch
The tyre-to-road contact patch is best described as the “footprint” that the tyre leaves on the road. This footprint can be altered by using different widths of tyres and by changing the pressure that you pump in to them.
With regards to tyre width, wider tyres tend to offer lower rolling resistance when inflated to the same pressure as a narrower sibling. This is because the shape of the tyre-to-road contact patch changes between the two with the wider tyre forming a shorter, more circular footprint as opposed to the long, thin area offered by the narrower tyre. The longer footprint of the narrower tyre means that the wheel loses more of its circumference because it is deformed to a greater extent. This bigger amount of deformation creates more heat, more energy loss and a larger amount of rolling resistance. This generally means that a narrower tyre has to be inflated to a higher pressure than a wider tyre to achieve the same, lower rolling resistance.
When looking at tyre pressure, shorter tyre-to-road contact patches are associated with running higher tyre pressures. By using a higher pressure tyre deformation will be less, with the already discussed energy, heat loss and resulting rolling resistance being minimised. The obvious trade off though is a loss of ride comfort as tyre pressure increases. This then seamlessly leads us on to the notion of “vertical tyre compression”
Vertical tyre compression is how much the tyre lowers under load. In other words it’s how “squashy” the tyre is when the rider is sitting on the bike with a heavier rider promoting more compression at the same tyre pressure. More vertical compression of any given tyre results in an increase in comfort as the greater amount of compression acts as a dampening spring with greater travel. With this in mind running lower pressures affords a softer ride.
The amount of vertical compression also widens the tyre-to-road footprint so gives the rider more cornering grip. However, running lower pressures also lengthens the tyre-to-road contact patch regardless of what size of tyre you are using so naturally increases rolling resistance. Lower tyre pressures and more vertical compression also increase the risk of the tyre “bottoming out” over rougher roads. Seeing this, rider body weight obviously need to be taken in to consideration when choosing the ideal tyre pressure.
When evaluating tyre-to-road contact patch and vertical compression together you will start to understand that there is a certain trade off between speed and comfort. The idea of tyre pressure is to find the perfect balance between these resulting in optimal ride characteristics for the width of tyre that you are using, your body weight and the roads over which you are riding.
So, how do you find your personalised tyre pressure? Read on below to find out.
THE WINNING FORMULA
Okay, so with everything that we’ve already covered in mind how do we arrive at the best tyre pressure, the one that is going to afford the best of both low rolling resistance and comfort? Fortunately here at HED we have already covered a lot of experimentation already so you don’t have to start from scratch. Using all of our findings we have arrived at a simple formula for tyre pressure which acts as a great initial reference. This reference pressure can then be altered slightly depending on your riding preferences and indeed the road surfaces that you are going to be riding over.
It’s important to note that these recommendations are for tyre pressures when using our “Plus” rim format and that pressures may differ for other brands of rim/wheel. Why is this? Well, our “Plus” clincher/tubeless rims have a wider internal rim measurement than most other road wheels available. This 21mm internal measurement increases air volume and permits a superior tyre cross sectional profile so will naturally promote a positive change to the tyre-to-road contact patch. Seeing this, we generally find that our rims can be used with slightly less tyre pressure.
HOW TO CALCULATE
Our tyre pressure formulas are simple to calculate and take into account a riders body weight and the width of tyre being used. To arrive at a great starting tyre pressure simply multiply your body weight in kilograms by the formula number show in the below chart. Note that this formula number is different for different widths of tyre because, as we have already discussed, tyre pressure should be altered depending on the size of rubber you are using.
We have found that these formulas give the optimal balance of comfort and speed but they can be adjusted slightly for the roads that you are riding over. For example, wet roads might need a touch less pressure, say 5psi less, to help with cornering grip whereas you could add 5psi if the roads you are riding on are glass smooth and you want a touch less rolling resistance.
As a final note we'd just like to say that our pressure recommendations are good guidelines but must be adjusted accurately for body weight and the roads that you are riding over. Tyre pressure that is too low can cause the tyre to bottom out potentially causing damage to the rim.