The Rule of 105 Calculator
Aerodynamics and cycling have always been intertwined, but modern tools and the sharing of data online continue to make it a more accessible field. The contemporary Obrees are not taking apart their washing machines, instead they're sifting through spreadsheets and 3D printing parts.
Far from an authority on all this we may be, but we love tinkering with our bikes. So allow us to try and explain the research conducted by some incredibly intelligence folks, and see if we all can't gain a few extra watts by taking a closer look at the relationship between your tyre and rim.
As you ride, airflow initially interacts with the tyre first, then the rim. But as the air passes through the spokes and makes its way to the back side of the wheel, it interacts with the rim first, before the tyre. In aviation parlance the tyre is acting as both the leading and trailing edge. This notion sparked the biggest wheel manufactures of the modern era to reconsider the conventional wisdom and instead think about the importance of 'recapture'.
As the tyre forges its path through the air, the flow is disturbed or separated. It's then down to the rim to smooth that disturbed flow and recapture it before it passes to the back side of the wheel, where the rims job is to create a flow structure that will navigate the tyre efficiently. Josh Poertner of Silca, who is an authority on all this, argues that the rim shape has an important job to do, but it can't do it unless the flow is recaptured, and this is where the rule of 105 comes into play.
The Rule of 105 states that the rim must be at least 105% the width of the tyre to recapture airflow as efficiently as possible.
In their testing Silca found this rule to be true for pretty much every tyre/rim width available, with the most contrasting results coming from setups that had a bigger disparity between tyre and rim size. A 32mm tyre on a 25mm rim will look like a lightbulb in a cross section view, causing a bigger separation of airflow that can't be recaptured by the rim. So although modern data shows wider tyres are faster in terms of rolling resistance, simply slapping a wider tyre on your bike may not provide overall gains.
Where this gets even more interesting is with tyre pressures. Between 87 and 115psi a tyre can grow by as much as 1mm. This additional size could alter the magic ratio and start to cause aero losses, as well as reducing comfort. This is one of the main reasons why we decided to build a Rule of 105 calculator. If the optimal pressure for aerodynamics turns out to be really high, it may be worth considering a different size tyre next time they need replacing.
So whether you're burning the midnight oil shopping for new wheels or evaluating the marginal gains and comfort sweet spot of tyre pressures, we hope our handy calculator can help. Using a set of calipers (or a bent wire and tape measure), gather some data, plug the numbers into the relevant boxes and hit calculate! We've made it accurate to two decimal places for precision when comparing small pressure adjustments, so don't be afraid to get nerdy.
/Happy Riding