When the brakes are applied, the kinetic energy of bike and rider is rapidly converted into heat through friction between the brake pad and braking surface. On a carbon brake surface, this heat can cause issues in one of two ways:
- Excess heat in the rim material can cause the temperature to rise above the glass transition temperature (TG) of the resin used in the carbon fibre. The resin is what holds the sheets of fibres together, so if it overheats like this it will soften, causing the rim to deform
- The heat generated through braking can cause the air inside the inner tube to expand, increasing the pressure and causing a blowout. Alternatively, the inner tube itself could melt or deform, causing a puncture or a different type of blowout
As a result, when looking at the rim design, we wanted to ensure we had the most heat-resistant options available. To do this, Parcours wheels use an industry-leading high TG resin. The brake track itself also uses a basalt coating, which is an excellent thermal insulator.
To put the braking performance to the test, we put our rims through a repeated braking cycle, using a thermal probe to monitor the rim temperature.
As with our aero testing, to keep things transparent, here is the test protocol:
- Test speed: wheel spun up to ~15kph each cycle
- Test load: 60kgf
- Braking load: 10kgf
- Deceleration: 1.5m.s-2 ±15%
- Testing cycle: 10s braking / 3s release, repeated over 3000 cycles
Below is a chart showing a snapshot of the results:
At no point does the rim temperature rise above 130°C, which keeps it well below the all-important TG of the carbon fibre resin (300°C). It is also well below the TG of a standard butyl inner tube. Note that latex inner tubes have a TG more towards 120°C, so worth bearing in mind if you’re planning on riding them when you’re expecting prolonged periods of braking.
As with our aero testing, it’s important to take into account real-world riding conditions. Parcours wheels have been tested across all terrain, ranging from short, sharp Lake District downhills in the rain to long, sweeping Alpine descents in summer heat. Each time they performed admirably, with no unpleasant surprises or issues.
So what does all the testing tell us, particularly about braking on Parcours wheels? Ultimately, our goal is to keep the rim temperature down, meaning we steer well clear of the critical glass transition temperature. To do this, you can employ a number of techniques:
- Most importantly, don’t drag your brakes all the way down a descent. Starting with smaller, controlled downhills, build up your confidence to release the brakes and fully enjoy the feeling that you’ve earned on the uphill
- Instead, use a brake/release method, often referred to as “pumping” your brakes. Even a short release allows the rim to cool significantly
- Alternatively, you can alternate between your front and rear brake when looking to reduce speed