Germany is parched, and nature is crying out for rain. The risk of forest and grassland fires is now particularly high, and in some parts of Germany the air even smells of smoke. You may well have heard that the hot exhaust pipe of a car parked carelessly in a meadow can easily set the dry grass alight and trigger a veritable inferno. What many people don’t know is that even a bicycle can start a forest fire.
Mountain biker causes forest fire – we’ve seen it all before
Actually, it has happened before. Perhaps – and hopefully – there has only been this one documented case so far, in Austria in 2018. BIKE magazine had also reported on it at the time, but unfortunately the article is no longer online. So today we’re reminding you once again of the danger posed by hot brake discs.
And they can get really hot! Anyone who’s ever accidentally brushed a brake disc with their calf – just as it’s been released from the constraints of a rapid descent down a mountain pass – knows this. If you’re in any doubt, take this:
Temperature measurements in a practical test
In tests on our test bench, we have often caused brake discs to glow bright red. And yet the brakes still managed to slow the bike down. So you might not even realise that you’re already pushing your bike’s brakes to their limit. Depending on your weight and the speed you’re travelling at, many of us are likely to find ourselves with a truly hot seat after a hard, sudden stop.
On the notorious brake test track in Bolzano, BIKE – the mountain bike magazine – documented the actual stresses placed on bicycle disc brakes 15 years ago using extensive measurements. A mountain bike fitted with temperature sensors, GPS and a force gauge, and featuring 185 mm brake discs, recorded all the relevant data. The test track also includes a 400-metre descent on tarmac with gradients of up to 27 per cent. It was here that the most rigorous stress tests were carried out.
The results showed that during perfectly normal descents, where both front and rear brakes were used, peak temperatures of 477 degrees were recorded on the front brake pad of a 114-kilo system (95-kilo rider + bike + measuring equipment).
With a lightweight test subject weighing 70 kilos, the maximum temperature was still 346 degrees at the front and 269 degrees at the rear brake pads. In the extreme test involving a 95-kilo rider with an additional 28-kilo trailer, the temperatures rose to 550 degrees on the pads. The brake discs themselves became even hotter than the brake pads, reaching a peak of around 700 degrees and glowing red.
Critical limit values for dry material
And that’s more than enough! Dry plant material such as grass, leaves or straw ignites at temperatures as low as 200 to 300 degrees Celsius (newspaper even at lower temperatures). The exact ignition temperature depends on the moisture content, structure and density of the material. Finely distributed tinder, such as dry blades of grass or thin twigs, is particularly susceptible.
In the event of direct contact with a hot surface, a temperature of as low as 200 degrees may therefore be sufficient to cause ignition, depending on the circumstances. The brake discs on a bicycle can easily reach this temperature and ignite dry leaves or grass, thereby causing a forest fire. Peak temperatures of up to 700 degrees are even well above this threshold.
Braking technology determines temperature
If you brake correctly, you prevent the brakes from overheating quickly and thus Fading: It’s better to brake briefly and sharply, but with short pauses in between. Continuous braking prevents the brakes from cooling down, whilst intermittent braking – with the brake caliper open between braking manoeuvres – significantly reduces the temperature.
The front brake does most of the work when braking, but as long as the rear wheel is on the ground, it can help to brake too. So shift your weight back and make full use of the rear brake.
Sebastian Brust
Editor