The last week has been a bombardment of news outlets jumping on the 'mechanical doping' story after Femke Van de Driessche was found to have a spare bike containing a motor in the pits of the U23 World CX champs. My initial thoughts were great, finally motors in bikes can be taken as a serious cheating method. For years there have been armchair experts spouting rubbish on how this was impossible with reasons like the battery being too heavy, motor too heavy, technology not there etc. I never bought in to it. The technology has been there for decades. Any first year engineering student could do the maths showing it is possible. A lithium ion battery with enough capacity for an hour at 50w would weigh under 500 grams, about the same weight in mud a CX bike picks up over a couple of laps!

What has started to grind on me is news outlets jumping on the bandwagon of insinuating any rotating wheel off the ground is proof of hidden motors, and that when the wheel contacts the ground and the bike inevitably is propelled forward it must be driven by a motor. Newsflash, wheels spin and energy must be conserved! Crashes such as Ryder Hesjedal's and Ion Izagirre's aren't the smoking guns the media want them to be. Both crashes were on downhill sections at >50kmh. Time for some geekery and a quick bit of maths. Let’s say they were doing 50kmh at the time of crash (likely more), and a rear wheel has a moment of inertia of around 0.035kg-m2 ( The rear wheel is storing 27.5 joules of rotational energy, if that was all discharged in a single second that’s 27.5w of power. For simplicity’s sake let’s say the bike weighs 7kg and is put back on the ground vertically so it can move forward. The bike would act to reach a point of equilibrium where forces are balanced; therefore, the bike would accelerate due to the spinning wheel applying a force through the tyre to the road. 27.5 joules of rotational energy would then be converted in to kinetic energy and accelerate the bike forward. Assuming a tyre coefficient of friction of 0.9, the bike would accelerate at its limit of adhesion (4.4m/s^2) to 10.05km/h within 0.63s, at which point it will have reached energy equilibrium. Obviously there are a number of (reasonable) assumptions here, as well as the bike not being restrained by the rider, but it displays the point well that there is a lot of energy stored in a spinning wheel, easily enough to move a bike forward a significant amount!

If anybody want to check my maths, then I can post up my spreadsheet and a full work through, but hopefully this demonstrates my point quite well. I’m not saying motors aren’t used in the pro peloton. I’m a huge cynic on things like this and truly believe their use is quite wide spread but there a number of other things that any engineer worth his salt would point out quite quickly when these videos are spread around social media. If I were using a hidden motor system then I wouldn’t be using it on a downhill section, where both the Ryder and Ion videos were captured. That is the least efficient time to be adding extra energy to the situation, headwinds and climbs are where the biggest bang for your buck can be had. I wouldn’t design a system that is controlled by a rider on/off switch, it’s too risky. It would be designed to only work when the cadence is non-zero, and I also feel it would designed to be proportional to the rider’s power output. This would ensure it is only used when it is most efficient (assuming the rider is riding the course efficiently) and it would also stop the motor activating during a crash or when picking the bike up. These are simple safeguards; there is so much to lose if a team is caught, it would be silly to think these wouldn’t have been engineered in!

The UCI’s approach of using a magnetometer to detect motors is pretty cool; a neat solution to look for magnetic fields induced by any electric field but maybe somebody should’ve reminded the UCI that there are such things as cable shielding. Maybe Femke didn’t have the knowledge to protect her system as such which could explain why she was the first to be caught. I think the UCI needs to approach this problem in a number of ways: magnetometer, thermal imaging cameras, X-rays and a good old mechanical strip down. The one good thing about finding a motor in a bike is that there’s none of this ‘my drink was spiked’, ‘I ate some dodgy steak’, ‘I’ve been at altitude and was on antibiotics’ BS that is often banded around after failing an anti-doping test. However, the ‘not my bike’ excuse was quite cute, you’re still responsible for your equipment! I don’t buy it…