Muret (France) (AFP) - Chris Froome has more to contend with than doping allegations as the Briton seeks a second Tour de France title -- he's also facing accusations of using a motor hidden in his bike.
Technological doping, as it has been called, has not been proved -- far from it -- but it is widely suspected to exist, and to have been used.
And the speed of Froome when he accelerates has left some convinced it could only have been produced by outside propulsion.
"It seems like the bike is pedalling itself," said former cyclist Cedric Vasseur, now working for French television.
Froome is not the first person to face accusations of using a motorised bicycle, as Fabian Cancellara's 2010 victory in the Tour of Flanders also raised more than a few eyebrows.
He denied the accusations before, a week later, racing off into the distance to win Paris-Roubaix even more impressively.
Authorities have started testing for motors in bicycles, but the tests are rare and target only a few cycles.
And, much to the chagrin of many French sceptics at the Tour, Froome's bike from his stage 10 victory was not tested.
- Amusement and derision -
International Cycling Union (UCI) president Brian Cookson insists his organisation is taking the issue seriously.
"We've done some testing already for concealed motors," he told AFP.
"We understand that although this subject sometimes causes amusement and derision we know that the technology is available: we have seen examples of it in laboratory conditions.
"We have no evidence that it has been used in competition yet but sadly we do know that in competitive sport sometimes some people will try to find ways of cheating.
"This is one way that would be very damaging and dangerous to an individual's reputation, a team's reputation and to the sport's reputation, so we're taking it very seriously.
"We are doing some testing, we've done testing at Milan-San Remo, the Giro (d'Italia), Paris-Nice and from time to time we'll do tests during the rest of the season."
That includes the Tour, and Froome's Sky team had their bikes tested following Sunday's team time trial, with no anomalies found.
- 'No-one believed' -
Yet according to Hungarian inventor Istvan Varjas, the authorities buried their heads in the sand for too long.
He demonstrated a motorised bike he'd built on Italian TV in 2010 and in April 2015 he spoke to French newspaper l'Equipe.
"People took 10 years to believe in EPO, it's the same for motors. No-one believed it but it's been going on for 17 years," he said.
The problem is finding the dividing line between mechanical cheating and improvements in material.
At the 2012 Olympics in London the French accused the dominant British of gaining an unfair advantage from the wheels their track squad used.
In an attempt to dissuade the use of motors, the UCI has banned the changing of unlisted bikes during a race.
But there is also the issue of detection as one theory has it that motors that can be activated remotely.
Italian police swooped on the Giro in May testing for magnetic fields that such motors could produce, with no results.
Former Tour winner Greg Lemond suggested the use of thermal detectors whilst the idea of a power passport, which tracks a rider's efforts -- and which could potentially identify whether or not an acceleration was accompanied by increased output -- could be used alongside biological passports.
That final idea is one backed by Froome's Sky manager Dave Brailsford.
But so far, no such motor has been detected, no such bike produced at a race, and no proof of cheating exists.