Advertisement

Sign up for our newsletter

Advertisement

How fast can you go on a bike?

James Witts
31 Jul 2020

You think you’re fast, you know you could be faster, but what is the fastest speed physically possible? We find out

There you are, speeding downhill like your life depended on it. Crouched over the bars, white knuckles gripping the drops, you look down at your bike computer and you see the figure click up to 70kmh. Oh yes, you’re really flying now. But before you can gain any more speed, the road sign signals a junction ahead and you squeeze the brakes to bring you safely to a halt. 

But what if that junction wasn’t there? What if there were no obstacles or bends or dogs wandering into the road, and the slope was as long and smooth and steep as you could possibly wish for?

How fast could you go then? Let’s begin to answer that question by looking at what’s holding you back.

Life’s a drag

 

‘That would be terminal velocity,’ explains Rob Kitching, founder of online aerodynamic outfit Cycling Power Lab. ‘In cycling terms, this is the point where the joint stopping forces of aerodynamic drag and rolling resistance equal the forces provided by gravity and power output.’

How much impact gravity has depends on the severity of the slope. ‘If you set the slope to infinite – in other words, a wall – there’d be no load on the tyres or the bike’s structure,’ says Ingmar Jungnickel, R&D engineer for Specialized.

‘Effectively that would make both redundant and you’d be skydiving.’

Or more technically ‘speed skydiving’, where the goal is to achieve and maintain the highest possible terminal velocity. Drop a human out of a plane belly down and they’ll reach speeds of up to 200kmh; head first and we’re talking 250-300kmh; head first and wearing specialist streamlined apparel allows for speeds of up to 450kmh.

‘But that’s not cycling, so let’s ignore that and use an actual road,’ continues Jungnickel. Scanning the world’s streets, Baldwin Street in Dunedin, New Zealand, holds the dubious honour of being the steepest road on the planet at 35-38°, depending on who you believe. 

‘On this road’s gradient – but lengthened beyond its 350m distance – assuming calm conditions and a power output of 400 watts, a rider in the road position could reach 89.48mph [144kmh],’ says Jungnickel.

That’s some speed, but still nearly 80kmh shy of the world downhill speed record, set last year by Frenchman Éric Barone when he reached 223.3kmh on the snow-covered Chabrières speed track in the French Alps in 2015. 

So perhaps to reduce rolling resistance our slope should feature an icy platform? Not necessarily, according to Jungnickel. ‘At these speeds, air resistance is around 99.5%.’

That compares to around 50% when riding at 12kmh. Air resistance increases the faster you ride, so what methods should our imaginary cyclist employ to reach maximum speed and defy air resistance?

Keep it aero

‘Clearly position is important,’ says Jungnickel. ‘So I undertook calculations with a rider optimised in the time-trial position and, using our lengthened Baldwin Street analogy, the 400W rider could reach 200mph [322kmh].’

When Jungnickel says optimised, he’s talking the full aerodynamic menu. That means a teardrop helmet and a position that sees the helmet’s tail flow naturally into a smooth, streamlined back.

A tight-fitting skinsuit is also a must to reduce air resistance. 

 

‘In fact, this is vital,’ says Rob Lewis of computational fluid dynamics specialist TotalSim. ‘Type of material, seam placement and surface treatment all make a huge difference. You could be talking a 12-15% difference in drag between a good and bad suit.’

Lewis also suggests that pulling up your socks as far as possible is more aerodynamically effective than booties, while a narrow grip on those aerobar extensions will slightly cut drag too.

You’d also want teardrop-shaped tubing because, as above, it helps to reduce the co-efficient of aerodynamic drag (CdA). This covers an object’s slippiness and size plus its frontal area.

Physics says that an object with a drag co-efficient of zero can’t actually exist on Earth – everything has some form of drag – but the numbers can be very low.

Teardrop-shaped handlebars on a top-end bike, for instance, can register a figure of 0.005. That’s pretty aero.

CdA examples of elites using aero-shaped bars might come in at the 0.18-0.25 mark, versus a good amateur athlete’s 0.25-0.30.

This figure becomes even more important when aligned with power output. When German pro Tony Martin won the 2011 Time-Trial World Championships in Copenhagen, his power output and aerodynamic drag (expressed as watts/m2 CdA) was calculated as 2,089.

This compared to 1,943 for Bradley Wiggins in second and 1,725 for Jakob Fuglsang in 10th. 

‘All riders can work to improve this figure,’ says Kitching. ‘But also hugely important to top speeds is air density, which is clearly less controllable.’

Coming up for air

At sea level and at 15°C, air density is around 1.225kg/m3. However, factors such as temperature, barometric pressure, humidity and altitude affect air density, with density reducing the higher up you are.

‘It’s why riders like Sam Whittingham head high when attempting to break human-powered land speed records,’ adds Lewis.

And why Felix Baumgartner floated up to the thin air of the stratosphere when skydiving to 1,342kmh back in 2012.

Canadian Whittingham has hit an incredible 132.5kmh on the flat, though that’s still shy of the world record for human-powered speed, recorded by countryman Todd Reichart last September.

Reichart left the rest in his wake, clocking a top speed of 137.9kmh. We say ‘the rest’ because Reichart registered that speed at the World Human Powered Speed Challenge on State Route 305 just outside of Battle Mountain, Nevada.

It was the 16th consecutive year that the competition was held in Nevada, and that’s down to two key factors: it’s 1,408m above sea level so air density is low and the course provides an acceleration zone of 8km leading to a 200m speed trap.

Both assisted Reichart’s maximum speed, as did his vehicle – a recumbent bike enshrouded with fairings. ‘I’ve undertaken further Baldwin Street calculations,’ says Jungnickel, ‘and with a fully faired bike, terminal velocity would be 369mph [594kmh].’

It would be even higher if you could do something about the tyres, with Jungnickel stating that more drag is produced by the tyres poking out than the entire vessel.

‘Also, at extreme power outputs, you’d eventually run into the maximum grip the tyres could elicit, which is a function of downforce,’ he says.

‘You then reach a catch-22. You could add spoilers to increase downforce, which add drag, which would require more power again (and so on). Beyond this, I don’t believe any structural concerns would be a factor as you could just build the bike sturdier with more material.’

There you have it. To reach your maximum speed of almost 600kmh, commission Graeme Obree to build you an aero Beastie bike, head to New Zealand, ask Dunedin council to extend Baldwin Street to around 10km long and generate a power output akin to Tony Martin. Simple…

Always stick to speed limits. Cyclist takes no responsibility for anyone's riding or the outcomes of it

Read more about: