How steep is too steep when cycling uphill?

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We’ve all struggled up slopes of 20%, 25%, even 30%. But just how steep would a road have to be before it was unclimbable by bike?

It’s a strange thing – our pursuit of steepness. It’s common to hear riders boasting of conquering Alpine cols that drive relentlessly skywards at gradients of 20% over several kilometres, or spikes of up to 40% at the centre of the hairpin.

But remembering that a 100% gradient only makes for a 45° slope – one metre vertical for one metre horizontal – surely a steepness approaching 100% wouldn’t be impossible to ride. Would it?

We decided to seek out the experts to find out.

First things first. When we say the ‘steepest gradient’ we’re not talking about those freakish spikes in the road’s steepness, or the vertical platform of a half-pipe.

We can only consider a persistent incline that a road cyclist can attempt to ride for a reasonable length of time.

Oddly, power is not a limiting factor to tackling the steepest of climbs, says Rhett Allain, professor of physics at Southeastern Louisiana University and long-standing blogger for Wired magazine.

‘If you don’t care about speed you can go up any incline with very little power as long as there’s enough friction,’ he says.

‘For instance, you can get tiny little motors to raise a very heavy load if you use enough pulleys.’

If you can create the right gear ratio on your bike, then even with a minuscule power output you could climb any gradient – in theory.

Reality is different. A gear ratio that allowed you to climb insanely steep inclines would require you to spin your legs like crazy while only creeping forward. You would soon topple over.

Allain puts the minimum speed you’d want to tackle a climb at as walking speed, or around two metres per second. By his calculations (which are a little too convoluted to discuss here), he places the maximum incline for a power of 422 watts at a speed of 2m/s (4.5mph) as being 40%.

So 40% might be where human power finds its match in an incline – beyond that you might as well walk. But for those of us less interested in practicalities, and more interested in proving that we can’t be beaten by gradient, it must be possible to climb an incline greater than 40% if we’re prepared to go slow enough.

What we want to know is at what angle the laws of physics will prevent us from being able to climb regardless of our power output or gear ratio.

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