HOW DOES HEADWIND COMPARE WITH GRADE?

Riders who have flat windswept roads often say, “We may not have hills, but we have the wind.” The idea behind that phrase is that the increased air resistance is comparable to the effects of gravity. But how exactly does headwind compare with grade?

If you look at the chart provided by Andrew Buckrell at Stac Performance, you can see that a 10 km/h headwind is equivalent to a 1.3 per cent grade. A 30 km/h headwind is like a leg-burning 5.4 per cent grade. Heading into a 50 km/h wind is like riding up Monte Zoncolan.

But what’s the connection between those winds and grades? How can you say a 50 km/h headwind is like an 11.1 per cent grade?

In the Power vs. Headwind chart, you’ll see that a 60 kg rider needs to put out roughly 60 more watts to maintain a speed of 25 km/h in a 9 km/h headwind compared with no headwind. What if that rider pumped out those extra 60 watts, roughly 130 watts in total, on flat ground with no headwind? He’d travel at, well, something faster than 25 km/h because there’s no wind or incline to slow him down. What we want is an incline steep enough to keep that rider at 25 km/h even though he’s putting out more power. It would be the hilly equivalent of a 9 km/h headwind. That incline has a grade of 1.2 per cent. That grade throws enough gravity at the rider to keep his speed at 25 km/h.