Cycling Power

Most of us don't want to spend large amounts of money on a Cycling Power Meter for our bikes, and unless we are training very hard and with particular goals it is more of a luxury than a necessity.

But it is still useful and interesting to know how much power we can generate when we cycle and an understanding of cycling power also makes it clear why we can all increase our speed quite quickly when we start, but it becomes progressively harder to make small improvements as we get faster!

The basic idea is that we need power to overcome air resistance (more important on flat / downhill sections) and gravity (important when going up hills!). There is also a small amount of friction to overcome, both in the bike and between the bike and the road (hence the use of smoother, thinner tyres).

As a guide, below are some approximate cycling power figures for various speeds of cycling:

Speed - kmh (mph) Power (watts) Increase in power needed
to increase speed by 2.5kmh
20 (12.5) 75  
22.5 (14) 95 20
25 (15.6) 120 25
27.5 (17.2) 148 28
30 (18.7) 180 32
32.5 (20.3) 218 38
35 (21.9) 262 46
37.5 (23.4) 311 49
40 (25) 366 55

Figures assume a flat road on a windless day. The exact figures will change according to rider and bike weight, air temperature and position on the bike but the principle is the same and the figures are quite similar.

The numbers explain immediately why cyclists find it hard to keep getting faster - the faster you are going, the larger the increase in power needed to keep improving - for example, increasing your power by 50% (no mean feat) will only increase your speed from 15.6 mph to 18.7 mph.

Similarly, increasing your average speed from 20 kmh to 25 kmh requires that you produce an extra 45 watts of energy, while the same increase in speed, from 35 to 40 kmh, needs an increase of over 100 watts - and you need to already be producing over 250 watts, which is already quite an achievement for most of us apart from shorter stretches of road!

This is because, in maths speak, 'wind resistance increases with the square of the velocity' - in non-maths terms, going 25% faster needs much more than a 25% increase in the power you produce!

At lower speeds air resistance is much less important (ie much easier to overcome) but the faster you travel the more of your energy goes in just pushing the air out of the way. Hence of course the reason to try and be quite aerodynamic on your bike - the faster you are riding, the more important this becomes.

It is useful to find a flat stretch of road a few miles long if possible, and every month or two see what average speed you can maintain on the same stretch or road - this will give you a guide to your power output, so you can compare your own performance over time. It is also worth noting your weight at the same time, so you can see a 'cycling power to weight' figure.

You can also use a 'cycling power calculator' (eg this one here) to come up with an estimate of your power.

It is this number that gets you up the hills faster and that you want to improve! For example,if you are gaining speed on hills but losing lots of weight over the same period it is possible that your actual cycling power is unchanged, but your 'power per kilogramme' is increasing.

Maximum power output is of course much higher than the average you can sustain over a long distance, at least 50% extra would be typical - so if you can cycle at 15.6 mph for a long distance it is probable that you can manage at least 19 mph for short distances.

Note: It is said that top riders produce 450 - 500 watts over reasonably extended stretches (e.g. a hill climb) and that sprinters at maximum output during the last 100 metres of a race produce 1000 - 1200 watts. Most of us will have to settle for something rather less!