I spoke at a camp in Saudi Arabia a couple of weeks ago and a triathlete asked me what he should do on downhill portions of a bike course. Should he pedal hard, pedal easy, or coast? That was a great question and one that also applies to cyclists doing time trials. It does not apply to runners as their speed is not significant enough to cause substantial drag. And drag is a key issue here.
On a bike as your speed increases linearly (a straight line from, let’s say, 20 to 25 mph), the power required to go faster increases exponentially. It’s not a straight line but rather one that curves upward. This largely is because of drag – wind resistance. The energy required to overcome this resistance (drag) is a function of land speed to the third power. So while it is only a 25% increase in speed to go from 20 to 25 mph, there is something like a 75% increase in the energy required to achieve that additional 5 mph.
Why am I telling you this?
Because as you go downhill and your speed increases, if you want to go even faster the energy “expense” of the additional miles per hour is going to cost you dearly. For a long race with enough such steep descents this could be the difference between a poor race and a good one. Who would have thought that going downhill could be such a performance determiner?
The bottom line solution is an old cycling TT adage which says that if you are riding on a fast portion of a course (downhill) ride easy; but if you are riding on a slow portion of a course (uphill) ride hard. So, when riding fast on a downhill don’t expend as much energy as when riding uphill. The longer the event, the more important this is. For a sprint-distance triathlon or a short time trial you can go much harder downhill than if it was an Ironman or a long TT.
The best advice I’ve seen for monitoring this came a few years ago from my good friend Alan Couzens, an exercise physiologist and triathlon coach from Colorado. He tried to nail it down for Ironman triathleteswith his “50-40-30-20-10 rule.” It goes like this…
At greater than 50km/hr (31mph) get aero and coast. Do NOT pedal.
Pedal easy at >40km/hr (25mph).
Pedal steady at >30km/h (19mph).
Pedal moderately hard at >20km/h (12mph).
Pedal hard at >10km/h (6mph).
Alan’s Ironman rule numbers may not work for your race distance, but the concept remains the same: Conserve energy when the bike is going fast; expend extra energy when the bike is going slow. How much energy depends on how fast the bike is going, how long your race is, and how fit you are. The less fit you are the more you will need to conserve energy on downhills. Based on this concept you can come up with your own rule for each race distance you do.
So the answer to the camp athlete’s question starts with (you guessed it) “it depends.” It depends on speed, race duration, and your fitness.
I have a new “Training Plans” tab (above) on my blog where I post my most popular plans. Here are one-week samples from my two most popular plans for half-distance triathlon races.