The Physics of Descending on a Bicycle

When a solo rider or a group of riders descend any hill, particularly a steep hill, why are some of the riders so much faster than others? 

The fastest descenders, whether by experience or instinct or learning, are the ones that sense or know the laws of physics and do everything they can to use them. 

When a rider descends, the motion of the bike is governed by a series of variables: 

--The grade of the hill 

--The total frontal area of the bicycle and rider 

--The air speed of the bike 

--The total mass of the bike and rider 

--Momentum: the combination of ground speed and mass 

--Spoke count of the wheels 

--Rolling resistance 

The grade of the hill is the most important variable. I have descended eight percent grades in the Alps and in the eastern US and never hit 50 mph, even after two or three miles. But I have gone 55 to 59 mph on half-mile hills with 15-20 percent grades. 

 

The frontal area of the bike and rider determines the top speed on any given grade. Wind resistance increases by the square of the speed. Double the speed, quadruple the wind resistance. At 11mph a rider is mostly pedaling to move the mass of bike and rider. To maintain 22mph, the same rider is putting 80% effort into moving air. The riders who descend the fastest, especially above 40mph put their crotch on the top tube and their sternum on the handlebars and pull their elbows and knees in. 

Related to wind resistance is air speed. I worked seventy miles east of my home for many years. I would ride to work once a month between April and September. I would wait for a day with a 20mph west wind and ride that 70 miles in under four hours, under 3.5 hours on the best days. When the wind was exactly behind me there were times it was quiet. I was going 22mph in a 20mph tail wind. My air speed was 2 mph. I was flying. 

I am the wrong size to be a bicycle racer. At nearly six feet and 185 pounds, I am 20 pounds heavier and several inches taller than many top racers. But descending, every pound is to the good, because… 

Mass plus ground speed makes momentum. The higher the speed and the greater the mass, the more force pushes the bike down the hill. When I pull out of the draft and sail past a 160-pound rider, momentum is my friend. 

One variable every rider can control is spoke count. Every revolution of the wheel, from the perspective of the wind, whips the spoke from no speed to twice the speed of the bike and back to zero. Low-spoke-count wheels with thin or bladed spokes reduce the wind resistance and the turbulence of spokes. The faster we ride, the more wind we whip through the spokes in our wheels. 

On a road bike with fully inflated 23 or 25mm tires, rolling resistance is negligible, but not zero. 

In summary, to go really fast downhill, find a steep grade, make yourself as small and narrow as you can, ride low-spoke-count wheels with fully inflated tires and hope the wind is behind you. I love going fast. My Strava KOMs are downhill, not up.