![]() However, there is a simple computational approach for computing resistance by multiplying air velocity squared by some values. Real world rolling resistance will be different, mostly because the actual road surface has a big effect.Īerodynamics is a fundamentally complex and difficult subject. There are big tables of coefficients you can find on the internet, yes, but those measurements are from tests conducted on steel drums. While this seems simple, there is a problem: it is difficult to make the measurements required to arrive at these coefficients. Everyone knows there is more resistance from a knobby mountain bike tire than a 23mm road bike tire - the MTB tire has a higher coefficient of rolling resistance. ![]() Rolling resistance is figured by multiplying weight by a number that gives the force, the coefficient of rolling resistance. ![]() GPS readings for elevation are highly inaccurate, however - the satellites are rarely in a good position. A good barometric altimeter can work well, too. Measuring grade with a ruler and a level is not such an easy thing, but the combination of a reasonably long distance and a topological map will provide a decent value. An inconvenient thing about "weight" is that kg is a unit of mass, not a unit of force, so it must be converted to newtons. For example, if you and your bike weigh 200 lbs and you are riding up a 1% grade, there is 2 lbs of gravity force holding you back. It turns out that the "grade" of a road - the rise divided by the distance traveled - is a quantity you can multiply by your weight to get the force of gravity. The only force in cycling that is easy to figure out is gravity, but I mean only the force you must resist as you climb, or the force that pushes you along when you descend. In the U.S., we all know that horsepower is a unit of power. It's more common in engineering to use a unit of power that comes from electrical units - watts. In metric units, we're talking about newtons (a unit of force) times meters/second. The sum of those forces can be multiplied by speed - this is the quantity known as "power". There is also some frictional force in the chain, gears, and bearings. The three major forces that must be overcome to maintain a certain speed are air resistance, rolling resistance, and gravity.
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