Project SSR, Part Two: The Science of Singlespeeds

Singlespeed bikes are the most efficient bikes in the world (except highwheelers). From the hour record , to Jennie Reed’s 2008 Keirin World Championship, when looking to getting the most return for your effort nothing beats one gear. Most hillclimb records have even been set on singlespeeds. Why? Drivetrain efficiency and weight.

track crank

What is efficiency? Scientists have a number of ways to define efficiency, for our purposes we will use “the amount of energy that the rider applies to the bike, which moves it forward.” The greater the forward movement you get out of your bike, with the same amount of effort going in, the greater it’s efficiency. A singlespeed drive bike is more efficient then a geared bike because less of your energy is “lost”, meaning used up by the bike without helping you move father forward. This is due to rear derailleur tension and the weight of the drivetrain components; the chain, cogs, chainrings, and derailleurs. Surprisingly, research has shown that there is very little friction added to the chain due to routing it through the pulleys of the rear derailleur. The decreased efficiency seems to have more to do with the tension applied to the chain, and the added weight. A bicycle chain is extremely efficient, about 98% of the energy applied to the chain is used to move you forward, and this is barely decreased on a geared bike. So we must look at the whole drivetrain to find the rest of the difference.

As any weight weenie will tell you, the weight called “rotational weight” is the most important on your bike. Rotational weight is the weight of the parts which spin, like your wheels and cranks. This type of weight is so critical because any effort we put into spinning it around does not help move us forward. The most important thing about rotational weight is that the further it is from what it is spinning around, the more energy it takes to spin it. You can easily feel this by taking a bike part and putting it on a board with a can, or beer bottle, under the middle of the board. First put the part near the bottle and push down on the other side a few inches away, pretty easy right? Now move the part to the end of the board and push down in the same place as before, see how much heavier it feels? When we look at wheels in a future post this will be a critical concept. In terms of a geared drivetrain this idea is important too. The chainrings, cogs, derailleur pulleys, and longer chain on a geared bike are all rotational weight and decrease its efficiency. That’s why just riding a geared bike without shifting feels nothing like riding a singlespeed bike. The added weight of the derailleurs, shifters, and cables is not rotational, but still adds to the overall weight of the bike. Overall weight is also important because a lighter bike accelerates faster and climbs with less effort. This is due to inertia, gravity and mass. Inertia is the resistance which must be overcome to get something moving. The greater the mass of the object, roughly the same as its weight, the more resistance there is. Greater mass also makes it harder to overcome gravity, the effect you encounter climbing. Yet, since all objects near earth fall at the same rate a light bike will not make you descend any slower. Without as many drivetrain components a singlespeed is more efficient, and lighter.

cranks

Being lighter and more efficient then a geared bike is a good start, but how do we get the most out of these advantages? We can start be further increasing drivetrain efficiency, reducing more weight, and adjusting our riding style to take advantage of these differences. We can also make some changes which apply equally to all types of bikes. All of these topics will be looked, in detail, in future posts.

For more about this series, see the first post here.

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