Bicycling Science (3rd edition) by David Gordon Wilson.
Bicycling Science (3rd edition) by David Gordon Wilson is a book that many BMXers will want to read. It begins with a thorough and well-documented chapter on the history of the bicycle, followed by chapters on the physiology of human-powered travel and how bicyclists keep cool. Part II of Bicycling Science has seven chapters on the physics of the bicycle. Part III is a look at the present and future of human-powered machines. Part II on the physics of bicycles is likely to be of interest to BMXers. For those engaged in the design of racing frames and parts, Bicycling Science is likely to be required reading. However, what about the BMX racer who wants to know how they can set up a racing bike so it can go faster?
Although Bicycling Science is aimed mostly at the road racer (picture Lance Armstrong in the Tour de France), there are a lot of ideas in this book that can be applied to BMX racing. According to Wilson, the two most important ways to improve cycling speed are cutting aerodynamic drag (wind resistance) and training. "Tight clothes, a good body position, and an aerodynamicaly clear bicycle can, in combination, cut [drag] by 50% or more" (p. 150). Other ways to reduce wind resistance are to use the typical road-racing position with the middle of the rider's back being the highest part of the body and bringing arms inward. Drafting behind another rider is always seen in televised road races. The typical road-racing position is impractical for BMX racers but paying some attention to wind resistance may give a BMX racer that critical edge in a close race. It seems that there are three possible ways to reduce wind resistance of the typical BMX racer. First, look at the design and size of the number plate. Smaller number plates installed so they tend to bow out in the center might be better than a large, flat number plate. When practical (i.e., when it's not too hot), somewhat tighter race clothing might offer less wind resistance. Finally, if the rider is not in first place, drafting behind another rider might allow the rider to save enough energy to make a move toward the end of the race. Then again, I always instruct my son to "not follow" when in a big race so he can get around any crashes. All in all, it seems pretty unlikely that wind resistance will affect the outcome of a sprint like a BMX race but a little thought directed at making the bike and rider a bit less wind resistant might make a difference.
The second greatest potential for speed improvement is training. Even in a sprint lasting about a minute, the rider who has trained hard will be faster, other things being equal. In bigger races, the rider may need to get through qualifying, eighth-mains, quarter-mains, and semi-mains, before they get to the main event. The rider who has trained hard will have the most stamina and be able to race their hardest through these races.
The third best way to improve cycling speed is through properly inflated tires. Generally, higher inflation pressures result in less rolling resistance and therefore a faster bike. Of course, this applies to a hard, smooth track. If the track is soft, or hard and bumpy, it may be best to experiment with tire pressures to see what works best. Slightly softer tires on a hard, bumpy track might allow the rider to remain more stable and have better control. Also, a tire that is bouncing off of the track is not getting the traction needed to go fast.
Gearing is a major variable for most BMX racers and Wilson has a lot to say about it. Some of his conclusions contradict standard practices in BMX racing. First of all, Wilson concludes that a minimum of 21 teeth are needed for greatest efficiency and that sprockets with 12 teeth were "unusually inefficient (p. 321). A freewheel (rear) with 16 teeth which is the typical size for a BMX racer is right in the middle between the least efficient and most efficient. Thus, riders and mechanics who are comfortable with such a switch might consider whether a more efficient gearing might result if a freewheel of 17 or more teeth was used instead of the typical 16-teeth sprocket. A gear chart would be needed to select the correct front or chain-wheel size. Chain alignment and lubrication were found to have "negligible" effects on efficiency.
Finally, what about the weight of the bicycle? Wilson concludes that small reductions in bicycle weight (say around 15% of bicycle weight) do not result in speed improvement. Howeve, there is more benefit from weight reduction on a steep hill. Since the obstacles in BMX racing may have more in common with a steep hill than a flat road racing course, the total weight of the bike may have a noticeable impact on speed, especially for riders who need quick acceleration in order to jump obstacles. However, gearing and training may be the most important things for BMX racers to consider.
Summing up: Bicycling Science is a an excellent book that summarizes just about everything known about the physics of bicycling. The last two chapters on unusual human powered machines and the future of human powered machines were particularly entertaining. I highly recommend this book to anyone who is seriously interested in the science of biking. Most BMX mechanics will find a lot of information that may influence how they practice their craft.
