Small Wheels are Slower, Right?

Hey, loyal readers! Sorry to disappoint all six of you with the 4 day layoff, but it was a busy weekend of . . . well, nothing. Stuff went down, etc. . . in the future you can expect at least 3 posts a week, sometimes as many as 5 though, so . . . you got that going for you. Anyway, on to the question of the day: Are Small Wheels Slower? This presumption is a common misconception, with some basis in fact, but probably not for the reasons many would expect. The most common point was addressed last time -- different gearing ratios allow small wheels to propel you at the same speed as larger wheels. Beyond this issue, we have to flip back to the physics that govern how a wheel functions. (Before delving into this I'd like to thank Bike Friday, Alex Moulton, and Tony Hadland for contributing to my own knowledge of these issues. I'll summarize what I gleaned from them in this post.)

I'll kick off with some of the more easily understood advantages. 1) Smaller wheels weigh less; 2) they present a smaller frontal area, and thus less aerodynamic drag; 3) the shorter spokes also produce less turbulence as they rotate -- turbulence equates to aerodynamic drag, thus smaller wheel are doubly better in this case. (You can even use disc wheels or some sort of cover for the spokes to further alleviate this issue as in a time trial bike with a much lower destabilizing effect from crosswind)


Ok, so those 3 points are pretty easy to grasp; the next issue relates to inertia and involves both positives and negatives for small wheeled bicycles. When you accelerate from a stop on a bicycle, the vast majority of your energy goes towards putting the mass of your body and your bicycle into forward motion. In the case of the wheels, you are not just moving their mass in the direction you are traveling, but setting the whole mass spinning as well. The lower overall mass of small wheels means this is an easier task, but you also benefit from the fact that the mass is closer to the axle of the wheel.

Imagine opening a door. Normally, the knob is placed far away from the hinges (the fulcrum) and it requires relatively little force to open. Now imagine closing the same door, but instead of pushing near the knob, you apply force very close to the hinge . . . it will be incredibly difficult. The same concept applies to bicycle wheels -- a case in which you are always driving forward momentum by applying a force near the fulcrum point (the rear hub). Thus the larger the wheel, and consequently the further away the rim and tire are from the center of the hub, the more force you will have to impart to start the wheel rotating. (I think that may have been the most simplified explanation of torque and the physics of levers and wheels in all of human history; for a more detailed discussion go here, or here, or check out this book.) The end result of all of this is that is is easier to accelerate a bicycle with smaller wheels. This by the way is not just a hypothetical difference, it is noticeably easier. The converse though is that at speed, you have a smaller flywheel effect from the mass of the wheels tendency to remain rotating. This means that after speeding down a hill on your small wheeled bike, you will experience a little less momentum carrying you up the next hill. I cannot say I have personally noticed this to the same extent I notice quicker acceleration at low speeds, but science says that it is there.

I am sure you as you read all this, you are thinking "That physics stuff is great, and I conceptually understand these effects, but what concrete numbers can I apply to actually riding?" Thankfully someone else wondered the same thing and the end result is that the smaller rotating mass of small wheels is more efficient below 16 mph, and the greater gyroscopic effect of a larger wheel is beneficial above 33mph; between the two speeds the difference is minimal. I am sure that we all think of ourselves as speed demons, but how often do you really go above 33 mph? Not that often right? (If you disagree, then you are deluded -- the record speed in an individual time trial in the Tour de France is a 34.27 mph average over a 4.47 mile course). So in the case of normal riding, small wheels will either benefit you or be no different from regular wheels speed wise. If you live in a city and commute like my self, smaller wheels are especially nice because you are constantly stopping and starting with the flow of traffic.

If you are still skeptical, keep in mind that virtually all human powered vehicle speed records are set with small wheels. Check out the International Human Powered Vehicle Association (IHPVA) for further info.

Varna Diablo II - the fastest human powered vehicle on earth at 81 mph

Moulton Liner II - The fastest upright bicycle in the world since 1986 (51.29 mph)

All in all, I think that should answer any doubts about the speed of small wheels. Next time we look at the ride quality / comfort of small wheels and folding bikes. As for now, get up and go ride.