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My understanding is that air resistance is where most of the pedal power goes to at high speeds, and that this is a concern not least for the wheels: What you pay for in a road bike wheel is aerodynamic and/or light wheels, not so much to other losses like rolling resistance and bearings.

So if air resistance is the thing to optimize a wheel for, the wheel should be tiny. Obviously in width, but also in radius, because even shortening the length of a surface reduces boundary layer drag, and the top half of each wheel is basically a surface that moves faster than the rest of the bike. Has anybody thought of this?


Postulate: The faster angular speed of a small wheel is irrelevant to air resistance: If the forward speed is given, every point on the wheel at a given fractional wheel radius has a speed compared to the ground that is independent of the wheel radius. In other words, shrinking the wheel does not speed up any point on its surface; you just get less surface. Which should be good!


Some speed record bikes have had small wheels, including the current record holder.

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    Uh, what is the single largest (by far) source of both weight and wind resistance for a bicycle (that's being ridden by a human)? – Daniel R Hicks Apr 22 at 22:03
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    And concoct an adult-sized bike with 16-inch wheels. Take it out on a highway and ride it 50 miles. Then come back and explain why wheels should be small. – Daniel R Hicks Apr 23 at 0:34
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    @Mazura yes, that's what a wheel is. The question was, why one should not be optimized for air resistance and how to explain the wheels that are commonly used in racing. – ojs Apr 23 at 5:37
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    @DanielRHicks Or just roll a piece of furniture on casters down the street. – David Richerby Apr 23 at 8:35
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    The rule book never explicitly states that bikes are banned for being too good or aerodynamic. They have rules on tube shapes and bike geometries which are to keep the spirit of the sport. A bike being raced today must be recognisably similar to bikes of the past. A recumbent is banned because of this rule. The rule would also prohibit pennyfarthings, so it's not performance. – Carbon side up Apr 23 at 8:44
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+50

The boundary layer drag (skin friction) is pretty small at large Reynolds numbers and even if still significant, most of it does not happen on the wheels, but also on the frame and on the rider, so the wheel causes only a small part of the drag (and even of the boundary layer drag). For bluff bodies, and a bicycle (and a rider) is a bluff body, the decisive part of the drag is the form drag, not the skin friction.

As explained in other answers, all other practical factors favour the design of bicycles as we know them. Especially for off road bicycles the aerodynamics is not very important and you can see how bike are designed for handling other capabilities.

Road aerobikes don't really choose to use smaller wheels (although 650 mm wheels are readily available). What is MUCH more important to the drag of the wheels is the tyre width, because that directly changes the form drag. The wider the tyres, the larger the form drag. Other forms of drag are obviously different. I DO know that racers use larger tyres for today's races, that does not invalidate the argument (I use 40 mm tyres myself for touring and gravel). But thin tyres DO offer lower aerodynamical (not rolling) resistance.

  • In road racing there has been a shift to wider tires over the last few years – ojs Apr 23 at 7:31
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    @ojs Indeed, what does that change? Is anything I wrote wrong? Wider tyres lower the rolling resistance, but they definitely DO increase the aerodynamic drag, that is simple physics. They are also heavier. In practice one aims for the fastest bike for the race, not for the bike with the lowest aero resistance or the lowest rolling resistance or whatever. Sometimes that is an aero bike, sometimes a superlight bike, sometimes a cobblestone special bike with suspension, sometimes maybe even a cyclocross bike. – Vladimir F Apr 23 at 7:46
  • The move to wider tyres in racing is much more widely discussed than implemented. While 23mm tyres are all but relics, it's still only the cobbles that will ever see the pros riding anything bigger than a 25. The switch to wider tyres is mostly at the bottom levels of racing still. – Carbon side up Apr 23 at 8:39
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    @user2394284 The difference won't be big. The air in the atmosphere is turbulent and even an artificial laminar inflow would quickly generate turbulence while hitting the wheel. – Vladimir F Apr 23 at 19:08
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    I'm in Australia, racing against the domestic UCI continental teams and a few Asian teams. I have a couple of friends in the Worldtour who say that it's the same there despite what you'll read online. – Carbon side up Apr 24 at 3:16
40

Assuming a bicycle with a conventional rider position, the rider cannot be positioned any lower because there has to be a certain amount of clearance between the cranks and the road. As wheels shrink, the frame has to extend downwards to reach the axles, so you are not really removing structure that causes drag, you are replacing it with something else.

Smaller wheels have higher rolling resistance, at some point that become significant compared to aero drag.

There are a host of other practical reasons, for instance:

As the driving wheel decreases in size the gear ratios have to increase to maintain the same distance travelled for a turn of the cranks. This is why folding bikes have huge chainrings.

Small wheels are less stable, as anyone who has ridden a Razor scooter knows.

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    Has anyone done research on what the optimum wheel diameter for bicycles on flat roads with smooth tarmac surface is? It’s rather strange that it’s pretty much only 622mm wheels for all disciplines and riders. – Michael Apr 23 at 17:13
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    @Michael This id a good question, and you should probably ask it as a new question, referring to this one. – Argenti Apparatus Apr 23 at 17:16
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    @Michael 650(B,C) are relatively common, though not vastly different from 700C. Many recumbents use smaller wheels, and of course they have different ergonomic aspects - the human factor that's part of the trade-off in any real system. Again, an interesting question in its own right – Chris H Apr 23 at 20:50
  • I'm aware that I'm replacing a structure that causes drag with something else, but I'm convinced that it has a (small) benefit! What I'm looking for in an answer are arguments for or against this. – user2394284 Apr 26 at 16:41
2

You also have to count for stuff you can get over - that's why 29-inch wheels are so popular with the mountain bike crowd these days. They suck at cornering, and that's why they came up with 27.5's. Smaller wheel diameters are great for accelerating quicker, hence the popularity of 650C wheel sets for triathlons. Scooters have tiny wheels because they ideally are ridden on paved roads or sidewalks, and can therefore get by without hitting any major potholes or bumps. At some point, people got together and determined an ideal threshold for wheel size that would prevent accidents by their sole ability to get over stuff. That's the best explanation I've got!

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    I understood that cycling leg in triathlon involves accelerating once and riding 180km in straight line. Acceleration would be more important in track and mass start racing. – ojs Apr 23 at 5:27
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    @ChrisH triathlon still does not have sprints or breakaways where acceleration would be far more important – ojs Apr 23 at 6:31
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    Triathlon is much more diverse than these comments suggest. There are many different distances and styles involved. An Iron Man triathlon has a 112 mile solo effort, where TT/tri bikes are allowed, but drafting is forbidden. Olympic distance and sprint triathlon both involve regular drop bar bikes, and bunch riding on shorter courses. Overall, short sharp acceleration is much less important across all tri disciplines than it is in road racing though. – Carbon side up Apr 23 at 8:36
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    Smaller wheel diameters are great for accelerating quicker, hence the popularity of 650C wheel sets for triathlons. I've got to jump into the chorus of everyone saying, "Huh?!?!" to that. Triathlons don't have attacks that racers have to suddenly accelerate to jump onto, they don't have breakaways where the riders covering the break have to work hard with lots of sharp accelerations in order to screw up any attempts to bridge up to the break, and triathlons sure don't have mass-finish sprints with rapid acceleration up to 40+ mph/60+ kph. – Andrew Henle Apr 23 at 10:19
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    (cont) And there's nothing in triathlons like a highly technical criterium in an urban setting with 8 or 12 tight 90-degree (or sharper...) turns for each .6 mile/1 km (or less...) lap - with a race being maybe 30 or 40 laps. No one racing a triathlon ever got "accordianed off the back of the pack" from repeated rapid deceleration/acceleration that causes riders to blow up and that can happen at the rear of the peloton in such a race. – Andrew Henle Apr 23 at 10:22
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20 inch wheels accelerate faster than larger wheels like 700c. However, the distance traveled is less at the same gear ratios.

-6

Have you ever played with a toy Gyroscope? The same principal is in play with a bicycle. A small wheel would have a similar affect to not moving, i.e. not stable.

As a larger wheel spins up, it keeps the bicycle upright.

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    Gyroscopic forces don't contribute significantly to bicycle stability, unless you're going very fast. If the gyroscopic forces really were significant, then trying to steer would throw you off. Kooijman et al., (A Bicycle Can Be Self-Stable Without Gyroscopic or Caster Effects, Science 332(6027):339-342, 2011) describe a bike modified to have extra contra-rotating wheels to cancel out the gyro forces. It was completely stable. – David Richerby Apr 23 at 13:42

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