RoadJerk

This may be a ridiculous question but I've always been curious and can't think of any answer, so I have to ask...

I can see why the tires are skinny for less rolling resistance, but why do road/track bikes always posses wheel diameters larger than any other type of bike?

fadi

Smaller wheels, all else being equal, have higher rolling resistance than larger wheels.
Rolling resistance increases in near proportion as wheel diameter is decreased for a given constant inflation pressure.

They also travel a longer distance on ground than their smaller counterpart for the same rotation speed
Unless you can keep up with a super high gearing range and pedaling rpm, you need the bigger diameter to go faster...

coasting 

why? i don't understand this but then again i'm no engineer or physicist.

gumbii

inb4facepalm...

jacksonhandy

I only minored in Mechanical Engineering, but rolling resistance is not related to the dimensions of the wheel. It is related to the friction coefficient and contact patch of the tire compound and the surface that the tire is rotating on in addition to other friction related losses in the wheel bearings.

Smaller wheels have a lower moment of inertia, because of the lower mass and smaller distance from the center of rotation. If you hold mass constant, then the smaller wheel will rotate faster because the mass is located closer to the axis or point of rotation, and is easier to change the rate of rotation.

The larger wheel has a larger moment of inertia and requires more effort to change is angular velocity, actually increasing the difficulty of rotating the tire as size increases.

However, the more I think about it, you apply the force to rotate the wheel on the smaller diameter cassette, which has a much lower moment of inertia, so its probably a wash.

I'm just an Army engineer, so maybe a real one can set us both straight, this was just based on my limited understanding of the principles at work.

fadi

Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when a round object such as a ball or tire rolls on a flat surface. It is caused mainly by the deformation of the object, the deformation of the surface, or both. Additional contributing factors include wheel radius, forward speed, surface adhesion, and relative micro-sliding between the surfaces of contact. It depends very much on the material of the wheel or tire and the sort of ground.

For example, Rubber will give a bigger rolling resistance than steel. Also, sand on the ground will give more rolling resistance than concrete. A moving wheeled vehicle will gradually slow down due to rolling resistance including that of the bearings, but a train car with steel wheels running on steel rails will roll farther than a bus of the same mass with rubber tires running on tarmac. The coefficient of rolling resistance is generally much smaller for tires or balls than the coefficient of sliding friction

simonaway427

Another thing to consider is hub bearing friction. Lets say your hub bearing creates 1 unit of friction for every complete rotation of the wheel. With a smaller wheel, that hub rotates more often over a fixed difference, say 10 meters, than a larger wheel does. So a 26" MTB wheel would create say, 10 units of friction in the hub over 10m than a 700c, which would only create 7 units in 10 meters.

(again, just pulling numbers out of the air to demonstrate the example - exact figures I have no idea - I'm a business major)

fadi

+1

AEO

very very minor amount for today's bearings, which are quite high precision, even for the low-mid end stuff. That's not to say it doesn't exist however. Just very minor.

AEO

All the components have been made with 700c in mind. The smaller wheels, like 650c, 26", 24" and 20" become very limited in tall gearing compared to 700c because there's a smaller selection for small wheels.

AEO

I like smaller wheels, they're stronger for the same spoke count and have a smaller frontal section. They also fit smaller riders better.
700c is great for taller riders who use frames in the 54~60cm range, since the frame doesn't have to be fudged to make the wheels fit.

StanSeven

29" wheels are fairly common now for mountain bikes.

Right before Schwinn went out of business, they made Paramounts with both 650 and 700 wheels for nearly all frame sizes. It was an experiment to see if the 650 size would catch on. Obviously it didn't.

coasting 

now i know why i dropped physics

martialman.45

Why are road wheels so small? If your 6'6" wouldn't a 750c be better?

merlinextraligh

Hacing actually ridden a bike with small wheels, I can tell you that this bike is actually pretty darn fast. While there may possibly be some rolling resistence, and hub friction issues, you gain advantage in less weight, and better aerodynamics. (i.e. smaller section into the wind, however, its debateable whether increased speed of the spokes offsets this.)



Bike Fridays are UCI illegal, alegedly because the smaller wheels create an unfair advantage.

As for why bikes have 700c wheels, Smaller wheels do make for twitchier handling, given that you have less of a gyroscopic effect. Howver, IMHO, history and convention play a greater role in the resulting standard size of wheel, than physical properties.

coasting 

that bike friday is just silly. i would be constantly laughing if i rode that and my open mouth would create extra wind resistance.

merlinextraligh

^ If a bike that packs in a standard sized suitcase (i.e no airline fees), adjusts for a stoker from 3'6" to 6' plus, is stable enough to descend at 55 mph, ride in a 30 mph paceline, is silly, then its silly.

For a bike to ride with a growing child, and to travel with, it actually was a pretty neat bike.

coasting 

at some stage the growing child would have to swap positions with you. that would be a dear diary moment.

Yellowbeard

Pretty sure the rolling resistance advantage comes into play when the surface is not smooth. Imagine a road bike rolling into a curb; not very healthy, but it may very well make it up. Now imagine a tiny kid's tricycle trying to mount the same curb. Roads are not perfectly smooth and free from bumps.

Case in point: 29" mountain bike wheels vs. 26"ers.

Also, road wheels may be large, but the tires are typically very small. The difference between a 2 inch (or larger) tire on a 26" wheel and a 23 mm tire on a 700c wheel is pretty small.

San Rensho 

What about the handling characteristics of a larger wheel? I have no scientific evidence, but I have noticed while riding scooters that a scooter with a very small diameter wheel, like some Vespas, seem to handle poorly compared to a scooter with a 16 inch wheel.

cyclezealot

How about. Because big bikes are big.

Vinci

Gear ratio is probably a significant concern. The smaller the wheel diameter, the smaller the cogs (or the larger the chainrings) need to be to keep the same gear ratio. Since cassettes are currently limited to a 11T cog on the small end, chainrings would have to get significantly larger to make up for the smaller wheel diameter.

AEO

yes, or a real 700mm wheel.

700c with 23mm tires ends up at 26"
622+23+23 = 668mm more or less.

but it would be a heavy wheel because bigger wheels are weaker and need more spokes and rim material to keep its integrity.

AEO

lower center of gravity = stabler, but less agile
really small wheels also have a steering design issue, such as length of steerer tube required and trail.

there's shimano capreo, which goes down to 9T.

wacomme

F1 cars, I believe, use 12" wheels. There was a recent article in an auto mag showing smaller wheels were faster and more efficient. The big wheels are mostly for looks.

Does this apply to bicycles too?