Niezgoda Fitness Systems Articles

The fastest wheel size is:

Would you believe that not all tri/road bikes have the same sized wheels? It’s the triathlete who flock to the size that is left of center (mostly because of under education on the topic or refuse to change with technology) and pushes the boundaries of conventional cycling. In enters the 650C wheeled tri bike (in the early 90’s tri-bikes needed 650’s). A bike with smaller than “normal” wheels makes roadies turn there nose up and shake there heads (why don’t they like us Trigeeks?).
First, let’s get the size thing right: A normal road bike has 700C wheels. 700 refers to the nominal outside diameter of an inflated tire on this rim. The “C” is a code that determines the size of rim that the tire size relates to (the 700 is actually a medium, rather than exact diameter). So a 650C has a 650 millimetre nominal tire outside diameter (the actual measurement is slightly less just like on a 700). To make things a bit more interesting 650C is often referred to as 26 inch and can be marked 26 x 1 to 26 x ¾. This is in fact different to the standard mountain bike tire rim, which is 26 x 1.0 to 26 x 2.1
So why all the fuss about such a small difference? Watched a figure skater then you may be able to figure out the answer. As a skater with arms outstretched spins slowly, the skater gently brings their arms into their side and slowly starts to spin faster. This is all related to the rotating mass. This is because the further the weight is from the axis of a turning object, the more force required to turn that object. In the case of the figure skater, by moving her arms in, her weigh is transferred from a point far away from the axis to a point much closer to the axis, the rotating mass is reduced so, the spinning becomes faster.
650C wheels (compared to 700C) benefit in two ways from this principle:
• 1. If you were to compare two wheels from the same manufacturer, one 650C and the other 700C, all other things being equal, the 650C wheel being a smaller diameter would have required less material to make, thus is lighter and has less rotating mass. For example, a HED “Aero/CX” wheel in 650C weighs 630 grams whilst in 700C it weighs 700grams.
• 2. As the diameter of the wheel is smaller, the weight of the rim is closer to the hub than the 700C wheel and thus the 650C wheel has less rotating mass. It follows that a 650C requires less force to reach the same revolution speed. In real terms this can be translated into a 650C wheel accelerating faster than a 700C wheel.
While a 650C wheel may well accelerate faster than its 700C counterpart, two things take away some of this apparent advantage: as a 650C wheel is smaller in diameter, to cover the same ground in distance terms it actually has to spin approximately 1.1 revolutions to one of a 700C wheel. As the wheel has to spin further to cover the same ground, other forces acting in a negative way are also increased (such as the resistance to rolling generated inside the hub). This also contributes to the fact that with fewer miles ridden a 650C tire will wear faster.
Being a smaller diameter, 650C wheels require fewer spokes to achieve the same strength. The actual number of spokes in a given wheel will further decrease its weight and provide less turbulence while spinning. Both of these advantages are small and are offset significantly by the fact that road shock in a 650C wheel is much more apparent than its 700C counterpart creating greater rider fatigue and more wheel hop.
Still, there are still more differences: a 650C wheel has a smaller frontal area than a 700C wheel and thus has a very slight aerodynamic advantage, less than 1 sec over a 20k. In more practical terms the availability of 650C tires, tubes and rims is somewhat limited. Most of the major brands all make products for 650C However, shops with a triathlon influence should all be able to offer a good selection. 650C wheels will require a cassette with fewer teeth to get a similar roll out as the 700C wheel. So you will not be able to have that down hill gear that many athletes require. Let me explain: Using a smaller diameter wheel affects the output of your bicycle’s gearing ratio. For weaker or beginner cyclists, this puts the gear ratio within the reach. For the average cyclist changing the big front chain-ring to a 56 tooth puts things back to normal. However this adds weight often enough to bring the 650c wheel to about the same weight, give or take, as the 700 wheel. This will also make shifting slower and more crunchy for the front derailleur. Another important point is that 650C and 700C wheels can only be ridden on a bicycle specifically designed for a wheel of that size. This includes the funny bikes, which have a 650C front, and a 700C rear wheel. 650C bikes fit small better than most 700C bikes, as the frame tubes of the bike can be kept in better proportion.
There is no clear winner here. Both size wheels have their virtues and faults. 650s are smaller, lighter, accelerate and climb better. However, they're less comfortable, decelerate faster, need a larger chain ring and are not nearly as available as 700s. In most cases, the 650 vs. 700 decisions will not affect the outcome of your race.
There are, however, two groups of people who should strongly consider one wheel over the other. Shorter riders who ride small frames can greatly benefit from a 650c wheel, since it lowers the bike height (thus giving you better stand over clearance) and reduces the chance for toe overlap. Tall riders, on the other hand, should stick with 700c wheels, since 650s on a large frame requires an insanely large head tube, and this will make the frame less stiff.
In real world situations there seems to be no significant differences in the 2 sizes in terms of speed.
Taken by themselves 650 wheels would be more aerodynamic and therefore faster in the lab, but since they are always used while attached to a bicycle and a rider this seems a rather academic point. Especially since you the rider will be more fatigued offsetting any aerodynamic advantage.
So if you are small and puny like Mickey Roonie go with the 650C’s but if you are big and bad like Alan Ladd than opt for the 700C’s

Bottom line- your wheels are not holding you back and if you are a shorter person they may be allowing you a better position which is important as the rider contributes the majority of the drag in a bicycle and rider situation.
For more information on this topic please read: Are 650c wheels relevant today?
by Dan Empfield 8.10.04 on www.slowtwitch.com