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650c wheels are slightly smaller in diameter and in circumference than 700c wheels. This smaller wheel size is a good alternative to match up with smaller road bike frame sizes, in particular from 49cm and smaller. Frame geometry is one of the main considerations, but another is what is called "toe overlap" - when a cyclist steers left or right, and is pedaling, their shoes should not strike the rear of the front tire. On smaller sized bike frames, and for riders with longer feet, this is a real bike handling and safety concern. For some, the riding comparison is tell absolutely no difference between 650c wheels and 700c wheels. Over a 10 mile ride, you may not ride any faster, and you may not descend - going downhill - any faster. There is one group of riders who made a case for 650c wheels, and that was triathletes. You would see tall riders on larger frame sizes outfitted with 650c wheels. The idea was that the smaller wheel size was an aerodynamic advantage, not to mention a weight savings. This played right into the "weight weenie" mentality that many triathletes have. Using 650c wheels for triathlon was common in the 1980's and 1990's, but current generation triathletes generally use 700c wheelsets. One other issue was spin-up speed for sprinting, and it's thought that 650c wheels spin up faster. On the flip side, there is maintaining angular velocity, and larger 700c wheels in motion will tend to stay in motion more easily, so it is thought. With a 650c wheel, you may get wobble on a downhill - but that may be due to your bike being too big, not the size of the wheels. Many bike guys in LBS will agree, there is little to no difference between 650c and 700c wheels in the grand scheme of things. Generally, smaller frame sizes will look proportionately better with 650c wheels on them. One example is the Velocity Uriel wheels. 650c wheels will help to avoid toe overlap and decrease standover height. Even smaller than 650c wheels, there are 24" and 20" wheels. Team time trial bikes were once built with 650c wheels because riders could draft that much closer to the rider in front of them. One thing about a 650c wheel is that it does limits your choice of bicycle tires. If you are on a group ride, even with SAG support, there may not be a spare 650c tire available should one be needed. 650c wheels feel a little different in tight, low speed turns and then on quick accelerations. Triathlon bikes that use the 650c wheelsets do so because for many of the frame designs, the larger 700c wheels will not fit in the rear triangle. Weight is arguably not really a factor for triathlon bikes because most of the racing is done at a relativly constant speed. Quality 650c tires are available from most online bike shops and your LBS should carry a few tire brandsl. There are arguments for 650c wheels, and you can do the math, which will show that you lose a little downhill momentum with 650c wheels. Aero wheels maintain their advantage until a road incline reaches a grade of roughly 4%. At that point, lightweight wheels gain the advantage. The steeper the grade, the greater the advantage of weight savings. A figure skater shows an example of why 650c wheels don't hold momentum as long. As the skater with outstretched arms spins slowly, the skater brings her arms in and starts to spin faster. This is related to the rotating mass. The further the weight is from the axis of a turning object, the more force required to turn that object. 650c wheels when compared to 700c wheels benefit in two ways from this principle. Most of the major bike brands make products for 650c sized wheels. Bicycle shops with a triathlon influence will be your best bet to find a selection of 650c wheels and tires. Note that the gearing with 650c wheels is different. If you use a gear calculator, you will find that you need a cassette with less teeth, when paired with a 650c wheel, to get the equivalent gear-inches rating as a larger cassette with a 700c wheel.