# Trek 1500 WSD with 650c tires



## runner1000 (Jul 15, 2005)

Because of the size frame that I need, this bike has 650c tires. Could this be a problem?


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## Mark McM (Jun 18, 2005)

runner1000 said:


> Because of the size frame that I need, this bike has 650c tires. Could this be a problem?


Well, a bike should be designed to fit a rider, so if a particular bike is much smaller than average, it could be more of a problem trying to fit 700c wheels to the frame than any problems introduced by 650c wheels. A bike with 650c wheels can be perfectly fine, as long as the geometry and overall design of the bike are adapted to the wheel size. I'm a little confused though, since the Trek web page shows that all sizes of the Trek 1500 WSD use 700c wheels ( http://www2.trekbikes.com/Bikes/Road/Performance_Road/Alpha_Aluminum/1500_WSD/index.php). 

As far as general issues with 650c wheels: There is not as wide a selection of 650c tires as 700c tires, especially in wider sizes (about 23mm). However, this situation is improving, as more bikes are being made with 650c wheels. All else being equal, a smaller wheel will be rougher riding and have higher rolling resistance than a larger wheel, but as the difference in diameter between the two wheels isn't very large, these affects should be reasonably small. These affects are also compensated by the fact that the smaller wheels will be lighter and have less air resistance.

All in all, for smaller riders, a 650c wheel bike is often a better solution than trying to build a small enough bike around 700c wheels.

Back to the Trek 1500 WSD geometry chart, I notice that Trek is playing games with the numbers, and the three smallest sizes are really effectively the same size in the dimension that really matters the most - the cockpit length (reach to the handlebars). Although the top length decreases as the frame size decreases, the seat tube angle increases as the frame size decreases. The affect of the steeper seat tube angle is to push the top tube forward, so that the effective reach to the handlebars stays almost identical for all three of the smaller frame sizes. I had thought that industry was moving away from playing these games, and were really building bikes with smaller cockpit lengths to better fit smaller women riders, but I guess not.


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## greenjp (May 8, 2005)

My wife has a 2100 WSD in the 47 cm size. It's got the 650 wheels, whose circumference is ~7% smaller than a 700. In simplified terms, the smaller wheels lower the effective gear ratio, so to keep a given speed at a particular cadence, she needs to be up one gear from somebody with 700s. This needn't be an issue, since you have 9 or 10 gears out back. 

Mark, could you elaborate on the higher rolling resistance? Seems to me that a smaller wheel will have a smaller contact area, thus reducing the rolling resistance. Or does something else come into play?

jeff


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## Mark McM (Jun 18, 2005)

greenjp said:


> Mark, could you elaborate on the higher rolling resistance? Seems to me that a smaller wheel will have a smaller contact area, thus reducing the rolling resistance. Or does something else come into play?


Yes, there are two things. Firstly, the size of a tire contact area is mostly related to inflation pressure, not diameter or width. When a tire in contact with the ground is loaded, it flattens out against the road, until the force from the air pressure on the inside of the tire balances out the force from the road on the outside of the tire. Afterall, a tire is basically a flexible fabric tube (the casing) with a flexible rubber strip (the tread) glued or molded on, and flexes easily under load - it is the air pressure that keeps its shape. The area of contact with the road necessary for the air pressure to support a load is equal to the applied force divided by the air pressure . For example, if a 60 pound load were applied to a tire with 120 pound-per-square-inch of pressure, the contact area would be 60/120 = 0.5 square inches. The diameter and width of the tire will affect the shape of the contact patch, but not its area.

But that's neither here nor there, because it is not the size of the contact patch that determines the rolling resistance (well, not directly, anyway). A tire does very little "scuffing" against the road, so there is little energy loss from the friction between the rubber and pavement - if there was much scuffing, then tires couldn't last many thousands of miles. The actual cause of rolling resistance is what is called hysteresis losses. When you flex a tire, some of the energy is absorbed and turned into heat, and not returned when the tire returns to it original shape. As a tire rolls through the ground contact point, it deforms at the "flat" spot at the bottom, absorbing energy. This is why thin, lightweight tires typically have lower rolling resistance - there is less materal that has to be flexed at the contact point, and therefore less energy lost in flexing the tire. This is also why rolling resistance decreases with increasing aire pressure - with more air pressure, there is a smaller "flat spot" at the ground contact point, which means the tire has to deform less to achieve this flat spot, and consequently suffers lower hysteresis losses.

Which gets back to the 650c tires. Because of the smaller diameter of the wheel, the tire must deflect more to acheive the contact area, and therefore a smaller whieel will have a higher rolling resistance (all else being equal). This affect applies to tire width as well - at the same pressure, wider tires typically have lower rolling resistance than narrower tires, because the tire has to flex less to achieve the same ground contact area.

There is plenty of information on this on the web - do a google search on rolling resistance and tire width.


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