Buying a Used Bicycle
This guide has the following information:
(1) Determining the size of a bike
(2) Determining your best fit
(3) Inspecting a used bike
(1) Determining the size of a bike
Don't trust what people say the frame size is. Insist on asking for actual measurements, and be sure to measure the bike yourself. Important dimensions to determine are: (A) the stand over height; (B) the top tube length; (C) the seat tube length.
(A) The stand over height is the distance from the ground to the top of the top horizontal tube. You need to know this in order to determine whether you can comfortably stand over the bike.
(B) The top tube is the horizontal tube. It is important to know its length because it determines how "stretched out" you will be when sitting on the bike and reaching the handlebars. There are two types of road bike frames: traditional and compact. On a traditional frame, the top tube is completely horizontal, whereas on a compact frame, the top tube slopes downward toward the seat tube. On a traditional frame, the top tube is is measured from the midpoint of the seat tube to the midpoint of the head tube. On a compact frame, you use what is called the "effective top tube length," where you draw an imaginary horizontal line where the top tube would be if it were horizontal. (Note: The seat tube is the vertical tube connected to the seat, and the head tube is the small vertical tube connected to the handlebars. The midpoint is the point halfway across the diameter of the tube).
(C) The seat tube is the vertical tube that connects to the seat post. I believe it is the least important of the three measurements to know, but it is still good to know it. It is measured from the center of the crank to the midpoint where the seat tube intersects the top tube.
(2) Determining your best fit
You will want a frame that you can comfortably stand over with about 1-2 inches clearance from your public bone. Here are instructions on how to measure your pubic bone height.
You will also want a top tube length that will provide a comfortable distance for you to reach the handlebars, not too stretched out, and not to scrunched up. A quick way to estimate the correct distance is to place your elbow on the front of the seat, and place your fingers on the handlebar stem. Your fingers should approximately reach the handlebars. Here is a demonstration of this. Minor adjustments can be made if necessary by replacing the stem with one that is shorter or longer (e.g., + or - 1 inch).
The Competitive Cyclist website has a great interactive Fit Calculator that helps to determine your correct frame size, and then lists the measurements according to three different frame size fitting philosophies. (Note: For most people who are not racing, their "Eddy fit" or "French Fit" will be most appropriate). Rivendell also has some good information about bicycle fitting. Here is another site that has good information.
(3) Inspecting a used bike
Here is a link to great information about how to inspect a used bike.
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http://www.ebicycles.com/article/
http://www.usedbicycleguide.com/index.html
Deciding On A Road, Mountain Or Hybrid Bicycle
Determining Bicycle Size And Fit
How To Inspect And Test Ride A Used Bicycle Before Buying It
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Avondale Cycles: 2754 E. College Ave., Avondale Estates. Phone 404-373-3850. http://decatur.patch.com/articles/bike-shop-opens-in-avondale-estates
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Starter Bikes (at Georgia Tech): http://www.atlantabike.org/starterbikes
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Atlanta Pro Bicycle: http://www.facebook.com/pages/Atlanta-Pro-Bicycle/298340712577
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Gear Revival: http://www.thegearrevival.com
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QUESTIONS TO ASK
(1) What is the standover height (from the ground to the top of the top tube)?
(2) What is the length of the top tube (measured from the midpoint at the seat tube to the midpoint at the head tube)?
(3) What is the length of the seat tube (measured from the center of the crank to the midpoint of where the seat tube intersects the top tube)? [The seat tube the vertical tube with the seat post at the top. The head tube the vertical tube that the stem and handlebars are connected to].
(4) Does it have centerpull (cantilever) brakes or sidepull brakes?