About Mountain Bike Frame Sizes

Significance

• The most common form of mountain bike frame sizing is a seat tube measurement. The seat tube is the part of the frame that runs from the seat post to the bottom bracket. These measurements typically range from 15 to 22 inches for adult mountain bikes. Some companies use a metric system and denote the frame size in centimeters or millimeters rather than inches.
  
  

Function

• Although most sizes are cited using a seat tube measurement, equally important is the top tube measurement. The top tube runs the length of the bike and will determine how comfortably that you are able to position your body. Too long of a top tube and you won't be able to properly reach the handlebars, too short and you will be scrunched. Though top tube isn't ordinarily cited in the size, you should be able to find it listed on the bike's specifications or geometry.
  
  

Misconceptions

• One of the main things that makes sizing a mountain bike difficult is that companies vary in how they measure the seat tube. Some companies measure from the middle of the bottom bracket to the top of the seat tube while others measure from the center of the bottom bracket to the center of the top tube. Since measuring standards vary, bikes of the same size from different companies could very well vary in fit. Further, mountain bikes have different geometry which will also affect how well a bike fits. Finally, some companies do not use a numerical system at all, but rather use small, medium, large and extra large designations, making it more confusing.
  
  

Prevention/Solution

• The good thing about sizing a mountain bike is that you'll be there in person--therefore, you can largely ignore the company-defined size. Mountain bikes are not a purchase that you should make over the Internet or phone. In fact, most major bike retailers don't ship mountain bikes. Therefore, you want to go in person to the bike shop to browse models and make your purchase. Since you are there, you'll be able to size the bike out in person, making the company-spec size of the bike irrelevant. In this way, you'll be sure that the bike fits properly and won't need to worry about measurement discrepancies.
  
  

Size

• Many different methods and theories on bike fitting exist. For the purpose of this article, we'll go with the most simple one. Check out the resources below for different methods and more in-depth discussion. Straddle the top tube of the mountain bike with your feet flat on the ground. Ensure that you have 2 to 3 inches of clearance for a cross country bike and 4 to 5 for a free ride or downhill bike. Sit on the bike and with your foot resting on the pedal at the bottom of the stroke, your knee should be bent slightly. Adjust seat post as needed. You should also have a slight bend at the elbows when holding the handlebars. You should not be stretched out uncomfortably or scrunched with drastic bend in the elbows. You can move the seat forward or backward to help adjust. Test ride the bike and determine if it is comfortable.
  
  

Considerations

• Remember that small adjustments are available to fit a frame better. If you're experiencing discomfort or performance issues after purchasing a bike, consider switching some hardware. The seat post can help to reposition your pedaling height and a longer or shorter stem will adjust the overall stretch of your riding position.