Mountain Bike Sizing and Fit
The most important measurement in getting a proper size mountain bike is the stand-over height, also called inseam clearance or crotch clearance.
Crotch clearance is the distance between the top tube and your crotch when you are standing over the bike in your bike shoes, with the small of your back touching the front tip of the bike saddle or seat. You need adequate clearance in case of a crash or you need to dismount the bike seat in a hurry.
Many mountain bikes today have a sloping top tube. The sloping top tube gives you extra crotch clearance without getting a frame size that is too small for your physique.
Since you are buying your bike at the local bike shop, someone there should be able to get you the correct size bike.
Ideally, you should have at least 3” to 4” of clearance so you can hop off the seat without hurting yourself on the top tube of the bike.
Another important measurement is handlebar reach. Handlebar reach is the distance between the seat and handlebar. Again, since you are buying your bike at the local bike shop, someone there should be able to help you with getting the proper reach.
A proper reach to the handlebars is important to your comfort while riding your mountain bike. If the handlebar is too far away from you while you are on your seat, you will likely be extending your arms too far, putting too much weight on your hands and arms, bending too forward at the waist and cranking your neck too far back. This will surely lead to hand, shoulder, neck and back pain. If the handlebar is close to you, you will put strain on your arms and shoulders, and your control of the bike will be diminished. An improper handlebar search will also cause your behind to shift forward or backward on your seat as you pedal the bike.
To get a rough estimate of the proper handlebar reach, sit on the bike seat with you feet on the pedals, grip the handlebar and look forward as if you are in a normal riding position.
Now have someone look at you from the side. Now, have someone look at you from the side. Ideally, the tip of your nose should be within an inch of the center of the handlebar and your arms should be slightly bent at the elbows. You should feel equal pressure on your hands and butt.
For small adjustments, you should be able to get the most comfortable handlebar reach by either moving the seat backward or forward. However, if the handlebar reach is too far off the ideal, you may have to change the length of the stem.
But ultimately, the ideal reach is what you found most comfort while you are riding the mountain bike.
Once your handlebar reach has bee determined, you need to adjust your seat to the proper height. Ideally, when you are sitting on the bike seat, your legs should bend just slightly at the knee when you are at the bottom of a pedal stroke.
Another way find your ideal saddle height is by placing your heels on the pedals and pedal backwards. When your legs are completely extended at the bottoms of the pedal strokes with your heels on the pedals, you have found your optimum seat height – for most relatively flat trails.
When you are riding trails with elevation changes, you should lower your seat about an inch or more from the flat trail optimum height.
But to get the full feel for how your mountain bike fits you, you will have to take the bike for an extended test ride before leaving the bike shop. Even than, as you ride you new bike more and more you will undoubtedly find that your mountain bike needs further adjustment to fine-tune its performance and comfort.