At 6 feet, you're a fairly tall guy. Selecting a bike that's correctly sized for your proportions is essential to being comfortable while you ride. A properly fitted bicycle reduces joint pain in your knees, soreness in your back and neck, and is much healthier for your body over long miles. Mountain bikes feature radically diverse frame designs, but they're all still fitted on a standard measurement scale that makes sizing the bike easy. Take a few key measurements, and you'll be able to confidently select the frame that's right for your proportions.
Getting a Leg Up
Your first step is to find your pubic bone height, or inseam, an essential measurement for sizing a bicycle frame. Your pubic bone height is the distance between the base of your pubic bone, located between your legs, to the base of your heel. When you take the measurement, follow the inside of your leg, and use a standard tape measure. You'll want to be wearing the shoes you intend to cycle with, since that's where you'll actually contact the pedals of the bike.
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Once you have your pubic bone height, you can match this to a manufacturer's sizing chart to find the closest match for a particular bike frame. For mountain bikes, the seat tube measurement corresponds to your pubic bone height and indicates the size of the frame. Mountain bikes use standard measurement, and a 6-foot tall man will fall somewhere between 18 and 20 inches, depending on your inseam measurement. If you're between sizes, it's best to size up, since you can make up the differences with a higher seat and longer stem.
To fine-tune the fit of your bike, you'll want to make sure your seat is at the optimum height. Lean the bike up against the wall and mount it, and keep adjusting the seat until you can sit comfortably with your foot on the pedal when the crank arm is pointing straight down. In this position, your knee should be perfectly straight, or just slightly bent. Next, lean forward and contact the handlebars, and adjust the seat's tilt until all your weight is situated on the bones of your hip, rather than the soft tissue between your legs.
For the most control and balance on the bike, check the height and stem length for your handlebars. You'll want to be comfortably leaned forward on the bike, with your elbows slightly bent, your head naturally pointed straight ahead, and your shoulders relaxed. Depending on the length of your top tube, you'll need a stem to make up the last inch or so between your headset and the handlebars. Try out a few different heights and stem sizes until you find one that is the most comfortable for you.