For newbies: Which frame size?

Thanks for explaining standover height. I'm buying a bike for a friend who is about 5'10. What size frame would fit best? —M

Good lord, tell me this is not going to be a surprise gift. The chances of buying a good-fitting bike for someone else are about as good as dropping a laser shot down the garbage chute of the Death Star.

Frame size depends not so much on the rider's overall height as on the relationship between leg length and torso length. Another way to think of it: The frame ought to accommodate you lengthwise as well as heightwise. Men tend to have shorter legs but longer torsos at any given height compared to women, and bikes for dirt-trail use generally are not as tall as bikes for use only on pavement. It all factors in.

Here are some tips for the rider, whom you should bring along for test rides. Otherwise—use the Force, M.

Before test-riding, adjust the seat height so that the knee is still a little bent even at the bottom of the pedal stroke. If the seat doesn't go up high enough (watch for the "minimum insertion" mark on the seatpost), try a bigger bike. While riding, pay attention to how it feels not only height-wise, but lengthwise. Do you feel too far "ahead of" or "behind" the pedals? Is the handlebar far enough way that you don't feel cramped, and your back is not arched or humped? When you pedal, do your knees seem to overlap your elbows a lot, or just a little?

Even (especially) if the verdict is bad, measure how long the distance was from the seat to the handlebar. That will give you a useful comparison number for screening other bikes. If it feels close but not quite, keep in mind that the seat can be adjusted a little fore and aft, and the stem—the piece that connects the handlebar to the rest of the bike—can be replaced with one that's longer or shorter.