BIKE EXPERTS::::::::: Is this a right BIKE FRAME? •easy best answer?

An 18″ will be very close. I would not go larger.

I actually know what I’m talking about and an 18″ is about right, as you said just an estimate. Even if you grow a little you should still be fine.

A quick answer is “No”.
At 5’11, the Trek 3700 18″ Medium frame has a top tube length (22.44″ ) that’s too short for you.
I’d say go for the 19.5″ Medium for a 23″ effective top tube length or even a 21″ Large frame with 23.4″ effective top tube.

However,based on the information you provided, an expert in bike size fitting would tell you that more information is needed to determine the “perfect” bike frame size for you. Information such as inseam measurement, reach, torso measurement and a rider profile on what type of riding you do (agressive trail, xc racing, long climbs, descents, etc.).

I am not an expert but I know that finding the “perfect” bike frame size is critical beacuase your enjoyment, comfort, handling, efficiency and skill-improvement depends on it. Therefore, I suggest that you visit a bike shop that sells the Trek 3700 so that you can demo several sizes, try them out and ride them (even only at the parking lot). The bike shop can also advice you on what frame size fits you. Also, before you make the purchase they can even dial in your correct saddle height/position, stem length as well as handlebar height. What I’m saying is that nothing beats actual testing and actual fitting.

I’m 5’8″ 140lbs. I have a Medium bike with 23″ effective top tube length which is perfect for me because it’s short enough for fast handling on technical terrain and long enough for cross country rides and climbs. I use a 90mm stem and a 16mm setback seatpost to center my weight on the bike. Sometimes I think even a 23.5″ Large frame will also fit me well (with a shorter stem and straight seatpost).

Your still growing,
An 18″ will be too small.
I’m 5’8″ and race on a 18″ in frame.
Look at a 19″ or 20″.
A good shop will let you ride both sizes.

Cycle Guy-
Nice to be called a Newbie, Semi Pro here.
I’ve been fit for all my bikes by Jeff at Intense, 18 For XC is perfect for me. My DH rig Is a 17″

Frame Size
Correct frame size is determined by straddling the bike in a standing position. (For basic mountain bike terminology, see figure 1 (not shown); for frame sizing, see figure 2.) Lift the entire bike off the floor until the top tube is pressing against your crotch. The distance between the bottom of the mountain bike’s tires and the floor should be 3 to 6 inches (1). (For a road bike, a 1- to 2-inch clearance is usually adequate.) Some newer mountain bikes do not have a traditional top tube; for them you have to rely more on comfort and balance to select the right frame size. If you are buying a new bicycle, test-ride as many bikes as possible and select the one that is the most comfortable.