I heard you can judge by if your stem covers up the front hub or not, then go from there.
I replaced my stock TT stem with an 80mm Cannondale stem, and it works for general riding. It also depends on what you want to do. My 80mm does not feel good when sprinting around in the drops tbh.
Originally Posted by Peter White Cycles
Next, where does the handlebar go? Just like the saddle, it all depends on what it is you're doing on a bike in the first place. The further forward the bar, the more power you will have standing and accelerating, the better the aerodynamics and high speed control. The lower the bar, the more you can pull up under hard acceleration and the better the aerodynamics. With the bar closer to you and/or higher, you can sit more upright and take in the view.
I like to use an adjustable stem that my customers can use for a few days to try different positions for a long enough time to be meaningful. But what about a starting point? For riders with drop bars, if you place your hands down in the drops at the forward most position, (the point that allows you to easily reach the brake levers), then bend your elbows enough that your forearms are horizontal, your elbow would be at a ninety degree angle for a good starting point. From there, try moving the bar in one half inch increments forward and back to find the best reach for you. Most people are quite comfortable just with the ninety degree elbow position. But that doesn't mean it's right for you. And of course this isn't a position you'd want to spend much time riding in, except on the occasional banzai descent down a mountain pass!
Racers generally end up with the handlebar height two to three inches below the saddle height, tourers will often like to have the bar at the same height as the saddle. Mountain bikers usually position the bar a couple of inches below the saddle. The important thing is to take enough time to find the best position for you. If that means setting up a touring bike with the handlebar four inches below the saddle height, so be it. I recommend the longest reach and lowest position you feel comfortable in, (with emphasis on comfortable).