After proper bike fit of saddle height the back itself takes optimal angle and hands itselves “find” the desired handlebar height. The main thing is does not impede for this.
Nevertheless, the inclination angle of the back is the next most important parameter after the height of the saddle. Unfortunately, the handlebar height can distort the natural slope of the back, so it is necessary to determine it independently. On the Internet it is believed that the more tilted back, the more sportive fit. Accordingly, it is assumed that the slope of the back depends on the type of bike, the farther bike from big sport, the straighter should be the back.
Unfortunately, it does not match nor my experience nor common sense. The angle of back is determined by the body structure, fitness, self-preservation instincts and generally not connected to the presence of bicycle at all 🙂
I think that is not harmful to highlight the issue from the begin. Where did come from vertical posture as recommended in the left picture for “commuters”? Seems it just the memory of riding a tricycle in childhood. And memory about sitting at a desk, a case that was described in the introductory note as not natural for humans. Apart from the fact that such position of the back is not convenient, it is also harmful. A rider sits not in an easy chair, but on the hard saddle and, most importantly, do not sit at all, but rides. With the vertical position of the back all the bumps in the road that passed through the suspension lead to spinal compression. The back starts to hurt, you should call in to doctor, the doctor does not recommend riding a bike (the real case of life as a result the carbon hardtail three years dusted in the garage). The problem is even deeper. Walking upright of human is in general bad for the back. When walking or running legs somehow absorb the load. But while cycling vertical back just should not be! The back must be tilted.
Now once again we return to the question: what determines the angle of the back? I think that the structure of the body, the physical form, habit and fitness. That is the optimum angle of inclination of the back does not depend on the type of bike. Moreover, the angle is not dependent on cycling. As for me, for example, the same angle as in cycling I have at the alpine skiing. In general, you could notice that the back trying to bend forward at any increase of speed. Even when driving the car is going. Apparently so instincts work for better balance on speed and better absorb the bumps. So each person has own optimum position of the back depend on external conditions. And it must be found. The angle is controlled by muscles that lower of back, near waist. The tone of these muscles determines the equilibrium slope of the back.
Unfortunately, the handlebar impedes to “hear” your back. You sit on the bike, put hands on the handlebar, and according to this some angle of the back immediately installs. And with this angle you are riding. And then you may understand, is it comfortable (if matched), or it is not convenient. Taking into account that discomfort may not indicate the back. The height of the handlebar could be adjusted in wide limits. Spacers, you can turn around the stem, and sometimes handlebars itself allows to change the height. Ironically, if the steering wheel is too high (as it usually installed in the shop), this does not mean that the arms and the back are of the minimum tension. Most likely you will hang on hands, because the back will try to go down. I recommend you start to reduce the height of handlebar and, “listening” to the back and arms, to seek optimum. Optimum – a minimum total tension arms and back, and indeed, once it becomes convenient. Tension is redistributed between the back and arms. That is, if to strain muscles of a back, there is no pressure on the hands while if relax the back, you push harder your hands on the handlebar. In fact, the hands are not so much press on the handlebar, how control the ride of the bicycle. If you want to raise the front wheel, the hand with the support of the back done it quickly. That is, at this moment hands does not put any pressure on handlebar 🙂
At the bike fit there are several more options that you can “tighten up”, but, in my experience, they are not so significant as the height of the saddle, angle of the back and height of handlebar.