Originally Posted by johngwheeler

Are there are guidelines to how you determine the correct set-back then?

I recently changed from a high-rise 105mm stem to a normal 120mm one (flipped up), and also moved my saddle forward 15mm to about 60mm set-back from saddle tip to BB.

I've found that I'm using my quads more, and have to get lower (with a straight back) in order to have a comfortable reach with the longer stem - which are both "good things", I think.

However, the negative is that my balance has been affected. With the saddle more to to rear I could more easily lift my hands from the bars and keep pedalling without shifting forward on the saddle (which causes some discomfort on longer rides). I think this is because there is was some part of the force on the pedals pushing the rider rearward, and so helped maintaini the rearward position. With the saddle forward, if I unweight by hands completely (supporting myself with my core) I tend to slide forward on the saddle or have to increase cadence to keep my butt to the rear.

So I wouldn't say that balance has nothing to do with it. I think it is an important aspect of bike fit that keeps you from sliding forward on your sensitive bits! The alternative is to put more weight on your hands to maintain the position. It's all a compromise.

I'm still not decided whether I like the 60mm or 75mm setback, but I do see advantages to the longer stem. The bike is less twitchy and feels a lot more stable in descents and when riding out of the saddle. I think I can output more power when out of the saddle, and my knees don't get anywhere near the bars. Also, having to adopt a low position with a straight back and relaxed shoulders and arms is going to be more comfortable on the neck & shoulders on a long ride, but possibly at the cost of somewhat reduced saddle comfort (although this may be my particular saddle and not a general rule!).