Originally Posted by
McBTC
The need for more setback is pretty much dictated by the size of the bike and how much room the rider needs to fit in the cockpit. If a bike was custom made there'd be no need for a setback post to accommodate the rider's dimensions because the seat tube would be at a more relaxed angle.
Unless ... you have the seatpost angle very steep to allow a rear wheel close to the BB, large tires and fenders and do not want to have to use a curved seatpost. My two ti customs were made with 74 and 75 seat angles. 72.5 would center a traditional seatpost in the middle of the rails of a correctly located seat for me. So I run seatposts with roughly 50mm setback. I find the rear wheel feeling very loose on downhill corners on bikes with longer chainstays. All the bikes that have felt really good have had short to very short chainstays. Short chainstays, 28c tires and fenders can be a challenge! Steep seat angles help a lot.
Ben