Seatpost Setback and Related Matters

A couple of days ago I replaced the seatpost on my Rivendell Sam Hillborne with one that has zero setback, and the change has been interesting. Before I go any further, I will warn you that this is a continuation of the long top tube post. So if that one gave you a headache, please stop reading now and save your sanity! Or, continue at your own risk.

For those new to the concept of setback, seatposts come with different amounts of it. One of the things the setback does is move the saddle clamp back, thus altering a bike's effective seat tube angle. Say your bicycle frame has a 74° seat tube, and you buy a seatpost with 2cm of setback. Unless you counteract the setback by moving the saddle forward along the rails, your bicycle's effective seat tube angle will be 2° slacker, making it more like 72°. And you can make it slacker still by pushing the saddle further backward. By contrast, a seatpost that goes straight up with no setback leaves your frame's natural seat tube angle unaltered. Seat tubes today tend to be steep, so it is rare that anybody wants to make them steeper still. But with a zero-setback seatpost, it is possible to make the effective angle a bit steeper by pushing the saddle forward on the rails.

The other factor influenced by a seatpost's setback is the reach from saddle to handlebars. The more setback a seatpost has, the further the saddle moves away from the handlebars. Here it is worth noting that bicycle fit experts typically warn against messing with seatpost setback and saddle positioning in order to alter reach. Instead it is advised that one's saddle position preference should be fixed in relation to the bottom bracket. At least that is my understanding.

Getting back to my bike, it has a 52cm seat tube and a 57.5cm top tube - the latter being unusually long given the former. Additionally, it has a 71.5° seat tube angle, which is atypically slack. In previous posts I explained that when I ride this bicycle, I feel as if my body is not sufficiently forward. The long top tube will not allow me to fit the bike with a stem longer than 7cm, and the slack seat tube puts me further back still.

Originally the bike was built up with a seatpost with generous setback, making the effective seat tube angle even slacker than its natural 71.5°. Eventually I replaced it with a seatpost that had only minimal setback, but even that did not feel as if I were sufficiently forward. I was reluctant to go with a zero-setback seatpost, because everyone I spoke to acted horrified by the idea. "Zero setback? What are you trying to do, turn it into a racing bike?" However, after the "long top tube" post I came to the conclusion that a zero setback seatpost is the most obvious solution. Far from making the bike "racy," it would simply continue the frame's already slack seat tube angle without slackening it further. Or, I could move the saddle a tiny bit forward and make the effective seat tube angle a rather normal 73° (as it is on my other two bicycles with drop bars). So, that is exactly what I did.

The welcome side-effect of the new saddle position is that the long top tube problem seems to be resolved. My reach has been reduced considerably and I can get a longer stem if I want. But even with the current stem I already feel myself positioned significantly more forward on the bike than before. The subjective sensation of this is greater than I would have predicted: I feel more in control over the steering, and I feel that the bicycle is distinctly faster to accelerate and to start from a stop. Although visually the saddle comes across as being too far forward now, its relationship to the bottom bracket is actually quite normal for a roadbike (off-the-shelf road frames in my size typically have 74-75° seat tube angles). I need to take the bicycle on a longer ride before I can say more, but I think this setup may be just the thing.

It's been exactly two years since I received the Sam Hillborne frame as a holiday gift, and this bicycle has given me over 2,000 happy miles. I've changed a lot as a cyclist over this time and the Sam's frame is quirkier than I initially realised. But I am going to try and make it work for me - hopefully learning a thing or two in the process.

Categories: comfort, diamond frame, Rivendell