Originally Posted by
Kedosto
Indirectly, yes. The slight forward rotation of the hips and flattened back causes you to rest on a more anterior portion of the sit bones and you'll scoot back on the saddle slightly in order to position the sit bones on the sweet spot of the saddle. The scoot back is what moves your knee back in relation to the pedal spindle. The slight forward hip rotation also (usually) means you'll probably want to take a bit of the tilt out of the saddle too, but not all of it. The saddle design requires a bit of tilt to function as a hammock.
Reach and saddle set back are really only indirectly associated. KOPS (as a starting point) has nothing to do with reach and everything to do with saddle setback and more specifically, your sit bone position on the saddle. In my case, I can use a zero setback post only on frames with 70-71 degree seat tube angles. For 72 degree seat tubes I go with about 12-15mm setback and with 73+ degree I go with 20-25mm setback. The set back has everything to do with my femur length and desired KOPS position (which for me is slightly behind spindle). As the seat tubes get more steep I need more setback to keep my femur length (knee) oriented to the pedal spindle at the distance I like.
Stem length comes in to play only to the extent that the length of the stem causes my upper body to either work with my saddle position, or against it. When my stem is too short my upper body wants to sit more upright. My back curves and hips roll back (more vertical) and as a result I tend to slide forward off the sweet spot of the saddle and onto the narrow section. The shorter my stem, the more I need to raise the tilt of the saddle to keep from sliding off the front. A longer stem creates a better reach which flattens my back, rolls my hips forward appropriately and scoots my butt back into the saddle. When the stem gets too long I stretch too far forward, roll my hips too far forward (pressure on the perineum) and my butt wants to slide off the back of the saddle.
Given the unusually short reach of the Vaya (considering the reach vs frame size), and the seat tube angle (72.5 for the 57cm), I'm wondering why they would spec a zero setback post. I'm not so sure the Vaya is the best choice for those Jones bars. The bars put your hands pretty much even with the steer tube. Frames deigned for flat bars have longer reach than usual, not shorter. In the end, you might find the only way you can use the SA is with the nose way way up.
-Kedosto
I agree with Kedosto's analysis - kudos for tying together so many of the aspects of fit and riding comfort! It frames my fitting problems perfectly.
I really cannot see the modern trend in favor of zero-setback seat posts. I've been able to adapt to a wide range of frames, but with seemingly outlandish saddle installations. My favorite rides are my Mondonicos, which somehow pedal beautifully. But the 53 cm (1984 or so) has a seat tube angle of 75 degrees and the 54 cm (2005 or so) has 74 degrees. With both of these frames I use a Nitto S-84 seatpost, which has a 35 mm setback and a very long-railed Specialized Toupe Gel or Selle AnAtomica saddle. My classic 1980 Masi has the same issue, but it's really too small (M53, which actually measures 51 cm c-c) and needs an extreme stem (110 mm extension. I adjust KOPS for the plumb line to the front of the kneecap to be 1 to 2 cm behind the pedal centerline. I cannot comfortably use Brooks saddles on these three bikes. I've also explored larger frames, a Trek 620 56 cm c-c, and found the setback was still difficult to manage. In terms of stack/reach, the reach was still very hard to manage, and I could not be comfortable on the bike.
So I've been disappointed with the fit of these high-end vintage-style road frames, but also with a few Treks with 73 degree seat tubes. In addition I have a 1984 610 with a 73 degree
In contrast I also have a 1952 Rudge clubman-style bike which has a 71 degree seatpost angle and a 55 cm c-c seat tube angle. Before I tore it down I checked the fit, and it felt ok (stationary on the floor, but not tested riding on the road) with the original-style straight pin seatpost and the vintage Ideale steel saddle clamp holding a B17 N up on top. In the tops with my seatbones comfortable I had the Brooks saddle centered on the steel clamp. It was comfortable enough to promise great things. I expect it to have a very comfortable ride, with its 105 cm wheelbase, long (45 cm) skinny chainstays, and similarly long, thin, and curved fork blades. Also, trail is only about 40 mm.
It looks to me like for a given riding position and a given set of cockpit gear (bars, stem, lever/shifter), the setback from the BB plumb to the sitbones contact is the same for all the bikes I own. Then it follows that the cockpit reach (distance from the BB plumb to the top of the levers) is also a constant, if you use the same levers, bars of the same reach, and stems of the same extension.
One frustration in all this is that when I've spoken to frame builders and fitters about getting a frame built with a similarly shallow seat tube less than 73 degrees, I get nearly outright refusals, saying hardly anybody needs an angle less than 73 degrees. I just don't get this, but I don't want a builder who can't believe in my requirements and denies my experience.