For bikes you don't lean on your elbows to ride, I am a big believer in forward set back limits. I phrase it that way because there really isn't anything terribly wrong with more set back, but moving forward too far rotates the pelvis past the point of working with an approximately level saddle and shifts weight onto the hands. So the maximum forward set back is the point where you are still balanced over the pedals reasonably and your pubic bone isn't burying itself into the nose of the saddle (or your sliding forward from a lowered saddle nose). That forward set back limit should allow the lowest bend to the handlebars, so it is preferred by sporty riders for the aerodynamic benefits. Touring riders who sit up more can be happy further back from that limit.

KOPS is a (now) traditional method of locating a set back for your saddle. In the late '70s, several industry coaches tried to tie bike position to performance, and found that many high performing riders had naturally found very similar stances on the bike. While trying to quantitize these relationships between saddle height, reach, set back and the like; someone noticed that good set back tended to be when the forward knee on horizontal held pedals was over its pedal axle. Lots of refinements to this rule followed, but that's the gist of it - get set back right and your knee will be coincidentally over the forward pedal.

Over time this method gained a life of its own and Dr. Andy Pruit - one of the first sports medicine folks to get into bike fit - created a study that demonstrated that KOPS goes with peak power. That might seem pretty great until you realize that being "over" something is a function of the direction of gravity, which has nothing to do with pedaling. And that's a good thing, or power would drop when we pedaled on an incline or while riding a recumbent. Unsurprisingly, no other bicycle physiologist were able to duplicate Pruit's results. But that didn't stop him from including his version of KOPS as an essential element of the RETUL system.

I trained on RETUL a few years ago, and just a few weeks later ran into the problem: Not everyone's legs are proportioned like 1970s European male racers. I was fitting a Persian woman, and KOPS had her sitting really far back on the bike, exasperating the reach problem many women already deal with. So I moved her saddle forward to a more common location given her seat tube angle and we went from there. It was a good fit. Same but opposite problem with an Asian coworker - KOPS moved him really far forward.

Alternatives:
Fitter Steve Hogg has long advocated for using balance method for finding set back. Essentially, you ride the bike in the drops position, and you try to take your hands off the bars. Too far forward and you can't lift yourself, too far back and you weren't leaned over much. Average them out and there you go.

This is a very reasonable approach. The only thing that I question is that it is all about weight distribution and (to a smaller extent) core strength. Should a guy with a muscular upper body need more set back just because he's top heavy? Should a women with a smallish upper body be moved more forward? What is the effect on pelvic angle? Maybe the big shouldered guy should live with a little extra weight on his muscular arms rather than have to sit back far enough that it limits how aero he can be?

The alternative I've been kicking around for awhile is to ignore the upper body and ignore the leg ratios. Instead, maybe a universal set back angle would be useful - at least as a start. The relationship between your pelvic angle and the location of the pedals comes down to the average pedal location (which is at the BB) and a line running up to the pelvis from the BB. Regardless of what your legs are doing, they average a certain angle, and that average angle determines the corresponding pelvic angle.

Proposal - find the sit bone contact point on people who's set back seems healthy for the pelvic angle and overall fit. Measure that point as the horizontal distance from a 73 degree line originating at the BB. (Why 73? It sits in the middle of most road bike geo charts for seat angle, and several brands have noted that there is no reason smaller or larger bikes should depart from it, as there is no data to suggest taller or shorter people need different proportional set back. So it is a good reference line.) I imagine that we'll find that most people with good fits are perching on sit bones that touch the saddle 2-3cm horizontally behind of that 73 degree line (but that needs verification). And if we start with their saddles around that point and adjust for pubic bone pressure and center of gravity we should quickly arrive at a useful set back that references pelvic angle and balance without falling into the trap of basing everything on shin to femur proportions.

(Keep in mind that every set back adjustment should be followed by a resetting of saddle height. As set back changes and saddle to BB height remains constant, the graph of those two is an arc that curves down and back toward the rear wheel. So they need to be made together.)

I think my approach might be useful in part because saddles don't come in sizes for horizontal length and sit bone location is closely tied to hip joint location. So the size or sex of the person shouldn't cause the horizontal relationship between angle and sit bone location to change much.


If any of that was unclear, please ask. I welcome any thoughts.