As many of you know, I’m working at JackRabbit’s new store in Manhattan as the lead bike designer / fitter. Our business model is to be one of the top shops in the country for custom road and triathlon bikes. To make that a reality, we are investing heavily in tools and processes that allow us to do a better job than anyone else out there. One of our many innovations is a new technique for determining proper saddle placement for road bikes. I think it’s more accurate than any of the other methods I’ve researched, and I don’t think anyone else does it this way, so I wanted to share what I’ve learned and get some feedback from the Slowtwitch community.
The Nutshell version of the method: Most fitting systems dictate the “Knee Over Pedal Spindle” rule, where the fitter drops a plumb line from the knee. I’ve read just about every document available regarding saddle fore/aft, and I’ve seen this method described in almost every one, but have never seen an adequate explanation for it. In our method, we set up the bike (in road bike fits) so that the rider’s center of gravity is directly above the bottom bracket, with a slightly more relaxed position for novice riders and a slightly more aggressive position for experienced riders. Results of a few dozen fittings and custom bikes thus far have shown that the majority of novice cyclists (many entirely new at road cycling) prefer a position which places their center of gravity (C.G.) just behind (0-1.25 cm) the bottom bracket, while more experienced cyclists have preferred the position which places their C.G. between 0 and 1.5 cm in front of the bottom bracket. The text below is from a document which will soon be posted on our website. I’ve also included a few pictures of our design set-up, which includes 4 laboratory scales for accurately locating our client’s center of gravity in a variety of positions and geometries. As you can see we’re using the Argon18 fit bike which allows us to rotate the seat tube angle accurately and easily from 70 to 80 degrees for both road and tri fits. I know the wires and tools are all over the place and the platform is dirty. I promise I’ll clean it up before you come in.
Road Bike Fore/Aft (Not Tri Bike!):
The fore/aft saddle position on a road bike is probably the single most important aspect of a comfortable and powerful ride. Unfortunately for cyclists, the popular method for determining saddle fore/aft is based on observation and is nothing more than a rule of thumb. The method is called the Knee Over Pedal Spindle (K.O.P.S.) method. The K.O.P.S. method says that the correct saddle position is the position which places the bottom of the knee cap directly over the pedal axle when the crank arm is in the 3 O’clock position (when viewing the right leg). K.O.P.S. can put a rider at a very shallow seat angle if they have long femurs, or a very steep one if they have short ones. There is no real science behind this method; it is based entirely on observation of what many riders deem a comfortable position. The leg is made up of several bones and joints which create a lever system that turns the crank arms. If one slides the saddle position 2 centimeters forward and adjusts the saddle so that it is at the same height as it was previously, then the lever system of the leg is exactly the same as it was before hand, just rotated a few degrees forward; the knee has no idea if it’s directly above, in front of or behind the pedal spindle. From a bio- mechanical standpoint it is identical, as shown in the diagrams below. 
The cyclist on the bottom has been rotated forward two degrees about the bottom bracket, note that the limb lengths and frame geometry have been drawn completely arbitrarily, but are consistent between the two diagrams.
The only thing that does change as one rotates the saddle fore and aft is the location of the rider’s center of gravity. If one’s center of gravity is too far forward, then an excessive amount of weight needs to be supported with the cyclist’s core and arms, and if one’s center of gravity is too far back, then an excessive amount of weight is placed on the cyclists sit bones. Bicycling is essentially a one legged squat performed over and over again. As one squats down, they bend at their hips pushing their torso forward and their rear backwards, this keeps their center of gravity directly above the ball of their foot, maintaining balance. The goal of the saddle position is to put the rider in a position that is as natural and as balanced as possible. If one were not pedaling at all, then their center of gravity would need to be directly over the bottom bracket in order to maintain balance. Pedaling however, creates forces which need to be balanced with the rider’s body weight in order to maintain a state of equilibrium. Simply put, a rider’s center of gravity should be slightly in front of the bottom bracket in order to maintain a balanced position while riding.
Think of a cyclist riding a bike with no handle bars, but maintaining their upper body position as if they were. If the cyclist does not pedal, then their weight will cause them to fall forward. The cyclists pedaling forces however, create balancing forces to keep them upright. Stronger cyclists generate more force at the pedals than weaker ones, which is why they should be positioned a little further forward (this lengthens the moment arm of the cyclists center of gravity).
Unfortunately for this method of bike fitting, it is impossible to establish one position that will work perfectly for every single minute in the saddle. The cyclist doesn’t always generate the same pedaling force and the pedal stroke has two major dead spots in it where the cyclist isn’t pushing down at all. When the cyclist’s pedal stroke is at the 12 and 6 O’clock positions, the pedal stroke is doing nothing to keep the cyclists upper body from falling forward. Furthermore, if the cyclist is fit using this technique while generating 175 watts at 90 rpm the pedaling forces are less than what they would be at say 175 watts and 70 rpm. The one thing that this method does have going for it is that there is some science to it and it can place the rider much closer to the position that they will feel the most comfortable in compared to a K.O.P.S. derived position. Ultimately, the rider needs to take the bike out on the road and experiment with a few different saddle positions to find the most comfortable one. The bike fitting will get the rider very close to where they feel they are positioned correctly; however it is the rider’s job to take the bike to the road to finalize the fit.
