Old 08-30-04, 07:44 PM
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Al.canoe
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For time trials and triathlons, you want a forward seating position to generate the max power for a relatively short period of time. The bike will have a large seat tube angle which effectively rotates the saddle forward about the crank spindle. That'll put your knees forward of the pedal spindle. You'll notice that the rider sits forward on the saddle nose which puts the knee even farther forward of the pedal spindle. The knee is still properly aligned with the pedal spindle, it just has nothing to do with gravity( or does it?).

For endurance and comfort, you want to balance your weight over the bottom bracket spindle. That puts my knee 2.5 to 3 cm behind the pedal spindle. It probably varies a lot as I move back and forth on the saddle a few inches to change my position every once in a while. Supposedly, having the knee behind the P spindle that much interferes with maintaining a high cadence. I spin easily to 110, cruise at 95 to 100, so it's not a problem too me.

The setback of the seat post effectively reduces the seat tube angle and provides a more rearward sitting position which often puts the knee behind the pedal spindle.

When my wife started riding 40+ miles on her bike, which is built on a touring frame, she started to complain that she felt too far forward. So I measured her knee position and found she was about 1" forward of the pedal spindle. That was a big surprise since her seat post has an inch set back. Something was wrong since touring frames generally have around 72 deg. seat tube angle. I measured her bike's angle and came up with 74. I checked the Trek site and sure enough, Trek 520's have aggressive angles and in her frame size it's 74. That's close to time trial territory I believe. My old touring frame, which I no longer ride, is 72.5 deg.

My fist thought was she needed a new bike, so I called a touring frame builder to see what gives and found out that smaller frames, which women often use because of short upper bodies, have to have steep seat tube angles to get adequate toe clearance with the front wheel, much less the fender. The only option was to get more seat post set back. There's only one, the Titec Hell Bent. It's advertised at 1.5 ", but I measure 2". Now she's happy. We left the stem length alone as she seems to be OK with the weight distribution. If she needs to go back more, the only option left is a men's saddle since they have longer rails which might allow another half inch or so. Sitting position is a compromise.

I bet there's at least a few short upper bodied women and generally just shorter people out there who are uncomfortable on their bikes because they are sitting too far forward.

Lennard Zinn, Zinn's Cycling Primer (p 5) makes two interesting points. (1) The center of rotation of the knee must be over the center of rotation of the pedals. That argues for the knee over spindle theory. Zinn says he's observed this from 3-D digitized videos of riders, (2) "the saddle is a tool for support only; it does not actually determine where the rider sits. Pruitt says, "the pelvis will find the place it needs to be to get the knee over the pedals""

There's plenty of theories, opinions and folklore out there for everybody. The best thing to do is to experiment and go for comfort. What's comfortable will likely change as the mileage and experience increases.

I take most of the stuff in most books with a big grain of salt. They tend to be race bike oriented. I find Peter White's stuff is more relevant to me as are books on randonneur cycling like the Complete Book of Long-Distance Cycling. They are not weight centric and focus on endurance and comfort.

Al
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