pakossa

While fooling around with my fit, I noticed something odd. At the height I had it, if my reach (tip-of-saddle to bar) was more than 53cm -- even by 5 - 10mm -- my speed/power would drop sharply while pedaling at the same perceived effort. Yet, when I lowered the saddle 5mm, my speed/power dropped if the reach was any SHORTER than 55cm! This happen regardless of the saddle fore/aft. (Or, at least as far fore and aft that the post and saddle rails will allow.) And, yes, I did make sure to keep the saddle height the same (measured BB to center-of-saddle). Any idea whether the higher saddle/shorter reach or lower/longer is more ideal? (It seems I can put out more power in the higher/shorter position, but I have yet to evaluate all possible saddle height/fore-aft combinations.)

Kontact 

When people are creating peak power in the saddle, they creep forward. Some pros are toying with this.

But its not a real sustainable position because cycling is also about being comfortable and pedaling below peak power.

Iride01 

It's not anything new. It's been known for a very long time that where ones butt is from the vertical centerline of the BB will affect how well their power gets transferred to the pedals.

Like most anything else about bike fit it's all about compromise. When you move forward to put out better power, you also decrease your comfort somewhat. So the the amount of time you wish to ride in that position over the BB gets shorter. That's part of the reason why typical seat tube angles differ between a TT bike, endurance bike (road bike) and cruiser style bikes.

TT bikes you can ride really fast, but you'd only want to be on them for a short distance. Cruiser style bikes you can ride comfortably but slow. Road bikes are a good compromise for overall for power and long distance.

79pmooney

Just a guess here but I think what is happening is that you are closing up the angle between your thighs and abdomen when you raise the seat, limit the ability to pull your diaphragm down and therefore limit oxygen intake. So you have to bring your bars back so you can sit up. Lower seat, bigger angle, more ability to lean further forward for aero benefit and still have good oxygen uptake.

And fun with numbers plus my pet theory. (The theory: the bars can be slid along an inclined path and not change your head/shoulder/back position or arm bend and have little change in comfort and+ power over a perhaps 4-6" range. For me that slope is 34 degrees; very conveniently two cm horizontally and 1 cm up along the headtube/steerer axis.)

So, lets say you raised your seat 1 cm. (Yes, that is a mile! I picked it to keep the numbers easy.) Just to keep all the same, the bars would need to come up with an additional cm of spacer (or a quill stem just pulled up 1 cm). But you didn't. So now effectively your stem is 1 cm lower which is equal to 2 cm more reach if you are like me. If it was right with the lower seat position it is now too low or far forward.

This is one reason I will never be a fan of shortening steerers to the minimum. Raising stems by sliding them up is much cheaper, faster and easier than swapping stems - if you left some excess steerer. (And another reason I like quill stems so much.)

Kontact 

People ride TT bikes for over 100 miles all the time. They have a forward seat position so the rider can have a flat back while leaning on their elbows - a position that would be impossible with a normal saddle set back. The forward seat position is for aerodynamics and comfort, not power.

Iride01 

The forward seat position is about power too.

And there are people that ride road bikes in a leisurely manner for short rides too. And maybe some that do leisurely rides on a TT bike.

There are always exceptions.

Kontact 

Just pointing out that Ironmans aren't short.

oldbobcat

It might be that changing the back angle was calling into play muscles your body hadn't had the time to learn to use. There's a acclimatization period that goes with any change in posture.