why use a Set Back Seatposts ?

[Mark McM]

Originally Posted by Kontact

I don't follow. "The cranks" have a center point, and that centerpoint doesn't change with crank length.

By center point, I assume you mean their central pivot - the bottom bracket. Yes, that is true. But we don't apply power to the center point of the cranks, we apply power to the end of the cranks (at the pedals). Furthermore, we get the most effective power when we push down on the pedals when the cranks are close to horizontal, in front of the center point. Therefore, we need to position our feet, legs and saddle to where the pedal is when we apply power.

This means that as the cranks get longer, we need to move forward, and as the cranks get shorter, we need to move backward.

If we could apply power to the pedal equally all around the circle, then we'd want to position the setback relative to the center of the circle. But since we get the majority of the power when pushing down on the pedal when it is forward of the center, we need to set the setback relative to this pedal position.

[wallymann]

to compensate for an overly steep seat-tube, allowing saddle to achieve desired setback.

[Kontact]

Originally Posted by Mark McM

By center point, I assume you mean their central pivot - the bottom bracket. Yes, that is true. But we don't apply power to the center point of the cranks, we apply power to the end of the cranks (at the pedals). Furthermore, we get the most effective power when we push down on the pedals when the cranks are close to horizontal, in front of the center point. Therefore, we need to position our feet, legs and saddle to where the pedal is when we apply power.

This means that as the cranks get longer, we need to move forward, and as the cranks get shorter, we need to move backward.

If we could apply power to the pedal equally all around the circle, then we'd want to position the setback relative to the center of the circle. But since we get the majority of the power when pushing down on the pedal when it is forward of the center, we need to set the setback relative to this pedal position.

We don't sit on the seat to just produce power. It supports our weight, balances us to take weight off of our hands and helps us control the bike.

To maintain a generally useful position on the saddle you have to reference everywhere the legs are going to be, regardless of whether it is under power or not.


However, some of this stuff is self regulating because of foot size, the use of KOPS, etc. Super long cranks are generally only 1cm longer than "short" cranks. 1cm is well below the 2.5cm difference between set back and zero seat posts.

[ripvanrando]

Swapping longer cranks requires lowering the saddle and also moving it back a little bit (not forward) or at least in my experience going back and forth from "short" 177.5mm cranks to the 200mm big ones. Moving it back to keep the same balance......there is some trig in there.

[azrider]

Originally Posted by pasadena

The rules changed recently, so many more pros are pushing saddles forward and tilting the nose down now that it is legal.

[BdaGhisallo]

Originally Posted by azrider

The best of both worlds!

[Ruimteaapje]

For me it is quite simple: for my ideal position on the bike the saddle is clamped 20 cm behind the perpendicular line from the centre of the bottom bracket. To achieve that with a non-setback seat post whilst keeping enough clearance between the seat tube and the rear tire, the wheelbase would be way too long

In the past I used a non-setback post on my Merlin but I had to clamp the saddle way too far to the front of the rails making it prone to snapping...

[colker]

Originally Posted by Mzilliox

Hey Charles, I used to wonder the same thing.
Here is my take, and im not sure how accurate it is, so take it with many grains of salt.

1. Setback seatposts i have heard are more comfy and absorb shock better...
2. From a geometry standpoint, you dont want too slack of a seat tube for a few reasons, but mainly to keep the wheels where they should be, and the rider's front center where it should be. but some riders still require being further behind the bottom bracket, so a bit of setback allows the rider to get in the correct position with regards to the bottom bracket. some riders do not require that, some riders do. I know that from riding different bikes i like a seat angle of 73-74 degrees. but to put me in the right place with regards to the bottom bracket, i need a touch of setback.
3. a set back seatpost should NOT be used to alleviate fit issues such as reach... as moving it for this purpose will put the rider in the wrong position for optimal pedalling.

why 5 pages of posts after this reply?

[cadence90]

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[mcteague]

I never had a straight seatpost until now. After getting a Selle SMP saddle I found it had to be moved up much closer to the bars than previous ones. On top of that, the angle of the saddle is super critical to making SMPs work. So, I ended up with a straight Thomson post. The saddle is now in the middle of the rails and it is super easy to dial in just the right amount of tilt.

Tim