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Seat angle(4 posts)

Seat angleEW
Aug 18, 2001 4:44 PM
Anyone have information on how much one can vary seat angle of the bike setup by adjusting the fore/aft position of the saddle? Can you change a seat angle from 74 degrees to 73 degrees by setting the saddle back?
re: Seat angleBirddog
Aug 18, 2001 4:49 PM
Yes, you can change the effective seat angle by moving the saddle back or forward. There are several variables at work here however. The length of the saddle rails and design and style of the seatpost and clamp are the limiting factors.
You get 'roughly' 1 deg/cm ..... nmJohnG
Aug 18, 2001 6:01 PM
not that simple....C-40
Aug 19, 2001 6:00 AM
Moving the saddle is strictly for the purpose of establishing a desired knee-over-pedal (KOP) position. Moving the saddle also changes the effective top tube length, which will require a proportional change in stem length to restore the original reach to the bars.

The formula used to determine the difference in saddle position from a change in seat tube angle is: saddle height x (cos73-cos74). Saddle height is measured fom the center of the bottom bracket to the top of the saddle, along the seat tube.

For a mid-size 56cm frame, with a 74cm saddle height, this example would yield a result of 1.24cm.

This formula comes in handy when comparing frames with different STA and TT length. Frames with a steep STA like 74 degrees can be accurately compared to frames with a 73 STA by adding the result to the TT length of the frame with the 74 degree STA.

What the formula won't tell you is whether the seat post and saddle rails will allow the saddle to be moved by the desired amount. Posts with wide clamping areas like Campy, don't allow much forward movement of the saddle. Posts like the Thomson straight-up model move the saddle forward by nearly 2cm and don't allow the saddle to be moved back far enough in most cases.

I like the new ITM post, which is similar to the Colango post on my C-40. It has plenty of setback to permit lots of rearward travel, and a moderate clamp width to permit a fair amount of forward travel.