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range of setback on seatposts(7 posts)

range of setback on seatpostsET
May 7, 2001 10:58 AM
We make a big deal about starting with the right Seat Tube Angle, but if a post is built with a setback of, say, .63 cm more than another, switching from, say, a STA of 72.5 to 73 could be handled by the setback (although one must keep in mind that the effective top tube length of the 73 bike will increase after moving the seat back to get it where it would be on the 72.5). This would allow one to choose from a wider range of bikes, I presume without introducing much, if any, performance and ride issues.

Does anyone know the average amount of setback, as well as the typical range, for all the standard seatposts? (I know there's some weird ones, e.g. Thompson, with a big setback due to a double-jointed post.) Also, are the specs for the amount of setback available anywhere? And can this be measured independent of STA?
re: range of setback on seatpostsLarry Meade
May 7, 2001 2:01 PM
According to Alber Eisentraut, the "standard" for setback is 20mm. This was established in the 50s when Camp introduced their two bolt seatpost. I don't know how many manufacturers adhere to this standard. As to the Thompson, despite their looks everything I have read on them says that they have less setback than a Campy or Shimano seatpost. Those two seem to be very similar in setback. I have one of each on identical frames with identical saddles and the saddle seems to be close to the same relative to the seatpost.

how do you measure setback?ET
May 7, 2001 2:41 PM
Just to be double-sure, when I refer to setback, I mean center of setback, not maximal setback; I presume we're on the same wavelength on that. So how do we measure the (center of) setback? Is it correct to presume to measure from the middle of the post, back to the middle of the setback? But doesn't it depend on the seat tube angle, or is simply the middle of the setback when the two layers are sandwhiched one exactly on top of the other (in which case there is an implicit angle)? Thanks.
A standard Thomson has no offset,smithers
May 7, 2001 2:55 PM
the laid-back model puts it in the standard range. Seats also vary with their range of adjustment.
no standard for setback measurement...dave
May 7, 2001 3:37 PM
that I've ever read about. You certainly never see a setback dimension listed for seatposts in any catlog I've seen. Traditional road posts usually have the front of the clamp located near the centerline of the seatpost. The front of the clamp is what limits the maximum saddle setback.

Many of the new models limit the amount of setback by positioning the front of the clamp further forward, which can be a problem.

Campy's new post design doesn't limit setback, but it does limit the amount the saddle can be moved forward, due to the wide clamping area.

The optimum seat tube angle is relevant, only when the style of seat post to be used, is also known. If a Thomson straight-up post is used, the seat tube angle would need to be decreased by about 2 degrees, to get the same saddle position as a traditional post.
your 2 degree theory also needs a frame size attached.smithers
May 7, 2001 4:22 PM
the bigger the frame the more difference this makes. Right?

And this only applies if you run the seat slammed all the way back. If you run it near the middle it might not have an affect.
May 7, 2001 5:13 PM
The two degree figure is just a rough number. On a midsize frame, every degree makes about 1.2cm difference in saddle position. If you want a fancy formula, here's a simplified version to figure the difference in saddle postion between two frames with different seat tube angles, in this case 73 and 74 degrees:

1.32 x(frame size)x(cos73-cos74)

This formula assumes that the frame is sized (c-t)by the formula .67 times inseam, and saddle height is .883 times inseam.