Seatpost Setback: An important dimension?

*Seems* like it's critical to a proper fit.

My body finds KOPS and my sit bones wind up somewhere depending on that. I can move them a little
backward and a little forward by varying the tilt of my pelvis - but mostly their position is locked
in by my other body dimensions and the fact that my body tends to position itself so that my
kneecaps are roughly over the pedal spindles.

To get a saddle where my sit bones are one needs a certain range of saddle clip adjustment - which
is constrained by seatpost setback and the length of the straight portion of a saddle's rails.

Sounds logical, right?

But if it's so logical, why don't makers specify setback in their frame geometry specs?
http://www.jamisbikes.com/bikes/spec_geo/03_steel_geo.html is typical. Lotta measurements there, but
nothing to tell me if my butt is going to be resting on the horn of the saddle, the rear edge of the
saddle, or some less-destructive place in-between.

Granted the fore-aft position of the saddle relative to BB changes as the saddle is raised/lowered -
but that change could easily(?) be computed given the seat tube angle.

SO, bottome line, why doesn't anybody talk about "setback" in the context of frame dimensions?
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PeteCresswell

 

Pete-<< But if it's so logical, why don't makers specify setback in their frame geometry specs?

Because setback depends on the rider's femur length, which varies, even with those of the same
inseam. Why for a given frameset, one rider may need a setback seatpost and another not, altho both
fit the frameset.

<< SO, bottome line, why doesn't anybody talk about "setback" in the context of frame dimensions?

For the same reason frameset manufacturers don't insist on a bike fit...most LBS don't know how to
do fits, most frame makers don't either.

It's all about a 'ride this here bicycle around that there parking lot, see how ya like it?, What
color ya want, how ya gonna pay for it-credit card??", type fits.

Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
(303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"

 

RE/
>Because setback depends on the rider's femur length, which varies, even with those of the same
>inseam. Why for a given frameset, one rider may need a setback seatpost and another not, altho both
>fit the frameset.

I think I've been there in the context of a surf ski I used to have.

I just couldn't get comfortable on the thing - my knees were getting in the way of my stroke.

Down at a Hatteras windsurfing shop, I ran into this guy who was precisely my height, an precisely
my measurement from hip socket to ground.

The ski fit him like it was made for him.

The diff was that the proportions of our upper to lower leg were different...a *lot* different.

I still think that a frame maker's specifying setback would help, though. I know what setback I need
at a given saddle height and if I knew a frame's setback at a certain height and the angle of the
seat tube I bet I could figure out the setback as it would be at my saddle height. If nothing else,
I could draw it full-size on some wrapping paper, extend the line, and measure...
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PeteCresswell