Raising seat while maintaining setback?

John Garbe said:

I would like to increase my seatheight 3 millimeters while maintaining its distance behind the
bottom bracket. Bike has 74 seat tube angle. If I raise it 3 millimeters I then would have to
slide it forward a small amount to maintain this distance. The overall effect is that I have not
increased the seatheight by 3 millimeters, right? When I raise my seat a certain amount how much
of that is lost when I slide it forward to maintain the distance from BB? Unfortunatly I do not
have a FitStick to give accurate measurements.

Assuming that you want to move your seat straight up by 3 mm:

Dist. to move seatpost = (desired height change)/(sin(seat tube angle))
= 3/sin(74) = 3.121 mm

Dist. to slide seat forward = (desired ht. change)/(tan(seat tube angle))
= 3/tan(74) = 0.860 mm

Hope you have a good micrometer.

John Garbe said:

I would like to increase my seatheight 3 millimeters while maintaining its distance behind the
bottom bracket. Bike has 74 seat tube angle. If I raise it 3 millimeters I then would have to slide
it forward a small amount to maintain this distance. The overall effect is that I have not
increased the seatheight by 3 millimeters, right? When I raise my seat a certain amount how much of
that is lost when I slide it forward to maintain the distance from BB? Unfortunatly I do not have a
FitStick to give accurate measurements.

Even if you do the calculation, for 3 mm up I don't think you'll notice any difference in the
forward/back of the saddle. I think that little things like the thickness of your shorts or whether
or not you are wearing tights will have more effect than 3 mm.

Of course, perceived difference is another story.

John Garbe said:

I would like to increase my seatheight 3 millimeters while maintaining its distance behind the
bottom bracket. Bike has 74 seat tube angle. If I raise it 3 millimeters I then would have to
slide it forward a small amount to maintain this distance. The overall effect is that I have not
increased the seatheight by 3 millimeters, right?

Why not? The setback is in the fore-aft direction, so changing that should not change the seat
height. Since the seat should be relatively level this is not a problem.

Besides, seat height is subjective. Who can tell the difference between raising it 3mm and
raising it 2.9mm?

Quoted message said:

Is there a simple formula that calculates how much you need to slide the seat forward when you
raise it a given amount?

In reality?  Measure the setback (distance between nose of saddle and stem) before you raise the
seat. Then adjust afterward.

Mathematically, for a 74-degree seattube you would move it forward .8269mm, about. That is
sin(16)*3.

Quoted message said:

With this factored in how do I raise my seat 3mm? I calculate my seats distance from BB by
measuring from the nose of the seat to the center of BB as determined with a T square(more like a
cross) extending beyond my horizontal top tube.

Oh, I never thought of doing that. So, your 3mm is vertical, not parallel to the seattube. That
changes things by cos(16) =0.9613, so to raise the seat by (your way) 3mm requires exposing
3/cos(16) = 3.12mm more seatpost.

If you can measure any of these dimensions more accurately than to the nearest mm you are very good.
Very obsessive as well.

--

David L. Johnson

__o | We have a record of conquest, colonization and expansion _`\(,_ | unequalled by any people
in the Nineteenth Century. We are not (_)/ (_) | to be curbed now. --Henry Cabot Lodge, 1895

John gets the answer-<< Assuming that you want to move your seat straight up by 3 mm:

Dist. to move seatpost = (desired height change)/(sin(seat tube angle))
= 3/sin(74) = 3.121 mm

Dist. to slide seat forward = (desired ht. change)/(tan(seat tube angle))
= 3/tan(74) = 0.860 mm >><BR><BR>

Or just raise it and measure the distance from the nose to the handlebars and make that the same.
Remember that you move all wround the saddle so .86mm is really not a huge deal.

Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
(303)440-3535 vecchios.comvecchios.comOpen ↗ "Ruote convenzionali costruite eccezionalmente bene"

Just play with it 'till you get it right..

John Garbe said:

I would like to increase my seatheight 3 millimeters while maintaining its distance behind the
bottom bracket. Bike has 74 seat tube angle. If I raise it 3 millimeters I then would have to
slide it forward a small amount to maintain this distance. The overall effect is that I have not
increased the seatheight by 3 millimeters, right? When I raise my seat a certain amount how much
of that is lost when I slide it forward to maintain the distance from BB? Unfortunatly I do not
have a FitStick to give accurate measurements.

Is there a simple formula that calculates how much you need to slide the seat forward when you
raise it a given amount? I never trust my measuring. With this factored in how do I raise my seat
3mm? I calculate my seats distance from BB by measuring from the nose of the seat to the center of
BB as determined with a T square(more like a cross) extending beyond my horizontal top tube.

Thanks for any suggestions. Remove NOSPAM to reply directly John