motorapido

Here is a question about seat tube angle and how the choice of seatpost setback and sliding the seat forward and back on the rails either changes or does not change the frame geometry. Suppose you have a frame with a relaxed, touring type of seat tube angle. Suppose you put on a seat tube with lots of setback and slide the seat as far back as you can on that big-setback post. Have you created an even more relaxed-feeling frame geometry? Then suppose you put on a zero-setback seatpost and slide the seat as far forward as you can. Have you negated the ride characteristic of the relaxed seat tube and create a much more stiff racing-style feeling?

For some reason I’m thinking that nothing you do with the seatpost setback choice or seat rail position will change the feeling of the frame, since the angle of the seat post isn’t being changed. Am I right or wrong?

bradtx

motorapido, The frame's geometry is set. While a saddle maybe moved more forward in the seat post clamp when on a bike with a 72 deg. seat tube angle compared to a frame with a 73.5 deg. seat tube angle (all other measurements equal). Saddle placement is determined by whatever is required for a proper fit.

The characteristic of a frame's geometry can be altered slightly by changing the amount of the fork's off set and if equipped with horizonal drop outs by reducing/increasing wheelbase.

Brad

fietsbob

+1, the seat tube is a frame builders choice.. cruiser bikes like 68 degrees ,

Racing , maybe 74+ degrees.. typically 73,72 ..

now seat Post offsets you can change.., like the ..
feel changes in body weight on your butt /hands, proportionally with seat setback.

bobotech

I have a taller torso but short legs so I have discovered that I like a seat that is almost as far back as it can go. I found a seatpost where the mounting point is almost an inch farther back than a normal post and I love it.

HillRider

About the only influence on handling changing the saddle's position can have is on weight distribution.

MichaelW

Does changing the saddle position with respect to the wheels alter the handling? Yes.
If you setup 2 bikes with the same riding position and points of contact, one with a steep tube and lots of layback, the other with a shallow tube but inline post, then the rear wheel on the shallow frame would be further back. The bike would track in a straight line better but corner worse.
This is a different issue to taking one bike and setting it up with two different rider position by use of alt seatposts.

If you look at any normal model of bike, the angles change throughout the size range. Big riders are sitting right over the rear axle, small riders have the rear axle way behind them. Same brand, same model but totally different rider experience. The problem is that neither xxl or xxs riders actually know what a proper-fitting bike should feel like so its all the same to them.

In the olden days of steel bikes, the (seat/chain) stays were of fixed diameter so the shorter you could make them, the stiffer the bike. These days, you can have fat, stiff stays of any length so wheelbase is not really part of the stiffness issue.