Seatpost Alignment... So Easy!
 

File this under things younger riders probably take for granted-- after spending all that time, never questioning what I had to work with, eyeing the setup from various odd angles to make sure the seat was aligned with the top bar; and, not able to reproduce seatpost height with any degree of precision, who knew it could have been so easy all those years...

Voila!

?!

It's as easy as......

http://lh4.ggpht.com/-zleUZVfqw_I/UA...jpg?imgmax=800

joking aside, i'm surprised more seatposts dont have those markings on them....

For saddle centering, that works if:

1. Seat post markings are perfectly centered.
2. Seat tube slot is perfectly centered.
3. Saddle rails are perfectly aligned.

I'm yet to see a combo where all three are true. Don't forget either that misalignments at the seatpost get amplified at the saddle, so even if all three conditions are met it can be quite hard to center the markings in the slot well enough for the saddle to end up perfectly centered.

For saddle height it's OK, but if you're removing the seatpost often and want to always have it at the exact same height, it might better idea to get a clamp that goes on the seatpost. See for example the BBB Postfix.

Explain why 3 degrees off center would be *amplified* to more than 3 degrees at the saddle.

My seat post HAD all those markings, but they are worn off now.

Three degrees wouldn't, but half a millimetre would. Guess I didn't explain it very well, so to put it another way: for me it's harder to notice a 3 degree error by looking at the seat post markings in relationship to the seat tube slot, than by looking at the saddle in relationship to the frame.

Easy. Assume:

1. Frame cutout is off-center to the left by three degrees.
2. Seatpost markings are off-center to the right by three degrees.
3. Saddle rails are off to the right by three degrees.

That scenario nets you a saddle position rotated to the right by 9 degrees.

I do wish that there could be a fool-proof way to ensure a saddle is aligned aside from just eye-balling it.

Just ride the bicycle. Your anatomy will tell you whether it's in or out of alignment.
Jon

Aero seatposts self align.

Some believe an out of alignment seat will re-align your anatomy too.

I see what you're saying now.

So a millimeter at the back of the small seat tube is just a millimeter - but at the nose of the saddle it could be like 9 millimeters off. Totally makes sense now...thusly, adjusting at the nose of the saddle (aka eyeballing it) is going to give you a much smaller margin for error.

I personally prefer the saddle to be a little off from the line of the top tube. But when checking, I find it easier to stand behind the bike and sight down the saddle.