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Seat tube angle(5 posts)

Seat tube angleChonola
Jul 16, 2002 12:02 PM
What are the benefits/drawbacks of a steep seat tube angle?

What are the benefits/drawbacks of a relaxed seat tube angle?

Is 73.2 degrees considered relaxed?
this depends on what size framebuffalosorrow
Jul 16, 2002 12:23 PM
This depends on what size frame you are speaking of, typically larger frames have more relaxed angles and the opposite for smaller frames. But if you look at some frame builders such as Look, all their seat tube angles are 72.5 (the top tubes are on the long side).
Colnago seem to have tighter angles.
All your riding preference and body type come into play.
What size frame do you ride, I'll guess a 56cm.
this depends on what size framecycle63
Jul 16, 2002 1:57 PM
Seat tube angles are usually more upright on smaller frames and especially so (75 and up) on triathlon bikes. Tighter angles are usually seen like the above post said on larger frame bikes. Depending on what your body type is, the average person usually will benefit as a spinner and not a masher with a seat tube more upright. Sitting in a position that is more back will benefit climbing. If you have long thighs and moving the seat back all the way doesn't get you the ideal "knee cap over pedal axle" theory, then you may need a seat tube angle that makes you sit more back.
That being said, I'm only 5'6" with a 29.5" inseam but my thighs are long. When I went to have a professional bike fitting, it was determined that I need a 72 degree seat tube angle on a 50cm frame!
Hope I didn't confuse you.
body fit, chainstay length...C-40
Jul 16, 2002 1:53 PM
Frames with steeper seat tube angles (STA) may have a little shorter chainstays, without the tire touching the seat tube. This doesn't mean that they necessarily will have shorter chainstays, but they could.

The main purpose of adjusting the STA is to center the saddle on the seatpost, with the rider's knee in the optimum position relative to the pedal spindle. Riders with longer upper legs or a desire to place the knee further behind the pedal need a more relaxed angle.

I can ride a frame with seat tube angles anywhere in the 73 to 74 degree range. Angles closer to 74 best suit my needs. Most seatposts have 2-3cm of fore/aft adjustment. This adjustment can compensate for a less-than-perfect STA, but not without affecting the reach to the handlebars. One degree is equal to 1.0 to 1.5cm of saddle movement, depending on frame size.

The formula for the difference in saddle position due to a change in STA is (cosA-cosB) x saddle height. Saddle height is measured from the center of the bottom bracket, parallel to the seat tube.

TT length and STA must be considered together when comparing different frames. For example, a frame with a 74 degree STA and 54.3cm TT will have the same reach as a frame with a 73 degree STA and a 55.5cm TT.
And then there's comfortKerry
Jul 16, 2002 5:02 PM
A steeper STA (everything else constant) will make the frame better at translating vertical forces into your butt. Something to consider on long rides and/or rough roads.