Bicycle Mechanics - Effect of seat tube angle?

Hello... long time lurker, first thread blah blah blah.

It's my understanding that a steeper seat tube places one in a more aggressive position (track bikes, crit bikes, etc), and a slacker seat tube is, obviously, more relaxed (I'm thinking of my early '80s LeTour).

But why? Can someone explain what effect the seat tube angle has on various muscles in the leg, back, and core? Is one's weight placed differently on the bike (more stress on the hands, perhaps)? Why is it easier to sprint with a 75 degree seat tube than a 72.5? Is it practical/comfortable to ride a long distance on a bike with a steep seat tube?

jollysnowman

p.s. you can call me by my gangster name, jsnow.

I think it has to do with the hip angle. Once this angle becomes too acute, you cannot develop as much power. Kinda like how a half-squat is easier than a full-squat. So if you want to ride in an aero position *and* develop power, you need to rotate the entire body forward on the bike, thus the steep seat angle.

You've been misinformed. The STA only affects your choice of seatpost offset. I can place the saddle in the same position with a 72.5 degree STA and nonsetback post or a 74.5 degree STA with a 25mm setback post.

It's a common misconception to think that differing STAs force you into a different position.

Another thing that the STA does, is work with the TT length to create the frame reach. Reach is the horizontal distance from a vertical line through the center of the BB to a specific point of reference at the head tube. That point could be the TT/HT centerline intersection point or the point that Cervelo uses - the top-center of the head tube. Steeper STAs increase the reach, for a given TT length.

A related question is the effect of rider position relative to the BB. A more forward position increases the weight on the front of the bike (about 1% per centimeter) and promotes high cadence pedaling as might be used at the track or in crits. A further back position is often used for climbing at lower cadences.

Placing the knee over the pedal spindle is a common practice and a decent starting point. If you look at pros bikes, you won't see many nonsetback posts to promote a forward postion (except on TT bikes). Most have setback posts with the saddle relatively far back.

Hello... long time lurker, first thread blah blah blah.

It's my understanding that a steeper seat tube places one in a more aggressive position (track bikes, crit bikes, etc), and a slacker seat tube is, obviously, more relaxed (I'm thinking of my early '80s LeTour).

But why? Can someone explain what effect the seat tube angle has on various muscles in the leg, back, and core? Is one's weight placed differently on the bike (more stress on the hands, perhaps)? Why is it easier to sprint with a 75 degree seat tube than a 72.5? Is it practical/comfortable to ride a long distance on a bike with a steep seat tube?

jollysnowman

p.s. you can call me by my gangster name, jsnow.


You're wanting some big long theory on why? A steeper seat tube can place you further forward of the BB, making it easier to get on top of the pedals, so to speak. But, this does not work for everyone in practice.

A 75 STA is not as comfortable as say a 72. Why? I'm no engineer, from experience I just know it is. I sit way back myself. 72 degree ST, 185mm cranks and I'm inches, not cm's , behind the ol' plumb-bob over the knee thing. It soaks up road bumps real nicely:)

a steeper st angle allows for the wheel to be closer to the bottom bracket, before the tire is rubbing on the seat tube. In the old days when all frames were steel, a shorter chainstay meant a stiffer chainstay, hence better for sprinting. With a carbon frame you can design in whatever stiffness in torsional, vertical, etc with whatever chainstay length you want.

The important thing to realize is that the seatube angle and the top tube length must be considered together when comparing the reach on frames, eg the reach of a frame with a 57cm TT and 73 sta is the same as a 58 cm TT and a 72 Seat tube angle.

http://www.ravingbikefiend.com/bikepics/ccmpathfixed4.jpg

Here is an extreme example...

This bike has a very long rear stay / wheelbase and very slack frame angles but my position on the bike is the same as if I was riding my road bike and is moderately aggressive with the bars set nearly level.

My position relative to the bike is different as I am set farther back with less weight on the bars which makes for a very comfortable ride as shocks from the front wheel are dampened a great deal (despite the very rigid fork). The stock fork had much more trail and absorbed even more road shock but the bike steered like a bus and would not accept a front brake.

The ride itself is very smooth and comfortable and the combination of frame and geometry does well to dampen even the hardest of shocks... small changes in saddle height also result in much greater changes to fore aft position of the saddle.

For bad uneven roads this bike rocks and it is pretty nice on glassy smooth pavement as well.

It's a common misconception to think that differing STAs force you into a different position.
Right, the STA doesn't force you into a different position as long as an available seatpost's setback can compensate. However, varying seattube angles can allow you to have a position that a seatpost isn't able to provide.

The most common use of an extreme seatube angle is on triathalon bikes where the use of aerobars is made easier by a rather far forward rider position. For these a 78° STA helps position the rider without using an odd-ball seatpost.