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Seat tube angle question....(12 posts)

Seat tube angle question....opencl
Sep 16, 2002 4:37 PM
Can anyone describe the pros/cons of having a bike with a steep ST angle?
Do a searchKerry
Sep 16, 2002 5:06 PM
This question was covered just last week in some detail. Short answer is that if you keep the same position on the bike (seat relative to BB, seat to H'bar distance) and the rest of the bike doesn't change (chain stay length, BB height, head tube angle, fork offset, wheel base, front center distance) then the only thing you'll get from a steeper STA is more road shock through your rear end. In practice, bikes with steeper STAs typically have a number of other "features" in the geometry.
princess and the pea...C-40
Sep 16, 2002 6:59 PM
The difference in transmitted road shock will not be more the 0.5% for each degree of angle. If you can feel a change that's 1% or less you must be as sensitive as the princess with pea under her mattress.

The real difference is in the location of the saddle on the seatpost and the "effective" top tube length (or reach to the bars). Steeper angles increase the effective top tube length (when the saddle is moved back) and position the saddle further back on the post for a given KOP. If you run out of saddle adjustment, the angle is obviously too steep. A different seatpost with more setback and a shorter stem might be required.

Steeper angles also permit the chainstays to be made a little shorter, if desired.
exactly ...bianchi boy
Sep 16, 2002 7:10 PM
If you have two frames with the same top tube length but different seat tube angles, the one with the steeper angle will fit longer because you will have to move the seat back further to position your knee properly over the pedal. Conversely, a frame with a slacker angle will fit shorter, assuming you keep the same knee knee-over-pedal position.

I have a Gios with a relatively short top and steep seat tube angle (74). In size 56 seat x 55 top, it fits about the same as a Merckx frame in size 56 seat x 56 top with a 73 seat angle. The only problem I've encountered from the steep seat tube angle is finding a saddle with rails long enough (or positioned) so I can move the seat far enough back. You can get around that problem with a setback seatpost, or shopping around for saddles. Koobi saddles can be moved back a lot and fit my frame great. However, I can't use certain classic saddles like the Brooks leather or San Marco Regal on that frame without a setback post. I haven't noticed any harshness in the ride of my Gios -- it has the same sweet ride of any nice steel frame I have ridden.
Pros and conshayaku
Sep 16, 2002 6:40 PM
Pro: It may suit your riding.

Con: It may not.

It depends on your needs, weight and desired purpose.
Sorry I can't be any more help, but questions about fit are so individual that you really need to experience things for yourself. Try everything that you read or hear and you'll find out what works for you.
very, VERY general answer...Steve_0
Sep 17, 2002 3:05 AM
steeper seattube angle results in increased aerodynamics at a cost of decreased comfort.

as others stated; gotta consider the whole geometry.
so much baloney...C-40
Sep 17, 2002 3:45 AM
STA won't affect comfort to any noticeable degree and it sure won't affect aerodynamics.
relativity my simple-minded friend....Steve_0
Sep 17, 2002 5:07 AM
GENERALLY, steep STAs are complimented with shallow HTL.... a combination which DOES result allow for more aerodynamic body positioning, which GENERALLY reduces comfort for most people.
If you subscribe to John Cobb and Dan Empfield,sn69
Sep 17, 2002 6:31 AM
...both of whom are experts in triathlon/TT bike fitting and biomechanics, you will find that steeper STAs with shallower HTAs are desinged to allow for effective hip-to-torso angles when riding on aero bars, indirectly resulting in better aerodynamics, better breathing and more power. Also, the design is meant to compensate for weight distribution problems associated with aero bars in order to keep typically short-top-tubed tri/TT bikes better balanced and more stable. Finally, steeper STAs tend to lend themselves to the steady cadence spinner whereas shallow, "traditional" STAs lend themselves well to a variety of bicycling styles.

It's too much of a generalization to say that steeper STAs will result in reduced comfort. There are a lot of steep STA advocates who argue 180-out from you with equal zeal. Frankly, I think it's all personal preference based upon an individual's riding style and biomechanical characteristics.

As with everything else in bikes, one must get and out try the different styles to determine what works best for the individual.

Sep 17, 2002 6:54 AM
comfort is highly individualized; I never intended to imply otherwise.

However I thinks its safe to believe the average body under 'typical' riding conditions (not Tri format) is more comfortable on a more traditional road geometry.

100 years of geometrical evolution seems to have landed a 'comfort zone' around 73-74 degrees for the masses.
True. No argument with that. (nm)sn69
Sep 17, 2002 6:57 AM
more baloney...C-40
Sep 17, 2002 6:28 PM
The exact same body position can be achieved with any STA normally used on a road bike (72-74 degrees) with a simple movement of the saddle or at most a change in the seatpost setback.

You guys must be confusing road bikes with specialized time trial bikes that have 78 degree STA. With these bikes, all rules go out the door. You see all kinds of weird stuff being built.

The HTL has little to do with body position. Stem angle or spacers can be used to position the rider just as high on a bike with a short HT as one with a long HT. I've seen lots of TT bikes with short head tubes and a huge stack of spacers to raise the bars up (even on pro bikes at the tour). Apparently the builder and the rider didn't agree on the appropriate HTL.