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  1. #1
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    Road bike with steeper seat tube angle

    Hi all- I have several road bikes, some set up with a profile design fast forward seatpost. When I switch between bikes with and without the steeper seat post angle, I am always surprised how much better the steep seat tube angle feels,(in all riding positions) Is this steeper seat tube a more efficient position? It seems I can spin easier, and much faster than a shallow seat tube angle.

    If this is a better riding position, why don't bike manufacturers make road bikes with a steep seat tube angle/

    thanks in advance for your time

  2. #2
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    I think it's more of a fit issue than anything.

    Steeper seat angles work better for individuals with shorter femurs (which may be the case with you). I basically ride a 58cm frame with a 74.5 degree STA (custom frame) and have short femurs to go along with my short legs. They also tend to open the hips up a little more which makes it a bit easier to get a little more power and spin easier, which may be what you are feeling.

  3. #3
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    Reputation: asciibaron's Avatar
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    the steeper seat tube angle puts the rider in a more forward position. a more relaxed seat tube angle moves the torso to a more upright position - notice the angles on cyclocross bikes or touring bikes compared to agressive racing frames.

    you'd be surprised how much difference a 1.5 degree change in the seat tube makes. for me, it's the difference bewteen riding for an hour and riding for 4 hours.

    the sloped top tube frames are an interesting middle ground allowing manufactures to make up the difference in fit by adjusting saddle height and stem length. instead of having to build 12 frame sizes, they can get by with 4 or 5. look at the seat tube angles and you'll see that the sloped top tube frames offer a more relaxed, upright ride - much like a mountian bike frame.

    -steve

  4. #4
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    more to it...

    I can produce exactly the same riding position on a frame with a 72.5 degree STA as another with a 74.5 STA. The STA does not dictate where you sit relative to the BB. In this example, the frame with the 72.5 STA needs a no-offset seat post and the other requires a standard 20-25mm setback model to produce identical positions. Most saddles also have 2-4cm of adjustment on the rails, which provides the same amount of movement as 2-3 degrees of STA.

    Just because you spin faster with the more forward position does not mean you'll produce more power and ride faster. Power = torque x cadence. A high cadence without sufficient torque won't produce much power. The optimum position requires a balance of torque and cadence. Most often, a further back position will allow more torque to be applied.

    Another point to consider is the wheelbase of the bike. If the STA is made much steeper and the TT length remains unchanged (assumes the rider wants to move further forward), then the wheelbase gets longer It then becomes the typical TT bike with a 76-78 degree STA.

    As for sloping frames having a different STA than horizontal TT frames, I haven't seen many cases of that. LOOK, for example, has abandoned the 72.5 degree STA that they used to offer in all sizes. Now they use angles as steep as 75 degrees on the smallest frames (like most other brands have for years). Although some brands still try to cover a 3cm range with one frame size, more are offering the more common 2cm increments, just as they did before sloping TTs became prevalent.

  5. #5
    donuts?
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    [QUOTE=C-40]I can produce exactly the same riding position on a frame with a 72.5 degree STA as another with a 74.5 STA. The STA does not dictate where you sit relative to the BB. In this example, the frame with the 72.5 STA needs a no-offset seat post and the other requires a standard 20-25mm setback model to produce identical positions. Most saddles also have 2-4cm of adjustment on the rails, which provides the same amount of movement as 2-3 degrees of STA.
    [\quote]

    you realize you just contradicted yourself - adding a seat post with offset to get the same position shows that STA effects position relative to the BB. using the same seat post on the 74 and 72.5 STA puts the rider in a different location relative to the BB.

    if the 72.5 STA puts the rider back 1.5 inches from the 74 STA, then adding the 4cm of seat adjustment puts the saddle back more then is possible on the 74 STA frame.

    -steve

  6. #6
    Incredibly slow
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    Steve, you didn't read what C-40 said well enough. Re-read his second sentence, and then realize that he is saying that where you sit is independent of seat tube angle. Being new you probably don't realize that anyone here wanting positioning advice asks for C-40. And his geometry is correct in this case, you just need to re-read it.

  7. #7
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    like bwana said...

    You didn't read closely. I never said STA has no effect, I said it does not dictate saddle position. Seatpost offset and the normal range of saddle rail adjustment can both be used to counter a less than perfect STA.

    I've owned a frame with a 72.5 STA and 54cm TT (old LOOK geometry) that fit exactly the same as one with a 74.5 STA and a 52.5cm TT (new LOOK geometry). The only difference was the style of seatpost required to position the saddle in the same place relative to the BB. Once that was done, the reach to the handlebars was the same, with the same stem length.

  8. #8
    donuts?
    Reputation: asciibaron's Avatar
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    my point was given all things being equal but the STA, the STA would effect the riders position to the BB. once you start changing seat posts and stems, you can get close to the same position - but that seems like your trying to compensate for an STA that is not ideal for the rider.

    -steve



    -steve

  9. #9
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    maybe..

    In my example, let's say that the rider insists on using his favorite Thomson no-offset seatpost. In that case, a 72.5 degree might be ideal. If the same rider later finds that he really like an FSA K-force post with 25mm of setback, then his ideal STA becomes 74.5 instead.

    In the example I gave, the frame with 72.5 STA just happened to have a longer TT length, so the reach was the same with the same stem length. I didn't make this example up, it's a LOOK KG381 and a KG461, both in the 51cm size.

    When comparing frames of the same c-c size, the dimension that tells you the true difference in the stem length required, with the rider in the same position relative to the BB, is the frame REACH. Reach is the TT length minus the frame setback. Setback is the c-c frame size times the cosine of the STA.

    In the geometry chart I've linked, it's O minus Sc.

    http://www.trialtir-usa.com/2006-col...sizecharts.htm

  10. #10
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    Quote Originally Posted by asciibaron
    if the 72.5 STA puts the rider back 1.5 inches from the 74 STA
    Not even close, more like 1.25 cm back.

  11. #11
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    actually...

    The exact amount is the c-c frame size times (cosA-cosB), so you really can't say how much difference there is without knowing the frame size. For a 60cm frame the amount would be 1.5cm, if the frame is only a 50cm, then it would be 1.25cm.

  12. #12
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    Mondonico and Torelli stock geometries tend to be a little steeper in the STA.

  13. #13
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    I think it comes down to one of 2 things:

    Either you are not the most flexible guy and haven't enough back strength to keep the traditional road riding position, hence feel more natural/relaxed/comfortable with a more forward position and a smaller hip angle, which makes you faster. I know as I rode more I actually moved my saddle backwards by almost 2cm, and I feel very comfortable with the current position.

    The other possibility is you are able to achieve a much more aero position by moving forward -- think David Zabriski.

    I suspect it's the former case and it's just a matter of getting used to a riding position. I have a suggestion, since you are an advocate of steep seat angle, why not keep using that forward post. Maybe in 1000 miles you will have a whole new perspective on this issue.

    We are not exactly talking about 73 degree versus 74 degree here, but rather a more dramatic shift forward of the body, more like 6-7cm. Notice a forward position will also have handling complications due to change in weight distribution.

  14. #14
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    Quote Originally Posted by C-40
    The exact amount is the c-c frame size times (cosA-cosB), so you really can't say how much difference there is without knowing the frame size. For a 60cm frame the amount would be 1.5cm, if the frame is only a 50cm, then it would be 1.25cm.

    Absolutely correct, that's why I said "more like". Even 1.5cm is a lot less than 1.5 inches.

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