HT Angle / ST Angle: Any known mfgs w/ large angle differences?
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  1. #1
    RoadBikeReview Member
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    HT Angle / ST Angle: Any known mfgs w/ large angle differences?

    I'm trying to do some research to find out what frame manufacturers might have frames where the head tube angle is steeper than the seat tube angle. I'm playing around with the idea that if I can find a frame for example, with a HT angle of 75* and ST angle of 73*, that the cockpit geometries will be larger for a smaller sized frame.

    From what I can tell it seems like most frames keep these angles the same.

  2. #2
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    you're confused...

    The HTA makes very little difference in the reach or "cockpit length". As an example, even if you use the maximum 3cm of spacer under the stem, the difference between a 72 ans 74 degree HTA only lengthens the reach by a measly 4mm. The resaon is that the distance from the pivot point to the top of the stem is only about 13mm, compared to four times that much, or more for the ST.

    A steeper STA actually increase the reach. FWIW, most Treks have about as much reach as you'll find in a stock frame.

    Post some specifis on frame size and I can be of more help.

  3. #3
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    Maybe I am confused...

    Let me know if this line of thought is wrong. If the STA and HTA are 73* and you measure a hypothetical reach of 60cm from tip of saddle to top of steerer tube, would the value of 60cm increase if the STA dropped to 71* and the saddle height remained the same?

  4. #4
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    maybe...

    Quote Originally Posted by tmanley
    Maybe I am confused...

    Let me know if this line of thought is wrong. If the STA and HTA are 73* and you measure a hypothetical reach of 60cm from tip of saddle to top of steerer tube, would the value of 60cm increase if the STA dropped to 71* and the saddle height remained the same?
    If the saddle is not moved on the seatpost, then obviously the reach would increase.

    What you're not considering is that every rider has only one optimum position relative to the BB. When comparing the fit of different frames, it's usually assumed that there is a fixed saddle fore/aft position relative to the BB. Moving the saddle very far back is usually not a great idea, becasue it increases the weight on the rear wheel. Get more than 60% of the weight on the back wheel and the bike won't handle as well.

    If the STA is more slack, like 71 degrees, the the saddle would usually be moved forward to compensate. That's why saddle rails have extra length for adjustemnt and seat posts are made with different amounts of setback. If you just need more reach, moving the saddle back is not the way to it.

    As an example I have one frame with a 72.5 STA and 54cm TT and another with a 74.5 STA and 52.5cm TT, but they fit the same, once the saddle is in the same position relative to the BB. The first frame mentioned requires a straight-up seat post to achieve the same saddle position as the second frame, which uses a 25mm setback post.
    Last edited by C-40; 06-08-2006 at 04:14 AM.

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