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  1. #1
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    How much diff does Head/SeatTube Angle make in terms of reach from Seat to Bars?

    How much diff does Seat/Head Tube Angle Make in terms of reach from the seat to the bars? For example, if the top tube length was the same between two bikes, but the head tube angle and/or seat tube angle was different, how much difference would there be in reach to the bars between the bikes. In other words, if the angle of a head tube or seat changes by 1 degree, how much does that increase/decrease the reach from seat to bars?


    I ask this because . . .

    I have narrowed by choices down to a 2010 Novara Randonee or a 2010 Jamis Aurora in size 57. The Head Tube Angle is 69 for the Randonee and 72 for the Aurora. The Seat Tube Angle is 72.5 for the Randonee and 72 for the Aurora. The top tube size is 555 for the Randonee and 560 for the Aurora. How much difference in reach from the seat to bars will there be between the Randonee and Aurora due the the head and seat tube angles?


    I ask this question because I cannot find a size 57 in the Aurora to test ride.

  2. #2
    rhm
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    multimodal commuter rhm's Avatar
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    Well, since the Aurora has parallel seat and head, the distance will be constant regardless how high you have the seat and bar, while the angles of the Randonee are going to bring the two together a little bit, depending how high you have the seat and the bar up. So the Randonee will be a little shorter, but at a normal extension the difference won't amount to more than 10 - 15 mm. I consider this more or less negligible; you can easily make up for that difference by the way you adjust the seat. The length of the stem is a much bigger variable. and of course you can change either stem or seatpost, or both, if you need to change the reach more.

  3. #3
    ah.... sure. kayakdiver's Avatar
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    I had the 2008 Jamis Aurora in a 55 and after I crashed it I have the 2010 Medium Norvara Rondonee. Both fit me stock. I could have went with a large and made it fit as well with less saddle to bar drop but I like having more drop than most.

    I would say that the Rondonee Large is going to be closer in sizing to the 57 Jamis. Either bike is a nice bike. You do gain some clearance in the heal dept. on the Rondonee.

    Don't forget that the Rondoneee also has compact shallow bars. More so than the Jamis.
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  4. #4
    Primate Metzinger's Avatar
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    The angle differences affect reach very little.
    Picture sitting on a bike with an adjustable angle head tube. Then push/pull the bars to the new position. The seat tube diff is too small to worry about.
    Even if the bars are 15cm above the TT, the reach change will be roughly 8mm.
    {tan(72-69)*150mm = 7.9mm**

  5. #5
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    Thanks for the replies. In the end, I am just trying to find out how much longer the reach will be for the Aurora since I cannot find one to test ride. I have a difficult time finding a bike that fits due to a short torso and short arms. I am trying to compare the size 58 in Randonee to the size 57 in the Aurora.

    Since the geometry changed on the Randonee for 2010, it is pretty close to being a good fit. I might raise the bars up with a riser stem and it should be good for size 58 in the Randonee. The only concern that I had with the bike was that it seemed difficult to stand and pedal. It almost seemed like that the bars needed to be a little further away to get any leverage for standing to pdeal. I like to stand and pedal sometimes while going uphill or to give my tail a rest. My body shape is to blame probably. The bikes that feel great when standing to pedal are the bikes that have too long of a top tube that fits me comfortably while seated.

    I am still curious if the size 57 Aurora would work as well, but I cannot find one to test ride. I have test ridden a 55 in the Aurora. I know the 55 would not work. However, I am concerned that the 57 in the Aurora might be too long of a top tube to be ridden comfortably.


    Will REI let you swap a stem for a test ride?

  6. #6
    More Energy than Sense aroundoz's Avatar
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    RHM explained it well.

    I had to check because a HT angle of 69 is pretty slack. The geometry has changed with the 2010 model since it used to be 72 degrees.

    You also have to consider apparent TT length which is often overlooked. Sorry for using myself as an example. I know I need 82.5 cm from the BB to the top of saddle and 12cm from the tip of my saddle to the center of the BB (using a plumb bob at the tip and measuring parallel to the ground). Whatever bike I am looking at, that's my starting point.

    So considering you have different ST angles, the saddle should always be in the same place relative the center of the BB. Since the ST angle is steeper on the Randonee by .5 degrees, you would be moving the saddle to the rear to make up for this which would increase your apparent TT length. And to figure out how much, consider the distance from the BB to the top of saddle as a radius. In my case: 825mm x 3.14 x 2/360 x .5(angle difference)= about 7mm. Add this to the Randonee's TT length and you are at about 62cm. But like RHM pointed out, angles converge on the Randonee which will shorten your apparent TT length. 3 degrees difference may be pretty significant depending on how high the bars are. If you had a short stem on the Randonee and felt pretty maxed out for reach, the Jamis might not work, for example.

  7. #7
    ah.... sure. kayakdiver's Avatar
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    The stem is part of the fit. One stem does not fit all. REI will swap stems for you at time of purchase for free if equal value. So getting it to fit shouldn't be a problem.

    REI will swap out any parts retail for retail and free labor at time of purchase. When I purchased my Rondonee I had them swap out the RD for an XT instead of the 105 and also the cassette from the 11/27 to an 11/34. Total cost was almost a wash.

    I'd imagine that a Jamis dealer would do the same if they are a good shop.
    Last edited by kayakdiver; 03-09-10 at 08:44 AM.
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    The 2009 Randonee geomety seems closer to the Aurora. I think that I will see if I can find a 2009 Randonee and compare it to the 2010 Randonee and that will at least give me a ball park idea of what the difference in reach would be like.

    Thanks for all of the help!!!

  9. #9
    www.Click-Stand.com tomn's Avatar
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    I was told that a one degree change in the seat tube angle would have a one centimeter effect on the saddle set-back. The set-back in this case refers to the distance from the nose of the saddle to a line drawn vertically through the center of your bottom bracket (which is what aroundoz was discussing earlier). You can maintain a saddle set-back as the seat tube angle changes by sliding the saddle forward or backward on it rails. What you are trying to maintain with a known saddle set-back is a particular knee over pedal axle position. Custom framebuilders will try to match that set-back with a seat tube angle that will place your seat post clamp in the middle of your saddle rails. So unless you run out over the limits of your saddle rails, the difference in seat tube angles between the bikes won't make a difference in your reach to the bars. I don't know of a rule of thumb about changes in heat tube angles, though the change in handle bar locations shouldn't be as dramatic as the change in saddle location. The pivot point for seat tube angles is the bottom bracket, which is a long distance to the saddle. the pivot point for a the head tube angles is the top tube/head tube intersection which is a short distance to the handlebar. Anyway, as stated, that would be adjusted bu a longer or shorter stem.

    I hope that helps,
    Tom
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  10. #10
    Primate Metzinger's Avatar
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    Quote Originally Posted by tomn View Post
    The pivot point for seat tube angles is the bottom bracket, which is a long distance to the saddle. the pivot point for a the head tube angles is the top tube/head tube intersection which is a short distance to the handlebar.
    It's true that that's where the angle is defined in the frame, but the reach differences for a described top-tube relate to the saddle distance above the top-tube (or virtual TT, on sloping geometry bikes.)
    Otherwise, TT length numbers wouldn't mean anything.
    For a given TT length, various seat setbacks can be described as follows:
    {TANx * h = sb** (where x is seat tube angle, h is seat height, and sb is the setback from directly over the top of the seat tube)
    In other words, for reach related matters, imagine the TT as fixed, and the head and seat tubes pivoting off that.

  11. #11
    Primate Metzinger's Avatar
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    I just reread what I wrote there, and it's a bit difficult.
    Here's a better example:
    If you've got your seatpost bottomed out, or at the height of a virtual toptube (on a sloping geometry bike) varying seat tube angle will not affect reach.
    It's only as the seatpost is extended above this that there's any increase in reach beyond the length of the toptube.

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