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Frame Size and Tube Angles

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Frame Size and Tube Angles

Old 07-29-10, 10:22 AM
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Stickney
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Frame Size and Tube Angles

I have a question on comparing different frames' geometry -- would appreciate some guidance.

I have a bit of confusion in how to analyze top tube length in relation to seat tube and head tube angles. Here are three frames, what differences in fit would I need to take into consideration for each frame? Thanks!

Frame 1:
Top Tube = 59 cm
Seat Tube Angle = 72.5
Head Tube Angle = 73

Frame 2:
Top Tube = 59 cm
Seat Tube Angle = 73
Head Tube Angle = 73.5

Frame 3:
Top Tube = 58 cm
Seat Tube Angle = 72.5
Head Tube Angle = 72
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Old 07-29-10, 04:59 PM
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I presume you are concerned with comparing the effective reach to the bars among the three bikes. If you have the same rider position (saddle position and height) relative to the bottom bracket, same bars, same stem, same bar height adjustment etc. on the three bikes, then the variation in reach due to the frame geometry is approximately:

Bike 1: Reach is X
Bike 2: Reach is X minus 5mm
Bike 3: Reach is X minus 10mm

Head tube angle only makes a tiny amount of difference and can be ignored.
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Old 07-30-10, 07:46 AM
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Scirocco -- I was beginning to think that with all of the bike knowledge on the board, no one knew the answer! So thank you very much. I was trying to set up the "math" in my head (and you are correct, I should have spelled out the issue as effective reach) but kept second guessing myself.

Thank you!
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Old 07-30-10, 06:47 PM
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AnthonyG
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Um, I'm going to have to disagree with Scirocco here. Right concept but I think he went the wrong way.

Frame one is classic geometry so we will compare to this frame. Same conditions of comparison as we assume the bars and seat are at the same height and here it would be nice to know the head tube length of the 3 frames because this WILL make quite a difference.

Frame 2 has the same top tube length but with a steeper seat tube angle which means its LONGER in reach than frame 1 although the steeper head tube angle is eating into the difference slightly.

Frame 3 has a shorter top tube length at the same seat tube angle which means it is 10mm shorter however it does have a quite slacker head tube angle which is making the difference shorter so its not the complete 10mm shorter. Now if you had a really tall head tube or high handlebar height it would be negating any difference in a hurry.

Comparing head tube height between the 3 frames will tell you as much as the other specs.

Anthony
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Old 07-30-10, 08:03 PM
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Anthony is quite right and I got myself confused thinking 72.5 was steeper than 73. So I should have written:

Bike 1: Reach is X
Bike 2: Reach is X plus 5mm
Bike 3: Reach is X minus 10mm

The head tube angle differences also affect the reach as Anthony says, but the actual effect is small (probably less than 1mm). The seat tube angle makes a much bigger difference because the assumption is that the saddle is moved (either on the rails or via a setback seatpost or combination of both) in order to keep the rider in the same preferred position relative to the bottom bracket. So, with a slacker seat tube angle, more of the top tube length is "used up" reaching back to the seat tube, leaving a shorter length in front of the BB to affect the reach.

The geometry works out such that one degree steeper in seat tube angle equates to roughly 10mm increase in reach, if everything else is kept constant.

Last edited by scirocco; 07-30-10 at 08:07 PM.
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Old 07-30-10, 08:35 PM
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I just want to say thanks, and thanks for the clarification. In my head I was getting most of the way there, but I was a little unsure about how it worked. I appreciate the responses, they helped.
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