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Stem sizing question for bike fit experts
Bike A has 54cm top tube, 73° head angle, 73° seat angle, 115mm stem.
Bike B has 54cm top tube, 73° head angle, 74° seat angle. Bike B has a seatpost w/sufficient setback to push the saddle back as far as necessary to duplicate the KOPS of bike A, and slightly longer chainstays/wheelbase to compensate for shifting the weight distribution rearward. What stem length is needed for bike B to replicate the upper body extension of bike A? Is it (a) 115mm, (b)110mm, or (c)105mm? Thank you for your help. |
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Stem sizing question for bike fit experts
On 1 Sep 2005 16:22:02 -0700, "
wrote: Bike A has 54cm top tube, 73° head angle, 73° seat angle, 115mm stem. Bike B has 54cm top tube, 73° head angle, 74° seat angle. Bike B has a seatpost w/sufficient setback to push the saddle back as far as necessary to duplicate the KOPS of bike A, and slightly longer Which is how far? You need both the seat angle and the seattube length to do that calculation (actually, you need seattube length plus exposed seatpost length for the exact number). Assuming a 50 cm seat tube, the difference that the 1 degree of angle makes is about 500 * (sin 1) mm, aka 8.7 mm. So your seatpost lug will be nearly a cm further forward, by the time you're at the saddle it'll probably be a bit more than a cm, and you should probably go to a 105 stem to compensate. Jasper |
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Stem sizing question for bike fit experts
I have a hard time worrying about 3/8 of an inch difference in reach. I'd
go with a 110 MM so I would not be any less reach or just stick with the 115 mm. Ken wrote in message oups.com... Bike A has 54cm top tube, 73° head angle, 73° seat angle, 115mm stem. Bike B has 54cm top tube, 73° head angle, 74° seat angle. Bike B has a seatpost w/sufficient setback to push the saddle back as far as necessary to duplicate the KOPS of bike A, and slightly longer chainstays/wheelbase to compensate for shifting the weight distribution rearward. What stem length is needed for bike B to replicate the upper body extension of bike A? Is it (a) 115mm, (b)110mm, or (c)105mm? Thank you for your help. |
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Stem sizing question for bike fit experts
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Stem sizing question for bike fit experts
Jasper Janssen wrote: Which is how far? You need both the seat angle and the seattube length to do that calculation (actually, you need seattube length plus exposed seatpost length for the exact number). Assuming a 50 cm seat tube, the difference that the 1 degree of angle makes is about 500 * (sin 1) mm, aka 8.7 mm. So your seatpost lug will be nearly a cm further forward, by the time you're at the saddle it'll probably be a bit more than a cm, and you should probably go to a 105 stem to compensate. Actually, the exposed seatpost does not matter (since he has already located his saddle), only the ctc seattube length. An important factor is whether or not the two bikes have horizontal or sloping toptubes... I'm assuming they are both horizontal with 55cm ctc seattubes (just because that is pretty average)... and if we wanted to be precise (for no good reason) we'd have to know a bunch of other things, too... so I'll assume that the following are all the same between the two bikes; bottom bracket height, headtube length, headset stack height, fork length, stem clamp height above the headset, stem angle, and the handlebar reach (can you tell I recently wrote a spreadsheet to deal with all this stuff?). Difference = 55*(sin(17)-sin(16)) = .92cm which means you'd like a 1cm shorter stem (105cm) if you wanted it to be the same as before. But like Ken said, 1cm on the reach is not much... maybe you'd like it a bit longer than before? |
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