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Seat tube angle question

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Seat tube angle question

Old 02-08-08, 04:17 PM
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Seat tube angle question

I'm thinking about a frame that is 74 degrees compared to my current 73.5 with the same top tube length. How much does .5 degrees translate to in mm or cm? I'm wondering if I will need a new stem length. Thanks.
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Old 02-08-08, 09:58 PM
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Old 02-08-08, 10:26 PM
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Is your intent to sit further forward or are you leaving your KOPS position the same?
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Old 02-08-08, 10:45 PM
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Originally Posted by powpow
I'm thinking about a frame that is 74 degrees compared to my current 73.5 with the same top tube length. How much does .5 degrees translate to in mm or cm? I'm wondering if I will need a new stem length. Thanks.
If you intend to ride with EXACTLY the same riding position on the new frame with the 74º seat tube angle as on your old frame with a 73.5º seat tube angle then EFFECTIVELY the new frame has a LONGER top tube. That is assuming that you are currently set at KOPS then to achieve KOPS on the new frame you will need to move the saddle REARWARD to compensate. If you then want to maintain the same reach to the handlebars as before then you would need to fit a shorter stem to compensate.

This is ALL assuming that you are currently fitted correctly so as usual with these questions its very helpful to know what your correct fitting is first.

Regards, Anthony
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Old 02-08-08, 11:06 PM
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I am currently fitted correctly, and I intend to keep the same position. My current bike has a 73.5 degree seat tube with a 549 top tube and a 90mm stem. The frame I'm looking at has a 74 degree seat tube with a 55cm top tube. Head tube length, fork rake and most other measurements are the same on both. Thanks.
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Old 02-08-08, 11:50 PM
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Sounds like you should be fine as long as you don't have your seat set far back on your current bike. I can't go higher than 73.5 because I need my seat far back, but I have weird proportions.
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Old 02-09-08, 12:39 AM
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Originally Posted by powpow
I am currently fitted correctly, and I intend to keep the same position. My current bike has a 73.5 degree seat tube with a 549 top tube and a 90mm stem. The frame I'm looking at has a 74 degree seat tube with a 55cm top tube. Head tube length, fork rake and most other measurements are the same on both. Thanks.
Well you could make it work. I think that your old frame would be better balanced however. Basically with the new frame you would have to move the saddle and handlebars rearwards 5-10 mm to maintain exactly the same position however getting an 85 mm stem may not be the easiest. The front wheel will be a little further forward on the prospective frame.

Something that's useful to know is your saddle setback. That's the horizontal distance from the nose of your saddle to the centre of the BB. Place a rule against the nose of the saddle and towards the head tube and then drop a plum bob down carefully to the centre of the bottom bracket and measure what the distance is. This is REALY useful information to know if your moving the saddle around or changing bike set ups. When it comes to fit the BB is the 0,0 point. Measure everything horizontally and vertically from there.

Regards, Anthony
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Old 02-09-08, 02:45 AM
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Originally Posted by urbanknight
Sounds like you should be fine as long as you don't have your seat set far back on your current bike. I can't go higher than 73.5 because I need my seat far back, but I have weird proportions.
I am the opposite as I am a shortie and can't get the saddle far enough forward at less than 73.5

Like all bikes- Set the saddle up for riding position- Then look at the length of the stem. And 1 mm will not be a longer top tube. Well it will but 1mm is not long enough to say it is. Get the saddle poition right first.
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Old 02-09-08, 05:41 AM
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Originally Posted by powpow
I am currently fitted correctly, and I intend to keep the same position. My current bike has a 73.5 degree seat tube with a 549 top tube and a 90mm stem. The frame I'm looking at has a 74 degree seat tube with a 55cm top tube. Head tube length, fork rake and most other measurements are the same on both. Thanks.
You left out the the most important measurement..seat tube length. Let's assume it is 55cm or so. 0.5deg sta diff = 5mm change in TT length for the same position between the bike axles. Your TT is effectively 5mm longer due to having a more upright seat tube angle if you put yourself in the same position fore/aft on the bike. Most stem's don't come in 5mm increments. If you put your seat in the same position on its rails on the new bike, you will be in the same position from tip of saddle to handlebars but your body will be 5mm more forward of the BB spindle which in the grand scheme is not that big a deal and you could split the difference if you wanted by fractionally moving the seat rearward a couple of mm's.
HTH.
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