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PostPosted: Wed May 11, 2011 11:33 am 
Dirt Disciple

Joined: Thu Mar 03, 2011 4:41 pm
Posts: 43
A couple of tech questions to give me an idea for proper frame fit.
When you have an extended seat tube

i.e.


Image

What is the benefit of this and how far out of the tube should the seat post be ideally (obviously this we be determined by the riders height)?


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PostPosted: Wed May 11, 2011 1:18 pm 
rider | rBoTM Winner
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I don't really think there's a benefit as such, just a different way of building a low-profile time trial frame compared to a sloping top tube style. I've known riders have long seat tube extensions and very short seat pins but this does limit the size of rider who can use it. It's a bit like the current vogue for 'seat masts' on modern 'knitted soot' frames. At least the one you show does have a 'reinforcement' between the seat and top tubes. The overall seat tube length c-2-t could be slightly longer than you would use on a 'normal' frame.

Is the frame yours? It's nice.


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PostPosted: Wed May 11, 2011 1:36 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
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Old Ned wrote:
'knitted soot' frames.


Oh dear, coffee all over the keyboard... :lol:


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PostPosted: Wed May 11, 2011 1:55 pm 
Dirt Disciple

Joined: Thu Mar 03, 2011 4:41 pm
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Old Ned wrote:
I don't really think there's a benefit as such, just a different way of building a low-profile time trial frame compared to a sloping top tube style. I've known riders have long seat tube extensions and very short seat pins but this does limit the size of rider who can use it. It's a bit like the current vogue for 'seat masts' on modern 'knitted soot' frames. At least the one you show does have a 'reinforcement' between the seat and top tubes. The overall seat tube length c-2-t could be slightly longer than you would use on a 'normal' frame.

Is the frame yours? It's nice.


No unfortunately its not mine!
The reason being is I am looking at a TT frame with a extended seat tube. the center to top is 58.5. Not sure about C to C but I normally ride a 51-53 C to C. the top tube is the right size (52.5) its a low pro frame with 700c wheels. Just trying to figure out if its a ok size for me. I imagine my seat pin will end up being quite short and wanted to know if that is a problem..


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PostPosted: Wed May 11, 2011 2:33 pm 
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It's done for aerodynamics. They're trying to do away with the turbulent air created by the binder bolt. The seat masts of today actually succeed but the extended seat tubes didn't.

I suggest you get the c-2-c height for standover reference. The c-2-t would just have to be a little less than your current BB to seat rails measurement. just to make sure you can get your post and seat on.


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PostPosted: Wed May 11, 2011 5:28 pm 
Dirt Disciple

Joined: Thu Mar 03, 2011 4:41 pm
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stauqmuk wrote:
It's done for aerodynamics. They're trying to do away with the turbulent air created by the binder bolt. The seat masts of today actually succeed but the extended seat tubes didn't.

I suggest you get the c-2-c height for standover reference. The c-2-t would just have to be a little less than your current BB to seat rails measurement. just to make sure you can get your post and seat on.



Why did they not succeed in reducing the turbulent air?
What about the benefits of reducing weight as the seat pin can be chopped down?


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PostPosted: Wed May 11, 2011 11:05 pm 
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Extended seat tubes don't fix the aerodynamic problems of the binder bolt because there is still a collar and binder bolt sticking out unless your seat is slammed all the way down to the top of the extension.

As for weight, yeah I guess you could save a good number of grams cutting your seatpost shorter but I'd still not go all that short. Although the frame pictured above has a reenforcing gusset most do not. If there are no gussets the stress points are the same as on a standard frame and require the minimum insertion point to be down at the top of the top tube.


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PostPosted: Fri May 13, 2011 6:53 pm 
Gold Trader
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Surely the point was just to get a more aero riding position at the bars without an overly long seatpost (400mm seatposts weren't widespread BITD) or a downwards pointing top tube.


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PostPosted: Fri May 13, 2011 11:56 pm 
Dirt Disciple

Joined: Thu Mar 03, 2011 4:41 pm
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dbmtb wrote:
Surely the point was just to get a more aero riding position at the bars without an overly long seatpost (400mm seatposts weren't widespread BITD) or a downwards pointing top tube.


So in essence the rider would be riding a much smaller C to C seattube frame in order to raise the seatpost higher and get a more aerodynamic position.... ?


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PostPosted: Sun May 15, 2011 10:44 am 
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Basically yes - with a longer top tube than normal for that length c/c seat tube.


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