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Lemond frame - perfectly symmetric "dent" in seat tube

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Lemond frame - perfectly symmetric "dent" in seat tube

Old 03-14-16, 05:10 PM
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lineinthewater
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Lemond frame - perfectly symmetric "dent" in seat tube

I tore down my friend's bike for the first time. It is a junior Lemond bike (he's really short) - probably about 44cm frame. There is what appears to be a "dent" where the front derailleur would typically clamp. But, it doesn't match the footprint of the FD clamp (there's no way that clamp could make this dent). BTW, in this case, the previous user or manufacturer put a braze-on adapter clamp on the frame, and then used a braze-on FD. I believe everything on the bike is OEM. Could this "dent" possibly be a design feature? I highly doubt it. I'm sure you're scratching your head, that's why I'm posting. I can post a pic if that helps.
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Old 03-14-16, 05:16 PM
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Here's the best photo I can take.

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Old 03-14-16, 05:17 PM
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Also, there's no "dent" on the other side of the tube.
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Old 03-14-16, 05:20 PM
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That's pretty common actually.

https://www.bikeman.com/bicycle-repa...at-tube-dented
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Old 03-14-16, 05:25 PM
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Originally Posted by Andy_K View Post
Wow, that might be it. What really had me wondering is the dent isn't even perfectly parallel to the crankset. It seemed so random.
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Old 03-14-16, 05:27 PM
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That appears to be the location where a tab would be mounted for a "braze-on" front derailleur. There would be two tapped holes for such a tab to be attached, or,presumably, the tab could be brazed to the frame here.
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Old 03-14-16, 05:43 PM
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Originally Posted by lineinthewater View Post
Wow, that might be it. What really had me wondering is the dent isn't even perfectly parallel to the crankset. It seemed so random.
Yeah, it does look wrong if you don't know why it's there. I first noticed it a few years ago on my Excalibur frame. That one's carbon, so it ruled out the possibility that it just got dented. My first thought was that the derailleur clamp was meant to be mounted there, but it was too low. Now I've got a couple of aluminum cyclocross bikes with the same feature. It seems to be needed with 34.9mm seat tubes.
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Old 03-14-16, 05:52 PM
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Originally Posted by Andy_K View Post
It seems to be needed with 34.9mm seat tubes.
Perhaps useful on some frames but not "needed". i have three frames with 34.9 mm (aka 35 mm) seat tubes and none of them have any dents, intentional or otherwise.
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Old 03-14-16, 06:21 PM
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Originally Posted by HillRider View Post
Perhaps useful on some frames but not "needed". i have three frames with 34.9 mm (aka 35 mm) seat tubes and none of them have any dents, intentional or otherwise.
I can't imagine such a small "dent" (probably 1-2mm) can make such a significant difference.
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Old 03-14-16, 06:55 PM
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Originally Posted by lineinthewater View Post
I can't imagine such a small "dent" (probably 1-2mm) can make such a significant difference.
Actually it does make a difference. The dent provides clearance for the front der to swing in enough to fit the crank set and not rub on the chain. Andy
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Old 03-14-16, 07:03 PM
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Originally Posted by Andrew R Stewart View Post
Actually it does make a difference. The dent provides clearance for the front der to swing in enough to fit the crank set and not rub on the chain. Andy
I guess all I'm saying is it seems like a bandaid approach to the problem. I guess they are trying to accommodate the available components at the time. Don't such detailed frame designs require more $$$?
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Old 03-14-16, 07:41 PM
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The science of specing a production bike is far more involved then what the average rider could ever imagine. Stuff happens during the process that wasn't in the cards at the start. Component companies have a dimensional change. The braze on der isn't available at some point along the production. A tubing diameter is changed mid production. All kinds of stuff happens. Placing a small dent in a tube is pretty low cost and the tooling is also low cost.

This solution also allows a low Q factor, which in my world is a good thing. Andy.
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Old 03-14-16, 07:50 PM
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Originally Posted by Andrew R Stewart View Post
The science of specing a production bike is far more involved then what the average rider could ever imagine. Stuff happens during the process that wasn't in the cards at the start. Component companies have a dimensional change. The braze on der isn't available at some point along the production. A tubing diameter is changed mid production. All kinds of stuff happens. Placing a small dent in a tube is pretty low cost and the tooling is also low cost.

This solution also allows a low Q factor, which in my world is a good thing. Andy.
Thanks, Andy, for your insight. Hopefully this indentation was intended.
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Old 03-15-16, 08:36 AM
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Is the dent located such that the front der pivots will settle in it? If so then there's your answer. Andy
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