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Butt length

Old 01-06-23, 01:43 PM
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Butt length

Hello all.
As a hobby frame builder I have
made about 13 frames over the past 35 years.
Due to retirement, production has been increased to 2 per year.
I am now planning to make a cyclocross/gravel bike frame, fillet brazed.
I hesitate for the down tube between :
1) 35mm ( 0.7/0.5 ) with 60 and 114mm butt.
2) 38mm max (0.65/0.45) with 40 and 70mm butt.
My concern is taking "obstacles", resulting in a dent, wrinkle in the downtube.
I think the 35mm tube is the safe choice, but maybe this is all wrong.
Please advice
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Old 01-06-23, 06:21 PM
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Are those butt lengths before you cut? I never liked tubes that came with short butts. The diameter difference isn't going to be very obvious between 35 and 38mm
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Old 01-06-23, 10:24 PM
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It sounds like you are an older rider and if so I would think the ever so slightly less stiff 35mm DT would be better. That it has slightly thicker walls and possibly longer end butts (not knowing the rest of that equation) and your comment on denting seem to make this the obvious choice. Note I said slightly a few times

I would hope that any obstacles one tries with their grovel bike are not the limiting factor. If so one might have picked the wrong arrow out of their quiver Andy
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Old 01-07-23, 06:24 AM
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I know its a personal choice, based on my riding style, but I wouldn't build any off road frame with less than a 9/6/9 35mm down tube. The weight savings vs. durability isn't worth the risk for me. My MTB has a sizeable crease in the 9/6/9 35mm downtube, from a rock step-up, and it still shows no sign of cracking 3 years later.
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Old 01-07-23, 08:57 AM
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butt length

The plan is to build my 3rd stainless frame, this time for my son.

I found another 3rd tube from XCR that I can use:
-42mm ( 0.6 / 0.45 ) ; butts after cutting 40 mm and 111 mm.
-As for the 35mm tube, I can reduce the butt at the bracket to 40mm and the butt at the headtube then becomes 134mm.
-As for the MAX tube, I think the bracket butt (40mm) cannot be shorter.

KVA has 38.1mm tube ( 0.7 / 0.5 ), if I shorten the butt at the bracket to 40 mm then the butt at the headtube becomes 149 mm.

The KVA tubes come from 5000 miles, the XCR dealer is 5 miles from me.
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Old 01-07-23, 10:33 AM
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Isn't the difference between long and short butts that the longer ones are meant for brazing and the shorter ones for TIG? I would go with the 35mm tube in any case due to the risk of "coke canning" as you say. 35mm will be insanely stiff anyway.
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