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How can stack and BB be the same, but head tube length different?

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How can stack and BB be the same, but head tube length different?

Old 07-05-22, 09:13 PM
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Lizzi
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How can stack and BB be the same, but head tube length different?

Hello, There's something I don't understand about bike geometry. My old bike has a stack of 540, a reach of 372, and a BB height of 263. I bought another used bike with very similar geometry, including a stack of 541, a reach of 362, and a BB height of 262.

What I don't understand is how the head tube on the first is 140, and on the second, 130. Head tube angles are about the same at 71.5 and 71.3. With very similar stack, BB, and head tube angles, shouldn't the head tube lengths also be about the same?

Please help me understand. Is it that the forks are different lengths, with one bike having a shorter fork, and longer headtube, and vice versa on the other?

Last edited by ehallerm@gmail.; 07-05-22 at 09:24 PM.
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Old 07-05-22, 10:30 PM
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axle to crown length of the fork affects the stack. road bikes generally haven't varied greatly in ACL, but mtb bikes can vary quite a bit, depending on design.
I haven't done extensive checks on rigid fork 'Gravel' designs. But having looked at just a few bikes, a cm or a bit more variance was apparent. The variance is often, by design, to allow differnt tire sizes - that being a consideration as well as fork blade spacing width.
so, yes, bikes with different head tube lengths CAN have same stack (all else being similar/same). ACL is often not listed in Geometry specs, but if a frame is qualified for 50mm width 700c, it prolly has a longer ACL than one which might only be spec'd for 40 mm width. In either case BB Height might be the same. A longer ACL, compensated by a short HT...
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Old 07-05-22, 10:58 PM
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See https://www.sheldonbrown.com/rinard/forklengths.htm

Also, cyclocross forks have typically 25-30mm more crown to axle height compared to road bikes, probably to allow for more tire clearance. I expect the same for all bike formats moving to bigger tires.
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Old 07-05-22, 11:24 PM
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Thank you very much. That explains the discrepency, and also why two bikes with neary the same stack, reach and BB height look different. I appreciate your help.

Liz
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