How can stack and BB be the same, but head tube length different?
07-05-22 | 09:13 PM
#1
Thread Starter
Lizzi
Joined: Oct 2021
Posts: 10
Likes: 1
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?
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.
07-05-22 | 10:30 PM
#2
OM boy
Joined: Jul 2005
Posts: 5,328
Likes: 1,311
From: Goleta CA
Bikes: a bunch
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...
Ride On
Yuri
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...
Ride On
Yuri
07-05-22 | 10:58 PM
#3
Broken neck Ken
Joined: Oct 2008
Posts: 5,221
Likes: 3,517
From: Portland, OR
Bikes: Trek Domane SL6 Gen 3, Soma Fog Cutter, Detroit Bikes Sparrow FG, Trek Mt Track XCNimbus MUni
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.
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.
08-15-22 | 11:35 AM
#5
Newbie
Joined: Aug 2022
Posts: 17
Likes: 3
From: California, USA
Bikes: Cervelo R5ca, Giant TCR Advanced, Neil Pride, Cervelo P5x
Stack is the vertical distance from the center of the BB to the top middle of the head tube and that is all it is. Now if you mean Hx | Hy, then Hy is the vertical of center of the BB to the center of the HBs
