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Discussion Starter · #1 ·
hope all is well. I have a steel frame that has what I feel is a slightly longish head tube. The tube extends quite a bit away from the top tube and down tube welds. Is there any idea as to why a frame builder would need to do this? If I trim this down would that be the end of the world? I know it will have a slight effect on the geometry but I am ok with that.
 

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The Matt
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Rick,

Cutting the bottom of the tube will affect the HT angle, BB height, ST angle, etc... It might not make that big a difference. I have never done the math. Just as importantly or maybe more so is the clearance between your fork and the DT. Also the longer your HT the larger the distance between the headset bearings and so the less leverage a fork has over the HT. Cutting the top of the HT will only affect the height of the handlebars.

Matt.
 

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longer headtubes lead to a frame more resistant to buckling at the ht/dt junction
(all other things being equal)

During a hard frontal impact: Your body weight on the handlebars is a forward force, the fork being bent towards you is a force bending backwards, your ht/dt is the highest stress point on your frame during this period. In the fork it's the steerer/fork crown.
Fork going back and you going forward means the fork's steerer tube tries to rotate front to back in the headtube.

A longer headtube will resist buckling by increasing the length of the lever above the fork crown compared to the length of the lever below (the fork itself).
Remember, two forces, not JUST the fork, the fork and YOU.
If the length of the levers top and bottom were equal then the whole assembly (you included) would just rotate kindly around the fork/frame junction.

It will then tend to want to rotate the frame around the front axle instead of rotate the fork into the frame (which is basically what happens when your frame buckles, your steerer tube has transferred the impact force and your frame wasn't strong enough to resist that force).

Forks tend to be made with a higher safety factor than frames so it's more likely that the fork will remain intact.
This WILL direct more forces through your fork (maybe buckling that instead?!) but it usually results in rotating your ass over teakettle... :D
 
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