Bottom Headset Stack Height considered for building headtube length?
 

How much is this considered when cutting the headtube length?

New frames, or existing frame?

On a new frame, people usually aim for a measurement for the top of the head tube, one way or another. So if there is more stack height, the head tube is shorter and vice versa.

Bikecad takes care of it for me on a new frame. The effects of screwing it up can be calculated. As long as you have a reasonable number, you can substitute headsets without changing things much. If you design a new frame on paper and forget about stack height, the head tube angle is going to be more slack than you wanted. But other than a bit of standover height, it's not going to have much effect.

I don't worry about a couple of mms of lower stack height change (headset exchanges) assuming the overall length of the steerer is good. But forgetting to account for the intended headset stack heights on a new design is just poor planning or too much beer.

I grew up with external cupped headsets, frames included the fork and there was no Bikecad and the stack heights were thought about nearly from the start of a drafting (back then it was sign paper taped to the floor). We would start with the wheel radius, head angle and rake to establish the axle point then move up to the top of the tire. Add under the crown clearance, making sure the brake reach is right, add the crown thickness... and next comes the lower stack. Back then the Campy NR/SR lower stacks were about 15mm, about the thickest around.

These days I use Bikecad too and have the HS lower stack set at 13mm as the default. But measuring is the best policy. Andy

You should include it in your design (whether you do that on paper or inside the confuser) because it affects the head-tube angle, seat-tube angle, and everything else. If you don't know exactly what headset you're going to use I would use 12mm which is fairly average. But some really fat head-tubes use "zero-stack" headsets so keep that in mind.

It usually doesn't matter that much if the angles aren't exactly as you intended in your design. But the idea is to minimize the errors if you can!

"But the idea is to minimize the errors if you can" Guy153

This! No frame will be perfectly made and no rider will have a perfect body to power that imaginary bike. However we can (and I think most all here do) strive for getting close, as we can. Many have considered 1mm to be a reasonable tolerance for c-c lengths and .5 degree for angular dimensions. If you push the math (and I STRONGLY suggest budding builders do their own on paper calculations at least once without the help of Bikecad, Rattlecan or other programs/apps) you can see how much head tube lift (or also it's rotation around the rear axle) equates with how much head angle reduction and the increase of trail. IIRC it's around 1cm of lift = .6 degrees of HA reduction but depends on the length from that HT to the rear axle. For the seat tube- each degree of change will be about 1cm of set back over the BB difference.

When beginning frame making I have taught, the few I have, to draw out their frame and fork, at some scale that fits their paper. This gets them thinking about where stuff goes, how much space that takes and obvious interference issues. Not so much for ending up with the final miter to specs but to better match the macro with the micro, understanding how the details and the grand plan fit. The first few friends I helped learn how to make their frame (note I don't say become a framebuilder:)) was before the internet and online design programs, we used the Eisentraut formulas I was provided at his last east coast workshop. These calculations produced the final numbers, after the drawing and much thinking about the detail points. The last friend helped came after I got Bikecad Pro and used it for the final specs. It was this last friend that told me that after he did his first attempt at a drafting (likely late at night with "lubrication") and then began to play with Bikecad, changing one spec at a time, he was better able to anticipate the resulting changes elsewhere. Andy (who sometimes feels like one of those old men on a park bench)

I feel like OP should clarify what they meant. It was a very low effort post to begin with.

Low effort, but suffice. Andrew and guy answered my question. Thanks

thanks for checking back in

And if you do build the frame post a few shots here for our pleasure. Andy