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Old 10-10-13, 10:50 AM   #1
skagitteam
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Headtube extension space for headset

Hi All:

Does anyone know how far a headtube needs to extend above and below the top tube and downtube junction in order to have space to install an external headset cup? I would like to shorten these extensions as much as possible but don't want to find that I can't install the headset cup because, for example, the bulging bottom cup collides with the downtube.

Headset will be Chris King external (technically external/internal--Inset 7). Headtube angle is 73.5 degrees.

Am I missing any other reason that the headtube needs to extend further?

Thoughts appreciated--thanks!
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Old 10-10-13, 12:13 PM   #2
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On lugs, the HT lip is about 5-10 mm. For a braze-welded joint you need some room for the fillet so I'd use 10 mm at least. For a welded frame you can go lower but it might affect the amount of distortion.

If you want to know how low you can go you should be able to figure it out from the tube angles, cup diameter and a bit of math.
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Old 10-10-13, 02:05 PM   #3
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Thanks for your thoughts--it is a welded frame.
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Old 10-10-13, 08:41 PM   #4
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For MtBs with suspension forks one needs to be aware of the fork crown striking the DT.
For road bikes i use 15mm for bottom edge of DT to end of HT and 10mm for the TT top edge to Ht edge. Andy.
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Old 10-11-13, 10:37 AM   #5
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Why do you want to lower the intersection? Just wondering.
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Old 10-11-13, 11:12 AM   #6
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Thanks for the replies. I want to shorten these extensions because the headtube is already very long by the time I apply the other geometry I want. The uncut steerer on the fork I want to use is only 300mm long. Once you account for headset stack and stem clamp space, this does not leave a lot of extra length to work with before I run out of steerer length. I could fix this by buying a fork with a longer steerer, but I don't like the available options. (This is a road disc bike.)

If any of you believe I'm thinking about this the wrong way, please speak up. I'm thinking I will lower the top-tube, seat tube junction by a few mm while maintaining the top tube angle to give myself a bit of extra space to work with on the headtube extensions.

By the way, the reason for the long head tube is that I'm very tall--6'8". My fitter recommended a 234mm headtube.
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Old 10-11-13, 11:50 AM   #7
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I ran into a similar issue on my latest frame, and I had a 400 mm steer tube! (20" wheels).

With that fork your options are limited. The HT length is ultimately relevant and changing the TT-HT intersection will only change the standover by a few mm, which is not that critical. The HT-DT intersection will have no effect. Stem clamps are about 40 mm and HS stack 32 mm (1.125" I assume), and with your HT that adds up to 306 which is cutting it close. I'd prefer 2-3 mm over. Moreover that means a slammed stem and I would guess your fitter had a taller h-bar height in mind?
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Old 10-11-13, 12:32 PM   #8
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Quote:
Originally Posted by skagitteam View Post
Thanks for the replies. I want to shorten these extensions because the head tube is already very long by the time I apply the other geometry I want. The uncut steerer on the fork I want to use is only 300mm long. Once you account for headset stack and stem clamp space, this does not leave a lot of extra length to work with before I run out of steerer length. I could fix this by buying a fork with a longer steerer, but I don't like the available options. (This is a road disc bike.)

If any of you believe I'm thinking about this the wrong way, please speak up. I'm thinking I will lower the top-tube, seat tube junction by a few mm while maintaining the top tube angle to give myself a bit of extra space to work with on the head tube extensions.

By the way, the reason for the long head tube is that I'm very tall--6'8". My fitter recommended a 234mm head tube.
Make your own fork. Another option if you only need a couple of cms more steerer length is to add a section to it's top. Sleeve the add on for strength and alignment. Since the bearing alignment issues will still be on the original length and the stem will overlap the added on section I think all would be good. But I'm sure some will find this option to be wrong... Andy.
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Old 10-13-13, 04:11 PM   #9
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Quote:
Originally Posted by skagitteam View Post
Thanks for the replies. I want to shorten these extensions because the headtube is already very long by the time I apply the other geometry I want. The uncut steerer on the fork I want to use is only 300mm long. Once you account for headset stack and stem clamp space, this does not leave a lot of extra length to work with before I run out of steerer length. I could fix this by buying a fork with a longer steerer, but I don't like the available options. (This is a road disc bike.)

If any of you believe I'm thinking about this the wrong way, please speak up. I'm thinking I will lower the top-tube, seat tube junction by a few mm while maintaining the top tube angle to give myself a bit of extra space to work with on the headtube extensions.

By the way, the reason for the long head tube is that I'm very tall--6'8". My fitter recommended a 234mm headtube.
Andy -

May be best to just put an uncut headtube with the lower cup installed in the jig and slide the folk in to see "what you can see". Sometimes it comes down to that. The eye can be the finer gauge. One the side, if the crown in not sloping then the feedback from others about hitting the DT are real possibilities...but I doubt that a road frame would be planned with a crown that big.

On the other side; I am starting to cut tubes for a XXL/XL tandem frame. It is 36" SOH at the middle of the front top tube. Sorts out to need a 9" HT (as I build on a flat table style jig, I just leave it full length and work in the middle of the long HT as that aids alignment. Am starting with a 14" steerer measurement, but will leave it full length and just cut to measure after sorting out what is needed for the rider with specific stem and bars. Would have preferred a thicker walled HT wanted to find materials big enough for a 1.25" headset. Most likely I will end up with the larger and thicker HT destined for 1.25" and just sleeve it down to fit a 1.125" headset and steerer in the nearterm.

Sourced following from NOVA for now. I have used them before and they are pretty stout;

Headtube: Nov_COHT_37_700 ( 27.6" L by 1.6mm wall )
Steerer: Nov_COFS_460_AHD ( 18" L by 2mm/1.55mm wall )
Crown: EVR_FCCX_286_W70
Blades: Nov_Tand_CS_1.0

/K
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