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Thread: Head tube length and measurement below DT how does that affect Geo

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    Default Head tube length and measurement below DT how does that affect Geo

    I was just curious, I fit bikes with taller headtubes as in 200mm or so in order to get a reasonable drop. Most bikes have very little headtube sticking below the DT and more length sticking above the top tube. I always thought for visual aspects it would look better if it was more even say 3cm above and below. I'm just curious does it create problems with the geometry of the bike or anything I'm missing. Or does it make it difficult with say a lugged bike?
     

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    That measurement, on it's own, has no bearing on the "geo" of the bike. Taking the same bike and simply changing the angle and length of the down tube will produce more or less HT lower extension while maintaining bike geometry and rider contact points.

    There may be reasons to alter this dimension while initially designing a bike for clearance, joinery, room for a suspension fork, etc. Depending on design there may be an argument for having less leverage there, or a closer relation between the lower HS cup and the DT.

    Building with lugs, this dimension is generally determined by the length of the lug skirt. Like all things this can be modified during construction, but the actual gains may be limited/ non existent. Changing the upper extension (in isolation) can have an impact on fit and set up.

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    ...all that matters is STACK and REACH
     

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    Well, that's not all that matters.
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    Default Re: Head tube length and measurement below DT how does that affect Geo

    Only if you use a really stiff steel headtube.
    Weight Doper

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    I think it would look just as bad if not worse- It changes the angle of the downtube.
    Ever seen an mtb with a rigid fork, but a suspension corrected rigid fork.
    Something just looks off.
    The slickest tricks for maximizing stack and minimizing headtube length are:
    1.) Low bb drop- go to 8cm if you are not racing
    2.) Use a fork with taller axle/crown. A Reynolds fork is longer in this regard than an Enve fork- a fork Like a Seven 5E for long reach brakesis even longer than that.
    3.) Use a 11/8 King no thread set. Headset. This external headset has a total stack of 31.4
    4.) Use a zero rise stem. Looks better than a positive rise- a 12cm zero degree nets 12mm increased height over a 12cm -6 stem.
    That's all I got-

    Quote Originally Posted by Bantamben View Post
    I was just curious, I fit bikes with taller headtubes as in 200mm or so in order to get a reasonable drop. Most bikes have very little headtube sticking below the DT and more length sticking above the top tube. I always thought for visual aspects it would look better if it was more even say 3cm above and below. I'm just curious does it create problems with the geometry of the bike or anything I'm missing. Or does it make it difficult with say a lugged bike?
     

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    Does the increased downtube angle affect the attachment point at the BB given standard seat tube angles and BB heights for road bikes? If so, what affect does that have on a large bike where flex of the bottom bracket might be more of an issue than smaller designs?
    Jorn Ake
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    Default Re: Head tube length and measurement below DT how does that affect Geo

    It does changes the ST/DT angle, which could be an issue with lugged shells. That angle changes quite a bit across a range of sizes and frame types already- BB flex would be less of a concern to me than what's happening at the front end of the bike.

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    Quote Originally Posted by Eric Estlund View Post
    It does changes the ST/DT angle, which could be an issue with lugged shells. That angle changes quite a bit across a range of sizes and frame types already- BB flex would be less of a concern to me than what's happening at the front end of the bike.

    Yeah I kinda figured it would affect the seat tube angle possibly if say you just slid the headtube of say 200mm down 20mm in the sow tube/ht lug, just wasn't sure how much thought it might be an option to obtain more stack height while having a more balanced headtube look with the lugs. Ie 3cm below DT and say 4cm above DT instead of the standard 1cm below dt and the rest above it like a pegoretti. Just trying to work out that tall headtube without such a sloping top tube
     

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    I'm not at all sure that your premise holds.

    I build with a slightly longer than normal extension below DT to create sufficient bonding area for this highly stressed joint. People who are more visually attuned than I have commented that the corresponding large gap from DT to front wheel looks less "racy", no-one has said the HT looks more balanced.

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    One doesn't simply slide the tube down in practice. Adding an extension to an existing frame would change the ST angle. Raising the DT mount on a ht would close up the included ST to DT angle given a fixed ST angle.

    To get to what I think is the root of the question- DT location and HT lower extension are generally based on pragmatic necessity of design. Visual design is important, but that balance happens after the functional drivers.

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    Quote Originally Posted by Bantamben View Post
    3cm above and below. I'm just curious does it create problems?
    Yes.

    That's too far from the tube, leverage plays against you.

    - Garro.
    Steve Garro, Coconino Cycles.
    Frames & Bicycles built to measure and Custom wheels
    Hecho en Flagstaff, Arizona desde 2003
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    Default Re: Head tube length and measurement below DT how does that affect Geo

    I think 3 cm below the dt would look really funny. You can go play with the online version of bikecad or d/l rattlecad to find out for yourself.
     

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    Quote Originally Posted by zank View Post
    Well, that's not all that matters.


    Apparently id does, for Strava
    Andrea "Gattonero" Cattolico, head mechanic @Condor Cycles London


    "Caron, non ti crucciare:
    vuolsi così colà dove si puote
    ciò che si vuole, e più non dimandare"

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    To get the space for the look your talking about what you are really doing is just raising the downtube. In order to keep the same head angle you basically have to adjust the lugs slightly for the new angle and the downtube length will probably have to be adjusted ever so slightly.
     

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    Quote Originally Posted by Heinrich Sprietsterbach View Post
    To get the space for the look your talking about what you are really doing is just raising the downtube. In order to keep the same head angle you basically have to adjust the lugs slightly for the new angle and the downtube length will probably have to be adjusted ever so slightly.
    Or replace it with a cable and clevis.



    TH
     

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    Default Re: Head tube length and measurement below DT how does that affect Geo

    my LBS had a Downtube folding bike in for repair. The company is apparently run by a batch of jokers, because none of their bikes actually have a downtube. So there is a pretty big gap on both sides of the main tube
     

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