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Thread: Curvaceous down tubes

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    Default Curvaceous down tubes

    I know this bike is irrelevant for how MTB geometry has - ahem - taken shape, but never mind that. How come Moots specs these swoopy downtubes apart from the s*x appeal?

    29'' hardtail, cross country, mountian bike, race bike-Moots

    Is there a function to that form? I would think the uppermost bend would put either more or less stress on the downtube, I don't know enough to know which (I'm queasy about that part of bikes now for reasons well known in these parts). Seems it'd matter less with ti, but you see steel and al bikes with downtubes bent behind the headtube (toe overlap? What gives?).

    From an aesthetic standpoint 90% of the time, when I see that it quickens my pulse and I tug on my collar, but the other 10% of the time, I don't actually like it and prefer the look of a plain ol' straight ti tube going right into a beer can 44mm headtube, like this Spot: http://twentynineinches.com/wp-conte...0/IMG_6913.jpg

    I went and visited Jim Kish's shop and he had a cool tube bender - a big lever with gears mounted on the wall. He said it took his 200 pounds hanging on it off the ground to bend a tube. Didn't think to ask him when I was there.
     

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    Default Re: Curvaceous down tubes

    Well I ain't no effbuilder but doesn't everyone to some extent gain clearance for a suspension fork for 29'ers do this similar thing? The bike looks very cool btw.
     

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    Default Re: Curvaceous down tubes

    uppermost is for fork clearance.
    -Dustin

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    Default Re: Curvaceous down tubes

    Fork crown clearance, for when your MTB has a steeper front end than your track bike.
     

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    Default Re: Curvaceous down tubes

    That's funny - OK, this just occurred to me, too: could the trend toward zero-offset forks contribute to the need? I can't recall fork clearance being an issue on my pretty steep xc bikes, but they all had an offset fork (which I recognize is not the most XC thing in the world, but then again I see zero-offset forks on the new trail rippers, too).
     

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    Default Re: Curvaceous down tubes

    Quote Originally Posted by zambenini View Post
    That's funny - OK, this just occurred to me, too: could the trend toward zero-offset forks contribute to the need? I can't recall fork clearance being an issue on my pretty steep xc bikes, but they all had an offset fork (which I recognize is not the most XC thing in the world, but then again I see zero-offset forks on the new trail rippers, too).
    I don't think offset should change it at all; yes, the offset is usually built into the CSU, not the lowers, but the distance from the steerer center to the stanchion centre doesn't (really) change. The fork is still equally "wide," and it's the CSU at 90 degrees when you don't want the knobs to go whacking the downtube.

    Mostly I just think it's crazy that in this day and age, where 68 deg HTAs are starting to look steep even on world cup bikes, Moots would advertise stock geometry in the low- to mid-70s. That's just crazy, crazy steep for modern 29ers. The bike pictured, while the details look good, and the spec and finish are typical Moots, but honestly...the whole bike just looks bizarre. At least for a mountain bike in late 2019. Should everyone want a 65 degree headtube angle on a "trail" hardtail? Not necessarily. But I live in a place of steep, technical climbs, and equally steep, technical descents, and my TB4 feels like a normal bike. And I barely have tires on it with knobs: I'm not the guy with two-ply casings on his trail bike, I still run medium-sized brakes and skinny-ish, low-knob, fast tires.

    I mean, you don't know until you know...but I know that 71.5 degrees on a MTB is steep, especially once you blow through all 100 mm of fork travel.
     

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    Default Re: Curvaceous down tubes

    That last line above made it all make sense to me Mr. Apple-Zappa.
     

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    Default Re: Curvaceous down tubes

    I agree with you. But I think the Moots Moot-X RSL above is a n+1 bike.
    People will get this once they have an Ibis Ripmo as well. And use the Moots on days they want to go fast on non-techy gravely singletrack. :-)

    Quote Originally Posted by Applesauce View Post
    I don't think offset should change it at all; yes, the offset is usually built into the CSU, not the lowers, but the distance from the steerer center to the stanchion centre doesn't (really) change. The fork is still equally "wide," and it's the CSU at 90 degrees when you don't want the knobs to go whacking the downtube.

    Mostly I just think it's crazy that in this day and age, where 68 deg HTAs are starting to look steep even on world cup bikes, Moots would advertise stock geometry in the low- to mid-70s. That's just crazy, crazy steep for modern 29ers. The bike pictured, while the details look good, and the spec and finish are typical Moots, but honestly...the whole bike just looks bizarre. At least for a mountain bike in late 2019. Should everyone want a 65 degree headtube angle on a "trail" hardtail? Not necessarily. But I live in a place of steep, technical climbs, and equally steep, technical descents, and my TB4 feels like a normal bike. And I barely have tires on it with knobs: I'm not the guy with two-ply casings on his trail bike, I still run medium-sized brakes and skinny-ish, low-knob, fast tires.

    I mean, you don't know until you know...but I know that 71.5 degrees on a MTB is steep, especially once you blow through all 100 mm of fork travel.
     

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    Default Re: Curvaceous down tubes

    Quote Originally Posted by zambenini View Post
    ...I can't recall fork clearance being an issue on my pretty steep xc bikes...
    Yeah, but that bike had a 100mm fork and 26" wheels. The Spot bike you linked above is from 2012, also has a 100mm fork and probably a pretty high bb too... They're putting bends in their downtubes now.

    Now with 29ers with 140mm+ forks and BBs not rising at all to compensate (if anything, they're getting lower), the downtube has gotten quite steep, and in order to stop your fork crown from hitting the downtube, you can either bend the downtube, or weld it ~1" up from the bottom of the headtube, like this 44 Bikes. Personally I prefer the aesthetic of a smaller amount of headtube either side of top and downtube.

    This image I stole from the internet starts to tell the story.

     

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    Default Re: Curvaceous down tubes

    A bent downtube significantly increases force dispersion from frontal impacts.
    The downtube diameter needs to be significantly larger to have a similar effect as a radius bend, but then you may need to also beef up the TT to compensate.
     

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    Default Re: Curvaceous down tubes

    Quote Originally Posted by suspectdevice View Post
    A bent downtube significantly increases force dispersion from frontal impacts.
    The downtube diameter needs to be significantly larger to have a similar effect as a radius bend, but then you may need to also beef up the TT to compensate.
    That's what I was wondering. Seems like it would be easier to crack a weld, the closer to a right angle the dt is coming out from the ht? ... but seems like you'd be less like to crack or damage the tube itself in that instance.
     

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    Default Re: Curvaceous down tubes

    Quote Originally Posted by Applesauce View Post
    Mostly I just think it's crazy that in this day and age, where 68 deg HTAs are starting to look steep even on world cup bikes, Moots would advertise stock geometry in the low- to mid-70s.
    I thought the picture looked smooshed horizontally.
     

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    Default Re: Curvaceous down tubes

    IMG_8716.JPG

    Everything is give & take and this also shows why I stop at 120mm travel on bikes = to maintain a certain look and frame weight - FWIW, I use the same DT as most of these guys (38mm Columbus, 1/.5/.8) but I get them un-bent.

    Here are some of the factors:

    You need more standover because not everyone has a 34" inseam, but they think they want (or do want) 6" of travel so to milk it you go with a zero-stack headset but then the knobs hit the DT so you curve it.

    This couples with the same factors making HT's dangerously small, so small they can fail = zero stack gives you a slightly larger HT to attach a suitable DT and TT to - you need the HT big enough the bearings don't get torn out, and bars are getting really high, the whole bike(s) is/are really high, many small people, if you watch them, mostly use the dropper to GET ON the bike.

    An external cup head set gives you 13mm clearance so you can run a 'normal' HT lip = I like 15mm but could go down to 10 but 15 gives me good room to put a fillet and to polish it up, and there are other things too like the HAZ's of TIG v. brazing and esoteric BS and other shit but I gotta got to work.........



    - Garro.
    Last edited by steve garro; 12-26-2019 at 12:01 PM.
    Steve Garro, Coconino Cycles.
    Frames & Bicycles built to measure and Custom wheels
    Hecho en Flagstaff, Arizona desde 2003
    www.coconinocycles.com
    www.coconinocycles.blogspot.com

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