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  1. #1
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    Reputation: Eric Malcolm's Avatar
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    Fitted Seat Tube - what sizes and options are there being used?

    I have a design brief that requires the use of a seat tube extending 200mm above the Top Tube.

    For those of you that have done this style of construction, my question is to do with sizing of the tube that was used, both tube diameter, wall thickness and whether sleeving has been used.

    As to why, an example of using Columbus Zona External butt seat tube 28.6mm and seat stem (235gr) combo to yeild a saddle height of 810mm = 507gr.
    Using a 35mm x 750mm butted 9/6/9 down tube = 470gr plus fittings. A similar result, but bespoke fitting to client only and no adjustment for height.

    No doubt 28.6mm tubes are lighter as a combo, but my brief does not give me that option. Most seat tubes of the 28.6mm variety have .6mm wall with 200mm to go with external having .9-1.2 to work with at the upper 75-100mm. At any rate, can't get the 750mm required that way with off the shelf tubing.

    My proposed option is to use a 34.7mm Down Tube that has .9 x 150mm with a 40mm butt taper to .6mm. This leaves 10mm of .6 before connection to the top tube. This area WILL be SLEEVED. So no sweat on Strength as I have the insurance of 35mm dia. plus...sleeve.

    I would like to expand the information base as there is no info around to base decisions on.

    Thanks

    Eric
    BRAKES...? I'm trying to go FASTER!!!

  2. #2
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    I have not done this but your plan sounds basically fine to me. I assume you will sleeve the TT/SS joint area with some 1 5/8" x .058" or something? I'm guessing your end result will be heavier than a conventional setup when all is said and done, but as you said, this is what the person wants so...

    I would guess you want to reinforce the top of the mast for the clamp somehow. Maybe do some lathe work and weld in a slightly thicker plug? I can't imagine 35x.9mm will take kindly to being clamped, but maybe I'm wrong.

    That is going to be a really, really rough ride. Forget about any seatpost flex!

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  3. #3
    Eric the Red
    Reputation: edoz's Avatar
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    I've done it with 1 1/4" .035 4130. I ran the tube all the way up and drilled it for some old USE seatpost hardware. I brazed on a decorative sleeve at the top to reinforce the holes where the hardware went though. There was no seatpost, and no adjustment for height. It's not really something I'd do for someone else, but it worked out great for me.

  4. #4
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    Hey, thanks edoz and Walt.

    Edoz, I intend using the USE style hardware also.

    Walt, you had me thinking about seat post flex for a moment, but most seat posts are alloy, and they are not noted for giving any flex. Appreciate the overview though.

    This concept is in cantilever so the longer it is the greater the stress on the TT joint. 1 1/4 x .035 (31.7 x 0.9mm) straight gauge seems good. Don't want it to collapse at the joint, it would mean a wreaked frame.

    Eric
    Last edited by Eric Malcolm; 02-11-2013 at 06:57 PM.
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  5. #5
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    Actually, all things being equal, aluminum is flexier than steel. A 27.2 alloy post is going to flex WAY more than your 35mm mast. Whether that's a good thing or not is up to you, of course.

    Velonews did a good seatpost flex article about 6 months ago - I can email you a pdf if you're interested. They found that setback/layback was the main driver of flex, not the material used, as I recall.

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  6. #6
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    Quote Originally Posted by Walt View Post
    Actually, all things being equal, aluminum is flexier than steel. A 27.2 alloy post is going to flex WAY more than your 35mm mast. Whether that's a good thing or not is up to you, of course.

    Velonews did a good seatpost flex article about 6 months ago - I can email you a pdf if you're interested. They found that setback/layback was the main driver of flex, not the material used, as I recall.

    -Walt
    Walt, I would like to look at that article.

    Eric
    BRAKES...? I'm trying to go FASTER!!!

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