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Thread: Curved top tube

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    Administrator Allen's Avatar
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    Curved top tube

    What are the ramifications of a curved top tube, like those found on cruisers? Are they significantly weaker than a straight tube? Same questions about curved seat stays.

    Thanks,

    --A

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    Senior Member thatcher's Avatar
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    beach cruiser tubing is much thicker than a road bikes.

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    cs1
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    Quote Originally Posted by AllenG
    What are the ramifications of a curved top tube, like those found on cruisers? Are they significantly weaker than a straight tube? Same questions about curved seat stays.

    Thanks,

    --A
    Check out these: http://ingliscycles.com/bikes/retrotec_home.php He's been doing them for a while with no complaints. I'm thinking it might be my next bike.

    Tim
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    1984 Specialized Stumpjumper, 1986 Specialized Stumpjumper and just way too many projects to list.

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    Beauty Everywhere snowy's Avatar
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    Quote Originally Posted by AllenG
    What are the ramifications of a curved top tube, like those found on cruisers? Are they significantly weaker than a straight tube? Same questions about curved seat stays.

    Thanks,

    --A
    AllenG my boyfriend had two custom cruisers that he rides. Both are mountain bikes. He has never had any issues with the top tube. He also races on them.

    Here are some pics for your viewing, http://gallery.mtbr.com/showgallery....&ppuser=263629
    "RIDE FAST TAKE CHANCES!"

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    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by AllenG
    What are the ramifications of a curved top tube, like those found on cruisers? Are they significantly weaker than a straight tube? Same questions about curved seat stays.

    Thanks,

    --A
    When Frank Schwinn patented the frame design popularly known as the "cantilever frame" in the late thirties, the claims made in the patent were that the two small diameter curved tubes welded to each side of the down tube, then carried back to the seat tube where they are welded to each side the seat tube near the seat post clamp, and then finally transitioned to become the seat stays, "...resist certain strains, resultant from accidental or other overloads, which frequently destroy other bicycle frames, - i.e. those strains which tend to twist the axis of the steering head (now called the head tube - Stan) out of the plane which includes the axis of the saddle post mast (now called the seat tube - Stan), and those strains which tend to disrupt the union between the steering head and the reach tube (now called the down tube - Stan) which, in the normal use of the bicycle, constitutes a tension member acting between the steering head and the crank hanger."

    It is no accident that the first home made "mountain bikes" were built using these extremely rugged frames.



    Last edited by Scooper; 04-03-07 at 12:34 PM.
    - Stan

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    Administrator Allen's Avatar
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    Thank you all.
    I laid the contents of my parts bin out on the floor the other day and it just screams cruiser.
    I've also ordered the Paterek manual and a dozen or so other books recently, got my hands on an Henrob torch and a set of tanks, and today I'm heading down to the police station to pick up a few frames that did not sell in the last stolen bike auction. By the end of the summer I hope to have my first frame on the road.

    My parts bin


    and my {current** inspiration

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    As was said before, you won't have any strength concerns as most curved tubes are made from straight gauge stock. And considering you're planning a double TT cruiser, no worries. For some good info on bending your own TT's, check out Steve Garro's posts on the subject on frameforum- good luck!

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    If you take that rust frame, the beauty of the triangle is that whether under tension or compression the straight tubes are loaded in collumn. for instance the top tube there is loaded mostly in compresion so the curved tube is much less strong when it is extremely bent like that, and the welds are put under uneven loads if the tube flexes. Same thing with the rear triangle, the SS is going to work a little bit like a pole vaulters pole there. It's a sign of serious misalignment when the ends are out of line with themiddle of the tube, not even overlapping.

    Hey it all works as long as the matrials are stout enough. I like curved tubes. On real working bikes, they do seem to be a little lazy, but on lazy bikes they fit right in. I don't really like the double top tube design though, kinda looks like the designer just wasn't sure...

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    Administrator Allen's Avatar
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    Quote Originally Posted by Peterpan1
    If you take that rust frame, the beauty of the triangle is that whether under tension or compression the straight tubes are loaded in collumn. for instance the top tube there is loaded mostly in compresion so the curved tube is much less strong when it is extremely bent like that, and the welds are put under uneven loads if the tube flexes. Same thing with the rear triangle, the SS is going to work a little bit like a pole vaulters pole there. It's a sign of serious misalignment when the ends are out of line with themiddle of the tube, not even overlapping.

    Hey it all works as long as the matrials are stout enough. I like curved tubes. On real working bikes, they do seem to be a little lazy, but on lazy bikes they fit right in. I don't really like the double top tube design though, kinda looks like the designer just wasn't sure...
    The Rust bike is one of Mike Flanigan's (A.N.T.) 29er designs.

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    Senior Member jacobs's Avatar
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    your parts bin has a ROHLOFF in it??

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    Yet another vegan biker
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    And he could strap an elks haunch on that rear rack...

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    "The Rust bike is one of Mike Flanigan's (A.N.T.) 29er designs."

    I've seen a number of respected guys do that double top tube thing. When you figure you could do just fine with one straight tube... It's one thing back in the day when fun was a cardboard clacker in the spokes...

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    Administrator Allen's Avatar
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    Quote Originally Posted by jacobs
    your parts bin has a ROHLOFF in it??
    I have a work bike that I gave to my brother. I stripped the Rohloff off of it before hand. My brother has since given me the frame back, but I think the hub would serve me better on a frame other than the work bike's.

    The rear rack is a Tubus Cargo rack, basically the same as a Tubus Logo rack. The front rack is the huge one. It came off an Azor work bike. Heavy and rated for 50lbs of cargo, it however has a quick mount bracket, and can be removed/mounted in seconds.

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    the thing with this build is that you have to consider the geometry of the entire frame before assuming what will be strong. considering the points that are conjoined, the physics of it works out where it really doesn't matter as long as the geometry is proportionate to the rake

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    Curved top tubes begain as a way to lower the seat on bikes built for 28" wheels.This design goes back to 1900 or earler.if you look clost at a cruzer frame the tube does not curve up but drops to lower the seat by an inch or two---makes it easer for kids to ride. Curved seat tubes were used in the 1890s.Paramount,claud butler and others used a curved seat tube to get a very short wheelbase frame

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