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  1. #1
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    Double top tube cures shimmy?

    I was wondering if anyone had done this as a remedy for shimmy. Im about 6'5 and and would like to tour with bags in the front but I have had shimmy problems before. Anyone ever built a double top tube frame to eliminate that or added a second top tube to an already existing frame?

  2. #2
    Randomhead
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    shimmy is not caused by frame flex. It is caused by the interaction of the weight distribution, the steering geometry and tires with the road at speed.

  3. #3
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    Yes, but... A second top tube might change the vibration nodes (or something) so that the problem goes away. OTOH, it might change the vibration nodes (or something) and make the problem even worse. Fun, huh?

    For my money (and I knew about it before Jan Heine wrote about it!) a roller bearing headset is the closest thing to a guarantee you can get, and will last forever, too.

  4. #4
    Randomhead
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    the first natural frequency of a frame is significantly higher than the frequency of shimmy. What this means is that, as far as the frame is concerned, shimmy is effectively a rigid body motion. On a bike that is shimmying, you can see the head tube moving laterally and the wheel veering back and forth. I have a bike that shimmies badly when riding no hands, so I have plenty of experience.

  5. #5
    coprolite fietsbob's Avatar
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    I got a frame built with 2 side by side top tubes for my Load bearing touring bike.
    these 2 x.75" .049 wall tubes also thru mandrel bending form the rear triangle as well

    on prior tour it seemed the frame was flexing sideways, with every pedal stroke
    a Japan made Specialized Expedition, steel Lugged oversize a bit ,touring frameset.

    2 side by side tubes don't flex, with that Load.. , it seemed.
    but did need a bit extra gusseting at the 2 headtube ,
    top tube brazed joints, so i got the tubes welded up with solid pluga in the end
    and a plate gusset joining all 3 tubes..
    a + was having the frame pump between the 2 toptubes where it stayed put well.

    I wonder if a oval wide horizontal tube could do the same ?
    perhaps a vertical horizontal down tube. with the same tube shape..

  6. #6
    Randomhead
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    the question of how to stiffen up a bike that sees a lot of load is an interesting one. The kind of movement you feel on a loaded touring bike is going to be really hard to get rid of. I have seen larger touring bikes with mixte stays, I have never ridden a bike like that, and I don't know if it's really worth the weight.

  7. #7
    coprolite fietsbob's Avatar
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    Well the small Mixte stays added to a Diamond frame is not that much more steel..
    particularly the thin wall stuff.

    I set up a GF, (Ex).. with a 531 mixte, Mercian, I think, it was a noodle, that one..
    lifting a handlebar end pushed the front wheel sideways.

    touring.. too light , creates problems, in the long run..

    for my use, the SBI Expedition was on the verge of too light.
    but I was still off the back on a club ride i took leaving the panniers behind,
    in the middle of the tour.

    [fortunately it was not an all testosterone group,
    they waited for me to tag along..]
    Last edited by fietsbob; 07-04-11 at 12:26 PM.

  8. #8
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    I found this thread searching, because I am considering the same thing as the OP, for pretty much the same reason. I am just shy of 6'5" myself...but very long limbed, so my PBH is at 37.5".

    Quote Originally Posted by unterhausen View Post
    the first natural frequency of a frame is significantly higher than the frequency of shimmy. .
    This is the conventional wisdom, gleaned from normal height riders, on normal height frames. First off, frames don't shimmy at all, because without wheels they are just wall art. It is the resonance of the built, loaded, and mounted bike that needs considering, not just the vibrational modes of the bare frame. Torsional flexing of the frame is a small factor in many frames, but matters a lot in a tall frame.

    The OP is 6'5" tall. He doesn't ride a conventional size frame. Making the frame taller greatly reduces the torsional spring constant, and even a thin tall rider will carry weight that short riders would consider obese. At 6'5" and 205 lbs. would be pretty trim....185# at that height would look like a string bean. So you have a heavy rider on a more flexible frame, and now he wants to carry some luggage. The suprise would be if the bike didn't handle poorly.

    In the 70's bike boom days, most bike manufacturers offered a few models in 25 and even 27" sizes. These were diamond frames built with conventional tube sizes, and they were pretty limber machines. That ended about the time lawyer lips started showing up on front dropouts, for what I suspect were much the same reasons. Lenard Zinn has written much about those old bikes, and has gained much fame, and hopefully some fortune, making frames for tall folks that are stiff enough to work well. The Rivendell AHH is pretty much the only off-the-rack bike you will find in 69 and 71 cm sizes, and at those sizes, the Hillson sports a second top tube. Grant Peterson likes high trail bikes, so the AHH still might not be the best thing if you want to load up the front end.

    When you try to twist a normalish sized frame, the main tubes have to twist in order for that to happen. The short head tube is stiff enough that the frame can't rack much. When you lengthen the head tube, (and to a lesser effect the seat tube) the frame can now "rack" much more by flexing the head and seat tubes in torsion, and the taller frame allows more leverage from the riders butt. Adding a second top tube, if you ignore the upper top tube, creates pretty much a "normal" frame down below which is torsionally stiffer, and the upper top tube then adds to the torsional stiffness of the lower top tube. Flying Pigions add a second top tube at the taller sizes, as does Rivendell, as did the classic english roadsters. It adds weight, which nobody likes, but there are very good reasons for it.

    To repeat: The main problem comes from the longer head tube. A 69cm frame will have a head tube with twice the distance between the top and down tube as the frame a normal height rider would mount. Torsional stiffness drops with the square of length, so that frame has about 4x the flex. The longer seat tube doesn't help, but even on a normal frame the seat tube is so long that it doesn't contribute nearly as much stiffness as the head tube.

    I have an old 27" (27" frame AND wheels..about 68.5 cm in metric terms) Japanese made Schwinn World Sport that shimmys like mad hands off with even minimal rear luggage. (1 pannier with my work clothes without shoes) When I brace the bars against a door frame and bump the rear rack, it resonates at pretty much the same frequency of the shimmy. Interestingly for the OP, it handles nicely with panniers on front low riders...a little heavy on the steering at parking lot speeds, but no problem hands off at good speed. .371276732_6Xfgf-S.jpg

  9. #9
    Randomhead
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    that's a lot of words to say ... not much. Maybe you could distill your argument down to its core so I can understand it. If you watch a bike in shimmy, the front wheel is tracking back and forth. This is a rigid body motion. The size of the bike doesn't really factor into that.

    Yes, all things being equal, a larger frame is less stiff. Tall people also have a much higher center of gravity and it is easy for them to move a lot of weight too far back which is generally known to be a condition which promotes shimmy. Since the first natural frequency of even the largest frame is much higher than shimmy, I stand behind my previous postings on this subject. There is nothing in the dynamic system defined by the wheels, rider, and frame that changes the structural vibrations of the frame. That giant hinge at the front (headset) is going to rotate before the frame has a chance to bend.

    I'm not going to put a frame into an fea program and see what the vibration modes are, but just as a first approximation, I can't see how the head tube torsional stiffness plays in this at all. In fact, I would say that the head tube sees near zero torsion. Maybe you could make an argument for the torsional stiffness of the down tube and top tube, but they aren't all that much longer on a bigger bike. Even then, the overall stiffness is defined by all the tubes, so any given formula from the back of an undergraduate text isn't particularly useful in this context

  10. #10
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    Quote Originally Posted by kevbo View Post
    To repeat: The main problem comes from the longer head tube. A 69cm frame will have a head tube with twice the distance between the top and down tube as the frame a normal height rider would mount. Torsional stiffness drops with the square of length, so that frame has about 4x the flex. The longer seat tube doesn't help, but even on a normal frame the seat tube is so long that it doesn't contribute nearly as much stiffness as the head tube.
    Slightly OT (because I don't know that frame stiffness contributes to shimmy) but interesting.

    By your argument the most effective way of adding a tube would be to put that tube diagonally from the seat cluster to the lower end of the head tube, rather than having it parallel to the top tube. Do you know of anyone who has tried this?

  11. #11
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    I've had two bikes with shimmy problems. One was a sixteen year old steel ( Columbus SLX ) that never shimmied until it got old. The other was a new bike that had very little trail and was twitchy from day one. I think the cause of the problem with the first was too much flex as the bike aged and the cause of the problem with the second was geometry. Two very different causes with the same results. One solution for the OP might be to try building with double oversize tubes and incorporate a little more trail than the typical touring design. If I works, we won't really know why because two different variables were changed but he will be happy. I'm suggesting the bigs tubes because I just finished a bike with double oversize OX Platinum tubes and I love the ride. Laterally, it's very stiff but not harsh.

  12. #12
    Randomhead
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    Steel doesn't get old and flex more because it's old. It's more likely it got bent a little, or residual stresses released causing misalignment. My old Italian racing bike would shimmy sometimes, but not always. I definitely rode it for years at a time without evidence of shimmy.

    My current bike has never started to shimmy. Not sure why that is, it has a little less trail than the previous bike that shimmied. I'm pretty sure that certain cases of misalignment will cause shimmy to initiate more easily, and my guess is that wheel flop will cause it to be stronger.
    Last edited by unterhausen; 11-25-11 at 07:52 AM.

  13. #13
    Wrench Savant balindamood's Avatar
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    A couple of thoughts:

    1). I do not believe that shimmy has anything to do with tubesets. IMHO it has to do with frame alignment, wheel/tire issues, and headsets. I have had bikes with shimmy issues and have managed to correct all of them through working on one or more of those points. I may be the lone voice in the wilderness on this, but that is my experience.

    2). The most stressed frame on the bike is the downtube, not the top tube. I think you will get more rigidity by focusing on the downtube first.

    3). I cannot provide you sources (because I have forgotten), but I know that a number of frame builders (generally older ones) would not due mixties on heavily loaded bikes because they offered less torsional resistance than a single tube. The top tube and down tube also resist a fair amout of twist about the axis of the frame, especially when the rider is pedalling out of the saddle. The two small tubes, especially alinged down the axis of twist like a mixtie, actually provide much less torsional resistance.

    4). A double top tube will do little other than make your bike look wierd and add weight. Even if you are a very tall rider (6-6+)I think you will get more out of thicker tubes than adding another tube.
    "Where you come from is gone;
    where you are headed weren't never there;
    and where you are ain't no good unless you can get away from it."

  14. #14
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    Above explained in video on YouTube:

    http://www.youtube.com/watch?v=LYyGG...e_gdata_player

  15. #15
    Randomhead
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    Quote Originally Posted by kevbo View Post
    Above explained in video on YouTube:

    http://www.youtube.com/watch?v=LYyGG...e_gdata_player
    you really need to put the camera on a tripod and get the bike to shake like that so we can see what's moving and what's not. If that motion is in the frame, it's broken.
    here is his video for those following along at home:


    The fact that you got it to vibrate is pretty interesting to me. I might have to try to replicate your experiment. I have a high speed camera that would probably make for an enlightening video. I also have a boss that rides 27" frames, so a test subject will not be hard to find.

    In case I decide to do this, my understanding is that you are holding the handlebar up against a doorway and pulling on the saddle, is this correct?

    My understanding of shimmy is that the actual cause is not really well understood, so it would be nice to have good video.
    Last edited by unterhausen; 11-25-11 at 01:48 PM.

  16. #16
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    Frame isn't broken...just tall! Yes, I brace the bar on the door frame, using my hip to press it solid. Sorry for shinola phone video. I just realized I could use my bar mount on shop stand to hold phone steady.ETA: Please note that I do not claim this is THE cause of shimmy. It is but ONE, but in a tall frame it is the prime suspect. Also, modern 1-1/8" steerers use a biggerHT that adds useful stiffness.
    Last edited by kevbo; 11-25-11 at 02:29 PM.

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  18. #18
    Senior Member PaPa's Avatar
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  19. #19
    Randomhead
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    there are three aspects of this problem: what starts the shimmy, what sustains the shimmy, and what determines the strength of the shimmy. I think that fork symmetry definitely is one of the reasons why shimmy starts. However, most, if not all, bikes will shimmy given the right speed if the CG of the bike/rider/load is too far back.

  20. #20
    Senior Member PaPa's Avatar
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    Quote Originally Posted by unterhausen View Post
    ... However, most, if not all, bikes will shimmy given the right speed if the CG of the bike/rider/load is too far back.
    What is too far?

    Depending on rider inseam, the 'bike' pictured below has a calculated weight distribution of up to 80r/20f. It is also the exact same design that captured the 2009 RAAM 4-man, effectively beating the 4-man DF team. To the best of my knowledge, no unusual handling problems were reported during the event, or among the hundreds sold to the public.



    http://www.rans.com/ITR79.htm

  21. #21
    Randomhead
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    Shimmy is well known among motorcyclists, and there are no questions of frame flex or asymmetry, but it is known that moving the center of gravity back will cause the problem.

    I'm pretty sure a 'bent in that configuration is effectively immune to shimmy due to the long wheelbase

    Wikipedia has a decent article about shimmy
    Last edited by unterhausen; 11-30-11 at 09:11 PM.

  22. #22
    Senior Member Homebrew01's Avatar
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    Quote Originally Posted by balindamood View Post

    4). A double top tube will do little other than make your bike look wierd and add weight. Even if you are a very tall rider (6-6+)I think you will get more out of thicker tubes than adding another tube.
    That's always been my thought too ... Of all the tubes to reinforce (double, thicken, enlarge...), why the top tube ?
    Bikes: Old steel race bikes, old Cannondale race bikes, less old Cannondale race bike, crappy old mtn bike

  23. #23
    Randomhead
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    There have been a lot of utility bikes made with double top tubes over the years. That may just be because they wanted a lot of load capacity, the standard tube diameters couldn't handle it, and they weren't smart enough to use bigger tubes. If you want to make a bike laterally stiffer, adding mixte tubes is probably the way to go

  24. #24
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    The practice evolved in the age when all good bikes were lugged.* By doubling the top tube, you didn't need special lugs in the normal spots, and only minimal to no modification for the added lugs.




    * An age we have yet to see end.

  25. #25
    Charles Ramsey
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    Bicycles with two top tubes are more likely to break the tubes have leverage against each other. Workman cycles had a problem with this. What makes more sense is to use a middle tube like on tandems the seat flexes enough to avoid breakage. You can then decide to an extra set of stays to the dropouts this makes good sense if you are using long chainstays.

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