High speed shimmy-can it be the head tube??

Discussion in 'Cycling Equipment' started by rayhuang, Dec 24, 2006.

  1. rayhuang

    rayhuang New Member

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    Hey all,

    I am concerned that my new 2006 bikes head tube is not stiiff enough, even though its a high modulus carbon monocoque frame (52cm) and has massive carbon top and down tubes around the head.

    Without dissing manufacturers, is it possible that these properties I will list can be caused by a frame thats not stiff enough in the headtube area or possibly in the steerer tube (mines carbon/carbon fork-no metal).

    -twitchy on center
    -when turning in, fast to change direction, but feels sloppy mid corner (almost like it wants to flop)and pushes on exit.
    -high speed descents over 40mph (65kph) front can start to shimmy.
    -in hard sprints, front seems to flop excessively when leaning bike.

    Thanks,
    Ray
     
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  2. alienator

    alienator Well-Known Member

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    First, high speed shimmy is a result of rider position/weight on the bike and how it couples with the bike frame to form harmonic vibration modes.

    What is "twitchy on center?"

    "Feeling sloppy midcorner".....I assume this means that it feels "vague" or maybe "loose?" This could be the result of wheels with poor lateral stiffness. Could be wrong tire pressure. However, a flexy fork or steerer will not cause high speed shimmy.

    As for the sprints...again, it could be poor lateral stiffness in the wheels. Could be poor preload in the wheels. Again, could be wrong tire pressure.

    If you want to know more, though, you're going to have to say what frame you've got. That in no way "disses" the manufacturer. For any given frame--no matter how good it is--there is a vibration mode for a given rider and rider position that will result in high speed shimmy. Heavy riders do tend to push that harmonic down into a lower speed range, but still, it's nothing that you can predict.

    If the bike feels twitchy, that could just be a function of the fork's offset. You could try a fork with less offset--which tends to make a front end a bit more slow steering and a bit more stable (with less offset, there's a greater recentering force on the front wheel.).

    As far as the bike "pushing on exit"....well, that could be a result of the weight bias being wrong on the bike, i.e. too much weight distributed to the rear (which will also tend to make the front end twitchier). It could also, frankly, be a result of the bike's handling making you more tentative and less willing to commit to a tighter line or higher cornering and exit speed. I know pushing ain't fun. I once rode for about 15 2-mile laps a Suzuki TL1000R that was bone stock. Talk about pushing. That damned thing required constant, high pressure on the inside bar to keep it from running off the track into the weeds. Conversely, when I bought my own TL1000R to race and I set it up properly (meaning jacked the rear end up to move the weight bias forward while also decreasing rake up front), I had a race bike that was anything but pushy (BTW, I called her "Jezebel," because she was a mean spirited bitch, kind of trashy, that enjoyed nothing more than a sweaty combat f*ck that resulted in bloodshed.). The point is that the wrong weight bias can really throw off a two-wheeled machine's handling.
     
  3. sogood

    sogood New Member

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    So what's the brand and model of the frame? :p
     
  4. rayhuang

    rayhuang New Member

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    2006 Fuji Team pro 52cm. I am 5'9" and 154 lbs. NO extremes on fit (meaning not way back on saddle short stem or long stem, etc.) and proffesionally built wheels (well tensioned). 95 psi in front and 105 psi in rears on OEM Continental 23c tires. Will replace with better tires come spring.

    Alienator-you hit it on the head about its cornering. It turns in sharp then mid corner it feels like it goes vague and I want to hit the brakes to make sure I dont go over center of the road which increases the push. I just dont like how it feels. I dont race motorcycles, but I did race karts for a long time and I never liked a loose kart. In fact I want a balanced steering feel from entry to exit with as much corner speed as possible of course (not bound up).

    Thanks again about your ideas on the shimmy.
     
  5. carpediemracing

    carpediemracing New Member

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    I used to experience what you're describing on some of my older frames. I didn't even know I was experiencing them till I bought my more recent frames/bikes.

    On smaller frames, especially those with shorter top tubes (your 52 has a 52 cm top tube), manufacturers need to address the toe/front-wheel overlap problem. If you have a "nice" geometry front end, your toe typically hits your front tire.

    A manufacturer can increase fork rake (offset) and decrease the headtube angle in order get some more clearance for the rider's foot. The front wheel flops a bit more - an extreme case of increasing rake and decreasing the headtube angle would be a chopper (and their front wheels definitely flop). This extra clearance comes at the expense of:
    1. "pushes" in turns
    2. "floppy" if sprinting out of saddle
    Sound familiar?

    The "twitchy on center" could be from a lack of trail (draw a straight line through your headtube to the ground, then measure back to the center of where the tire sits on the ground - the difference is "trail" - usually ). More detail here: http://www.dclxvi.org/chunk/tech/trail/

    Getting a less offset fork will increase trail (stability). It will also pull the front tire back up to 5 mm (most pro forks come in a minimum 40mm rake - your fork has 45 mm).

    I used to buy 50 cm traditional frames (for racing). They were afflicted with 72 degree (or shallower) headtube angles, massive amounts of rake, and questionable handling. These were "good" frames - Basso, Panasonic, Cannondale, Specialized M2, Specialized carbon. I got a Giant TCR a while back, after studying its measurements, and it was a completely refreshing experience - instant line changes while sprinting out of the saddle, absolute confidence cornering, etc. This was due to a normal headtube and fork rake combination.

    There was one other thing that happened - for the first time, I had a high speed wobble. I changed out the "aero" Giant fork for a Reynolds fork and it went away. The aero Giant forks were notorious for their wobbles. Whatever caused it, changing the fork fixed it. Flexy frames and forks exacerbate wobble - just watch the clip of Bruyneel winning his '95 Tour road stage in front of Indurain (after sitting on his wheel the whole break I might add). He raises his hands at probably 35-38 mph and it immediately wobbles like mad. He has to make some very un-pro-like corrections to stay upright - I believe he even has to reach down to steady the bars. And this in a relatively slow sprint on a flat road. He was riding a very flexible Look frame/fork.

    In your situation, without seeing your fork first hand, you might consider swapping out your fork. If that doesn't work, maybe a better-geometry frame would be good. (I bought the Giant only for its geometry).

    Finally, I found that when riding a very long stem (I use 130mm stems), it really helps plant the front end in turns and out-of-saddle sprints. Lower is better and I am currently on a size S Giant (vs size M) in order to drop my stem a couple cm. btw I use that size stem because it fits, not for handling purposes.

    hope this helps
    cdr
     
  6. rayhuang

    rayhuang New Member

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    cdr-I thank you. I was pretty green when i got back inot cycling again and didnt pay much attention to angles. I knew I wanted a longer top tube than 52 (My custom PDG is 51cm seat tube (traditional) 53cm top tube) and it handles really to my liking. Sadly the bikes 4.5lbs heavier than my Fuji and I am not giving up 4.5lbs.

    I can live with the Fuji-no doubt about it, but it irks me everytime I try to dive around a 90 or sharper corner and i feel like I am trying to hold the bike up and still carve the corner at speed. I'm worried when I start racing again that this will be in the back of my mind.

    I'll make some measurements tonight and post them.

    Ray
     
  7. ccdisce

    ccdisce New Member

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    Ray,
    you may also want to read the 'shimmy' articles over at Calfee's web page and at Dave Moulton's web page.

    I use the free version of BikeCAD downloadble from Bikeforest.com to look at things like trail, standover, reach etc. It also has a fit adviser to help.

    Ton.
     
  8. rayhuang

    rayhuang New Member

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    Thanks for the links!! Reading all I can before I spend more money!!
     
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