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PostPosted: Sat Apr 07, 2012 12:12 pm 
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Formerly known as wassertreter

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Location: Pedal Square
My bike is twitchy at speeds over 50kph. It's hard to describe how it feels though, it only happens when going straight, even on super smooth surfaces. Almost like hitting a resonance frequency or something. Once engaging in a turn it's very stable again. I have checked the cockpit, headset and the front wheel, and mechanically everything is ok.

Is there anything else I can try short of drastic measures like running a heavier front wheel or a fork with more offset?

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Posted: Sat Apr 07, 2012 12:12 pm 


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PostPosted: Sat Apr 07, 2012 12:51 pm 
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Are you sure you are relaxed at that speed. If so, you could try putting some tape on different areas and see if this has a dampening effect on your frame and alter the harmonic frequency .


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PostPosted: Sat Apr 07, 2012 1:41 pm 
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Cycling Tips Blog did a piece on this last year, worth a read :thumbup:

http://www.cyclingtips.com.au/2011/03/s ... -its-head/

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PostPosted: Sat Apr 07, 2012 3:14 pm 
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hansonator69 wrote:
Cycling Tips Blog did a piece on this last year, worth a read :thumbup:

http://www.cyclingtips.com.au/2011/03/s ... -its-head/" onclick="window.open(this.href);return false;


That video is insane.......I almost soiled my britches watching it! :oops: :thumbup:

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PostPosted: Sat Apr 07, 2012 6:16 pm 
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Put your knee up against your top tube.

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PostPosted: Sat Apr 07, 2012 7:26 pm 
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prendrefeu wrote:
Put your knee up against your top tube.

Knee is busy pedalling ...

Anyway the Cycling Tips thing is pretty much exactly what it feels like for me. Also I'm riding an "XL" frame and it came with an 48mm rake fork.

Cycling Tips says shorter trail should help -- is this the best justification (to self) for a 40mm Enve 1.0 fork? And all in the name of safety!

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PostPosted: Sat Apr 07, 2012 8:03 pm 
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Not sure if this is of any help, but I have also experienced a properly wobbly steering with a very light tubular front wheel where I have unintentionally placed a quite heavy spoke magnet almost directly next to the valve. The wheel was therefore badly balanced and felt quite sketchy at higher speeds. As soon as I changed the magnet the steering was fine again.


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PostPosted: Sat Apr 07, 2012 9:13 pm 
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wassertreter wrote:
prendrefeu wrote:
Put your knee up against your top tube.

Knee is busy pedalling ...

Putting your knees against the top tube is a known way to dampen speed wobble.
Cycling tips recommends putting your feet at 6 and 12 o'clock positions so you you shouldn't carry on pedalling.
It's certainly worth putting on another set of wheels just to eliminate them as a possible cause.


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PostPosted: Sun Apr 08, 2012 2:34 am 
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IF shorter trail would help, you wouldnt want a fork with less rake. however, it sounds like a shimmy issue, and not that it's "twitchy" as in extremely quick handling, so frankly any fork change, even one with the same geometry, may make a difference. you're trying to do anything that will change the bicycle/rider's natural/geometric frequency.
if you actually are looking to make the steering more stable and more resistant to changing directions, you want MORE trail, which means LESS rake, all else being equal.
im curious what bike in an XL has 48mm rake. must be a pretty slack bike to begin with. im also curious how upright your position is. a very high handlebar position does increase the possibility of shimmy (anecdotal)


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PostPosted: Sun Apr 08, 2012 3:28 am 
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Less fork rake (offset) means more trail, which is more stable.
Image
https://en.wikipedia.org/wiki/Bicycle_a ... e_geometry" onclick="window.open(this.href);return false;


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PostPosted: Sun Apr 08, 2012 4:10 am 
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You can change forks, and/or just balance your front wheel very carefully.

Assuming it is a resonance frequency shimmy. I had a bike that did that. I noticed that if I balanced my front wheel carefully (and true of course) there was no vibration to initiate the shimmy. later, I switched to a different fork (same geometry but stiffer) and the problem went away completely.

Lighter bars and shift levers might also help. The key is to just change enough masses and stiffnesses that the resonant frequency moves to a place whre you don't notice it.


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PostPosted: Sun Apr 08, 2012 7:39 am 
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As has been said, it's all about resonant frequancy/ies so you need to change a bit at a time for longer/shorter, stiffer/bendier or lighter/heavier or add some damping, also as has been said look at the trail as the self centring that gives helps damp it out, although it's effect is small.

For example try rotaing the levers on the bars slightly to move them out/in which will affect the polar moment of inertia of the whole fork assembly.....

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PostPosted: Sun Apr 08, 2012 11:13 am 
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It could be geometry but could also be the front wheel. It could be not still enough and some tire profiles can be more nervous at speed.

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PostPosted: Sun Apr 08, 2012 12:18 pm 
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50kph isn't very quick. I'd be looking at a lose or bust headset, then the wheels..
Are the bearings shot? Is there play in em.?
Also tires... Are the they worn out? Or are you running too much pressure.. Eg 120psi?
I doubt your bike was built to be twichy it sounds like a problem

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Posted: Sun Apr 08, 2012 12:18 pm 


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PostPosted: Sun Apr 08, 2012 9:25 pm 
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thisisatest: strictly spoken it might be an 48mm offset or trail (straight) fork, but enve calls it rake on their straight forks as well, so I went with that.

"Unfortunately" I can rule out most of the mechanical issues.

About the rider position, I have a rather sporty one, check the "Raceur" link in the signature if you're interested. My saddle is back far enough so there's not a lot of weight on the handlebar. Anyway there'll a 17° stem in the mail soon, maybe that changes anything.

Thanks for all the input so far, that's much appreciated. Yes the weird thing is that the (tendency to) wobble starts at rather low speeds already, 50-60kph, but it goes away over 80 :mrgreen:

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