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wobbling front end(10 posts)

wobbling front endmilkman
Oct 3, 2001 2:27 PM
Question for anyone. I have a Moots road bike with a titanium stem and a Look Carbon fork. When I am going down a hill with my hands off the bars (drinking, or stretching) and I begin to pick up speed, the front end of the bike starts to wobble and progressively gets worse if I do not return my hands to the handlebars. What does this mean?
I have had other road bikes, and this never happened. Is it the fork? thanks in advance.
re: wobbling front endHighgear
Oct 3, 2001 3:48 PM
Sounds to me that the rake on the fork doesn't work well with the head tube angle. I think http://www.anvilbikes.com has some info on this at his site. You could always ask Moots.
Try this too for a better understanding.Highgear
Oct 3, 2001 3:59 PM
http://www.sheldonbrown.com/gloss_tp-z.html#trail
Harmonic resonanceKerry Irons
Oct 3, 2001 4:08 PM
This is a relatively common but poorly understood phenomenon. The only thing you can say for sure about it is that the total mechanical system (including you) is at a harmonic frequency when you are coasting hands-off downhill. It can be triggered by flexy frames, poorly adjusted headsets, out of true wheels, out of balance wheels, and many other things. While there are those who claim to have full explanations, in fact these are theories, not facts. At any rate, it is NOT that "the fork doesn't work with the head angle." The short answer to solve this is to check your headset adjustment, etc. and if that doesn't work, don't take your hands off the bars in circumstances where this is likely to happen. All else constant, stiffer frames will reduce this problem. Given your Moots/Look combo, I would look somewhere else than the frame and fork.
Harmonic resonance (ie. flexy frame)GTlover
Oct 4, 2001 2:51 PM
Could be the wheels that start it but its a flexy frame that get the wobbles.

Keep your hands on the bars on the descents.
Harmonic resonance (ie. flexy frame)grzy
Oct 4, 2001 4:17 PM
So just how much do you think the frame is "flexing" and why if the natural frequency of the frame is quite high (give it a wack to see) is it contributing to something at such a low frequency. To simply say that the flexing frame is the culprit is pretty misleading.

You gots to look at the whole system and ultimately the biggest variable on an established design is the organic connection between the pedals, seat and bars, i.e. the rider. Ghost ride that bike down the hill and it won't have any wobble. You've got the driving force, the frequency response of the system and the dampening. BTW there is a steering dampener made under the name of Hopey. Not such a radical idea when one considers that cars and hot motorcycles use them. It's getting rave reviews in the MTB world and starting to cross over into the roadie world.
Mine too but just recentlyMick
Oct 3, 2001 4:50 PM
My Giant CFR started showing this tendency a couple of weeks ago. It has a Giant carbon fork and until then it was as steady as a rock, hands-on or hands-off.

I've been puzzled but not overly concerned because I can stop it by putting my hands on the bars.

The only thing I can think of that's related that would get progressively worse would be the headset adjustment. But that feels okay.

Honest, it used to ride straight and true. I'll check the headset again but can it be anything else?
re: wobbling front endCartman
Oct 3, 2001 5:55 PM
I had a problem w/ a high speed wobble. I had my stem changed for one a bit shorter, and it went away.
re: wobbling front endguido
Oct 4, 2001 1:16 PM
Front wheel can wobble when too much weight is on it. You can slide back in the saddle to get rid of it. If that doesn't work, check wheel alignment (tracking) with a straight edge or string. Will your bike scribe a straight line with hands off the bars? Brinneled or non-parallel headset races can cause wobble too. It's always scary, because it happens when you least expect it. That's why in a steep descent, it never hurts to load up the rear wheel by scooting back on the saddle, and lower your center of gravity by crouching over the bars.
re: wobbling front endgrzy
Oct 4, 2001 1:34 PM
Manauverability and stability are inversely related. You want one, then you give up the other. Trick is finding an acceptable trade off. Used to be able to ride all do w/out hands on my older touring style frame. I don't do that much on my current bike. I also run a C'dale Jekyll that allows me to change the suspension setup via moving the rear shock through a yoke and still have full travel. When the bike is super upright XC mode it's very twitchy, but I drop the suspension for blasting down hill and it becomes super stable - riding it no handed is a breeze. Ultimately you only have so much control over a built road bike - changing body positions and forks is about it. You could throw a heavy set of wheels on with a high moment of inertia to get a bigger gyroscopic effect, but that's counter productive - (adding weight so you can ride w/out hands more often). You wanna see twitchy check out a full-on tri bike in a stiff cross wind....