downhill wobble

gearedhed

help! everytime i go down a steep hill and reach about 25mph the front of my bike starts to wobble viciously. Ive tried 'relaxing'(not easy),breaking(just makes it worse) and gripping the frame with my knees (with a little success)
Has anyone got any advice? Will upgrading the wheels/tyres help? Is it me or is the bike not right for me? My bike is a Specialized allez triple.

Erudin

One of my old rigid mountain bikes does it a bit at a certain speed, it did not do it with the previous tyres, so try some different tyres if you have them.

More links:

http://www.bikeradar.com/forum/viewtopic.php?p=16119043

http://www.bikeradar.com/fitness/article/qa-speed-wobble-14776

http://www.sheldonbrown.com/brandt/shimmy.html

Keith47

I'm no expert but it sounds as though it could be either your front wheel is out of true or maybe the aheadset bearings have too much play in them?

andy81

Sounds like it could be speed wobble. It's caused by the frequency of the vibrations in the road reaching the bike's 'natural frequency', causing severe oscillations. It's the same concept as when you oscillate a pendulum from side to side really fast and the pendulum doesn't move very far, but slow the frequency of your movements down and the pendulum swings in a big arc.

There's not much you can do to prevent it, but when it happens, try giving the top tube a slap with your hand. This should disrupt the oscillations enough to change the frequency of the vibrations away from the 'natural frequency'.

Hope this helps!

Ron Stuart

The shimmy as it is known, happens with some motorbikes as well (known as tank slap). The above advice is generally good. Don't worry about you basic bike, it will be fine just isn't happy being married with some other component.
Try placing the computer sensor opposite to valve on front wheel and check for play in headset.
If that doesn't work try different model of tyre on front wheel.
If that doesn't work try different wheel.

If it starts up again try pressing on the top tube(crossbar) with the inside of your upper leg and easy on the brakes. It will strangely disappear at a speed below 16mph normally and it is all about frequencies.

We have all been there and it can be quite alarming I know.

IShaggy

25mph sounds way too slow for frequency-induced speed wobble. Usually I'd expect this at speeds in excess of 40mph. My bet would be on something mechanical such as a loose head-set or wheel bearings.

kev77

i had the same on my pinarello, but in the end it was the set up of the bike that caused.
( position on the bike ) in the end it was noticed i had to short a stem and there was not enough weight over the front wheel.

vorsprung

IShaggy wrote:

25mph sounds way too slow for frequency-induced speed wobble. Usually I'd expect this at speeds in excess of 40mph. My bet would be on something mechanical such as a loose head-set or wheel bearings.

I agree, 25mph is just a normal speed. If you got the bike from a local shop, take it back, explain the problem and ask them to service it