wheel/head shake

[oldgreyandslow]

I was descending at about 35mph when a car in front decided to slow down, we were in a 30 mph zone, I hit the brakes fairly hard and got a serious wheel wobble and the bike was shaking around the headstock. In motorcycling it's termed a tank slapper I believe.

When I let off the brakes it calmed down, reapply and it shakes again. This hasn't happened previously but then again I can't recall braking hard on a descent before, it certainly seems to run true otherwise.

The bike is a Boardman team carbon, the stem and bars were changed when I got a bike fit, the bars are now 42cm from 44cm and the stem was replaced with one 5mm shorter. It certainly made the bike more comfortable and I managed the ride London in relative comfort and hit 42mph on that ride with this set up although I don't recall hitting the brakes. I also have Shimano C24 wheels and these seem to run nice and true.

I don't know if any of these changes from the standard bike is likely to affect the handing that much, or is it?

I have replaced the stem with the original Boradman item and will try the same descent later to see if it makes any difference.

Any thoughts?

 

[ColinJ]

Commonly known as 'shimmy' by cyclists - read this.

 

[Smurfy]

Try reading this.
http://en.wikipedia.org/wiki/Speed_wobble

The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire.

You need to change the damping, and that means changing your grip on the handlebars (in the absence of changing components/frame/fork).

 

[fossyant]

Commonly known as 'shimmy' by cyclists - read this.

Or brown trousers moment.

It's quite complicated how this can happen as there are many factors. The only times I've had it are when at speed with mudguards on one bike, and when hit hard by a cross wind at about 55 mph - just braked gently, and went for it again !

Some bikes are more prone to it. As to which, its impossible to say.

 

[oldgreyandslow]

Thanks folks, well I put the original stem back on and made sure everything was nice and tight, despite the links from Colin and Tim stating it shouldn't have anything to do with it, I managed to do the same descent twice without problem at slightly higher speed than before. Although I only tried gentle braking on the first run down I definitely felt there was a tendency for it to wobble when the bike was gently leaning into a bend, so I'll persevere with the original stem and find some steeper hills to test my nerve and brakes.

 

[Kestevan]

Only time I've had this was when slipstreaming a HGV down a steep hill. At one particular point the bike shook violently. Suspect the airflow from the HGV was causing it, as it stopped as the truck got further away, and it's never happened since (I ride the hill most days).

 

[Fab Foodie]

Commonly known as 'shimmy' by cyclists - read this.

Yep, that's the baby!
It seems hard to do, but relaxing and moving back on the saddle is the best way to restore a sense of poise when hurtling downhill. The death-grip on the bars will only end in woe ....

 

[glasgowcyclist]

The only bike I experienced a head wobble on was my Bontrager Privateer which was found to have a manufacturing fault that meant the head tube was very slightly ovalised.


GC

 

[sazzaa]

Maybe just go a bit slower down that hill? Why are you going above the speed limit anyway?

 

[oldgreyandslow]

Maybe just go a bit slower down that hill? Why are you going above the speed limit anyway?

I only noticed the speed afterwards when I strava'd it (sad I know) but I wasn't really looking at how fast I was going at the time, I was more concerned with not falling off. Anyway speed limits don't apply to cyclists