Tank Slappers what causes it???
 

Riding into work this morning had a bit of a numb left hand so took it off the handle bar to shake it out only to put it straight back on again as the front started twitching (real scary stuff)!!! the only thing i think it can be is that i have recently adjusted the rear preload for when the wife is on board would this affect the front end by making it lighter??? but she was not onboard this morning.

Help would be greatly appreciated as this was a scary moment for me and don't want it to happen again

Check your steer head bearings adjustment. Might be loose.

Cheers
Ben

The bike is supposed to run straight, even with no hands on the bars.

Either the road had a crappy surface or something is wrong with your bike. :idea:

Try a search of the site.There are a few long threads on this.

I had a scary tankslapper on Sunday whilst accelerating hard on the Blackbird overtaking and hitting a bumpy surface. Scared the bejesuz out of me!

This site gives an interesting overview of what can cause them.

The front of the bike is a castor, just like a supermarket trolley wheel, and we all know what they can be like. It's exactly the same physical system.

Whether the system (tyre/wheel/forks/head bearings/frame/rider etc) becomes unstable depends on a variety of factors, not just the sort of things described in the link.

Things which you can do something about are the tyre selection, tyre pressure, wheel balance, fork springs and oil (to a minor extent), steering head bearing adjustment, your riding position. What you can't do much about is road input, fork stanchion/yoke stiffness, frame stiffness, bike mass (inertia).

The common things which makes a bike prone to steering wobble are tyre stiffness (i.e. tyre make/model) and as 21Quest says, steering head bearings being loose. If they are too tight the bike tends to weave slowly left/right as you correct the steering. Pillions or loaded top boxes etc can make things worse sometimes (mass/inertia effects in a damped spring-mass system).

Check tyre pressure, tightness of wheel spindle/clamps, yoke clamp bolts, general fork geometry (twisted forks), and head bearings.

Having the rear adjusted very high or dropping the forks through the yokes can make it more prone since it alters the rake and trail, thus changing the natural frequency of the system by altering the input/restoring forces and moments when the steering is turned.

A steering damper reduces the tendency, it doesn't significantly change the natural frequency of the system (only very slightly), but can be enough to make the system manageable. This kind of business
http://hyperphysics.phy-astr.gsu.edu/hbase/oscda2.html
Steering dampers are really a last resort, and should be selected with care. You usually want a mildly underdamped system, which will give slight wobble but will enable the steering to be changed quickly. Overdamping leads to weaving.

Whats the front tyre like? Stepped BT010?

..and they didn't seem to take long for them be become stepped. Drove me nuts. I think it was kind off a Bridgetones trait as the BT45 on my bros were the same especially ran at the recommended pressures.

Cheers
Ben