The uncontrollable shaking of the handlebar at a certain speed is called headshake. This is mostly a balance problem and can be cured quite easily. It gives an indication to the balance of the bike. Don't ignore it or use a steering damper to cure it. That will only mask the problem.
When a bike is balanced I know it immediately because I think "wow, I am such a good rider!" It holds straight lines beautifully without headshakes. At the same time it is nice agile and easy to manoeuvre through the tight corners. Hard to say: is it the rider or the bike?
As much as I would like to take credit for all that skilful riding it is probably 80% the handling of the bike. Sure a good rider can make up for the bad handling but will lose valuable seconds on the track just the same.
Headshake is caused when
These are two different scenarios even if they cause the same problem. We have to separate them in order to find the right cure for each one of them.
As you might know by now you have to get the basics right when faced with handling problems. If you haven’t checked them out yet or are not familiar with setting sag then get right into it. You will need it! There is no way around that if you want your dirt bike to handle well.
Again and again you will find that throughout Dirt-Bike-Secrets because it is just so important and totally underrated. If you are not familiar with your click adjusters take your manual and check it out. Normally you close them clockwise and then open them counter clockwise to the amount of clicks indicated in your manual to set them to standard.
if your bike is new and you haven’t changed the clicks it is good to
check if they are set to standard. There might have been a production
problem or an over motivated mechanic somewhere along the line.
It is not always easy to distinguish between the 2 cases (front lower than rear or rear higher than front). Sometimes it is obvious that the front is lower like in the picture above. If in doubt, start with the adjustments on the fork.
Only change one thing at the time or you lose track of what has the most influence. Never change more than 3 clicks at the time before testing it again. Don't go more than 9 clicks away from the standard click position. Do the adjustments during an outride and test the result before changing the next.
If you change the fork position in the clamps make sure you tighten the lower triple clamp to the recommended torque as over tightening has serious negative effects on your fork performance.
If you have tried the above and it didn't bring the results you wanted then try the following also in the indicated order:
If all that didn't help or it only helped very little there might be other problems in the balance that cause headshake.
Maybe you have changed the back spring to a harder spring for your weight but not the front. More unlikely but also a possibility is that your fork spring is too soft in comparison with your shock spring.
The bike comes with the correct spring rate front and rear for riders between 70-85 kg. If you need to change springs always change front and rear at the same time.
Another possibility is that your shock has been modified incorrectly. If you (or the pre-owner of your bike) had some setting change done and it is not done properly it can cause bad balance problems.
If you are not sure then get a professional suspension technician to check it out for you.
Headshake normally goes hand in hand with oversteer meaning that the bike falls into the corner. By doing the adjustments you should have eliminated those problems.
If you did changes and the headshake is gone, the bike holds the line incredibly well but now you experience understeer (the bike doesn't want to turn) you went too far with your adjustments. Partially reverse them and keep on testing till it feels right.
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders