How can I eliminate Headshake?

[Duwvesmith]

I have an 03' KTM 250 EXC RFS that wants to headshake on fast choppy straights. What causes this? How can I remedy the problem?

I don't what to drop another $500 or more on a premium quality bike that should not have this problem to start with.

Right now the settings are as such;

Shock - 32mm static sag, 20 clicks both R & C (of 40 clicks)
Forks - Compression 18, rebound 10 (from fully closed, 30 clicks available)

Any thoughts?

 

[euro gasgas]

Make changes that will keep the front end up and/or let the rear end squat. First check to make sure your spring rates are correct and balanced front and rear by checking loaded and static sag for both fork and shock.

After that, I would check and adjust the following to get the bike balanced and feeling right (the ktms are generally quite stable - so I would guess suspension settings will solve issue).

1. Increase compression dampening on the front forks.
2. Check race sag and set to 100mm as baseline.
3. Reduce rebound on front forks a little.
4. Increase rebound on rear shock a little.


Making one change at a time I would go in 4 clicks on compression on forks (to 12 clicks out) and take another run down the same straight (run over same terrain repeatedly then go ride in another area to see if it works okay there too).

If you haven't changed your front fork oil in a while, I would freshen it up with spectro SX.

jeff
www.gasgasrider.org GasGas Rider's Club
www.lt-racing.com GasGas Performance Products

 

[sharp]

Lower the forks in the tripple clamp.

 

[euro gasgas]

Note that I edited my post above - meant to say "go in 4 clicks on compression" instead of "go in to 4 clicks compression" on forks...

jeff

 

[SFO]

increasing your sag will decrease trail.
It will also increase rake which is oft confused with stability.
The reduced offset triple clamps increase trail and pull the front wheel rearward.

Increasing trail is normally associated with increasing stability.

Rear ride height creates the most dramatic adjustment of rake and trail.

Try this book for some very concrete definitions of chassis dynamics....

http://www.tonyfoale.com/

 

[SFO]

also try this


http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

 

[Jaybird]

What sharp said, and also make sure your stearing stem nut is not too loose, but especially do what sharp said. Probably flush would be a good position.
Try lowering your forks in the clamps before you drive yourself crazy with clickers.

 

[TM-Frank]

Has anybody ever put a KTM on a weight scale and checked how the front wheel/rear wheel weight distribution looks like in comparism to other bikes? Would be interesting. I believe that KTM for whatever reasons likes to put more weight on the rear and less on the front (compared to my TM for example). In addition the frame geometry is completely different. We put my TM 300 side by side to my friends KTM 300 and noticed that the KTM has a noticable shorter wheelbase and a less steep steering angle.
Could be the reason why I find KTMs always very wobbly on choppy sections. When I rode someones' KTM I always thought this would be the bike that needs a steering damper.

 

[Patman]

Yup what TM-Frank said, steering damper

 

[MikeS]

I have been told that a properly set up KTM will not require steering dampner... I just didn't ask what defines properly set up and how much $$$$.

I think the suggestions are good here. Adding preload to the front may help some to keep the front up a tad.

We found the steering dampner helps too on certain models.:)

Good luck.

 

[ktmboy]

Try riding an '80's era Honda and the KTM will feel very stable in comparison! :laugh:

 

[Duwvesmith]

Thanks for the information. I am going to try every one of these suggestions if that's what it takes to get it right.

I'm going out Saturday morning. Will report back on what mods I made and the response.