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I need some help figuring out headshake.

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Posted by: cantrell24x---------------------

I have been riding a 2008 yz 250 for about 6 month now. A good bit of riding actually at least two hours a week not counting the races.

The suspension is a showa a-kit that was lowered and set up by RG3.

I have always loved this suspension. It is very predictable and helped me build a lot of confidence.

Recently however the bike has begun to head shake, and there is a slight problem with the front end not holding a line like it used to.

I want to say this all started after I had the suspension serviced, a few weeks ago but I could have been before.

Anyhow my problem is headshake under acceleration. Its really bad on a bumpy section but will headshake even on flat gound, even slick flat ground.

My sag and fork level are set as they always were so nothing has changed there.

One thing I found is that the shock may have originally been charged to 150 psi and now it is only at 130psi. But I do not know for sure that it was at 150.

I guess what I really need help with is what different things can cause headshake.

I have been told to lower the forks in the clamps but to me that seems like a patch fix. The bike handled great before with the forks at their current level.

If anyone has some insight I really appreciate it.


Posted by: ws6transam---------------------

Besides pressure, sag, and fork level, what else have you checked? Head stem bearings okay? Both forks adjusted to the same compression & rebound settings? Have the forks twisted in the mounts due to a crash? Are your swingarm bearings okay? Rear tire still tracking straight ? Head shake can be caused by problems in the rear as well, from stiff swingarm/ linkage bearings, bent axle, to misaligned chain adjusters.


Posted by: whenfoxforks-ruled---------------------

Head-shake bad! It seems to me, by lowering the back, you have increased the front rake. So that should lessen the chance of head-shake. There is something else going on? Have you contacted RG3, that would have been the first call? Vintage Bob


Posted by: cantrell24x---------------------

Thanks guys, I did call Rg3. They were like you guys. There are just so many things it could be. I am going to tear down the bike to the frame and check and rebuild everything.

Thanks for the suggestions.


Posted by: whenfoxforks-ruled---------------------

I have never abused RG3, but a lot of others. With your issues, and their answer, I would find another tuner. I know if I asked Jeremy about this company, he would get this half smile going. He has NEVER let me down, and 1 of the greatest guys in any aspect to deal with, MX Tech. Vintage Bob


Posted by: cantrell24x---------------------

I must have misspoke. RG3 has been extremely helpful. But just like the answers I have gotten from others, there are countless things that could cause my headshake problem. RG3 even took my shock in and checked it out and serviced it for free. Well except for shipping and I paid them for the oil. I have also been helped by mx-tech and was very happy with them as well.

As for tearing the bike down it just seems like the easiest way to cover everything quickly.


Posted by: Matt90GT---------------------

headshake is 90% of the time from the rear sag setting. I would discuss your sag setting with RG3 in depth now that things are lowered. More than likely you will not be able to use the standard 100-110mm of sag. You may need to just experiment with say 80mm and then try 120mm see if either amount helps/hurts things. .


Posted by: whenfoxforks-ruled---------------------

Stock front rake, then add the lowering link, that has to be the same as putting 27 mm clamps on? How sharp did it corner? When you say head shake, the bars are bouncing off the stops? I would carefully look over the front end, have the forks serviced. If the top and bottom clamp is not torqued properly, it sure could make for some shake. Wheel bearings, spokes, or loose axle. Bent rotor? You do bleed the air with the front wheel off the ground? Vintage Bob


Posted by: truespode---------------------

I would check for binding in the triple clamps. Since you just got it back from RG it is possible that you did not install the forks correctly in the triple clamps.

I would also check your clicker settings. Although most suspension shops try to get the clicker settings back to your original settings when they do the work sometimes they may be off or not get it correct.

I would also double check the SAG and clicker settings on the rear suspenders too.

Ivan


Posted by: cantrell24x---------------------

Thanks for the replys guys. Fox, I imagine the rake is stock but the forks and shock were both lowered internally, I did not use what is commonly refered to as a lowering link. Is that what you meant?

THe problem just about has to be something I did either riding or reinstalling the suspension.

Because when I first got the stuff on the bike it was very very good. And it was working right for over six months.

I did learn that originally my shock was charged to 150psi. When it was serviced it was only charged to 130psi. Is that 20psi change enough to cause medium to severe headshake?

I have had four instances of headshake in three rides since putting the suspension back on the bike. All are under acceleration, one was in some chatter type bumps and very uickly became tank slapping headshake. The other three where on smooth ground and not nearly as bad but still noticable by spectators, and enough to put a big dent in my confidence.

Other than the nitrogen psi I havent found anything that is different about the set up.

I do have a Ti front axle, it does not appear to be bent though. I am headed out to the shop now to completely tear down the bike. Thanks for all the help and I will let you guys know if I find anything.


Posted by: whenfoxforks-ruled---------------------

Most full service shops can gas the shock back up. I can't see an under loaded rear doing this to the front end, but? Packing in the rear? Vintage Bob


Posted by: RM_guy---------------------

Like Matt90GT said, recheck your sag. I'm not sure about the affect of the pressure on sag but I imagine that setting the sag would overcome it--I may be wrong on that so ask RG3.


Posted by: helio lucas---------------------

Quote:
Originally Posted by cantrell24x
THe problem just about has to be something I did either riding or reinstalling the suspension.

one was in some chatter type bumps and very uickly became tank slapping headshake.

this kind of head shake can ocur if the circunstancies are right, with any kind of suspension.

i do believe that severe head shaking on straight line, smooth ground has nothing to do with bike geometry (sag and heigths). maybe some woble or very light shaking but not severe!

in fact your point on it just started after the rebuild and it not hold the line just make me more towards some binding of forks (some axles need the forks to be compressed several times before tightning), steering head not properly torqued, bad front tire or very low pressure, suspension not smooth on stroke or both forks are not equal.

while a major rebuild of the frame is great you have to be sure what is the problem. it woth nothing to put a set of bearings on the steering and have a fork that is not working properly.

hope this helps and good luck


Posted by: cantrell24x---------------------

Thanks. All of my bearings were in great condition. Since I have the bike torn down I will regrease them but that was not the problem.

Is it normal for the forks to be very sticky when at a stand still with the rear brake applied?

After my last race I was sitting there talking with some friends when I realized that with the front brake applied the fork are very smooth, but with the rear brake applied they barely even work.


Posted by: helio lucas---------------------

Quote:
Originally Posted by cantrell24x
Is it normal for the forks to be very sticky when at a stand still with the rear brake applied?

yes. to compress the forks in that way all bike have to turn along the rear wheel spindle. since the brake is applied neither the bike will turn or the rear wheel will move.
hope this make sense.


Posted by: whenfoxforks-ruled---------------------

Your wheelbase changes as the suspension is moved, but not when the tire is locked. Settle in on the bike, no brakes, does it feel equal? Vintage Bob



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