Head Shake???



10 replies to this topic
  • Tom_N

Posted March 27, 2001 - 10:35 PM

#1

I have a 98 yz400 I purchased used in September of last year. I am new to dirt biking and I am having a great time riging and learning to ride an off road machine..

I am finding that when blasting down rough straightaways that I get a tremendous amount of head shake. It's to the point the wheel will shakes left and right and out of control. Many times the wheel has almost turned a full 90 degrees sideways and bucked me off.. Scarry deal for sure.

I have the front forks set equally at about 6 clicks out from full on hard and I try to run around 12lbs psi in the front tire. \
I don't know if this right or not nor do I know how much impact this has on the head shake.. Is there anything I can do in the set up and or in my riding position to make this more managable or is a stabilizer my only option? Oh yea I am 6' 175lbs

Thanks!

  • enmerdeur

Posted March 27, 2001 - 10:51 PM

#2

I run 14 psi in my tires. This would not have much effect on headshake but is pretty much the standard everyone uses.

What do yo have your rear sag set at?

------------------
SHN
99'YZ-400 (Mine)
00' TT-R 125 (Hers)
91' KX-125 (Son #1)
93' CR-80 (Son #2)
99' PW-80 (Son #3)

  • DaveJ

Posted March 27, 2001 - 11:56 AM

#3

Tom,

Headshake is an oscillation that takes place starting at the tire. For all motorcycles, it's normal. However, the problem you are having is that the oscillation is not canceling out which leads to further and further dramatic headshake.

In other words, it's okay to get a bit of headshake every now and then, but it should rapidly diminish with little input from the rider.

Since you purchased the bike used, I'll give a list of things to check.

In most cases, this is caused by in inconsistency between damping effects of the two forks. Setting the rebound and compression clicks on each fork equally is a good start, but it does not mean that they are in balance.

Before you rip your forks apart, check the oil level and take a sample from each tube. Make sure the oil is consistent between the two in level, appearance and smell. If not, it's time for some servicing.

The next thing to check is if the forks are aligned. Since the right fork can be moved about the axle, you'll need to make sure they are perfecting parallel. Remove the wheel and make an aluminum or wooden dowel that fits perfectly between the two lower fork tubes. Then reassemble, (you may have to take your covers off). Tighten the axle nut down and the left fork axle bolts. Leave the right loose.

Use a soft hammer against the lower casting of the right fork to move it about the axle, while the dowel is between the spokes of the wheel and between the tubes. Move the fork until you have perfect alignment then lock down the bolts. This is critical so take your time.

By the way, check to make sure your axle is not bent when you have it off.

You can also push up on each fork tube with the wheel off to see if they are responding the same, but sometimes the smaller inconsistency are not felt. Try different settings as well to see if they feel the same throughout the range of clicks.

The next concern would be the tire and wheel. Check the rim for bends and trueness. Examine the tire for unequal wear.

Even though a loose headset will seldom cause this problem, check it anyway. Just sit on the bike with the front brake on and try to move the forks about by rocking back and fourth.

If everything checks out, replace the tire and tube.

If this doesn't fix it, you'll need to have your forks looked at. At it may take more than just a simple service so make sure the person doing this for you understand your problem and knows what to look for.

Let me know what you find.

DaveJ

  • F-Pilot

Posted March 27, 2001 - 01:30 PM

#4

Check the sag on your rear shock,(3 3/4- 4 inches) if it's too little it loads the front end, also make sure your fork springs are not sacked, that will affect the balance too.
If these check out ok I would bet your rebound on the forks is waaay off.
Try running the rebound at about 10-12 clicks out. 6 is too slow.
Did you get a manual with the bike?
[This message has been edited by F-Pilot (edited 03-27-2001).]

[This message has been edited by F-Pilot (edited 03-27-2001).]

  • motojunkie

Posted March 27, 2001 - 10:41 PM

#5

If you're riding on rough ground, and compression (or spring rate)is too stiff, the front wheel will want to skip off everything which will amplify head shake. If you're significantly lighter (or slower) than your bikes previous owner then this might be the culprit. I agree with F-pilot improper sag can cause headshake also.

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  • Tom_N

Posted March 28, 2001 - 05:47 AM

#6

DaveJ - Thanks for all the detailed info, I had the fork seals replaced when I got the bike and I am wondering if they were properly aligned but I will check everything as you say.

FPilot - my sag is about 3 1/2 to 4 inches when I sit on it but I am not sure how fast it should rebound.. You also mentioned "if your forks are sacked" what do you mean??

I really need to learn a lot about the suspension set ups.. any advice on where I could get some good step by step how-to info on this? I need dumb guy terms.. I am mech inclined but not up on all the lingo..

Oh yea as alway's everyones posts are great, I spend ton's of time just reading all the different stuff trying to learn.

Thanks!

  • YZ400Court

Posted March 28, 2001 - 06:15 AM

#7

Tom,
Don't forget to check the steering stem bearings for wear. The bearings will dimple the races, causing the bars to want to self center. This can easilly cause violent headshake. Put the bike on a stand with the front wheel elevated. Turn the bars side to side slowly and feel for any sense of a stop in the middle. This is a suprisingly common problem with YZ4**'s. The oil sump in the frame gets very hot, thus the sttering stem bearings also get very hot, causing premature bearing failure. Re-pack the stem bearings every few rides, and there should be no problem. Also don't forget the YZ is a motocross bike. Steep rake angle = good turning ability = poor high speed stability.

Once your comfey with the bike, do yourself a favor and buy the greatest invention ever...a scotts steering stabalizer. Next to good tires, the best aftermarket part you'll ever buy. GPR is also good, and a little cheaper, but Scotts is hands down the best.

YZ400Court

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Pretend it's flat and give it the gas.

  • YZ400Court

Posted March 28, 2001 - 06:19 AM

#8

Tom,
One other thing. When I bought my 98 YZ400 used about 6 months ago it had a violent headshake in 70mph sand washes. This was with the stabalizer installed. The previos owner had just replaced the steering head bearings, and had left them loose. Once tight the bike is perfect (except for the intake velve broken two weekend ago).

Good Luck
YZ400Court

Soon to be KTM 520 EXC Court

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Pretend it's flat and give it the gas.

  • Guest_Guest_*

Posted March 28, 2001 - 07:34 PM

#9

All of these are great posts but dont just focus on the front end.
Try adjusting the rear maybe a little quicker rebound may help or soften the dampning In the rear.
but try only one at a time.
sometimes when you think that softening something will help it is actually worsening the problem and usually at a higher rate than you would think.
Your front end may be PACKING causing it to be running at a stiffer part of the susp and shorter length which throws the whole geomotry of the bike off!
Give it a try see what happens.
Let us know how it goes.
sometimes these things are like a toothache. It feels like the bottom aches but in reallity it is the top tooth wiht the cavity and the true pain!
also read the post and my reply on Gold Valves............
They are wwll worth the $$$$$$$$$$$
$.02
G4

------------------
NOPE..........BOY.......
That's not thunder ridin' your BUTT .........
It's a whole Damn HURRICANE!!!!!!

[This message has been edited by g4racing (edited 03-29-2001).]

  • vznx1w

Posted March 29, 2001 - 05:37 AM

#10

TomN,

Assuming that your components are in good shape, your head shake is being caused because the bike's suspension setup is loading the front too much. You have many ways of correcting this. I'd try the following in order:

Rear sag- Increase (Don't go over 4")
Front forks - lower until flush w/ top clamp

Alter fork dampening to hold front end HIGHER or rough ground.
Front rebound - Decrease (turn clickers out)
Front Compression - Increase

Alter shock dampening to hold rear end LOWER over rough ground.
Rebound - Increase (turn clicker in.

I guarantee that these will fix the headshake. However, all will also tend to degrade the bike's ability to turn. Good luck!

  • yzernie

Posted March 29, 2001 - 06:04 PM

#11

Tom,

Nobody has mentioned this yet so I will. I have found if I do not keep my knees close to or squeezing the tank/seat I get some headshake.

The headshake I have experienced is nothing like what you are describing but this is just another thing for you to consider.

yzernie





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