Bad Headshake at High Speeds


37 replies to this topic
  • Diode663

Posted June 17, 2009 - 10:51 AM

#21

I tend to think the more you lean back and lighten the front wheel, the more headshake you get, so yeah I disagre.

  • husqy360

Posted June 17, 2009 - 11:50 AM

#22

i know its probebly not the problem but, id check if the rebound and the comp knobs are on the same settings.


try to loosen all the triple clamps bolts, close and see if it helpet.

this things that fixet my nasty shake.

rear shock make big diffrent.

  • FinchFan194

Posted June 17, 2009 - 01:47 PM

#23

I tend to think the more you lean back and lighten the front wheel, the more headshake you get, so yeah I disagre.



Well then what do YOU do when you get headshake?

I lean back and pin it.

Rather than nitpicking me maybe you and Gray should try to help this guy out. . .

  • Greg_T_M

Posted June 17, 2009 - 02:16 PM

#24

I had the same problem with another bike. Couldn't get over 35 mph on a hard packed surface without the bars trying to shake out of my hands. I tried adjusting the sag but turns out the springs were too light for my weight. The more I increased the pre-load on the shock the worse it got. Replaced the springs with the appropriate size, set the sag correctly and no more head shake.

  • grayracer513

Posted June 17, 2009 - 02:21 PM

#25

Rather than nitpicking me maybe you and Gray should try to help this guy out. . .

Did already. :worthy:

There are a lot of things that can feed into this problem. Head Shake, speed wobble, tank-slappers, all describe an oscillation of primarily just the front end (front wheel, fork, and handlebars) at speed, while the rest of the bike remains more or less unaffected. This instability occurs mostly at high speed and is similar to that experienced by shopping cart wheels and such.

The frequency of the oscillation is independent of bike speed, which doesn't mean speed doesn't cause it, just that speed doesn't have any influence on how fast or hard it shakes. Gyroscopic effects are also not a factor. The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damping, height of bike center of mass, distance of bike center of mass from the wheel, and cornering stiffness of the front tire.

Wobble starts when some otherwise minor irregularity, like a slight crease or change of angle of the road surface, accelerates the wheel to one side. The restoring force of the bike's natural caster effect is applied in phase with the progress of the irregularity (the farther off to the side the wheel is pulled, the stronger the force pulling it back), and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until control is lost. The oscillation frequency can be altered by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider's arms are a main component.

Wheel alignment can set this off by creating a slight pulling force at the front tire when the tire is stepped off to one side. A front wheel off center in the fork can do the same thing. Also, most dirt bikes are set up with several strikes against them if you look at the list above; they're tall, have a high CG, and usually run low air pressure, reducing the stiffness of the tires.

Loose steering bearings are another culprit. And, of course, nothing gets rid of this like a good damper.

  • MOTOPHILE

Posted June 17, 2009 - 04:22 PM

#26

Rather than nitpicking me maybe you and Gray should try to help this guy out. . .


Not to sidetrack or hijack but... you seemed very sure of your statement and I just was wondering what others thought. It's all good.

  • FinchFan194

Posted June 18, 2009 - 01:14 PM

#27

Not to sidetrack or hijack but... you seemed very sure of your statement and I just was wondering what others thought. It's all good.


Its all good I am just giving the guy my opinion is all.

A lot of people on here are right or wrong its mostly a matter of personal preference or opinion. What I don't get is the need to point it out to someone so harshly and try to "gang up" on them.

I still stand by my opinion from my experiences which were from my 125 days and I have never had headshake with a YZ450f 426 or 400.

  • TIG88

Posted June 24, 2009 - 02:11 PM

#28

Thanks for all the input guys. I have never changed fork seals, this oughta be an experience! I'll do that first, after confirming the rear wheel is straight.

I should have time to tear into it this weekend.

  • Wyatt

Posted June 25, 2009 - 11:25 AM

#29

Make sure your forks are not too high in the clamps. You may want to even lower them if you do a lot of high speed riding. Check your rear sag. If you are getting headshake, increase the rear sag. Both of these should dull your head angle and help with headshake at high speeds. If you arent concerned with carving inside ruts on a mx track, it wont mater to you.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • grayracer513

Posted June 25, 2009 - 11:49 AM

#30

Head angle can have an effect, but it wouldn't be the cause in the range in which a YZF would be running, regardless of the fork height. They are built at or near 27 degrees, while an R1 is set at 24, and is stable at speeds that are a little higher than those generated by a YZF. Some of the flat trackers I used to ride were at 22 and never shook, even at 110+ mph (unless encouraged too).

  • Wyatt

Posted June 26, 2009 - 07:31 AM

#31

R1's and flattrackers dont ride on the same type of ground as dirtbikes do. Blasting down a whooped our straight on an MX track at 50 mph is much different. I can certainly tell a difference when I get on a bike that is sitting too high in the rear. I rode a YZ125 a couple of months ago that scared me with high speed headshake. It is sorted out now.

  • grayracer513

Posted June 26, 2009 - 10:11 AM

#32

The OP was not talking about instability in the whoops:

Above say, roughly 50 mph or so, on any hard surface (road, dry field) I get a terrible headshake.


It's true that the ability to operate on extremely uneven surfaces is the reason fro the shallow head angle on MX/Off-road bikes, but that is done for suspension function, and to reduce the tendency to "tuck under" when striking an obstruction at a relatively nose down angle, not to prevent or reduce head shake.

As you saw me state, head angle is a contributing factor, but not the cause in and of itself. Any time you have a problem caused by multiple factors, you can reduce one of them and reduce the problem overall. But you're as likely to fix the problem with tire pressure as head angle. Remember that trail feeds into this equation, too.

On my own YZ450, and the '03 that preceded it, I have or had the forks pulled up at least 5mm (9mm on my current '06) and the sag set at 95~97mm, and have not had any such problem with either bike. That's particularly interesting, because the '03 was very prone to going into a killer tank slapping fit when the front wheel hit something at an angle and deflected out, but had no tendency to wobble from speed on flat ground.

Not all flat tracks are billiard table smooth, and some of the old clay mile tracks would develop pot holes at the end of the day that would rattle your teeth at 100 plus. And if you imagine that even a California freeway is a sheet of glass, try a lane change at 160. 50 is, um, different than 100 or 160.

If you look back to my response to FinchFan's contention that leaning over the front causes head shake. You'll see that I told him it would not. Yet if you look at the list of contributing factors, you'll see that having the CG farther from the rear, and a soft front tire are both on that list, and both of these would be aggravated by leaning over the front. But at the same time, it is only the "cause" if it's the last thing added to a stack of factors that existed beforehand. Likewise, your experience with head angle, in my opinion.

  • Aka.Goose

Posted June 26, 2009 - 10:32 AM

#33

Watch it be something completely simple...
Like your fork clickers aren't adjusted equally...

  • FinchFan194

Posted June 29, 2009 - 03:42 PM

#34

Watch it be something completely simple...
Like your fork clickers aren't adjusted equally...


Maybe this is a stupid question. . .Would the clickers even matter? I know I have two different springs in my bike. I was told no matter what you do to the front forks they work together as one unit.

  • grayracer513

Posted June 29, 2009 - 04:01 PM

#35

Maybe this is a stupid question. . .Would the clickers even matter? I know I have two different springs in my bike. I was told no matter what you do to the front forks they work together as one unit.

You're basically correct. One of Dave Johnson's very successful solutions for the single chamber KYB forks dealt with a basic design problem with that fork by putting all of the compression damping in one fork, and all the rebound damping in the other. Only if there were a huge amount of flex in the fork at the axle lugs would any such thing as mismatched adjusters make any difference.

  • Aka.Goose

Posted June 29, 2009 - 04:49 PM

#36

Lol...I'm filled with an incredible amount of useless wrong info...
I was kind of just kidding about the clickers, but now I'm intrigued...
If I had different springs on each side and the clickers uneven, I wouldn't suffer any performance issues? I can understand how it works as one unit, but seems like at high speed you'd notice something...

  • grayracer513

Posted June 29, 2009 - 06:30 PM

#37

Lol...I'm filled with an incredible amount of useless wrong info...

...I'm intrigued...
If I had different springs on each side and the clickers uneven, I wouldn't suffer any performance issues? I can understand how it works as one unit, but seems like at high speed you'd notice something...

There's a preponderance of misinformation afoot in the world (pick up any newspaper), I don't why you shouldn't have some of it.

The fork is supposed to let the wheel move up and down so it will float along the uneven terrain surface, and at the same time, it is supposed to hold the wheel in a very much fixed relationship to the steering axis, and in alignment with other chassis components, most importantly the rear wheel.

In the interest of meeting this second requirement, the wheel is held in place between the two fork legs as rigidly as possible to prevent any tendency for the wheel to tilt left or right at all. The two tubes act as a pair, and their individual spring and damping rates are additive, even if not entirely equal.

You should have no handling problems as a result of having different damping settings (witness Johnson's Phase 4 fork), or different springs. Some suspension builders seeking a particular spring rate that isn't available will mix springs in order to get it.

  • Aka.Goose

Posted June 29, 2009 - 09:18 PM

#38

Hahaha...Thanks Grey...It's so funny, I've even taken off the Ride adjusters a few times to make sure the clickers are even...
No mas...





Related Content

Forums
Photo

BIGBORE FOR YZ450F 2006? by Taattie94


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies
Forums
Photo

Megabomb Fitment by 288yz450


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply
Forums
Photo
Supercross

Wilson the Blue Privateer by jetfuel


Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  50 replies
Wiki
WR Camshaft Swap Info - last post by jamesm113

WR Camshaft Swap Info


Articles
  • 0 replies
Forums
Photo

Is it worth it? (Cam and High compression piston) by macgi77


Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.