Front Chatter under braking

[riopko]

I'm a new racer and I need some help. At VIR last weekend I seemed to have  a front wheel chatter under heavy braking for turn 1 and less pronounced at turn 7. I checked the run out on the rotors when I got home. .002 an one side .0035 on the other. I think this is OK. Any ideas on chatter, compression damping? I have an FZR 400 with stock forks new springs for my weight. What are acceptable nubers for run out on rotors. Any Ideas are appreciated. ???

[mdr14]

All else being equal, Are you banging your downshifts too soon locking up the back wheel?

If you are , stop doing that.

If not, what is your current fork oil level?

[riopko]

Fork oil at 120 mm per race tech sheets. 10wt. No rear wheel lock-up on downshifts. That much I have learned(at least most of the time).

[MightyDuc Racing]

You mean when I go into the tight turns with the rear bouncing and just stuff it in because I was a little too hot that's bad?  How am I gonna pass anybody from now on? ;D

[tcchin]

I believe the service limits for lateral runout is in the .010-.012" range, so .002-.0035" lateral runout is not a problem. In fact, it's hard to straighten rotors to better than .003".

Do you have the RT Gold Valve Emulators installed, or are you just using the oil level recommendation from RT? Also, have you checked the condition of your steering head and wheel bearings? Is your front axle true?

[Gumby647]

How old are your tires? I know they can last a long time on light weight bikes. Back when I was racing a Hawk I had a bad front chatter in four at RRR and nothing I did would make it go away. After a new tire it was perfect again.

[Late_Apex_Racing]

All of the things mentioned before this post are good comments.  I had the same problem in the same turns last season.  For me the solution was to have my front end redone.  I could have gotten away with just changing springs and oil/level but my mechanic (Ed Carpenter, who had his first races this past weekend at VIR, Go Ed) installed Racetech kit.  Apparently, my riding/braking style caused me to "bottom" out my front suspension under very hard braking.  I guess I get on them a little too hard initially and that would collapse the front suspension.  I tried to stiffen up the stock front but that would ruin the "sag" and I would have problems in the corners.  Once the racetech stuff was in it really handles well.  Now if I could just save some cash to get a rear shock.

Ken A. AM#318
2000 GSXR750

[riopko]

Checked all of the things suggested here, steering bearings etc. All was in order. On the FZR board several people told me they have had the exact same problem. Emulators were the unanimous fix. Guess what I am ordering today.

Thanks for all of the suggestions. It's great to have a body of knowledge like this to draw on.
 :)

[Litespeed]

YOu may also want to check the condition of your bushings.  They get heavily loaded and bind up under braking...

[james-redsv]

Front brake chatter like you described is from the forks being bottomed out under braking and causing the tire to slightly start sliding. You need stiffer springs to go along with the emulators. ;D

[Bernie]

Did you try the zip-tie on the fork slider trick to see how much fork travel you are using?

[tcchin]

Front end chatter can have many sources, not just fork bottoming. In fact, stiffening the springs can make chatter worse in some cases.

[unforgivenracing]

Other then the noise, does it handle OK?  ???
 IF it ain't broke don't fix it.  ;)
 Its probubly a combo of things,  Shoes make alot of noise on warped roters, bottomed out forks make for a louse ride any way you look at it.  but if it dont feel like its floating in the corners, it might not be the springs, how about a stearing damper?   :P

[MightyDuc Racing]

I had some pretty heavy chatter into T1 at Jennings, even after getting off the brakes.  Tightened the steering head a bit, added a 1/4 turn of compression, and raised the ride height before it went away.  Thanks to Kent at GMD Computrack.  I recommend getting a pro to help you out.  Knocked another 1.5 secs off my times cuz I was more comfortable into the fast turns.  Just my .02.

[am_#65_john_deere]

may sound silly,but this happened to me too :o

is your front wheel balanced correctly, cause if its not, you will have chatter under braking. :-/

[MudDawg]

More than likely it was the front end rebounding too fast.  Either add more compression...which could make it a bit harsh.  Or thicker fork oil.  Don't worry....you'll see that problem again when your fork oil needs changing.  :)

[unforgivenracing]

Fork springs are too soft

[riopko]

Heres an update. Installed Emulaters, new tires, wheels balance. Much improved but still some chatter. Checked everything again, front wheel has a slight warp. I just picked up another wheel. I think this will be the fianl fix.