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.