Help please - severe "chatter" under hard braking?

[EXTremist]

Hi Guys,

I'm not a racer but figured you guys might be able to shed some light on this phenomonon I experienced this weekend.

I had the opportunity to put about 15 laps on at a local 1/2 mile oval this weekend as part of a charity ride. I rode the track mostly in 3rd gear going down to second occassionaly when I needed to slow down for traffic.

The corners were high banked but the apex's fairly flat. I ride a stock 2007 R6s.

Several times under hard braking I would get the sensation and noise that the bike was severly chattering (felt almost like the fornt end was bouncing slightly) it was scary and dangerous feeling. I would let off the brakes slightly and pull in the clutch to get it to stop. Once or twice I slightly skidded the back tire. I just don't know what to make of this...the bars felt tlike they were jumping a bit in my hands and it seemed like it would escalate until I let off and pulled in the clutch.

What was I doing wrong, or what could this have been? Any ideas?

Thanks!

[Mantis]

How hard were you downshifting? I would experience the rear wheel skidding if I downshifted without matching revs or just downshifting to the wrong gears. I don't understand completely, is it the front or rear wheel "chattering"? If you pulled in the clutch and it stopped you weren't matching revs or just downshifting into too low of a gear.

[GSXR RACER MIKE]

Sounds like rear wheel hop to me as well (like Mantis said) by the info you've provided so far.

If it truely were front end related it could be from low tire pressure or improperly set fork damping (I would guess compression damping too stiff/slow).

More info please.

[Mmelmann]

It sounds like your suspension isn't set up properly to me. I used the specs off of sportrider.com under their tech/suspension section as a base line setting. I then visited a suspension guy to fine tune my settings... and there is still room for improvement. If you really want to learn about your suspension, I'd recommend reading Twist of the Wrist or Total Control. They are both pretty dry reading material, but it helps if you actually want to learn how to setup your suspension to meet your weight and riding style requirements. A street setup will obviously be different than a race setup.

[EXTremist]

Thanks Guys!

It sounds as if the rear tire skipping is the most probable explanation, althought I seemed to feel it heavily in the front as an effect.

Thanks for the suspension setup site tip/s etc. It's a bit overwhelming with all the adjustments. this track I was on had tons of rubbber laid down from car racing on it and was extermely grippy so I could see how engine braking combined with a heavier foot would make the rear tire skip rather than just slide.

Do most of you machine/alter your rear brake to make it less effective/prone to locking the rear up? I always adjust my rear lever downward on a new bike so it takes more effort to apply the rear.

Any other tips are greatly appreciated.

Thanks again!

[Mantis]

Originally Posted by EXTremist

Thanks Guys!

It sounds as if the rear tire skipping is the most probable explanation, althought I seemed to feel it heavily in the front as an effect.

Thanks for the suspension setup site tip/s etc. It's a bit overwhelming with all the adjustments. this track I was on had tons of rubbber laid down from car racing on it and was extermely grippy so I could see how engine braking combined with a heavier foot would make the rear tire skip rather than just slide.

Do most of you machine/alter your rear brake to make it less effective/prone to locking the rear up? I always adjust my rear lever downward on a new bike so it takes more effort to apply the rear.

Any other tips are greatly appreciated.

Thanks again!

I'll put some air in the rear brake, but to be honest I never touch it when racing unless I'm in off the track in the grass

[GSXR RACER MIKE]

I've never used my rear brake while racing. Something I was told is always keep the original rubber brake line on the rear brake because it will allow some expansion of the brake line, that way the rear brake isn't quite as sensitive as a braided line would make it.

[Luvin' Twins]

Originally Posted by Mmelmann

It sounds like your suspension isn't set up properly to me. I used the specs off of sportrider.com under their tech/suspension section as a base line setting. I then visited a suspension guy to fine tune my settings... and there is still room for improvement.

I took mine to the same specs as a baseline. This winter may be a time to go through the suspension and get it set up for me. When I got the bike initially, I felt the same kind of hop or almost chatter, it did feel like the front end to me and tire pressure was fine.

[Mmelmann]

Originally Posted by GSXR RACER MIKE

I've never used my rear brake while racing. Something I was told is always keep the original rubber brake line on the rear brake because it will allow some expansion of the brake line, that way the rear brake isn't quite as sensitive as a braided line would make it.

Agreed. I don't use the rear brake at all... even on the street. Unless you are riding a 2-stroke, the engine will provide most, if not all, of the rear braking forces that you'll need. My rear will dance around a little bit and basically act like a rudder to keep the bike in line when I'm braking at full force when entering a corner... so applying the rear brake isn't going to do anything but make matters worse off... possibly giving you a high side. Try to focus on solely using your front brake and keeping your body weight as low and aft as possible... focusing on sqeezing the tank with your legs and keeping as much weight off the handlebar as you can. Thighmaster workouts for the win. j/k