Front-end chatter

[gursesl]

Good Day NESBA Riders,

I have severe front-end chatter issues and I am puzzled to the degree that I've lost confidence in the ability of my bike to handle various straights and turns. I hope someone with experience can help.

The issue started three weeks ago in Road Atlanta. My 2011 GXSR750, under heavy acceleration on the back straight would have front-end vibrations - up and down. I could not honestly open up the throttle past 140 mph. The first suspicion was out of blanace wheel or a lost weight. Took the tire off and one of the guys put it on a balancing machine. It appeared balanced.

The follwing weekend at VIR N and the issue reappeared - on the front straight. Some visual inspection later the rotors and the wheel appeared straight. The bike has never been in an accident. So, we started suspecting the front tire - Diablo Supercorsa SP.

I changed the front to a Dunlop Ntec race takeoff, in a pretty decent shape.

This past weekend at NCBIKE, the bike had the same front-end issues on the straight, but now it also started chattering more in turns 2 and 11.

As it stands, I have no idea what's causing the chatter. The front and the rear suspension is bone stock, just the preload set to my weight. I am not confident riding the bike in the current setup.

Any insights will be highly appreciated.

 

[steve gould]

Chatter is typically felt mid turn. The front wheel is momentarily losing then gaining traction. Is your chatter just on the straight when you hit a specific speed.

Edit just re read your post. Your problem could be from a rear tire that grows at speed and throws your geometry outta whack. Is the rear shock shimmed up? I would put a matched set of tires on NTec rear and front. If the chatter is occurring in turns as well you may want to check to see if your steering head bearings are loose.

If none of that works, I would have someone check out your suspension and then set it up properly.

 

[gursesl]

Hey Steve,

Thanks for chiming in.

Yes, midturn chatter turns T2 and T11. Separately, it happens on the straight, when going over 100 mph.

 

[steve gould]

Check out my edit

 

[betarace]

Check steering head bearings

 

[steve gould]

Is it head shake when you are going on the straight? Chatter doesn't happen going in a straight line. My first action would be changing out that rear tire. That would isolate the problem to a setup/equipment problem and eliminate tires

 

[daddyfriedrich]

make sure your forks are set exactly the same height above/at/below your triple tree. I chased the same problem for two months before checking this.

 

[gursesl]

Yes, changed tires, issue persisted

steve gould;281954 wrote: Is it head shake when you are going on the straight? Chatter doesn't happen going in a straight line. My first action would be changing out that rear tire. That would isolate the problem to a setup/equipment problem and eliminate tires

Hi Steve,

Thanks again for chiming in.

Yes, I changed the tires and the bike now has a set of Dunlop NTec front and rear, both AMA race takeoffs, in a fairly decent shape.

I am not sure if this is related, but the rear tire shows severe signs of shredding after only a day at NCBIKE. I was told my riding style may have something to do with the tire wear. I get on the brakes too much, slow down way too much in the turn, and then to compensate I tend to overdo the corner exit throttle.

We tried the following to combat the rear tire shredding, but none helped:

• Adjust the rebound compression on the rear (it was maxed out, so no more turns)
• Confirm the correct tire pressure - 22 PSI hot off the trackAdjust my riding style to have less braking into the corner, carry more corner speed, therefore reduce the need to get real hard on the throttle at corner exit

To the degree that I tried to change my riding style, I did not see any smoothing out of the rear tire. The front was in a perfect shape and remained pristine at the end of the weekend. The rear is gone.

The GSXR750 has stock front and rear suspension and with the preload and the rebound/compression settings set for my weight.

One more piece of possibly irrelevant information. In the two days at NCBIKE I swapped the GSXR-750 with my street ride, a 2012 Ducati Panigale. I rode the bikes back to back to see the difference. Apart from the obvious power advantage of the Duc, one thing in particular was the tack sharp stability in all of the corners. I mean absolutely no chatter - on the front straight or in any of the turns. Total line control and obedience. I rode in Race mode with TC dialed in 2 and with the standard race DES settings, except that we dialed in the rear rebound to 15. All corners were smooth and easy; even T6 was a breeze compared to the GSXR. T2 and T11 - no front-end issues. Interestingly, despite the rebound compression change (adjusted through the computer on the dashboard) the Ducati rear tire wear pattern was strikingly similar to the GSXR. In summary, in a single day the rear tire was shredded.

Again, this may all be totally irrelevant to the GSXR front end chatter issues.

 

[gursesl]

Fork heights

daddyfriedrich;281991 wrote: make sure your forks are set exactly the same height above/at/below your triple tree. I chased the same problem for two months before checking this.

I'll do just that. Now, this is an interesting issue. See, I've done about 20 TDs just this season and I always strap the bike with a canyon dancer onto a trailer or to the bed of my truck. It has traveled about 5K miles in that position. Is it possible that all that shaking could have changed the fork geometry? One higher that the other?

Either way, I will check it out and let you know. Thank you for the tip.

 

[shad0ws]

Don't quote me on this but if I remember correctly I think NCBIKE's surface is extremely hard on tires. Like I said, I'm not really sure but I feel like I read that somewhere on the R6 forum.

Sent from my HTCONE using Tapatalk 2

 

[gursesl]

shad0ws;282002 wrote: Don't quote me on this but if I remember correctly I think NCBIKE's surface is extremely hard on tires. Like I said, I'm not really sure but I feel like I read that somewhere on the R6 forum.

Correct. For the record, this is the second TD where the rear tire was shot after only a day of riding. Does not happen on any other track - VIR S/N, NYST, RA, Barber, Summit, etc. Only at NCBIKE.

That's what makes it a challenge to understand what's causing the shredding. No idea if other riders had similar issues, however tow fast A guys were OK and the two I guys that got the bump to A were also OK. So, it must be me - tires, suspension setup, any other issue such as forks or bearings, or my riding style. Really, no idea. Clueless as a mushroom. :notsure:

 

[Lonewrench]

Is your suspension stock? Where is the geometry at? ect......

 

[gursesl]

Chuck

Lonewrench;282316 wrote: Is your suspension stock? Where is the geometry at? ect......

Hey Chuck,

Yes, both forks and rear shock are stock. The fork geometry is also stock -- i.e no bar raisers and the forks are at the edge of the upper triple clamp. Maybe I take a picture and post it when I get home.

 

[daddyfriedrich]

gursesl;282003 wrote: Correct. For the record, this is the second TD where the rear tire was shot after only a day of riding. Does not happen on any other track - VIR S/N, NYST, RA, Barber, Summit, etc. Only at NCBIKE.

That's what makes it a challenge to understand what's causing the shredding. No idea if other riders had similar issues, however tow fast A guys were OK and the two I guys that got the bump to A were also OK. So, it must be me - tires, suspension setup, any other issue such as forks or bearings, or my riding style. Really, no idea. Clueless as a mushroom. :notsure:

My pit-partner, Yohoho in B-group, rear rim looked like it had been dipped in ice cream sprinkles. Dunlop Qualifier sprinkles I should say. I've never seen anything like it. I was on a CBR1000rr, Mich PP-2CT's. Rear at 28psi and front at 29. Here's what they looked like after 8000 street miles, one track day at Summit Jefferson and two days at NCCAR: ( no sprinkles either )

 

[gursesl]

Those wear patterns don't look bad, at least to my unediucated eye.

Here are the tire wear patterns from the Ducati 1199. The Gixxer750 has similar lines. Both tires are shot and I am confident the tire pressures were more or less within the acceptable range (+/- 1PSI).

Any ideas? Cold tear? Hot tear?

 

[Henry]

I think its more suspension setup/failure issue. Id check your front end valves and or seals. you may have damaged them compressing the front end when strapping it down. thats why i like those front wheel lock in set ups where your strap goes over the front tire only. I could see front end chatter happening on exceleration into a straight because it unloads, transferring weight to the rear, and with no damping or unbalanced damping in your front end, it will cause a rapid rebound, cause your front springs to chatter, then causing an unstable load applied to the rear because your bouncing weight up and down, unsmoothly back to front from the chatter shredding rubber.

 

[physicistkev]

This is an older post, but I will put in my 2 cents because I would like to know what the fix was, if it's solved and lend a hand if it's not. My first suspect would be over/under torqued bolts/nuts.

Going through the front triple tree with a torque wrench would be the first order of business
Check fork leg symmetry, as has been suggested
Verify that the steering head bearings are not notched/worn
Checking to see if the rear axle bolt is over/under torqued
Checking to see if the front axle bolt is over/under torqued
Check for a bend in front or rear axle
Checking wheel bearings for wear
Checking swingarm mount point for play and proper torque
Checking rear shock linkage/mount for play and proper torque

This should cover everything that attaches the wheels to the bike. If at this point nothing is out of the ordinary, it's time to look at geometry of the bike. Maybe something has moved or changed. After that, its time to look at the innards of the forks and shock to see if there is something wrong there.