Rear tire chatter
- Thread starter ahastings
- Start date
On my second race weekend (6th race) on the bike, I was experiencing rear end chatter. Actually, it was more like rear-end coming up off the ground mid corner. It was a particularly bumpy section of the track. I had raced 5 times prior to this and done several full practice days. But this was the fastest I'd pushed the bike around my monster of a track (Loudon) so I was expericing new symptoms.
I am on a JRi shock 700# spring. We added pre-load and compression. We did this 3 times that weekend and each time seemed to relieve the symptom, some. I am going to stiffen up the rear spring (going to a 750#).
Probably a longer story than you anticipated but you must know *why* you're having rear-end chatter.
green_bread
Member
I get chatter on my rear end when I am pushing it as hard as I can. Talking to my suspension guy, we have come to two conclusions:- Not enough weight on the rear- Rear end raised too highFor me, I actually describe the symptom as feeling like "over steer", because I can get it to happen even when Im not on the gas. The rear end feels like it is both sliding and hopping and the rear end is trying to come around. This is why we believe we need to get more weight on the rear. I was told to add 2 turns of preload to the front so it doesnt compress as much and the weight would transfer better to the rear. I tried it out this weekend and it seemed to help, though I was having trouble getting the bike to finish corners after getting on the gas. I backed out a half turn of preload and that seemed to be the sweet spot for the track I was at.For reference, I am also running a JRi shock but I think its a 600# spring.FWIW, the bike seems to favor the front end, even from the factory. I raised the front and rear, both, trying to get the geometry in order but raised the rear more, so Im guessing thats where my issues with not getting enough weight back there come from.DISCLAIMER: I am not a suspension expert. I am just repeating what Ive been told about my own bike/issues.
I described the same symptom to my suspension guy!
Our solution was go to 6 on compression and add 3 mm of pre load on the rear. We added one full turn of preload up front and add one turn compression. My set up is JRi with 700# spring in the rear and GP suspension with 0.9 springs. We're going to be trying out a 750 spring this weekend.
Did you mess with the rear suspension settings? Compression and preload helped a ton.
Good to know it wasn't just me. I just picked up the bike last Saturday and raced it a week later. Haven't had much time to set it up. I did get andreani cartridge kit in and threw on an extra Race Tech billet shock from my gsxr 600. It was a little long. I've read Ari Hennings articles in Motorcyclist about setting up and racing one. He said he had to switch to race compound rubber to get rid of chatter. But my tire sponsor doesn't make a full race compound for the little bikes.
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What tire do you run? I run Pirelli DOTs (supercorsa v2) which is their race version and not a fully committed street tire. I cannot run any sort of slick in my classes. Wasn't Ari running strictly street tires? I don't know to what degree tire choice has to do with chatter. I would think chatter would be universal across compounds. But maybe some are softer than others and can take the bumps better.
green_bread
Member
I havent messed with preload on the rear. I initially set the sag for my weight and have only made adjustments to compression/rebound damping depending on whether Im getting rubber raising on the front or rear of the sipes of the tires. Changing the preload on the front seemed to help quite a bit, so I havent messed with it a whole lot since then. We will be going back to the track where I was having issues with geometry tearing on the rear and where I slid out/saved it, but then crashed in the wet grass, next month. Im hoping that the adjustments Ive made will help. Here is what my rear tire looked like after 3 practice sessions and 1 race at that track back in April:
a) re bold statement: I'd be weary on making adjustments based on what the sipes are telling you. It could just be how the rubber is folding down on itself as you get on the gas
b) that's a nasty looking tire... I have not had that issue on the RC. But a buddy did on the R3. Both running Pirelli SuperCorsa V2 SC1 rears. I know the compound on the SC1 this year is not 100% the same compound as last year. So I don't know if that has anything to do with it. But it's awesome how you're getting on the gas early if it's tearing that far.
green_bread
Member
What's weird is that my font tore like that last year. They repaved the track, now my rear is doing it. I went to another track after only making front preload adjustments, and the rear tire cleaned up fine. I had to change it out after one day because so much had eaten away, but the new tire is wearing beautifully.
As far as the comp/rebound, you're right. I only make one click changes, but even that can be pretty big. Luckily, I haven't had to touch it since last year and the bike is feeling pretty decent now that in getting the weight transfer figured out.
As far as the comp/rebound, you're right. I only make one click changes, but even that can be pretty big. Luckily, I haven't had to touch it since last year and the bike is feeling pretty decent now that in getting the weight transfer figured out.
Sorry for my poor English,I'm not expert on the suspension set up yet,I'm not sure is it your setting causing chatter but your tyre wear looks like under inflated, pirelli SC have the soft and low side wall, suppose run with high pressure, maybe is your rear shock too hard and kick it out of the traction during u apply the throttle too aggressive in low lean angle,my friend is amature racer and he use scv2 on his rc before,as he said this tyre is soft enough to suck out all the pummpy road surface and give u massive grip even u apply the the throttle aggressive when mid turn...I'm not sure for it but u can have a try,soften your rear shock and add the rebound make it bounce back more fast,add a few psi pressure,let the rear shock working smooth but not giving more pressure on the Tyre,I'm still using the stock bouncing rear shock with bridgestone s21,body weight 55kg,rear shock preload 7 clicks, front fork still stock but change the fork oil to motul 15w medium hard,400 ml on each fork, maximum travel near bottom it,so far the rear and chatter only happen during hard brakingand will a little wobble during exit Conner,looks like too hard my front setting,any idea for solve my problem too pls...thanks
Fangfactor
Member
Hi Martin85
How do you find the standard fork action with 15W motul?
Did you drop to 400ml in each leg due to maximum travel not being achieved due to not enough air space in the forks? as this is what Ive found.
I can't remember how many times I disassemble the fork,drain and refill the fork oil, measure the air gap to get the correct measurements after I test it,my opinion 400ml is the perfect capacity to reach the maximum travel(43mm),u can found the rubber sitting on the spring rod is perfectly landing on that rod bleeding hole and block the oil(sorry I'm not sure what's that rubber name and the part I just mention it is it correct ) 400ml the travel around 45mm,left 2mm to the bottom...390ml already bottom and tapping it,and I just realized the oil capacity only control the dive amount of travel,more oil will less travel and become stiff because already change the geometry angle,if follow the user manual 460ml...Damn it the dive amount just 50% of full travel even u already hard breaking like Marquez for the oil thickness on the rc OEM stock is control the rebound speed,how fast it bouncing,I already try ELF 5W,MOTUL 10W & currently trying the MOTUL 15w medium hard,result like just now I mentioned,capacity control dive amount, thickness control rebound,now I'm headache because 15w looks like a little bit stiff for me,and difficult to match the OEM rear shock to get the chassis balance sweet spot...Too light on front causing wobble during exit conner and high speed pumpy road,I just drop the front fork around 2mm(the 1st line on top of fork) try to change the geometry...And now it's looks more stable then before with my rear shock preloaded 7 clicks on my 55kg body weight,but I think still can more good,i‚ll try again and again until I found that sweet spot,if not I can't say this is MY BIKE,
Fangfactor
Member
Thank you Martin85 for the feedback.
I also found the recommended 460ml of oil in each leg only allowed approx 100mm of travel. (65mm bottom tube unused)
I have been reducing the oil amount as well and now have 115mm
With 400ml what is the oil level from the top of the fork?
Im 100kg and running 10w and would like to try 15w.
I have an stiffer rear shock setup so 15w may be ok for my weight and setup.
