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Discussion Starter #1
I've always had front chatter at Grange, but somehow it's gotten worse after the recent tire change. For those familiar with the track I attached the picture where it's happening. Previously I only experienced it at the second location. I'm on the gas in both turns, so I'd classify it as post-apex on the gas front chatter. Both time I'm at a good (peg-dragging) lean angle.There are some pavement irregularities and the forks react to them rather harshly to the point of losing the front traction (not the best feeling).

The forks are in good condition: no leaks or air buildup and pushing the front and rear while standing still feels normal: moderately fast compression and moderately slow rebound. The sag is correct. The settings are appr. in the middle. I asked one of the friends (Earl) to adjust them and he only turned it out a couple of clicks. The ride is generally plush.

The only change I did recently is replacing the rear with the new 170 SuMo-specific tire, while the front is still old UK-spec 120/70, which is not SuMo-specific and thus has a slightly different profile. It shouldn't matter at SuMo speeds IMO. The bike feels stable and confident in other corners. It doesn't run wide or resist turning and has no headshake whatsoever.

So what should I try to eliminate that chatter? Cause as I get faster it becomes really annoying.
 

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Change tire pressure. Atleast 24 psi front and rear.

If that doesnt work, try go even higher. If it still doesnt work, slow the rebound of the front fork, ride it, slow the rebound of the rear.

You guys generally use a lot softer set up and faster rebound than we do in europe. On a asphalt only track theres HUUUGE time-savings to make with stiffening up and using higher tire pressures.

*Edit, when I get my suspension back from refreshing I can bounce it for you to show what difference it is to your bikes
 

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Bite the bullet and get your suspension done. That helped a lot for me. Also, do you got a slipper clutch? That'll keep the year from bouncing.
 

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Try increasing pre-load on your shock.
 

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Discussion Starter #6
So most of you are saying it bottomed out? I'll post the front sag after I get home, as I've never measured it but from the KTM's manual the preload is not adjustable and they're only concerned about the rear sag. I have appr. 90mm rear sag (in spec) if it helps to diagnose it.

I was running 20-21psi cause the track was relatively cold (it's still warm here in SoCal in the winter). I don't understand how a harder tire would heal the harsh suspension. If anything it'd make everything bouncier IMO

The chatter was front, not rear. I wasn't braking or backing it in, so it has nothing to do with the rear hop.
 

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So most of you are saying it bottomed out? I'll post the front sag after I get home, as I've never measured it but from the KTM's manual the preload is not adjustable and they're only concerned about the rear sag. I have appr. 90mm rear sag (in spec) if it helps to diagnose it.

I was running 20-21psi cause the track was relatively cold (it's still warm here in SoCal in the winter). I don't understand how a harder tire would heal the harsh suspension. If anything it'd make everything bouncier IMO

The chatter was front, not rear. I wasn't braking or backing it in, so it has nothing to do with the rear hop.
Trust me, I practice in north sweden (we have 2 feet snow right now), you need atleast 24 psi. The only thing that might make it possible to use anything less is if theres a lot of dirt to cool the tire down.

I have never ridden the track but from what I can understand its on the spots where the tire is the hottest you get the problem, after these corners you've got the long straight. Chatter is most often a tire problem. I would bet on that its overheating for you.

The front preload can be adjusted with spacers but it is not important or atleast not as important as the rear end. Dont mind the rider sag, check the static sag (which is the difference between the suspension unloaded and the suspension loaded by the weight of the bike only). I think you should use 10mm static sag but I dont think this is the source of your problem.

We're not saying that its bottoming out, its just that when you have a soft suspension on asphalt it will move very much from brakes to throttle, this shifts the weight balance too much and the front gets either too light or too heavy. Still, if you get it going out of the corner with the throttle on its not that its too heavy or too light, its either overheated or the rebound isnt working properly. It could be that the rebound is too slow for the compression you are using.
 

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Anyways, put 24-25psi in the tires before you go next time, and if it doesnt work just release some air. I'm pretty sure it will atleast help the problem
 

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you have NO idea how much the rear has to contribute front end chatter
I got rid of most of my chatter on Crappy arse old tires
by adjusting the rear



and yeah go higher
i run 26 psi on mine COLD
 

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Discussion Starter #11
What kind of lap times are you running. It would help to know your skill level.
I'm slow: 1:08 now, after I broke my ankle 5 months ago (excuses, excuses). Before the injury I was running 1:05s. That is w/o the dirt section. The chatter is happening at anything below 1:09. Thanks, guys. Looks like a lot of things to try next time, hopefully under the expert's supervision as always (that's you, Rudy :)).
 

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I'd try the tire pressure first see If that helps.

The bike was ok prior to the 170 rear ?
The rear being 90 mm of sag...Is that total race sag ?
Do your forks have a pre load adjustment ? or compression and re bound only. ?


If the tire pressure doesn't work then I'd crank up the sag by 25 mm as mentioned previously

I'd up the compression and slow down the re bound some more

do one thing at a time to find out what is going to solve the problem....If nothing really improves, is it possible the front tire is gone to crap....?

Does it also do this on other tracks ??
 

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I had put new tires on PP2cts and had the same feeling ? , tires broke in it went away.Don't know if it was balance of the tire or just the tire seating. That feeling makes you think WTF. I thought it was alignment also, because the back tire was moved.
 

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I'm slow: 1:08 now, after I broke my ankle 5 months ago (excuses, excuses). Before the injury I was running 1:05s. That is w/o the dirt section. The chatter is happening at anything below 1:09. Thanks, guys. Looks like a lot of things to try next time, hopefully under the expert's supervision as always (that's you, Rudy :)).
Hayden has the record of 57 or 59 on that track right?

Who has done your suspension?

How much throttle are you using when it occurs?
 

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This is going to sound a bit nuts, but check out the article in Supermoto Racer magazine about the front wheel clamps. Sounds silly, but it helped cure the little bit of front chatter that I had. That and running 24-26 cold PSI.
 

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thats what I was referring about with the forks binding......dunno If that will help, buts its a free experiment
 

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Trust me, I practice in north sweden (we have 2 feet snow right now), you need atleast 24 psi. The only thing that might make it possible to use anything less is if theres a lot of dirt to cool the tire down.
I've been combing the board looking for solutions to chatter issues (mine is the typical mid-corner off-throttle one), and this caught my eye.

When road racing, I was told to run whatever pressure is the one that keeps the tire at operating temperature (180F). Which usually was somewhat lower than what most people used because I'm light. Is supermoto any different?
 
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