nos rob
Novice Class
Posts: 63
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posted July 08, 2004 11:11 PM
Can the chain jump a tooth on a cog?
Hi guys,
On very hard launches at the strip I hear and feel a terrible clunking type sound, there is a mechanical type surge that makes me wonder if the chain is jumping a tooth, most likely on the rear sprocket.
The bike is on the lowest hole on links and stretched about 6" with extensions. The stock rear shock is maxed out on pre-load, compression and rebound.
Is it possible that when I take off the back wheel comes up far enough so that the chain loosens off (due to geometry) allowing a lot of chain sag at the bottom, the rear cog doesn't pick up the chain cleanly thus allowing it to jump a tooth?
Something is happening and i'm buggered if i know what it is, this has occured at least a dozen times, even the guys i'm racing against can hear something strange.
Any thoughts?
NOS Rob
  
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entropy
Moderator
Posts: 8671
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posted July 09, 2004 12:52 AM
very likely yr shift fork(s) are worn and the trans is jumping out/in of second gear. New forks, and possibly the 2-6 gearset & shift drum need replacing. This happened to me and others.
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swft
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Full throttle!
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posted July 09, 2004 12:52 AM
Check the teeth on your front sprocket. They should show signs of slippage.
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Hells Dark Lord
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posted July 09, 2004 03:14 AM
I have never had a streeet bike with the chain adjusted properly jump a tooth, I have had numerous dirt bikes do it when the chain wore out...had one jump and split the crankcase and fuked up the tranny....on and old Montesa dirtbike I had.....
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entropy
Moderator
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posted July 09, 2004 03:53 AM
I'm w/HDL & swft; if the chain jumped a tooth you'd see damage on the teeth.
BUT, in any event, the symptoms are classic shift fork wear; poppin' out/in gear under hard accel. Prolly 20 different people have posted this sort of thing over the last 3 years (including me) 
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MadMike
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posted July 09, 2004 09:04 AM
Ditto! either or loose chain will click, tight chain and still clicking is your tranny!
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RAC4IT
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Bergie
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posted July 09, 2004 02:18 PM
A loose chain doesn't have to skip a tooth to make an incredible racket when launching at the track, just being loose can cause it to smack around enough to sound like it's skipping.
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nos rob
Novice Class
Posts: 63
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posted July 09, 2004 03:04 PM
Thanks everyone, I have a closer inspection.
Still curious if the shain slackens off when the swingarm comes up?
Rob
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wannabe
Pro
Posts: 1931
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posted July 09, 2004 03:38 PM
If you're just thinking about the geometry of the swingarm, the chain actually gets tighter when the rear drops. That's the whole purpose of the slack called out in the books. If your swingarm didn't move (i.e. you had a strut in place of your shock), you could run your chain a little tighter.
So, what I'm saying is that if you set up your rear shock like most guys do, you will be dropping the rear end on the launch, which will tighten up the chain and not loosen it.
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nos rob
Novice Class
Posts: 63
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posted July 09, 2004 04:56 PM
Thanks wannabe, that answers my question and I now find it very hard to believe there is any chance of the chain jumping a tooth on either sprocket.
The chain and sprockets are all in top condition.
The problem only arrises in 1st gear about the time I hit WOT at about the 60 foot mark.
Looks like I need to have a look at the shift forks.
Rob
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BA
Pro
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posted July 09, 2004 05:23 PM
Also look at the bottem of your rear shock and dog-bones to be sure that they are not hitting the track.
You might also want to look ONLY at the tach the next time you make the problem happen.
A friends bike did something like you describe, only under launches and eventually they found a wire for the MPS box that was shorting out when the rear of the bike squatted a lot. It caused a very "on/off/on/off/on" kind of a launch. Not unlike a chain skipping over sprocket teeth.
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Hells Dark Lord
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posted July 10, 2004 04:43 AM
I think on the 12 if you let the chain get bad enough, or the sprockets bad enough to skip a tooth, that you will have much bigger problems than just the hesitation and clunking noise of the skip.........I am seeing broken engine cases, broken trannys, adn a whole assortment of stuf that would go wrong as well.....not to mention you souuld see worn teeth on the sprockets.........
These guys are right I think, you need to start looking at other things....the shift forks I think would be my first guess....then I think you should take a look at possible hard parts catching on your launch......it doesnt sound electrical to me, but i am not a mechanic.....
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nos rob
Novice Class
Posts: 63
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posted July 10, 2004 05:23 AM
I think I have solved the mystery, BA gave me the clue.
I have a Holeshot PowerShifter installed and I think when launching very hard that the plunger in the solenoid is being pulled towards the back of the bike causing the micro switch to trip thus causing the delay box to activate.
The Powershifter is installed very parallel to the ground, meaning not upright, so it is relatively easy for the plunger to slide towards the rear of the bike on hard launches.
I hope to confirm this within the next few days.
I appreciate the feedback I have received on this issue, I think if anything was "jumping" I would have blown the shit out of things by now.
NOS Rob
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Hells Dark Lord
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posted July 10, 2004 06:21 AM
Good luck with it Rob, lets hope it something as easy as relocating the box.....
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entropy
Moderator
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posted July 10, 2004 06:56 AM
nosrob
I have a PS3 installed parallel to the ground and have not had that problem. You could move the microswitch back just a titch (a few thousanths) if you really think that's the culprit.
The plunger can't move without the shift lever also moving so it seems improbable, but not impossible. Good luck.
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