[KaiMana]

I really need help here, am about to throw the bike in the trash compactor!:angry:

Ok, I replaced my entire drivetrain(chainrings, chain and cassette) 3 weeks ago, the bike is a '03 giant AC with dual chainrings. On the 2nd ride after the work the drivetrain slipped once under extreme torque(think knees and teeth into handlebar/stem/uppercrown). By the fourth ride it happened 4-5 times.

The chain remains on the correct sprocket on the cassette but jumps or mostly jumps to the small chainring. Other than the slip under high torque everything shifts and runs smoothly.

So, I figured must be the freehub letting loose. Nope, swapped for different rear wheel with new cassette still happened. New chain and it happened under less torque. Different chainring and it still happens.

Whats left? Frame doesn't seem to be flexing any more than normal. I've never had rear derailler issues that had a complete slippage feel.

 

[binary visions]

Spray some degreaser or WD-40 on your chain, clean it off, then lube it up again. I was having a similar problem and chaulked it up to that sticky crap that comes on new chains. Don't know if that was the correct diagnosis but degreasing and re-lubing fixed it so...

 

[Kornphlake]

I had a similar problem and it ended up being one of the plates on my chain had come loose and was pushing itself off the sprocket under extreme torque. It was most likely the pin I had just pressed in on my new chain and since I didn't really know what I was doing it came out. Now I use SRAM Power links so I don't have to fuss with pressing pins in.

 

[DßR]

Originally posted by KaiMana

The chain remains on the correct sprocket on the cassette but jumps or mostly jumps to the small chainring.

Sounds to me like your chainline might be HUGELY f*cked. Did you replace the BB with anything other than the exact same stock one, or replace the cranks at any time? That could throw off your chainline.

Is the BB loose? Crank loose? Spider bent? Just throwing stuff out there....check all those.....

Does your lower swingarm hit the front derailleur cage under compression (hard sprint) forcing it to move over?

 

[KaiMana]

chain was degreased with wd-40 and relubed with teflon based and it is a sram powerlink so only pressed a pin out. I also tried a new chain.

 

[Transcend]

either your crank/BB or chainrings are horrendously bent.

You HAVE to have a horrible chainline for this to be happenning.

 

[KaiMana]

Originally posted by DßR
Sounds to me like your chainline might be HUGELY f*cked. Did you replace the BB with anything other than the exact same stock one, or replace the cranks at any time? That could throw off your chainline.

Yeah, I was thinking maybe frame flex was affecting chainline.
Didn't replace the BB or crank

Originally posted by DßR
Is the BB loose? Crank loose? Spider bent? Just throwing stuff out there....check all those.....

checked BB, crank bolts, chainring bolt for tightness, doesn't appear that spider is bent

Originally posted by DßR
Does your lower swingarm hit the front derailleur cage under compression (hard sprint) forcing it to move over?

nope, the ac has dropped chainstays

 

[KaiMana]

I think the bottom bracket is the next thing to try, it is a year old and that is usually what I get out of one. If the bearings are going I guess it could cause spindle movement?

 

[ssaddict]

Originally posted by KaiMana
chain was degreased with wd-40 and relubed with teflon based and it is a sram powerlink so only pressed a pin out. I also tried a new chain.

What chain, Sram/Shimano?

8/9speed?

powerlink matches the chain right?


re-check and tighten every bolt on your cranks and rear suspension

 

[KaiMana]

Originally posted by ssaddict
What chain, Sram/Shimano?

8/9speed?

powerlink matches the chain right?


re-check and tighten every bolt on your cranks and rear suspension

Sram 9speed powerlink chain, I tightened rear suspension bolts.

 

[ssaddict]

Originally posted by KaiMana
Sram 9speed powerlink chain, I tightened rear suspension bolts.

I'm out of suggestions then cause you really shouldn't be having any issues.

Worst case, maybe buy a chain guide?

 

[KaiMana]

Yeah, I've been drooling over the evil drs and thought it might help, but if it didn't I would still have a broken bike and be out more $.

 

[ssaddict]

Originally posted by KaiMana
Yeah, I've been drooling over the evil drs and thought it might help, but if it didn't I would still have a broken bike and be out more $.

Very true, my only thought is maybe the extra tension, and the lower chain not hopping around might keep it on more?

 

[binary visions]

Originally posted by ssaddict
Very true, my only thought is maybe the extra tension, and the lower chain not hopping around might keep it on more?

Not to mention the chain could be kept on by the sheer coolness of the bashguard...

 

[OGRipper]

From what you describe, sounds like the problem is coming from your front end: big ring, chain, front der, maybe crank/bb. Chainline hasn't changed so that's not it, crank is not bent, and it's not jumping in the rear, so it's prob not the freehub, cassette, or rear der.

You've already replaced the chain so assuming you installed 'em both right that's probably not it. May seem obvious, but you might double check to make sure you don't have any bent or tight links.

If that's not it, carefully check every tooth on your big ring. You may have one rogue tooth that is tossing your chain under heavy load. You didn't say what brand rings - depending on the manufacturer, production tolerances might not be tight, and a little variation can make a big difference. Plus, the more it happens, the more a bad tooth can get bent out of shape, which means (as you are experiencing) the problem will get worse, and could give you bent chain links to boot. I've had to file imperfect teeth from time to time. Because even the smallest imperfection can screw you up, if you can't see anything to fix, you should try a different ring, maybe something from a different manufacturer. And this sucks, but because a bad ring can mess up your chain, to be sure you might toss a new chain on at the same time...and so it snowballs...

Only other thing I can think of is that you are a monster and are flexing the crank/bb enough so that on the downstroke, the big ring flexes outboard and into the front der, causing an unintended shift. If it never happened before, you might double check the front der adjustment, your new rings may be mounted a little more outboard, enough so that when crank flexes you get a shift. Oh, and make sure the crank bolts are tight, loose cranks wobble.

Best of luck, that's all I can offer without getting my hands on the bi-otch.

Damn all mystical mechanical goblins!!

 

[Msisle Dad]

check your front derailleur, i had a bike in our shop that did this but it turned out that the fromt derailleur was bent

Morgan

 

[KaiMana]

I think you guys got it, will check it out tomorrow. But I went from a 32 to a 36 middle chainring and had to go to a bigger bashguard which meant I moved the front derailler up so as to clear the new bashguard.

So maybe the front der. is too close and when cranks-BB flex it is causing a downshift.

That would sure be an easy fix.

 

[OGRipper]

Let us know if it works out, chasing these kind of goblins is fun but not nearly as fun as catching one now and then.