Bicycle Mechanics - Derailur Suck? Is my theroy correct?

lets cut to the chase. Top gear, that is small in back, big in front. I was getting this nasty noise. I looked down and back, and saw that my deraillur was doing funny thing. The Derailur looked like it was trying to go up into the rear cluster. The teeth from the top pully were grinding against the small sprocket. If I backpedaled a little, then continued to pedal, it would work out ok. In 23rd gear, things work smoothly.

It appears to me that my deraillur is not keeping proper tension. I was going to attempt a fix by increasing the b tension on the bike.

Is this the right path to be going down?

Chain might be too long.

Limit screw not set right? Cassette too close to the dropout face so the chain is rubbing there in the smallest cog? Cable tension not set properly? Stiff link in the chain? Lots of possible causes.

BTW, what does "doing funny things" mean. A more detailed description would be useful.

Is the noise coming off front or rear derailler? Big in front, small in the back combo should be fine in the rear unless the chain is WAY long. I can imagine rear derailler not being able to take the slack and folding on itself in small ring, small-ish cog.

It appears to me that my deraillur is not keeping proper tension. I was going to attempt a fix by increasing the b tension on the bike.

Is this the right path to be going down?

In a word, no.

The issue sounds more like an improperly adjusted limit screw on the rear derailleur (specifically, the one marked "H"), a bent derailleur hanger or a worn small cog. The B-tension screw adjusts for pulley clearance on the large cog, but this should only be done after everything else is adjusted properly and/or worn parts are replaced.

When diagnosing or tuning rear shifting issues, I go through things in this order:

Check chain for wear, replace if needed
Clean drivetrain
Check chain for length
Check derailleur hanger alignment (nothing else will do a thing if this is off)
Inspect cogs for wear or damage
Inspect cables and housings, lube or replace
Check shifter function and release all tension
Lube derailleur pivots, inspect for damage
Check outer stop adjustment
Bottom out cable adjusters and pull slack from the cable before tightening anchor
Shift to large cog and adjust inner stop
Adjust cable tension for proper indexing on small cog
Shift one gear and check indexing again
Check function in all gears
Adjust B-tension

What's "doing funny thing?"
Does it happen pedaling or coasting? How about when in a small/small combo?
Is your drive train clean and lubed? That's the first thing to check IMO.

the "funny things' is when I looked down, it appeared that the derailur was being sucked into the rear cluster. And it happened once per chain revoution. it would be fine for 3/4 of the chain length, then it would get sucked in. I could pedal through it, but the noise was rather disturbing.

The chain and bike are only 150 miles old or so. I will clean everything tonight. The chain works fine in all other gear combinations, including crosschaining from the small front ring to the small back ring (this makes me think the chain is not to long). The shifters still respond with authority.. there's half the adjustment left in the cable adjuster.

The bike works in ALL gears except for 24th.

lets cut to the chase. Top gear, that is small in back, big in front. I was getting this nasty noise. I looked down and back, and saw that my deraillur was doing funny thing. In 23rd gear, things work smoothly.

It appears to me that my deraillur is not keeping proper tension. I was going to attempt a fix by increasing the b tension on the bike.

Is this the right path to be going down?

"Nasty noise". "Doing funny thing". Not much to go on here. Sounds like stand up comedy to me.

"Nasty noise". "Doing funny thing". Not much to go on here. Sounds like stand up comedy to me.

Sorry, I tried to describe it better in my last post, and I edited my first post to clarify as well.

Tight chain link?

Get off the bike and pedal slowly in reverse. Watch the rear derailleur. If you have a tight link it will show up.

Tight chain link?

Get off the bike and pedal slowly in reverse. Watch the rear derailleur. If you have a tight link it will show up.

+1

Well then. Once my ride dries off, I'll be cleaning and lubing the chain. :-) I should have taken better care of my chain. The whole bike needs a good cleaning anyway.

Well, we have a result. We were all wrong. I cleaned, then lubed my chain. (I have a motorcycle, I ended up using my motorcycle chain lube, very low fling)

SO... I did some testing. it was definitely suck of some kind..... Under load, the chain would get stuck on one of the teeth on the big sprocket. The chain would wrap all the way around the big sprocket before releasing, if it released. I'll need to file that tooth down sometime soon. At least I know what the problem is now.

It seems that whenever the chain has any sort of misalignment (say being anywhere but the 8th cog) it's enough of a change to help pull the chain off. Watching the chain carefully in the 7th cog shows the chain catch on the big sprocket, a little. But it never gets to the point that it sucks the chain up around like it does when the chainline is straight.

This may just be an excuse for me to get a new crankset for the bike. :-) This set already shows that it's bent a little. It's not exactly a high quality crank. *grins* Yey, an excuse to buy new bike parts.

Well, we have a result. We were all wrong.

Not to put too fine a point on it but...YOU were the only one wrong. :rolleyes: Not one response agreed with YOU that your problem could be resolved with the B adj screw, which is what you asked. WE tried to help point you in other directions by guessing more likely problems based on your vague description.

Glad that WE could help you solve our problem.;)

My bike also started doing that after I bent the hanger a bit.

Not to put too fine a point on it but...*snip*Glad that WE could help you solve our problem.;)I'm not sure how to take that. Your post starts off with an "Well we can't be wrong" and ends with a sarcastic twist. I'll definitely agree my original description ended up being comically poor. It was written at work, and I managed to omit three sentances I thought I had already written. I did go back to correct that.

I think I did make proper use of the word "we" as my guess was wrong, as were all the other (wide ranging) suggestions. That makes all of us wrong. Nobody even hinted that it could be the the chain clinging to the front sprocket. I don't know if it was clear, but I took all the suggestions to heart. I went ahead and cleaned then rechecked everything. That's to say I started with the suggestions that the forum made rather than going down my own path.

The closest we came was Wordbikers suggestion to "inspect cogs for wear or damage." The bike has less than 200 miles on it, nearly all of them in granny gear. And most likely a grand total of 10 minutes of time in the top ring. The paint wasn't even chipped off the cogs teeth. The tooth that is at fault, doesn't look wrong in any way. But this is a stamped chainring, which is an indicator of the price point my crank was made to.

If I wanted to be mean about it, I could have just said everything you guys said was useless, because your suggestions were wrong. I didn't, for a myriad of reasons, most importantly that I do appreciate all of your efforts in helping me on this issue. Incorrect or not, you tried. Thanks.

Maybe I should have used more grins in my post, thought that was the point of my opening phrase.

FWIW, since you seem to see right and wrong in black and white terms, several responses did not "guess" the problem, just offered suggestions for better diagnoses - so everyone wasn't wrong. Regardless, everyone that did respond took the time and effort to help you.

But back to your real issue: you can probably bend the tooth/chairing into alignment. An adjustable wrench works well for that.

Getting it wrong is part of life. A little wrong, or a lot wrong. We all are from time to time. Though I'm beginning to think we have personalities that just don't get along. No, I don't see the world in blacks and whites.

The culprit tooth isn't bent, it just has a very poor shape. The face of the tooth is nearly vertical, which isn't right for a sprocket, and would explain why the chain is hanging on with such tenacity. It just needs to be knocked back a little bit, and it will let the chain go.

The chain rub happens both on the little ring, and the big ring, and has happened since new. I haven't checked to see if the rubs happen at the same place on both rings, so I don't know if it's just a really crappy machining job on the spider, or if it could be the square taper in the crankarm itself, or even a badly machined bottom bracket.

A tight link in the chain will also cause this type of chain suck, in fact it's quite common.

Al

In a word, no.

.....The B-tension screw adjusts for pulley clearance on the large cog....

According to the Shimano tech docs, you set it on the large cog, but then you check it to make sure it's not interfering with the small cog.
A RDER designed for an 11-34 cassette probably won't exhibit any problems, but put on a 12-23 or similar and it might.

"4. How to use the B-tension adjustment screw
Mount the chain on the smallest chainring and the largest
sprocket, and turn the crank arm backward. Then turn the
B-tension adjustment screw to adjust the guide pulley as close to
the sprocket as possible but not so close that it touches. Next,
set the chain to the smallest sprocket and repeat the above to
make sure that the pulley does not touch the sprocket."

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Deore-XT/RD-M751/SI_5VV0B_En_v1_m56577569830604982.pdf

Al1943: First thing I did was check, clean, then relube the chain. Before I even went about trying to replicate the conditions where the noise was happening. The chain is a little dirty and gritty, and I should clean it again. But there are no stiff links.

Bill Kapaun: Hah, :-) very interesting bit of documentation there. Looks like I wasn't completely off base. I would have still been wrong though.

I filed the tooth. viola, problem fixed.