Bicycle Mechanics - Whirring Sound from Drive Train on LWB

So I built up a Mach 2 and it rides pretty well, but I can't get rid of a whirring sound coming from the rear of the drive train. It's most noticeable in high gear, though I can hear it a bit through out the gear range. Guessing the noise is coming from the rear derailleur. Adjusted the B-tension on the SRAMx7 so that it could handle the largest cog on the 7-speed MegaRange cassette, without jumping gears on the smaller ones. Also adjusted the cable length so that the guide pulley aligns with each cog and the chain does not rub on either side. Could the problem be that the SRAM derailleur cage is too narrow for a 7-speed chain? I don't see any links touching against the inside, but must admit it is a pretty close fit. (Don't hear any noise in the drive chain of my DF with the same size MegaRange freewheel and a crappy Tourney derailleur, whose B-tension screw is merely cosmetic.) Incidentally, the rear fork was spread by the frame builder to accommodate a tandem wheel, can't tell if the chain line has anything to do with the noise. Wondering now if maybe the derailleur hanger could be bent or misaligned.

Any ideas, folks?

Thanks.

http://farm3.static.flickr.com/2475/3970081169_e8cec4a294.jpg

So I built up a Mach 2 and it rides pretty well, but I can't get rid of a whirring sound coming from the rear of the drive train. It's most noticeable in high gear, though I can hear it a bit through out the gear range. Guessing the noise is coming from the rear derailleur. Adjusted the B-tension on the SRAMx7 so that it could handle the largest cog on the 7-speed MegaRange cassette, without jumping gears on the smaller ones. Also adjusted the cable length so that the guide pulley aligns with each cog and the chain does not rub on either side. Could the problem be that the SRAM derailleur cage is too narrow for a 7-speed chain? I don't see any links touching against the inside, but must admit it is a pretty close fit. (Don't hear any noise in the drive chain of my DF with the same size MegaRange freewheel and a crappy Tourney derailleur, whose B-tension screw is merely cosmetic.) Incidentally, the rear fork was spread by the frame builder to accommodate a tandem wheel, can't tell if the chain line has anything to do with the noise. Wondering now if maybe the derailleur hanger could be bent or misaligned.

Any ideas, folks?

Thanks.


Bunches of different potential problems, and you probably scared a couple people with the picture of your 'bent. It don't scare me- I have to look in the mirror every morning!

I'd go after the derailleur hanger and dropout alignment first, since it's a cheap-ish adjustment.

I've listened to a couple noisy SRAM derailleurs. A lot of times they sound like Shimano derailleurs with cheap aluminum pulleys installed. Standard Shimano "Centeron" pulleys are nice and quiet- I don't know if they work on SRAM derailleurs, though.

Is the idler pulley new? A lot of time drivetrain noise can resonate through the frame and sound like it's coming from a completely different area. How's the bike sound with the chain off the idler?

Thanks Jeff.

Yeah, all the components on the bike are brand spankin' new, it's just the frame/fork that have been recycled. Sounds the same with the chain off the idler too. Thing is, the noise gets more noticeable the more force I am applying to the pedals, which would suggest the problem is with the cassette, rather than the derailleur, right? Because the RD cage only handles the chain return, so should not be affected by how hard I am pedaling, other than when I am shifting. (And as I have it set up now, with SRAM indexed grips, the shifting is perfectly reliable.) If the derailleur hanger is out of whack, would that cause the noise under the same circumstance as I described? Or would the drive chain be whirring no matter how lightly I pedaled?

What I'm worrying about now is that the problem might be in the rear hub, or in the freehub. If the internal assembly is giving under strain then the cassette might be angling slightly against the chain. Or it could be that the bike itself is flexing enough as I pedal to alter the alignment in the drive.

Hope to get a chance to play around with it some more soon, am down in the city for a spell and the bike lives upstate.

What I'm worrying about now is that the problem might be in the rear hub, or in the freehub. If the internal assembly is giving under strain then the cassette might be angling slightly against the chain. Or it could be that the bike itself is flexing enough as I pedal to alter the alignment in the drive.
.

I really doubt the rear hub is the problem. (I had a Shimano rear hub self-destruct on me this summer, but it was 15 years old and had gone through several cassette body transplants. The threads simply won't hold the body on anymore.)

I'd guess that the frame could be flexing, particularly since the noise varies with pedal pressure. If it were my bike, I'd give it a while longer to see if things wear in.

Is that one of ADC's ( http://www.bentrideronline.com/messageboard/member.php?u=28 ) ??

Is that one of ADC's ( http://www.bentrideronline.com/messageboard/member.php?u=28 ) ??

You betcha.

And I think you're right, it's the frame that's flexing. Because I heard what sounded like brake rub too and it went away when I rotated the left brake shoe away from the rim, even though it had not even been close to touching.

Thanks again for your insight, Jeff.

Make sure the bolt the idler pulley rides/slides on is clean and lubed. Idlers can be noisy. Also, make sure that chain isn't too long. It looks like there is a lot of slack in it. bk

Make sure the bolt the idler pulley rides/slides on is clean and lubed. Idlers can be noisy. Also, make sure that chain isn't too long. It looks like there is a lot of slack in it. bk

Well as I mentioned earlier, it makes the noise whether or not the chain is on the idler. But I should check the idler anyway, it's the one part that I have not fully inspected.

I think the chain looks slackish in the picture because it is on the innermost chainring and only the middle cog-- I followed the instructions pretty carefully when I prepped it. But I'll have to give it another look, with the chain on the largest ring, to see if there is still too much slack.

Thanks.