Rahzel

Our 2008 Cannondale Road Tandem 3 has been making some unusual drivetrain noise recently. Whenever I shift into the second largest rear cog (30t, cassette is a 9spd 11-34), regardless of which chainring I'm in (53/39/30) I get a grinding noise from the rear portion of the bike--either the RD (Shimano Deore LX), cassette or hub (Shimano Tandem 40h).

No other gear combinations make this sound. To the best of my knowledge, I have adjusted the derailleurs and limit screws correctly. This is evidenced (IMO) by the crisp, quiet shifting in every other gear combination. There seems to be no debris either in the RD or near the 30t cog, and the chain is clean and relatively new.

Lastly, the grinding sound only happens under load--i.e. when I put the bike in a stand, shift into the 53/30, 39/30 or 30/30, and use my hand to turn the cranks, I can't reproduce the sound. However, when pedaling under load, the grinding sound happens whenever I shift into the 30t, regardless of whether I'm shifting from an easier gear or a harder gear.

Any advice would be greatly appreciated. Thanks!

merlinextraligh

How many miles on the cassette? How many miles on the chain? It may be its time to replace the cassette.

Measure your chain and see if it's "stretched" (actually worn) If you've ridden with a stretched chain it will wear out the cassette. And even with proper chain replacement cassettes wear out over time.

The fact that only the one cog is making noise, and it happens in each ring suggest that the cassette is worn, and its just starting to show up in that cog first.

Rahzel

That's a good thought, I didn't consider that. However, the chain and cassette are all relatively new, with each having about 250 miles of wear. At any rate, I just checked the chain for stretch and measured no perceptible stretch, and I didn't notice any wear on the 30t cog.

We're going on a ride this morning, and I'll continue to play with the derailleur adjustment--maybe I simply missed something in my adjustments of the barrel adjuster, limit screws, etc. Knowing my mechanical skills, it's most likely a user error issue :-)

specbill

If you have another cassette in the shop swap it out and see if the noise goes away or move that cassette to another bike and see what you have...clearly something is up with that 30 gear or chain ...wear or damage...let us know what fixes it

Bill J.

cornucopia72

+1 ^

Did you save the old cassette? The one you are using now may have a problem even if it is new. I would stay away from that cog until the problem is solved.

Rahzel

The cassette and chain are both stock--the bike is relatively new--so there is no "old" cassette. However, I do have a well-used (5k miles?) 11-26 9spd cassette that I can swap with the 11-34 cassette. I will do this sometime soon and update this thread when I can figure out what's going on.

On our ride this morning, I was able to slightly decrease the grinding noise by fine-tuning the RD, but there was still some noise. I will play with the RD some more as well.

joe@vwvortex

Make sure the rear wheels is seated correctly in the dropout and that the rear derailleur hanger is not bent. Both those could cause odd sounds in certain gears, but otherwise the bike shifts ok.

Rahzel

Okay, so I re-seated the rear wheel in the rear dropout, checked the RD hanger (it was not perceptibly bent), and re-tensioned/adjusted the rear derailleur cable.

On our ride this morning, the grinding sound was still there, but it was much quieter. This tells me that either (a) re-seating the wheel helped, or (b) the RD cable adjustment helped. Progress! I will continue to play with the cable adjustment, and give all relevant parts a good cleaning (the bike is due for one anyway), and hopefully the grinding noise will continue to subside.

mkane77g

Check the freehub, bearing preload may need adjustment

Rahzel

I think I finally got this fixed. I completely cleaned and adjusted the drivetrain. That's all it took. DURRRR. Stoker and I did a 10 mile time trial the other day with no noise coming from the drivetrain.

All the same, we're going to switch from our 11-34 cassette to a 12-26 cassette, as the 52/39/30 chainrings provide more than enough gearing range to tackle the central Florida flatlands.

Quick question--How many links should I remove from my chain when I go from the 11-34 to the 12-26? (I know I'm supposed to replace the chain when I replace the cassette, but the chain shows zero stretch, so I'm going to see if I can get away with not replacing the chain at this time.)

specbill

Here is one easy way to nail the chain length issue. I copied it off of Sheldon Brown's site under 'chain measurment'. This is just one of many ways to determine the length.

''The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that they could be connected (outer link meets inner link), then make the chain one complete link (one inch) longer than that. In almost all cases, this will give the optimum length.

This link will take you to more chain info: https://sheldonbrown.com/chains.html

Bill J.

merlinextraligh

I would consider not removing any. The new smallest combination is now 1 tooth larger, than with the old cassette. Thus the derailleur will able to wrap the chain just fine.

The upside to not resizing the chain is that you can always put the old cassette back on anytime you want, like when you go do Six Gap.