Bicycle Mechanics - Tandem drive-train lockup

Help me out so I don’t have to schlepp this beast back and forth to the dealer. The bike is a 2002 KHS Tandemania Milano. Components are Shimano 8 speed 105 STI. Rear wheel spacing is 160 mm.

This is what happens:

Occasionally, when standing and climbing, the cranksets will just lock up - unable to pedal - on the downstoke. You have to quickly backpedal to free it up, and then you can pedal again. The first time this happened, it caught me completely off guard. We were climbing a short, steep ‘sprinters hill’ in the middle ring/big rear cog (42/26). The chain locked up and we lost speed so fast I lost my footing coming to a stop and dropped the bike and my gurl!

I figure the problem is some sort of chain suck. But, I can’t see what is going on at the back crankset, so I am not sure. I checked the chain and could find nothing wrong with it, but replaced it anyway. Thought I cured it until a few weeks later when we were standing and humping up a super-steep little number in the small ring, maybe 2nd or 3rd cog on the freewheel when it does it again. I stop us without dropping but we have to walk up the dang hill!

So, I email the shop and the owner thinks it is the shifting ramps/pins on the 105 chainrings catching the chain. He says any burr on the chainring can cause suck. Now, the granny does not have any ramps. But, I replace the middle and big rings with plain chainrings and take a link out of the chain. But guess what? It does the same thing! Just last Sunday we had a good big-ring roll on a flat section of road. A little rise is up ahead, so when we hit the bottom I shift down 2 cogs so I am in the big ring/3rd cog (53/21). We stand and start climbing and the thing just locks. Since we are rolling at about 18 MPH, I call for a backpedal which frees the chain and we continue on. I stop at the top of the hill and examine the drivetrain, but can’t find anything wrong. The only time this has ever happened when seated on the bike was one time when we were really hauling arse on a bumpy flat road.

What could it be????

EDIT: Writing this got me thinking, so I checked the bike out some more and I am not sure that chain suck is the issue. There are only 2 small scratches on the drive side chain stay, no big gouges like the chain was getting whacked against it or jammed against it. I wonder if the cassette could be the problem or maybe the rear hub. The hub is a Shimano tandem hub with a plain threaded axle. Could the axle be bending? The cassette binding???

That's a scary problem. FYI, I have a 2003 version of the same bike. I've never experienced this problem, although we have some different components. Have you checked that the rear chain is the right length? Perhaps its too long and the rear derailleur is having a tough time handling it. Also, double check the B-tension adjustment to make sure your rear derailleur is not hitting any of your cogs. How about the alignment of the drive train. Rear derailleur isn't bent, is it?

I shortened the chain once, and could take out one or two more links. If I took out 2 links, the chain would be too short to cross-chain in the big-big combination (which, of course you don't want to use, but if you accidently shift into that gear with insifficient chain length, you can destroy the derailuer). Chain bounce/swing may be part of the problem. The derailleur hanger is strait and the bike has never been crashed. It shifts gears cleanly. The derailleur clears all the cogs with room to spare (biggest cog is a 26t).

I am thinking of going out with the bike and intentionally trying to get it to lock up. Then, coast to a stop while keeping presure on the pedals so it stays locked and having my stoker hold the bike while I dismount and see what is binding.

I am thinking more and more that it is not chain suck at the chainrings as I just can't see how the chain would stick to the 53 tooth ring when the chain line is almost dead strait.

My guess is that a chain side plate catches on the back edge of the front derailler, due to excessive flexing of the chainrings. Maybe you would avoid the problem with a stiffer BB or a lighter stoker.

My guess is that a chain side plate catches on the back edge of the front derailler, due to excessive flexing of the chainrings. Maybe you would avoid the problem with a stiffer BB or a lighter stoker.

Maybe, but I don't think this so, as the chain would rub before it caught and would make a niose. When standing and climbing, if the front derailleur is not centered over the chainring, I can hear some chain rub on the front derailleur. But when I have the lockup, there is no noise at all.

Also, I would think that if the chain were catching on the front derailleur, whe would have at least tweaked it if not ripped it completely off the bike.

Help me out so I don’t have to schlepp this beast back and forth to the dealer. The bike is a 2002 KHS Tandemania Milano.

This is what happens:

Occasionally, when standing and climbing, the cranksets will just lock up - unable to pedal - on the downstoke. You have to quickly backpedal to free it up, and then you can pedal again. The first time this happened, it caught me completely off guard. We were climbing a short, steep ‘sprinters hill’ in the middle ring/big rear cog (42/26). The chain locked up and we lost speed so fast I lost my footing coming to a stop and dropped the bike and my gurl!

My guess is that the synch chain is loose. The bottom (slack) run is swinging back and forth, and the end of the stoker's crank can sometimes snag it.

You can adjust this with the eccentric.

See: http://sheldonbrown.com/synchain for specifics on this.

Sheldon "Two Wheels, Four Legs" Brown


+-----------------------------------------------------------+
| There are essentially three entities riding a Tandem: |
| The captain, the stoker, and the spirit. |
| It is the spirit who likes in-phase cranks. |
| --Osman Isvan |
+-----------------------------------------------------------+

My guess is that the synch chain is loose. The bottom (slack) run is swinging back and forth, and the end of the stoker's crank can sometimes snag it.



Dayum Sheldon, I never thought of that.... I may have a little too much slack in the timing chain. When I get home tonight, I am going to see if there is enough side play that it could catch on the stoker's crank...

Thanks!

Dayum Sheldon, I never thought of that.... I may have a little too much slack in the timing chain.


Sheldon is a genius.

Sheldon is a genius.

If he is right, I have some hardly-used Shimano 105 chainrings for backups....