Shayne

This one's a little puzzling to me.

The tandem is an 05 Co-Motion Primera.

When we first got it late last summer it would drop the chain to the inside
when we went into the small ring and applied power. Couldn't duplicate on
the stand, so the LBS installed a Deda chain watcher thingy. Since my
daughter hates the granny gear, we really have not tested it. I know that
sounds silly, but we can climb most hills around here in the middle ring and I
kind of just forgot about it.

So, the other day, it occured to me that we haven't tried it and should give it
a shot. The stoker agreed but wasn't to happy as we almost wrecked when
the chain dropped.

As I approached the climb I shifted down to the small ring plenty early. No problem.
When we got to were the grade started to increase and we started to apply some
power to the cranks......bang, crunch, snap, pow.....the crank arm goes straight down
with no resistance, making a horrible sound. End of test.

Get back home and I put it in the big cog out back and the granny ring up front. Works
fine with the rear tire held up off the ground. I put the bike on the ground, hold the front
brake and apply preasure to the crank......bang, crunch, snap, pow.....the crank arm goes straight down
with no resistance, making a horrible sound. End of test.

I couldn't see behind me to figure out what's happening and I don't want to do it agian.
It's almost like the chain is slipping up and over the teeth on the cog or ring, but that can't
happen can it? Could it be something internal in the hub? I don't know anything about the hub internals.

Any help would be great. I'll most likely take it in to the LBS but I'd like to be armed with as much info
as I can get.

thanks
Shayne

skinny

Could possible be the wrong front derailleur. They have maximum capacities. What are the front chainring sizes? Front derailleur manufacturer? Front derailleur model #?

There are many factors that could cause this including things like chainring spacings, chain alignment, frame alignment, etc... Start with the derailleur and chainrings and make sure you have the right set up first.

TandemGeek

How many miles are on your chain or, put another way, have you checked your chain for wear?

If you have a brand-spanking new, hardly been used small chain ring and the rest of your drive chain has seen a lot of use, you can create a problem where the chain has worn so much that it will no longer properly seat in the small chain ring's sprockets and will skip across the teeth (fitting the sound and crank action that you describe) when placed under a load. Without the load, the chain just doesn't fully seat but it will not ride up over the chainring's teeth, hence the problem will not be replicated sitting in a workstand.

Putting a new chain on will solve the small chain ring problem, but you will probably find that your chain was so worn that you'll also need a new cassette as the sprockets you use the most will have become elogated. As a result, with a new chain on a worn-out cassette you'll see the same thing on the cassette that you saw with the granny ring when the chain is riding on the cogs used most often: once placed under rider loads, the chain will skip/slip off of the cassette sprockets. Note: It's not usually a problem putting a new chain on old chain rings, it's the other way around that creates the problem. So, unless your middle and big chain rings are really screwed up, you shouldn't need to replace those.

Bottom Line: IMHO, expect that a new chain and cassette will likely be the recommendation IF your chain is worn out (which is what I suspect to be the case).

To prevent this type of domino effect I change out my drive chains once they have stretched beyond 50% of their useful life, but not more than 75% when checked with a Park Tool chain checker. Using a ruler, that's about 3/32". To learn more about chain wear, please refer to Sheldon Brown's Website: https://www.sheldonbrown.com/chains.html

Now, if your chain isn't worn out, it is possible that your rear hub's engagement mechanism is skipping. However, that would normally show up any time that the rear hub was put under a heavy load and not be germane to the granny ring UNLESS you were attacking a really big climb that you had attempted in the past. A bent rear axle could also cause the engagement mechanism to skip. In either case, both are easy to check and correct. Your LBS should be able to pull the rear hub apart to clean and lubricate the engagement system and should also be able to pull the rear axle and make sure it's not bent. If it's bent, a new axle should solve the problem.

Those are the obvious things to check based on the information you have provided.

Shayne

Guys thanks for the replies,

We bought the bike new late last July. We've only put 400 miles on it. I wouldn't
be surprised that a chain would wear out quicker on a tandem but 400 miles and
maybe some demo rides while it sat in the shop seems odd. I normally get between
2200-2600 miles on my single bikes chains (PC-59) before they're 50% worn and I replace them.


I'll deffinetly check the chain tonight when I get home and report back.

thanks
Shayne

TandemGeek

Interesting... The only thing I'll note is that I've seen some Demo tandems that were used to log hundreds of miles by the shop owners before being sold to their eventual owners.

Let me know what you discover. Again, those are the only obvious things that come to mind aside from a cheap or a mis-spaced granny ring that's close enough to the middle to ride up on the middle chain ring's ramps. However, I've only ever seen the latter when replacing the OEM chain rings with aftermarket models that have different chamfering.

Shayne

I hear you there. They have a Co-Motion Speedster on the floor that I know was ridin by the shop team at some race they were calling the Tandem Nationals. You can tell just by looking at that bike that its been ridin, while ours was spanking, shiney new. All the little cad and zink plated pieces were still sparkling and fresh. Still, you never know.

Shayne

Hermes

I suggest going to the LBS with your stoker and have them put the tandem on the stationary trainer. Then you and the stoker can apply load and let the mechanic observe the operation.

dbohemian

My thought is a worn chain or improperly manufactured/worn granny ring.

You mentioned it only happens in the granny up front and the big cog out back. This would reduce chain wrap on the small chainring to a minimum. If something about the chain or more likely the granny ring then there may not be enough tooth engagment to hold onto the chain.

I would look at the ring closely. Many rings today have low teeth profiles and shifting ramps to facilitate shifting. Since you don't really use it much you may consider more of a single speed ring that has taller teeth on it and will hold the chain better.

Just a thought.

Dave Bohm
Bohemian Bicycles

Shayne

Ok, I'm back from the LBS. The problem was easy to duplicate in front of the mechanic. Simply hold the front brake and step on the pedal hard. What's happening is the chain is skipping over the teeth of the small ring. All three rings and the cassette look great. The chain is only showing minimal stretch, about 1/32". However, the mechanic was not happy with the chain quality. He showed me how it was very loose side to side. You could even grab the chain and twist it up-side-down.

What he wants to try first is to replace the cheap chain with a good one. We'll see how it goes. He's more than willing to make it right and in the past has treated me very good.

Thanks for everyones comments, seems like you guys were right on track. I'll let you know how it goes.

Shayne

dbohemian

Chains are really important.

I personally use Rohloff chains because I can get them when I order Rohloff hubs and they are top quality.

This was apparent on my recent tandem. I used KMC chains on the timing rings and a Rohloff final drive chain.

The KMC chains stretched so much that I had to adjust the tension 3 times until it stopped. The Rohloff has not required any adjustment to date and neither have any of my others. That alone tells me there is some sort of manufacturing difference.

I have had good luck with most chains, even inexpensive ones. One chain I will no longer use is Wipperman. I have had so many bad experiences with breakage and poor quality that I stopped spec'ing them all together.

Dave Bohm
Bohemian Bicycles

R900

What chain was Co-Mo using? I would say anything 105 and up should be fine. Mechanic might need to start from scratch and set the bike up as if it were new. Best of luck.

Shayne

I've been using sram PC-59's on my single bike. I like the little gold link that comes with it. When I replace a chain, I keep the link to carry in my saddle bag just in case.

The mechanic thought the chain on the tandem was a KMC at first, he's had issues with them. Hopefully he'll get me fixed up. The wife and daughter are off to girlscout camp this weekend so I told him to take his time. Looks like I'll be on my own for a few days, wonder howmany miles I can get in .

Shayne

Shayne

After looking at it, the mech. said it was a low end Shimano. I was very flimsy side to side. The bike is pretty much at the 105/XT level with everything else.

Shayne

mikefranktroymi

I had a similar problem with a police model trek. The local lbs tried everything (Chan, adjustments, multiple chain saver devices) with no luck. Eventually the lbs kept my bike for a couple of weeks and with the help of trek determined that there was a problem with the bottom bracket. They ended up building up the bottom bracket with a spacer. In laymen’s terms I thing it was an alignment problem. P.S. this took a couple of seasons to fix and not once did I get charged.

Shayne

Just picked it up. He put the new sram chain on. He said that helped but he could still make it skip over the top of the ring gear teath, all be it just barely and with a lot of force. So he swapped in a new campy chain ring that he had and that took care of it. Even me, at 210 lbs couldn't budge it. He didn't charge me anything so I slipped him enough for a lunch.

Thanks for everyone's replies, seems like we're ready for some big climbs now.

Shayne

skinny

TandemGeek

IMHO: It should have taken about 1hr to diagnose and fix... given that they could put their hands, eyes, and ears on the problem. Who ever claimed to be a technician didn't know what they were doing and attempted the shade tree mechanic's approach of "remove and replace" in the hope that somehow they might magically solve the problem instead of doing a proper diagnosis. Again, I would suggest that you paid dearly for that level of service if only measured in the cost of your mileage to and from the shop and the value you place on your time spent doing so, never mind the loss of use / reduced enjoyment of the bicycle while the problems were on-going.

Please bear in mind, troubleshooters working via forums and email:
Therefore, when presenting problems for diagnosis please recognize that most of us who try to help have only the information that you provide to go on and our past experience with "similar" problems, bounded by whatever additional facts that you provide or subject to the caveats included in the initial diagnosis. Speaking for myself, I will usually provide the "most likely" and then work down to one or two "possible things to look at" and will rarely resort to the "well, it could be most anything" (aka, the laundry list) since that's usually of little help and tantamount to the shade tree mechanic approach or a SWAG.

Anyone in a bike shop who claims to be a technician should NOT be perplexed all that often, notwithstanding the occasional bent derailleur hanger or frame (which, by the way, are easily checked by gauges that a professional bike shop should have on hand). If they can't fix it in two attempts, chances are they don't know what they're doing. With tandems, even good technicians who haven't worked on tandems will get stumped... but if they're REALLY good technicians they'd better get it right the second time.