throwing the chain in a sprint
 

this last weekend on our local race training ride (150-250 riders) i took out the tandem with a friend and fellow racer. it was the first time captaining in a group ride like that as opposed to a recreational group ride. i have stoked many times on tandems in training rides but not in about 15 years. anyways when we came to a sprint point we found ourselves in a pretty good position to sprint but had never sprinted on my tandem before. we moved into the right spot behind a good cat 1 racer who went a little too early and started to launch our sprint. just and we were really pushing the 53x11 the chain flew off the big ring. we stopped pedaling and shifted the chain back on but it happened two more times in the sprint.

i'm not sure if it's a simple as an adjustment to the front derailleur limit screw or something larger. i can't make this happen on the repair stand or even riding hard just in the sprint. i did swap the bb from the 118mm to something narrower like a 113mm or 110mm (i don't remember the width) and am wondering if the chain angle is now too much in the 53x11 with narrower bottom bracket.

Probably a combination of flex, chainline, and a "wave" developing on the chain due to pulsing. Certainly bringing in the limit of the FD could help, as the chain is coming off the outside, I presume. It might also be worth checking the chain tension at the top gear combo.

Sprinters need stiff frames.

You could also try OOP to change the dynamics on the frame in the sprint.

We have a hard time triying to sprint out of the saddle OOP

Some questions:

1. What's the Bolt Circle Diameter (BCD) of your crankset: 130mm or 110mm?
2. How many miles are on the big chain ring?
3. How many miles / how much wear is the chain showing?
4. How many miles are on the cassette?
5. What was your cadance like when the chain came off as you 'started' your sprint: 60's, 70's, 80's, 90's?

Another way OOP can help prevent throwing a chain :lol:

We threw the chain off in a much smaller group approaching the sprint zone hill a couple of months ago and now intend to adopt this technique as a new strategy.

We were in the middle, knowing that a sprint was coming up and waiting for someone to jump, when we decided to go for it ourselves. We kicked it up and when I went for the big ring I accidently over-shifted and tossed the chain outside.

The team behind us saw it happen, attacked, and got a length or so on us while I trimmed it in, got on their wheel and let them pull us close to the top of the small hill where we passed them quite handily.

This is just between you and us so I'm trusting you won't tell anyone else about this "secret strategy".:notamused:

Frame flex with 2 guys stomping on it, that's why you can't replicate that on the repair stand; also check limit screw on der. and chainring/chain wear.

are you trying to say that i can't put out as much power with one arm pedaling the bike on the stand as four legs of racers trying to sprint? how dare you:p bike racers are famous for their arm strength:roflmao2:

1. 130mm basic shimano tandem cranks
2. we bought the bike used so i'm not exactly sure, but the big ring is in good shape, we've put over 2000 miles on the bike. with my wife we don't use the big ring much
3. chain is a few hundred miles old, maybe 500
4. cassette is recent about the same as the chain, the 11 doesn't get much use
5. 90 to 100, we were above 35 mph at the start of the sprint

Most likely the combination of chain stay deflection (rear triangle flex, as we're talking about a Duet right?) exacerbated by your narrowed bottom brackets / bad chain line between the 53t chain wheel and 11t cog. It was essentially derailling itself off the top of the 53t. I'm not sure the front derailleur even if moved in closer would do much to overcome the loading on that chain.

Solutions? How about a Robusta... or correct the chain line by going back to a wider BB or perhaps using a +5 / offset bottom bracket as a compromise.

correct. it's a duet.
my guess was that it is frame flex and the bad chainline is not helping.
i could put the wider bb back in the rear bb. i'd like to leave the front bb narrow.
robusta, i have been dreaming/drooling about that bike for a while. i'll tell my wife you said that it's okay to get one. no matter that it's more than we've paid for any of our cars

OK, go with the Roadster and spec. it with calipers instead of V-brakes. Same frame as the $7,850 Robusta for $5,595... Add the go-fast goodies when you find them on sale and as the OEM parts wear out.

Or.... find yourself a second-hand Co-Motion Big AL, Roadster, Robusta, or perhaps even a second hand Cannondale RT3000 and sell the Burley to off-set the majority of the expense. Heck, even a new or second hand Co-Motion Speedster would be a quantum leap forward in frame stiffness (not to mention handling) vs. your Duet.

Seriously, it really sounds like your riding habits could warrant something a bit more 'robust' and it's all about having an upgrade strategy and plan when you realize what you're riding might not be ideally suited for the way you're riding.

Yes, I know you know all this but if you're going to blame this on me when you speak with your beloved I want to make sure you have some talking points and alternative strategies for the negotiations.

in reality the majority of my riding on the tandem is with my wife, so the need for a robusta is not nearly as strong as the want. but i will drool on one at interbike next week. i would probably keep the duet as the touring tandem and add another as more of a race type bike, some day...

We recognized similar problems with our C'dale (purchased brandnew in May 2008; only 400km), but only in case of starting after stops at traffic lights with high torque and big gear (e.g. 53x13)...but never during high speed riding. I cannot imagine that the ultra-stiff C'dale frame is causing this behavior. My stoker says she could reproduce this by strong pressure on left pedal.

In my opinion the chainline is the reason. We have a FSA triple crank set and Campagnolo (Chorus and Triple-) components (12-25 cassette). We can run the 53x23 without problems. A wider rear bottom bracket could help...

But what about changing the chainrings from 53-39-30 to 55-42-30?

I asked our bikeshop for an appropriate offer.

The 55t chainring should cause a better chainline by usage of a bigger sprocket. And we will no more run out of gears in fast downhill rides...

Greets from Germany

i like the idea. actually our chhainrings are 53- (42 or 39)-26. i could just get a drive side crank since i wouldn't need a tandem specific one. i thought for a second about a double but that would probably make the chanline worse. but i could put the big gears on the second drive side crank.

i think the chainline is a big part of the problem and the slightly narrower bb makes it worse. i'm not sure i understand how the bigger chainring will help the chainline except that we could sprint in the 55x12 instead of the 11.

You are right...for a throwing chain in sprint scenarios the 55t chainring will probably not help, because you need the biggest gear regardless of which chainring. Maybe some bottom brackets are truly too narrow. But our FSA crank set should be tandem specific...!! It would be depressing to replace the crank set after only 500km. That's why changed chainrings will probably be an inexpensive solution.
I will replace the components of my single road bike next year. I'm interested in the compact crank set Specialites T.A. Carmina with 185mm crank arms (I'm 6'9" tall) combined with the new Campagnolo Centaur 10s group. The well known tandem crank sets are mostly limited to 175-170 crankarms. Maybe there will be a way to introduce the Specialites T.A. Carmina to our C'dale Tandem...

What happens with your chain in case of high torque ride starting (with strong stoker pedal force) in the 53x12 or 53x11 gear? (I know this big gears are not the adequate starting gear...)

Yesterday was our first testride after introducing a new 55t Stronglight TT ring (CT) and a 42t FSA middle ring. I'm really surprised, because it seems that the problem is fixed. We tested some high torque ride starts also in 55x13 (our biggest gear)...absolutely no problems. I'm also satisfied with the new gear range and the shifting is also perfect.