Rear cassette tandem failure
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  1. #1
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    Rear cassette tandem failure

    Riding up a steep incline on the tandem on the weekend and something started to make noise. As we crested the top, the chain was dancing on the rear cassette. Upon removal of the cassette, I find that the carrier has cracked at each of the four arms at the "rivets" which hold the individual cogs onto the carrier.



    This is a 9-speed mountain cassette, 11 x 32, a CS-M750 made by Shimano.

    Common failure mode? We're probably 350 pounds all-in, including the bike. The hill was 19% for maybe 50 feet. Our gear is a 28 up front with the 32 in the back, often can do the short climb in the 28 in the rear, but we weren't feeling that spunky and did it in the 32 instead.

  2. #2
    tlg
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    Can't say if it's a common failure mode. But 350lbs on a 19% climb.... dang that's some torque on the cassette! Not surprised at all.
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    I've never seen this before.

  4. #4
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    Me either but I'm also not too surprised. Two strong riders on a tandem can lay down a lot of torque. On steep climbs my wife and I would regularly flex the frame of our MTB tandem enough to cause chain suck with brand new drive chain components in good adjustment.

    You may also have had a small nick/gouge at the rivet on one arm that acted as a stress-riser. Once that one arm broke the others didn't have the strength to hold together. I once had a crank arm fail because of this.

    Also, I think part of the design strategy for XT parts is to shed weight. But while a lightweight part may be strong enough for a single rider it might not be up for tandem use. Maybe replace that with a lower-end non-spider cassette?

  5. #5
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    Quote Originally Posted by dir-t View Post
    Also, I think part of the design strategy for XT parts is to shed weight. But while a lightweight part may be strong enough for a single rider it might not be up for tandem use. Maybe replace that with a lower-end non-spider cassette?
    XT seems to be sold at many tandem specific retailers, so I doubt it is a common problem. But it might be a known issue for advanced and strong tandem riders.

    Might want to browse/ask on some tandem specific boards for this one. Tandems are their own thing in many ways.
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  6. #6
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    Quote Originally Posted by bike867 View Post
    Riding up a steep incline on the tandem on the weekend and something started to make noise. As we crested the top, the chain was dancing on the rear cassette. Upon removal of the cassette, I find that the carrier has cracked at each of the four arms at the "rivets" which hold the individual cogs onto the carrier.



    This is a 9-speed mountain cassette, 11 x 32, a CS-M750 made by Shimano.

    Common failure mode? We're probably 350 pounds all-in, including the bike. The hill was 19% for maybe 50 feet. Our gear is a 28 up front with the 32 in the back, often can do the short climb in the 28 in the rear, but we weren't feeling that spunky and did it in the 32 instead.
    Many years ago my then 14 year old daughter and I drove the pawls of a freewheel right into the body, deforming the metal. That is hardened steel at the ratchet surface. Tandems combine a heavy load with a large power source. Component failure often results. If companies made "tandem specific" components for these high-load applications, hardly anyone would buy them because they would be "too heavy."

  7. #7
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    350 lbs, 19% grade... and still less torque/force than one 160 lbs pro sprinter. I'd chalk it up to a fluke. Or just fatigue from thousands (?) of miles.

  8. #8
    tlg
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    Quote Originally Posted by OldZaskar View Post
    350 lbs, 19% grade... and still less torque/force than one 160 lbs pro sprinter.
    Power maybe. Torque... no.

    A pro sprinter putting out 1500w at 120rpm generates 119Nm Torque (1050 in-lb). With a 172.5 crank arm, that's a force of 154lbs. (The max force a 160lb person could exert on a crank arm is ~160lbs and 1100 in-lbs)
    Torque (N.m) = 9.5488 x Power (kW) / Speed (RPM)

    Two grown men weighing 150lbs each, standing on a climb, would generate about 2000 in-lbs of torque. The forces on the cranks, chain, and gears are significantly higher than a pro sprinter. But at 30rpm it's only 700w so the power is much less. Power is force over time.
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    Quote Originally Posted by tlg View Post
    The max force a 160lb person could exert on a crank arm is ~160lbs and 1100 in-lbs)
    Why... because he weighs 160lb?

  10. #10
    tlg
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    Quote Originally Posted by OldZaskar View Post
    Why... because he weighs 160lb?
    Yea. Maybe a little more using the bars as leverage.

    The math is pretty simple. A pro sprinter putting out 1500w at 120rpm has a crank force of 154lbs.

    If a pro sprinter was putting out the torque of two standing riders, at 120rpm, it'd be 2,900w.
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