Many tandems have a front bottom bracket that looks like this:

pinch bolts:

The front bottom bracket is mounted off-centre in an aluminum shell called "the eccentric." When the pinch bolts shown in the picture are loosened, the eccentric can rotate within the bottom bracket shell to tighten or loosen the tandem's timing chain (that joins the front and rear cranks).

If the pinch bolts broke, or if the braze-ons they fit into sheered off, would the bottom bracket shell pop open? It seems to me that the integrity of the bottom bracket shell is totally dependent on the bolts; that you'd bend the frame quite badly if they broke.

  • If you're on the bike and you shear off the braze ons, I'm guessing you have larger problems to worry about.
    – Tha Riddla
    Jul 1, 2012 at 14:55
  • Right. Keep in mind that the "braze-ons" are fastened as securely as the frame tubes. Not many people do technical downhills on tandems. Jul 1, 2012 at 22:06

2 Answers 2


From the photo, it is possible to notice that the lower part of the front downtube, and the lower part of the bottom bracket boom tube (the one connecting both bottom brackets) surround the eccentric at an angle quite larger than 180 degrees. The following exagerated ilustrations shows it:

enter image description here

That means, even if both bolts brake (If one breaks, the sudden load increase could indeed cause the second one to break), most probably the bottom bracket tube wouldn't fully open, and the eccentric wouldn't pop out, because the said tubes would avoid it, or at least avoid it enough that the rider would notice the abnormal behaviour and safely stop the bike.

Hope this helps


First off, in order for there to be any sort of a "crisis", both bolts would have to fail, an unlikely possibility on a reasonably well-maintained bike.

But, beyond that, the integrity of the frame doesn't depend on the BB shell resisting twisting loads. Rather, due to the "truss" nature of the standard bike frame (tandem or not), the load on the shell is almost totally "radial" -- force into or away from the shell. While this force would tend to "pry" the shell open, it's very unlikely to be substantial enough to cause catastrophic failure.

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