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Cracked eccentric BB on Tandem

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Cracked eccentric BB on Tandem

Old 06-05-17, 02:45 AM
Big H
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Cracked eccentric BB on Tandem

Hi, we have a MTB Tandem that is built on a 6061 Kinesis aluminium frame. We are not “true” mountain bikers in the sense of the word, but we cycle urban roads and rural gravel roads and not the mountain roads. When returning from a recent cycle I noticed a play in the front eccentric BB. Upon inspection I found that the block with the grub screws to hold the aluminum insert has cracked along with a crack that separated some of the BB shell. Tandem frames are scarce in this part of the woods and I need to repair this. I will most likely follow some of the methods below:-
  • Remove the lug holding the grub screws to assess the crack damage to the BB shell. From inspection and probing with a feeler gauge the crack does not seem to have propagated very deep.
  • Have the crack welded and then weld a fabricated gusset plate over the crack.
  • Alternatively fabricate an aluminum gusset plate and fit by using two us bolts that fit over the bottom tube and the down tube of the frame.
  • The crack will be filled with a proprietarymetal epoxy and the gusset plate will be stuck to the existing bottom bracketplate.
  • Fit the Niner Biocentric II bottom bracket.
Constructive comments will be appreciated.
Attached Images
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Cracked BB.jpg (65.3 KB, 86 views)
File Type: jpg
Niner.jpg (29.9 KB, 85 views)

Last edited by Big H; 06-05-17 at 02:55 AM.
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Old 06-05-17, 06:54 AM
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does not look good , removing the cracked material, almost ready to separate, surface prep for welding,
and re welding the area will leave the BB shell so distorted*, by the work and welding heat
You'd need , re machining the hole to fit the eccentric insert..

* whole frame portion could be distorted by the welding... often they normalize, heat whole frames, to a softer state
to let the counter forces from the welding of the aluminum settle out.
Then re heat treat it, as a whole

major money , and logistics , finding a machine shop, etc. to make it work again,

consider shopping for a new rig.


Last edited by fietsbob; 06-05-17 at 07:03 AM.
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Old 06-05-17, 11:37 AM
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I am with Bob. Aluminum has a finite fatigue life and it looks like yours has come to the end. The crack started in the heat affected zone of the weld and grew.
Some manufacturers will warrantee their work on something like this. I'd try them first.
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Old 06-05-17, 08:20 PM
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I have to agree with Bob and David.

Salvage the components and get a new frame is the most cost effective route. Properly repairing that frame will take many hours of machine shop time.

The down tube will peal off the BB shell soon.

The failure location is not surprising given how the frame was originally fabricated as David alludes.
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Old 06-06-17, 12:27 AM
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Tandem frames has an oversize BB shell that is 55mm in diameter in front that used a eccentric BB that is used to tension the drive chain on the left hand side of the tandem. The aluminium insert in which a normal bottom bracket is situated is kept in place by two grub screws as I wrote in OP. The tension and stress induced in this area is caused by the point load of the two grub screws keeping the aluminium insert in place. Some may say it is a bad design that will eventually cause a crack but it is one of the ways to keep the eccentric insert in place. Other ways are the Niner BioCentric II BB that I included in a photo that clamps to the sides of the BB shell as well as the Bushnell BB that is a wedge type eccentric.

Other advice I have received is that I should remove the threaded grub nut block to inspect the crack that peeled away part of the BB shell. If aluminum corrosion helps to propagate the crack there is not much I can do. A non-welding option has been suggested where two machined rings should be manufactured that will be pressed in on both sides of the BB shell. I have a small metal lathe and will most likely be able to make the rings myself.

Thanx for the comments so far.
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Old 06-06-17, 11:38 AM
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You'll proably want to remove the paint from the edges of the bottom bracket if you "ring" it. Ideally, you'd find a way to use 4340 or some such (need a bushing of very stiff material to avoid corrosion between steel and Al frame). I think using AL is a losing proposition, as 1) the AL has already broken, and 2) the rings will not run full length. You also could make your own eccentric BB of solid AL (assuming you know how to do this, and have a 4 jaw chuck), get your kit set up with proper chain tension, and then drill holes through each side of the broken BB into your solid eccentric, then drill out the bb holes to >major pitch diameter, then tap the AL eccentric.

You may find it easier and cheaper to find a good tandem frame and replace...

Regarding the welding, the Lincoln Electric (very good US made welding machine mfr) suggests " If after welding, the structure is given a complete heat treatment (i.e., solution treat at 1000°F [540°C], quench, age at 400°F [205°C]), all of the material properties (even in the weld) will be recovered and T6 properties will be obtained. This practice is frequently followed on small structures such as bicycle frames, but it is impractical for larger structures. Furthermore, the quenching usually causes enough distortion of the structure that a straightening operation is necessary before aging."

T6 is a specific, high-strength condition of the heat-treated 6061 alloy. It's the state you want your frame's alloy in. Obviously the heat treat destroys the paint, and so you would have to weld, partially heat-treat, straighten, final heat treat, and then paint...

Last edited by WizardOfBoz; 06-06-17 at 01:14 PM.
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