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Chin Haur ISIS Bottom Bracket Axle Failure

Two Chin Haur CH-95 ISIS bottom bracket axles. Both failed in center section.

     

One BB was used on an MTB and lasted a few hundred kms of trail riding. Failure may have been accelerated by pedal strikes — although pedal strikes are common in MTB riding. The other BB was used on a road bike and lasted about 15,000 kms.

Both failed under one rider, weight about 75 kgf; bike weights and riding habits unknown.

Rider noticed failure when the crank and pedal fell off. MTB crank started creaking a few hundred kms before failure.

Failed road spindle led to injuries requiring elbow surgery.

This bottom bracket is designed to clamp together as a unit, rather than having separately adjustable cups. The right bearing is "captured" between the center sleeve and a shoulder in the bore of the the cup. The other cup has a smooth bore, allowing the bearing to "float", thus avoiding bearing preload that varies depending on cup tightness.

The smooth left bore means that failures on the left (non-captured) side allow the crank to fall off the bike. In comparison, a failure on the "captured" side would leave the crank rotating freely, but without falling off the bike.

     

Having the pedal suddenly separate from the bicycle sometimes leads to serious falls. A better version of this BB design might use a small lip that does not touch the bearing in normal use, but which captures the bearing in the event of failure.

As an aside, note the bottom bracket axle is a tube which is largest at the ends. To fit the berarings, aluminum spacers reach from the axle's outside diameter to the bearing's inner race diameter, so the bearing can slide over the outside diameter of the ISIS spline.

Both failures occured at or near the transition from bearing retainer shoulder to center section of the axle. Probably a more gradual transition would be more durable.

The axle center section is about 17.4 mm outside diameter and 15.9 mm inside diameter (about 0.75 mm wall thickness). Most other bottom brackets either have a thin spindle with a thick wall, or a thick spindle with a thin wall. This has a thin wall despite being about the same outer diameter as many square-taper bottom brakcets.

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