The BMW design is also problematic as it relies on one 8mm locking bolt to secure the RHS slider to the axle. The locking bolt is in the centre of the fork rather than the side as in a multi bolt system providing less rigidity.
The axle has visibly pulled through the RHS fork slider in more than one failure.
The majority of known failures have been of the RHS fork leg and fork brace only. The failures have been across the entire production period up to the introduction of the revised castings and on machines of both high and low mileage, with the lowest mileage known of 1390 miles. The speed of the machines at times of failure have ranged from 20 kms/h through to highway speed limit speeds. The usage of machines has ranged from sealed road usage only through to rough bush track usage.
This has effectively eliminated the possibility of both a batch related problem and fatigue induced by the length of the period of usage, as well as speed or type of road usage.
Visual inspection of the machine which suffered failure of both fork axle lugs shows the RHS as a clean break and the LHS is peeled open like a can opener anti clockwise. This is due to the fact that the fork brace did not break and the LH fork leg was unable to steer/turn anti clockwise following the failure of the RHS. See pics below.
Metallurgy test have been conducted on failed axle mounts by both BMW and owners who have suffered failures.
BMW has not been prepared to release to owners the results of their testing, only being prepared to say there was no fatigue or metallurgical defects in the castings.
NHTSA states the failures are "forced failures". This means some combination of forces is exceeding the strength of the axle mounting lug.
The precise causes of failures are likely to vary on an individual basis with a combination of an under strength casting, manufacturing tolerances, manufacturing defects and metal fatigue.