How can this [1] piece fail so many times?

[1] The piece that links the rotation of front wheel axle to the cable
that goes to the speedo, which costs some 90 quid for a Thunderace or
a YZF750R.

It failed after 45.000Km, then again at 85.000Km, and recently after
only 10.000Km. According to the factory guy, it is not a common
problem, which is proved by the fact that few replacement pieces have
been sold (or people CBA and put a push bike digital speedo).

Any ideas? Tyre service guys applying too much torque, may be?

 

Is it the usual component, that has two "ears" that engage in slots in
the cover? It's possible that it has not been correctly aligned after
the wheel has been replaced. The speedo drive might then work, but
failure would definitely occur. The trouble is I can't believe that
anyone could replace it wrongly three times.

Alternatively, someone may have left out, or incorrectly replace, a
vital washer or spacer. This is rather more likely, as if it was
omitted or wrongly positioned the first time (at 45k km), then the
other times anyone replacing the wheel would have carefully replaced
the parts in what they thought was the correct order, not knowing that
they were wrong..

<Pause while TOG has a look at the schematics on www.cmsnl.com>

OK: http://www.cmsnl.com/yamaha-yzf750rf-1994_model10164/partslist/D-11.html

Is it the component marked 5 that fails (I suspect it is) or the
component marked 11? Are all the other components assembled in the
correct order?

 

Sir is right.

I haven't dismantled it. Only removed the speedo cable (well greased) and
noticed tha the inner part of #11 didn't turn.

 

I'll pay some beers to Mechanically Skilled Friend and check it ourselves.

Guy at the shop said #11 needed replacement, but I'm not sure about what he
did.

I'll keep in mind your hypothesis, which make sense.