I didn’t get in any riding last week due to inclement weather, but I did collect the last of my outstanding PowerParts from Alfie Cox Racing the previous Saturday, i.e. the Akrapovic slip-on and a set of rear crash bobbins. I will do a separate post about the Akrapovic slip-on later, but I’m going to focus here on an unexpected problem with the fitting of the bobbins.
I’d previously fitted the front crash bobbin set without any hiccups at all – it really is a 5 minute job – the only delay caused by having to go off to the nearest good hardware store to find a 27 mm socket, which is needed to undo and re-torque the spindle retaining nut. For the back wheel a 32 mm socket is needed to secure the rear retaining nut.
The kit (part no 75610945000) comprises 6 items – left and right bobbins (parts no 61310945001 and 61310945002 respectively), two M6 collar screws (30 mm and 45 mm long respectively) for attaching the bobbins to the mounting axle, the axle itself and a replacement retaining nut. After removing the spindle retaining nut which comes with the bike I replaced it with the kit item, and tightened it to the prescribed 90Nm.
Now for the bobbins – left bobbin is pre-mounted to the axle, then threaded through the spindle and the right bobbin then attached. The kit provides no torquing information for the bobbin screws, but I guess around 5 Nm is more than adequate. Anyway, before reaching the point when I had to think about torque values, it became obvious that it was going to be impossible to seat the right bobbin properly. It appears that there is a design fault with the inner mating surface of the bobbin which means that it cannot sit true on (or over) the spindle retaining nut, but will rather cause it to slip out of alignment in either the horizontal or vertical planes – or both. The photo below (apologies for quality) clearly shows the effects of the problem.
As mentioned earlier the problem seems to be caused by the design of the inner surface of the bobbin.
You can see from the photo above that there are six ‘arms’ with raised ‘tabs’ where the arms meet the cylinder wall and that the point of contact between the bobbin and the retaining nut, must therefore be at the tabs. I can’t really work out why the tabs are there at all, their effect is that the bobbin lines up with a narrow raised portion of the retaining nut in just such a way that it is inevitable that one or more of the tabs will slide off and down that raised portion, leading to the misalignment described.
The tab to opposite tab distance is 28 mm and so would perfectly hold the bobbin in place on a front wheel retaining nut which is 27 mm across the flats. Could the problem be so simple that the wrong retaining nut was used when designing the bobbin??
Anyway, I can’t fathom it out so have asked KTM to look at the issue. I’ll report back when I have an answer.