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Drive Sprocket Failure

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
3 days back when in a tunnel on my Comet(the Melba on Eastlink) my bike lost all drive and I coasted to a stop, hugging the tunnel wall at trucks roared past, just a few inches from me. The motor was still running, I could change gear but no forward momentum.

Rescued by a breakdown truck, once home I eventually discovered that ALL of the splines on the inner of the gearbox drive sprocket had "disappeared"; the inner of the sprocket had the appearance of having been machined clean.

With the debris removed and all cleaned up a new sprocket has been installed and my Comet is back on the road again.

The failed sprocket was installed around 12,000 miles earlier. I can only (speculate) that the retaining nut was not sufficiently tight to prevent a very small rocking action that eventually wore the splines away. Fortunately, there is no apparent damage to the Burman gearbox output shaft or splines.

Has anyone experienced this type of failure before?
 

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
not helpful, but yes on a unit Triumph. There was a barely perceptable 'knock' on deceleration. On inspection the retainining nut had slackened off.

Your experience sounds like the thing of nightmares.

Surely on reflection you noted some associated increased mechanical noise before complete failiure.?

Cheap fix for a lesson learnt.
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
3 days back when in a tunnel on my Comet(the Melba on Eastlink) my bike lost all drive and I coasted to a stop, hugging the tunnel wall at trucks roared past, just a few inches from me. The motor was still running, I could change gear but no forward momentum.

Rescued by a breakdown truck, once home I eventually discovered that ALL of the splines on the inner of the gearbox drive sprocket had "disappeared"; the inner of the sprocket had the appearance of having been machined clean.

With the debris removed and all cleaned up a new sprocket has been installed and my Comet is back on the road again.

The failed sprocket was installed around 12,000 miles earlier. I can only (speculate) that the retaining nut was not sufficiently tight to prevent a very small rocking action that eventually wore the splines away. Fortunately, there is no apparent damage to the Burman gearbox output shaft or splines.

Has anyone experienced this type of failure before?



This was a common thing in early years with Japan motorcycles. Material and the heat treat of the sprockets was not up to par. The sprocket wears out so it is highly unlikely that the sprocket you installed 12,000 miles ago was new old stock and it would be any bodies guess where the sprocket was made . So actually in my opinion the longevity wasn't that bad , I've seen worse . It is unlikely that a loos nut caused the spline to go away. There are a lot of things on a Vin where inner and outer splines are working with no nut.

Regards Max
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Hi Martyn, When I bought my Comet, the nut was rattling around on the shaft....not even a single thread engaged, since then I try to safety lock wire all the important bits.
Having said that, on my first excursion around the block on the Comet, first corner and my brand new, shiny, beautiful muffler went sailing past, I had only pushed it on over the pipe.
Was the failed sprocket brand new when fitted 12,000 miles ago?

Kevin
 

Albervin

Well Known and Active Forum Website User
VOC Member
What a place to have a failure to proceed! On the plus side it wasn't Oodnadatta! Sprockets have been made by various people over the last 20 or so years. Just because you bought it X years ago doesn't mean it was made 10X years ago. I have seen generator sprockets lose all their teeth (on a twin) within 2,000 miles. That there was no apparent damage to the splines indicates the sprocket was a lot softer than the spline. Was a lock washer in place? Loctite? I would consider 12,000 miles less than satisfactory but there is no warranty on these parts. As an aside, in the early days of the twin there were numerous claims for gear and sprocket failures in the timing chest and elsewhere. Lots of warranty claims led to upgraded parts in the timing chest and gearbox. One would have thought the lessons had been learned.
 

Hugo Myatt

Well Known and Active Forum Website User
VOC Member
Some people believe that Loctite is sufficient to lock the nut and omit the lockwasher. My experience is that it isn't. Nowadays I always fit the lockwasher.
 

greg brillus

Well Known and Active Forum Website User
VOC Member
The trick is to make sure the sprocket spline is a good fit on the spline with no play...........no amount of Loctite, tabwashers, etc will help if this is not the case. Heck you can always weld it on to the spline, this works every time. Strictly for the rough buggers amongst us.......Oh that's right, Vincent owners don't do rough work do they.......???:p
 

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
Hi Martyn, When I bought my Comet, the nut was rattling around on the shaft....not even a single thread engaged, since then I try to safety lock wire all the important bits.
Having said that, on my first excursion around the block on the Comet, first corner and my brand new, shiny, beautiful muffler went sailing past, I had only pushed it on over the pipe.
Was the failed sprocket brand new when fitted 12,000 miles ago?

Kevin
It was a brand new sprocket, purchased 18 months back (yes, I DO use my Comet!) from the VOC spares Company. There was a lock washer in place and nut and lock washer were secured. After I removed the clutch basket I was able to observe the nut and sprocket rotating independent of each other!

FWIW at the same time I installed the (failed) sprocket I installed a new driven sprocket (49 teeth) on the rear wheel and a new final drive chain into the bargain. I have 2 final drive chains and swap them around every 1,000 miles covered. there is no discernible wear on the teeth of either the original drive or driven sprockets.

I now have another new sprocket in place and will be road testing tomorrow - Feb 13.

I cannot think of a method of examination to check for drive sprocket spline wear that does not involve removal of the clutch, ESA and inner case. Any ideas??

Martyn
 
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