Gear change Grrrrrrrrrremlins!!

Stephen_Spencer

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Awesome 150 km ride today, until the final half an hour! Pulled out to blast past a couple of cars whilst chasing my mates Ducati, went to change up - nothing! I knew exactly what it was straight away as this was my second such experience.

My gear change actuating rod had snapped at the forward threaded portion! For the second time; the first was inside warranty. Clutch in, freewheeled into a rural driveway, car horns blaring! Yup, snapped in exactly the same place:(.


So, 20km from home, no way to bodge up a repair. It’s RACQ home, or hand select a gear and try riding. The actuating rod snapped in 4th (I think). Although on a slight up hill section, she still pulled away, just; bags of revs and feather that clutch! Made it home easy enough. Felt the clutch physically slip a couple of times and there was some labouring occurring, obviously. In hindsight I should have tried to hand select 3rd.

Has this happened to you? If not, I suspect the likelihood is pretty high that it will. The design is obviously poor, so what’s the answer? Has anyone designed a more robust link bar? Should we all carry a spare and the tools to change it roadside?

Pulled into my garage over 5 hours ago and the engine casings are still warm to the touch. I’m hoping not to have damaged the clutch in any way, guess it’s designed to slip when under excessive load.

Any thoughts my fellow Nortonistas? I can accept one failure, bad batch right? But two?!

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Fast Eddie

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I think it’s the design. Due to the curve in the linkage the stress on that rose join is increased. It is not pushing / pulling ‘in line’ as it would with a straight linkage.
If it’s not possible to modify to use a straight linkage, then heavier gauge rod and joints would seem your only option.
Or, perhaps, lighter weight boots ??
 
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Split looks to be at a thread root, as Eddie says the bend is the problem leading to failure at weakest point. As you still need the bend to clear the case then a solid rod with no weak thread roots and larger diameter as extra insurance would work.

Other solution would be to lengthen both foot and gearbox levers the same amount until a straight rod would not foul the cover.
 

BLIGHTYBRIT/SF

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Mine was in second gear & I was only a mile from home ,Mileage was 16K at the time , over 20k now & ok , I do carry the old one with the Snapped of threaded stud Now reattached.
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I remember that John Snead (G81 Can Cycle) had made a 3/8" diameter shift rod for the 961 he was racing . So , the parts are available somewhere to do this. You must be doing a great deal of hard and fast shifting ? In 5,500 miles you broke yours twice , and I am going on 17,000 miles and yet to break it ? Is you gear change difficult requiring extra pressure for shifting ? Can you compare your bike to other 961's in you area ? Try to find that 3/8 " shift linkage , that should last ! Here they are in metic size , all the way up to 14mm ! You want the CS type with safety catch .




 
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BritTwit

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I think it’s the design. Due to the curve in the linkage the stress on that rose join this increased. It is not pushing / pulling ‘in line’ as it would with a straight linkage.
If it’s not possible to modify to use a straight linkage, then heavier gauge rod and joints would seem your only option.
Or, perhaps, lighter weight boots ??

How about modifying the linkage rod at the foot lever end, where the rod is straight?
Use a solid piece of rod from the rose joint back close to the foot lever, then insert a threaded adjuster close to the lever end.

Another bad choice by Dreer to "rethink" his original 952 shifter shaft position below and behind the clutch cover, and reposition the shifter shaft on the 961 back to position between the clutch and alternator cover. If the 952 didn't shift smoothly, he should have hired a mechanical engineer to design a better pawl and detent mechanism for the 952.
Water under the bridge now.
 
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Happened to me last year, with less than 2000 miles done. Got home in 3rd gear. Factory replaced it quickly and foc, but you've got me worried now
 
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or get a front ball with larger male thread, and weld female-threaded section over a cut back rod??
 
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Is it possible they are getting over tightened ? The jam nut may be part of the problem causing the installer to not realize they are causing stress and micro fractures . I see that the 6mm toe pedal bolt does not fracture like this .
 

Stephen_Spencer

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Thanks guys, all good input as ever. Always thought my gearbox to be particularly slick, although I’ve not ridden any other. Gear changes are relatively smooth with no unusual force required. I, of course, have the touch of a prima ballerina!

I do ride aggressively, riding with some serious sports bike mates who like to scratch a little. As we know you need to be in the right gear when hustling this bike. There is a beutiful, tight but smooth series of bends local to us on which I struggle because quite literally, 2nd is just to low to drive between the bends and 3rd is just to high to drive out of the corners so it’s up and down the gearbox we go. Point I’m making is that I use the CR as a Sunday afternoon thrash toy (pardon the expression). No highways, no distance touring with minimal gear changes. No doubt a significant difference in the way we each ride our bikes.

‘The 3/8“ shift linkage looks interesting Tony, but where did he get the rod, it must be unique to this family of bikes. Found a decent engineering firm I guess. That bend needs to be pretty spot on. When my original failed, the warranty replacement rod was the wrong profile and did not fit. We had to steal one from a showroom bike. Engineering a more robust linkage is obviously an option - I’ll have a look at this, maybe using the broken rod.

Over tightening is possible I suppose - I may have been over exuberant given that I did’nt want this to happen again. Unlikely though. Norton tightened the original failure and other bikes that have suffered this failure. Adjuster nut and lock nut are both regular hex jam nuts. May use a hex jam Nyloc nut as a lock nut instead. Less force required.

‘In the meantime I will order a 3rd time lucky assembly. Can somebody please provide name (Maria?) and best e-mail contact for spares at Norton? I realise this may have changed.

All of the above aside, I believe this to be a clear design flaw and a significant potential point of failure for all of us - so as a minimum it requires a Mark 1 eyeball on a regular basis. Depending on cost, I may carry a spare. My opinion only. Thanks guys.

Steve.
 

Clive

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This is obviously how the thread is cut creating an edge where a fracture will start to spread from day 1.

Option1 - ride with slippers on
Option 2 - drill it, tap it and threadlock a piece of stainless threaded bar in. Haven't looked - is there enough diameter? Any mechanic can do this.
Option 3 - trim the end of the rod and make a collar to fit over the end with a thread the other end for the ball joint. Loctite and pin.

Add it to the list of 'You should do this' preventative maintenance jobs.
 

Fast Eddie

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Option 4, weld the sucker!

Seriously, if the rod is stainless, I’d weld the rose joint to the rod at the engine lever
end and use the adjustment at the footrest end. You don’t need adjustment both ends.

It is such a small thread. If you look at the angle it’s expected to operate at, it is very acute. Imagine bending it like that in your hands, you‘d probably be able to snap it if you tried hard enough. Pulling it straight, and relying on it’s pure tensile strength, you’d have no chance.
 

robs ss

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Option 4, weld the sucker!

Seriously, if the rod is stainless, I’d weld the rose joint to the rod at the engine lever
end and use the adjustment at the footrest end. You don’t need adjustment both ends.

It is such a small thread. If you look at the angle it’s expected to operate at, it is very acute. Imagine bending it like that in your hands, you‘d probably be able to snap it if you tried hard enough. Pulling it straight, and relying on it’s pure tensile strength, you’d have no chance.
Similar to spokes - you don't "cut" spoke threads, you roll them.
Maybe this is the answer to this problem - if it's a problem.
Cheers
 

Clive

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If you look at the part which was fractured the ball joint and lock nuts were all tight together so no great amount of flexing. I reckon the tension when locked together has been pulling the fracture on the cut thread apart (yes - rolled thread is best) and the tiny amount of flex completed the job. Still reckon drill tap and, if you really want, a piece of high tensile bolt thread.

I hear that Nigel can tear telephone books in half with his teeth but I would challenge him to bend a high tensile thread even as small as that.;)
 

Fast Eddie

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High tensile threads prevent stretching Clive... not bending... that’s assuming it is a high tensile thread... which I would doubt.

And anyway, we don’t have telephone books any more !
 

Clive

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High tensile threads prevent stretching Clive... not bending... that’s assuming it is a high tensile thread... which I would doubt.

And anyway, we don’t have telephone books any more !
I guess that's why they are called high TENSILE but I would still challenge you to bend one.

No telephone books??? Here in leafy Solihull we recently burned someone as a witch for using a telephonic apparatus without any connecting wires!
 
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Hi tensile will snap easier as its more brittle, hence why shafts that get bent are case hardened to retain a soft core, its the thread root that is the point of crack propagation so you need rid of the root. What I have down in the past to repair a shaft is to have an 8.8 hi tensile bolt screwed into both halves and then, with a wide gap so the welder can get in to the bolt, had the shaft halves jointed with mild steel welding rod. You get a hi tensile core with a softer exterior which resists bending from the core but will not propagate cracks from the surface.
 

Clive

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It’s only M6... and that linkage rod makes a long lever...
The engineer in me likes to know real numbers - back to you with the results soon Flatulent Eddie.

Down to the workshop Igor we have experiments to do....
 

Fast Eddie

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The engineer in me likes to know real numbers - back to you with the results soon Flatulent Eddie.

Down to the workshop Igor we have experiments to do....

Well, let’s not digress too much. The point is that they do break, and as far as I can recall, all in the same place. So there’s sumthin to look at...
 
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