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ken_hosking

Beta "Clutch Noise"

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I am interested in opinions regarding the so-called clutch rattle that occurs when the engine is running, but which lessens when the clutch lever is pulled in. My view is that the noise does not come from the clutch at all, despite the commonly used name, because the noise is at its loudest when the clutch is engaged and the clutch plates are locked together. I suspect that the noise comes either from chatter in the primary drive gears or chatter in the gearbox; probably the latter as the gears in the gearbox should be stationary when the clutch lever is pulled in. It’s not just a Beta issue, as all the various brands that I have ridden have made the same noise. 

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The non-factory bikes have the primary gear mounted with rubber dampers on the clutch basket. You may be hearing the flex in that part of the assembly. In the past  when I started to think I was hearing something odd from the clutch/primary I’d open it up and typically find nothing out of place. Only exception was a screw backing out because some dumbass (me) didn’t loctite it and it hit the rivets on the back of the primary assembly and there was no mistaking that for “normal” engine noise.😀 I’ve done mods to the clutch and shifter so I may be a little paranoid. Now I just ignore the clunky clattery Beta sound since the mods all seem to be fine.

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Thanks for the feedback Dan: I felt certain that you would have a theory relating to the noise. However, I continue to be of the view that it is gear backlash that is causing the noise. The primary drive and gearbox gears are straight cut rather than being helical in all trials bikes (to the best of my knowledge) and gear chatter is to be expected when the gear train is not loaded. Just to add another variable to the mix, some trials bikes make more of a whining noise than others. Presumably this is a machining tolerance issue in the components of the drive train.

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I have never seen an Evo without the primary gear mounted with rubber dampers, factory or standard. I know that there are aftermarket clutch systems without the rubber dampers.

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My 19 evo done it from brand new  as does my Fantic so did my vfr800 and I’ve now got a Yamaha tracer that also does it so I wouldn’t  give it another thought and get on with enjoy riding it 

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10 hours ago, technowaldo said:

My 19 evo done it from brand new  as does my Fantic so did my vfr800 and I’ve now got a Yamaha tracer that also does it so I wouldn’t  give it another thought and get on with enjoy riding it 

My purpose in raising this matter was not that I am at all worried about the noise, but rather that I would like to understand the mechanism giving rise to the noise. Who knows what insights might be forthcoming from the discussion?

 

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I've always figured the noise comes from the engaged "block" of plates slopping back and forth in the basket, which pulses with the piston. When you pull the lever the plates have less combined inertia (they then effectively form two blocks with a degree of slip) so the rattle is reduced. When you give it a few revs the frequency of the piston pulse is simply way higher than the frequency the block of plates can move at, so the rattle disappears. Probably viscous drag from the oil and other friction also works to keep the plates anchored against the basket fingers at higher revs.

No empirical data to back that up though. A wrap of tape on the basket fingers would probably be a quick and dirty way to prove it out, if only for a very short time! If it's the fingers then it should go quiet, if it's the gears it will remain.

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On 7/15/2021 at 9:30 PM, bikerpet said:

I've always figured the noise comes from the engaged "block" of plates slopping back and forth in the basket, which pulses with the piston. When you pull the lever the plates have less combined inertia (they then effectively form two blocks with a degree of slip) so the rattle is reduced. When you give it a few revs the frequency of the piston pulse is simply way higher than the frequency the block of plates can move at, so the rattle disappears. Probably viscous drag from the oil and other friction also works to keep the plates anchored against the basket fingers at higher revs.

No empirical data to back that up though. A wrap of tape on the basket fingers would probably be a quick and dirty way to prove it out, if only for a very short time! If it's the fingers then it should go quiet, if it's the gears it will remain.

My view is that when the clutch is engaged the clutch plates can't readily move in the basket as the whole assembly is locked up. My best guess remains that the noise is caused by backlash in the primary drive gears, or the gearbox, caused by the pulses coming from the engine, as you noted.

I should perhaps have started this post as an all-bikes post, rather than a Beta post, as they all do it, as technowaldo rightly pointed out.

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23 minutes ago, ken_hosking said:

My view is that when the clutch is engaged the clutch plates can't readily move in the basket as the whole assembly is locked up. My best guess remains that the noise is caused by backlash in the primary drive gears, or the gearbox, caused by the pulses coming from the engine, as you noted.

I should perhaps have started this post as an all-bikes post, rather than a Beta post, as they all do it, as technowaldo rightly pointed out.

@ken_hosking I think I disagree that the clutch can't move in the basket when it's engaged.

 The plates all bind up together but the whole cluster of plates and pressure plates is still somewhat free from the the basket. Any tolerance (slop) between plate splines and basket splines still exists. The plates are bound up with the hub because it's all part of the pressure plates, but the basket isn't part of the pressure plate system. The basket is just sitting on the bearings free to do it's thing. It's only when the bike is in gear that the output gear exerts significant resistance to the hub and therefore drags it all back against the fingers of the plates.So I still think the noise is just the whole mass of plates, pressure plates, springs etc clattering about inside the basket (or the basket bouncing around over the plate assembly).

At higher RPM either oil and bearing drag etc provides enough drag that it all sits tight against the basket fingers or the mass of plates etc. simply can't resonate at the higher frequency, or both. Also at idle a single cylinder two stroke RPM is bouncing around like nobody's business, so the basket is always being accelerated and decelerated as the engine pulses and surges, while the plates etc are acting like a flywheel inside the basket. Constantly a different speed between the two.

Take a look at the pic here to see the exploded clutch assembly - the only bit missing is the output shaft which runs through the middle into the hub. You can see that the two pressure plates and all clutch plates are completely free from the basket except for the splines, and the splines have bucket loads of free play in them (in the scheme of manufacturing tolerances), I'm guessing there's probably something in the order of 0.2mm free play between basket and plate splines - that's more than enough slop to cause a ruckus!

It's presumably not a big deal - the driven side of the basket fingers are always the ones that wear and cause issues, if it was caused by the plates banging about at idle I'd expect both sides of the fingers to be similarly knocked about, which they never are.

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15 hours ago, bikerpet said:

@ken_hosking I think I disagree that the clutch can't move in the basket when it's engaged.

 The plates all bind up together but the whole cluster of plates and pressure plates is still somewhat free from the the basket. Any tolerance (slop) between plate splines and basket splines still exists. The plates are bound up with the hub because it's all part of the pressure plates, but the basket isn't part of the pressure plate system. The basket is just sitting on the bearings free to do it's thing. It's only when the bike is in gear that the output gear exerts significant resistance to the hub and therefore drags it all back against the fingers of the plates.So I still think the noise is just the whole mass of plates, pressure plates, springs etc clattering about inside the basket (or the basket bouncing around over the plate assembly).

At higher RPM either oil and bearing drag etc provides enough drag that it all sits tight against the basket fingers or the mass of plates etc. simply can't resonate at the higher frequency, or both. Also at idle a single cylinder two stroke RPM is bouncing around like nobody's business, so the basket is always being accelerated and decelerated as the engine pulses and surges, while the plates etc are acting like a flywheel inside the basket. Constantly a different speed between the two.

Take a look at the pic here to see the exploded clutch assembly - the only bit missing is the output shaft which runs through the middle into the hub. You can see that the two pressure plates and all clutch plates are completely free from the basket except for the splines, and the splines have bucket loads of free play in them (in the scheme of manufacturing tolerances), I'm guessing there's probably something in the order of 0.2mm free play between basket and plate splines - that's more than enough slop to cause a ruckus!

It's presumably not a big deal - the driven side of the basket fingers are always the ones that wear and cause issues, if it was caused by the plates banging about at idle I'd expect both sides of the fingers to be similarly knocked about, which they never are.

Yes, that is indeed the probable explanation. I had overlooked the fact that the hub and pressure plates can move independently from the basket, albeit by only a small amount. Thanks for that insight, bikerpet.

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