Author Topic: Knock not rattle  (Read 2061 times)

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Ivy

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on: August 10, 2022, 10:13:47 am
Hi all.

My new Meteor that has only 70 miles on the clock makes a knocking sound on tickover when fully warmed up.
I will add that I have 40 years experience of riding, repairing and maintaining my bikes and it is not a rattle i.e. valve clearance but a heavier knock, I think I can actually feel a vibration at the same frequency.
The oil level is correct and it is only on tickover, it runs great otherwise.
Any one else experienced this? Is it normal?
I would like to be prepared for the dealers excuses.

Thanks for reading.
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SteveThackery

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Reply #1 on: August 10, 2022, 12:20:43 pm
Yes!  I've done some research and can tell you exactly what it is.

I was really bothered by it on my bike, to the point of talking it back to the dealer to be checked.  It's a heavy knock that can also be felt through the bars and footpegs.  The dealer said it sounded normal.

Anyway, here is the straight dope.  The balance shaft is driven from the crankshaft by a pair of gears.  Now, the balance shaft is not under any permanent torque, so the gear teeth can rattle against each other as the backlash is taken up one way, then the other.

This happens at low speeds because the crankshaft rotation is not at a constant speed.  It speeds up during the power stroke, slows down gradually during the exhaust an intake strokes, and then slows down fairly suddenly during the compression stroke.

The balance shaft, having its own inertia, is thus accelerated by the crankshaft during the power stroke, and then when the crankshaft slows down it decelerates the balance shaft.  Thus the drive to the balance shaft reverses back and forth in time with the compression and power strokes - the backlash in the gear teeth is taken up one way then the other, which makes the knocking sound.

Traditionally manufacturers use a lash-less gear drive to eliminate the knocking.  It comprises a split gear and some springs which remove all the backlash.  However, those lash-less drives are expensive.

Presumably to save costs RE has decided not to use lash-less gears to the balance shaft, which is why they knock.  They've replaced it with a cush drive on the balance shaft, apparently to soften the shock loading on the teeth.  However, it doesn't seem to reduce the knocking sound, sadly.

You might be interested to know that Triumph have also just decided to drop the lash-less gears to the balance shaft, in order to reduce friction.  To compensate they've tightened up the manufacturing tolerances.  I don't know if RE have used extra-tight tolerances on theirs.

Interestingly, lots of owners don't even notice the knock, but for us sensitive types, well, we've just got to live with it.  😐
Meteor 350

Previous:
'14 B5
'06 ElectraX (Good bike, had no trouble at all)
'02 500ES (Fully "Hitchcocked" - 535, cams, piston, etc - and still a piece of junk)

...plus loads of other bikes: German, British, Japanese, Italian, East European.


Ivy

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Reply #2 on: August 10, 2022, 12:42:36 pm
Thanks for your input Steve.
Looking at this parts diagram https://shop.motogb.co.uk/parts-catalouge/royal-enfield/937/32896 it looks like the balancer has the springs you mentioned.
Or am I misunderstanding you?
I hope it is normal, your explanation makes sense if they are lash-less gears.
Moto Guzzi --Making mechanics of motorcyclists for 100 Years--


SteveThackery

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Reply #3 on: August 10, 2022, 04:30:50 pm
Thanks for your input Steve.
Looking at this parts diagram https://shop.motogb.co.uk/parts-catalouge/royal-enfield/937/32896 it looks like the balancer has the springs you mentioned.
Or am I misunderstanding you?
I hope it is normal, your explanation makes sense if they are lash-less gears.

That's the cush drive I mentioned.  It uses springs but is nothing like a lash-less drive.  You can probably see that the gear on the balance shaft (the "driven gear") connects to the shaft through four springs.  This does nothing to remove the backlash between the gear teeth of the crankshaft gear (the driving gear) and the balance shaft (driven) gear.

Lash-less drives use a "split" gear with springs between them, like this picture.  Only one of the pair of gears needs to be made like that.

« Last Edit: August 10, 2022, 04:38:24 pm by SteveThackery »
Meteor 350

Previous:
'14 B5
'06 ElectraX (Good bike, had no trouble at all)
'02 500ES (Fully "Hitchcocked" - 535, cams, piston, etc - and still a piece of junk)

...plus loads of other bikes: German, British, Japanese, Italian, East European.


SteveThackery

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Reply #4 on: August 10, 2022, 04:32:25 pm
See how the springs try to rotate the "split" gears with respect to each other, so the teeth try to move out of line with each other.  When the teeth are engaged with another gear wheel, the teeth are forced apart by the springs and take up all the backlash.

You can probably also imagine that this arrangement will, indeed, result in extra friction, because the teeth on the split gear are being forced into the gaps between the teeth on the unsplit gear; the force being required to overcome the splitting force from the springs.

Is that OK?  Can you now see that the cush drive doesn't do anything to reduce the backlash between the two gears?  It's tricky to explain just with words!

Also, I hope I've convinced you that the knocking is nothing to worry about, and pretty well inevitable to some extent.
« Last Edit: August 10, 2022, 04:42:46 pm by SteveThackery »
Meteor 350

Previous:
'14 B5
'06 ElectraX (Good bike, had no trouble at all)
'02 500ES (Fully "Hitchcocked" - 535, cams, piston, etc - and still a piece of junk)

...plus loads of other bikes: German, British, Japanese, Italian, East European.


Ivy

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Reply #5 on: August 10, 2022, 05:09:22 pm
Thank you Steve, I have learnt something new today.

Every day's a school day :) :) :)
Moto Guzzi --Making mechanics of motorcyclists for 100 Years--


KENNRIDE

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Reply #6 on: August 12, 2022, 02:11:21 pm
Yeah thanks for a great explanation and picture of the pic of the lashless gear. ;)