LCGB Forums

Any input appreciated on the following.
I have just returned from a 100 is mile round trip. On the outgoing half the scooter seemed to be acceptable vibration wise, however on the return leg the amount of handlebar vibration became pretty appalling. Is the likely cause a twisted crank? It's been in the engine 18 years. I don't think it is the engine mounts as they look ok. I haven't taken the engine bolt out for a close inspection though. The forks are not loose. A new mag housing has just been fitted. Any ideas appreciated!

Super

Tracking down vibration causes can having you chasing your tail and be pretty frustrating that’s for sure

You mention you’ve changed the mag housing recently and the vibration got worse over a ride
I’d be checking the flywheel area (cowlings, etc) and flywheel 1st

I assume you refitted the same bearing into the new mag housing else changed the inner track on the crank if you fitted new?

There’s thread here with a list of stuff to go through
http://www.ilambretta.co.uk/forum/viewtopic.php?f=10&t=2200&p=42086&hilit=Vibration#p42086

Start by checking the easy stuff 1st though

[quote="nsaints"]Tracking down vibration causes can having you chasing your tail and be pretty frustrating that’s for sure

You mention you’ve changed the mag housing recently and the vibration got worse over a ride
I’d be checking the flywheel area (cowlings, etc) and flywheel 1st

I assume you refitted the same bearing into the new mag housing else changed the inner track on the crank if you fitted new?

There’s thread here with a list of stuff to go through
http://www.ilambretta.co.uk/forum/viewtopic.php?f=10&t=2200&p=42086&hilit=Vibration#p42086

Start by checking the easy stuff 1st though [/quote
Same bearing, will try your suggestions. Many thanks! Super

Are you certain that the new flange has its bearing and spacers in the correct place?
Was a new inner bearing race also fitted to the crank shaft?
What brand of bearing did you use and did it come from a reputable source?
Was the crank removed from the motor to fit the new inner race or was it done in situ? Tell us what engineering methods were used here as incorrect ones could certainly lead to twisted crank alignment.
Whilst replacing the mag flange, did you re-new the woodruff key?
If so, then double check it to make sure it isn't bottoming out in the flywheel boss keyway.
Some woodruff keys can sit too tall in the crank if their diameter is slightly oversized. This results in the flywheel taper not locking concentrically on the crank taper making the flywheel run slightly wobbly, which will induce vibration and eventual shearing of the key with possible splitting of the flywheel boss keyway.
With the spark plug removed, turn the flywheel over by hand and check to see how concentric it is with respect to the magflange edge. A twisted crank or a binding woodruff key will usually cause the flywheel to wobble "out" at one point and conversely "in" 180 degrees opposite.

Fast n Furious wrote:Are you certain that the new flange has its bearing and spacers in the correct place?
Was a new inner bearing race also fitted to the crank shaft?
What brand of bearing did you use and did it come from a reputable source?
Was the crank removed from the motor to fit the new inner race or was it done in situ? Tell us what engineering methods were used here as incorrect ones could certainly lead to twisted crank alignment.
Whilst replacing the mag flange, did you re-new the woodruff key?
If so, then double check it to make sure it isn't bottoming out in the flywheel boss keyway.
Some woodruff keys can sit too tall in the crank if their diameter is slightly oversized. This results in the flywheel taper not locking concentrically on the crank taper making the flywheel run slightly wobbly, which will induce vibration and eventual shearing of the key with possible splitting of the flywheel boss keyway.
With the spark plug removed, turn the flywheel over by hand and check to see how concentric it is with respect to the magflange edge. A twisted crank or a binding woodruff key will usually cause the flywheel to wobble "out" at one point and conversely "in" 180 degrees opposite.

Wow that's a lot to go through , to busy at work to have a look until the weekend. Looks like fixing it may be abandoned until after euro lamb. Lucky me I have another scooter to go on. Many thanks, will post results when fixed. Cheers Super

Finally got round to fixing scoot. Turns out the offset cones had moved from a vertical to horizontal position causing the inlet manifold to be hard up against the frame.
Thanks again for advice.
Super

Actually Roy, you have over simplified this explanation.

The real problem was that your engine mounts had come loose and were able to rotate in their respective lugs in the engine. This allowed the offset cones to rotate with the engine mounts (all still torqued-up correctly).

Anyway, I refitted Roy’s mounts with a dollop of Loctite bearing retainer and he will have to monitor them for any further rotation.

If this fails we will have to try the dot-punch method of improving the interference-fit.

Knowledge wrote:Actually Roy, you have over simplified thus explanation.

The real problem was that your engine mounts had come loose and were able to rotate in their respective lugs in the engine. This allowed the offset cones to rotate with the engine mounts (all still torqued-up correctly).

Anyway, I refitted Roy’s mounts with a dollop of Loctite bearing retainer and he will have to monitor them for any further rotation.

If this fails we will have to try the dot-punch method of improving the interference-fit.

I've never tried it, but others have recommended grub screws and I thunk it's a better idea than dot punching

Knowledge wrote:Actually Roy, you have over simplified this explanation.

The real problem was that your engine mounts had come loose and were able to rotate in their respective lugs in the engine. This allowed the offset cones to rotate with the engine mounts (all still torqued-up correctly).

Anyway, I refitted Roy’s mounts with a dollop of Loctite bearing retainer and he will have to monitor them for any further rotation.

If this fails we will have to try the dot-punch method of improving the interference-fit.

The bizarre thing was that the cones were pretty well stuck in the tube and took a little persuasion to remove.
Odd!

really when using offset cones, you should weld one off the nuts onto the pivot bolt, marking top posn on the nut to suit the cones, then just tighten the other nut up keeping a looksey on the posn of the welded nut

As good a place to mention this as any

Many years ago, Dave Webster told me that the conventional tool for engine mounts - a large tube with cut-outs to suit & M16 studding/nuts - worked a lot better by overcoming the inherent friction in the device, thus transferring the majority of the force to the engine mount itself.

The way he did it was just with the simple inclusion of a roller thrust bearing between the M16 nut/washer & the tool. It needn't be a roller thrust bearing as a deep groove ball bearing - even a worn one - will serve the same purpose. You just need the addition of a large washer so that the outer element of the bearing is taking the force direct to the tool cap face. Or just have a bearing & washer between the mount & the M16 nut/washer. Better still, combine both. I'm sure you'll get the gist

Since then, with a number of varying size deep groove ball bearings, I have always pulled in or out engine mounts without ever resorting to heat. In fact, my tube is just that. No welded on cap with a hole to suit M16. I just use a deep groove ball bearing that the outer fits onto the face of the tube, then use a Christmas tree stack of smaller ones to get down to the M16 clearance hole size.

Why would anybody want to risk heating up the rubber & change it's properties or risk distorting/enlarging the precise fits of the mount to the case? The steel used in the mounts is 'swaged' smaller after the moulding process to add desirable, further properties to the rubber, so that steel is a grade 'very friendly' toward being shrunk in diameter yet again (& please. Don't think that the application of heat will not encourage it to shrink smaller) That appears to be what has happened here

I have literally changed dozens in my time & the only extra applied might be a good penetrating oil.

Great info guys!