Balancing a lambretta crank/top end??

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Sat Jun 02, 2018 6:03 pm

Need some advice folks,
I'm building a 135/140 Vega. From what I'm told, when this is done the machine has a tendency to vibrate a touch. This gave me the idea that perhaps balancing the top end in some way may be an idea?? Now with a scoot where the flywheel and clutch are on opposing sides of ones crank this would seem to be a matter of matching these two sides and lo and behold you basically have a balanced situation. However, as Lambrettas have the flywheel and front sprocket on opposing sides it has me wondering how to move forward?? Do I simply weigh each half of the crank with their respective attachments and adjust to suit, or would I need to take into account the chain links that would be attached to the front sprocket at any given time as well?? The flywheel I'll be using is rather light in it's own right (I'm using the same basic flywheel as is on most Variable ignition systems but atm with only some sort of plastic flywheel attached for cooling)

I'd appreciate any help, advice or pointers in the direction of relevant information.


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Sat Jun 02, 2018 11:57 pm

This is a art unto its self and in order to end up with a system that could be considered “balanced” you’d need to do more than just making sure everything weighs the same. Rotational mass’, ratios and RPM all need to be taken in to account.

I’m not saying this to be discouraging but more of this rabbit hole goes deep.

At the end of the day you still have a single cylinder two-stroke, it will vibrate.

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Sun Jun 10, 2018 7:56 am

Interesting idea.

I would assume that the drive side components are fairly balanced as is the flywheel. And if they're not, they should be. The same for the crank. That would put the rotating assembly at a "zero balance" or neutral state, typically referred to being "internally balanced". Now hanging a chain off one side is going to screw that up slightly, but not any more than the transmission in your car does. The stress imparted on the drive side under heavy acceleration as opposed to that on the mag side is the big factor that contributes to a vibration issue, i would think. That exacerbates itself into a scenario where the crank is twisted "out of balance".

You are correct in thinking that all the rotating parts should be balanced, but it gets to a point where the "rabbit hole" goes too deep and beyond the scope of what's appropriate for a moped. :D

But I like what you're thinking! Best of luck!

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Sun Jun 10, 2018 9:54 am

when i build my rotax 290 i had the crank made by martin at chiselspeed, he built it from scratch, but i had to weigh the piston, rings, circlips and pin so he could balance the crank, it had holes drilled into the inside of the webs near the big end pin, i would guess he worked it out with a formula or a computer programe, end result is a 64 stroke and a 76 piston that runs pretty smooth, even at high revs, the crankshaft design is key to a smooth engine, con rod length plays a part as well i think.

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Sun Jun 10, 2018 12:14 pm

Is there such a thing as 'harmonics' (if that's the right word?). For example, I get vibration at something like 3,500rpm that clears by 4,000 rpm and I'm wondering if that is just the combining effect of the frequencies during those engine speeds. Outside of those figures the engine is very smooth.

As a test I have a BGM flywheel to swap with the current elemec one, so I'll see if that changes the behaviour at all.

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Wed Jul 29, 2020 11:19 pm

There is no need to match rotational masses of primary drive and flywheels this has nothing to do with crank balance BUT the flywheel should be finely balanced, ideally dynamically balanced. Crank balance on a single is always a trade between forced axially up and down the cylinder (forces along the bore, reciprocating masses) and forces across the block (rotational out of balance masses). The rotational out of balance counteracts the reciprocating mass. Consequently as the piston descends the crank counter weight rotates upward and as it descends the counter weight rotates downwards. Consequently at TDC and BDC the rotational out of balance mass swings cross the block which creates the force across the block. Crank balance is the % bias of forces across block or along the bore. The lowest values are when the force across the block is equal to the force along the bore. The simple fact to know is that the heavier the piston the greater the force along the bore. On Li engines this isn't usually a problem as the increased force is fore and aft in the engine so you tend to get away with a wide piston weight range. On Lui Vega engines where the piston is vertical it might be different. As George mentioned harmonics above it is also worth mentioning "specific frequency". Engine mounts will all have a specific frequency so certain rev ranges might "feel" to vibrate more than others even on a piston and crank that is well matched. Other tricks like inertial mass damping (heavy engine cases, and the reason your washing machine has a concrete slab bolted to the back of the drum housing) can be used to counteract specific frequency problems.

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