Mathematical formula for balancing 69cc Chinese Bicycle motor

[Fabian]

G'day all

I've stripped down my Happy Time style 69cc Chinese bicycle motor.
It has a 40mm stroke and 47mm bore.

The crank has been split to remove the big end bearing and to allow for accurate phasing of the crankshaft halves.
I've found high quality replacement big end and small end connecting rod bearings, also high quality replacement left and right side crankshaft bearings - these also happen to be the same specification as the clutch shaft bearings.

Ok, i am not knowledgeable about crankshaft balancing or the mathematical formula to create a bob weight for dialing in crank balance.

Can someone with expert knowledge in this area advise how (or give a tutorial) to complete this task.

Fabian

 

[Flapdoodle]

Not sure I qualify as a "knowledgeable expert", but I have balanced small engines and reciprocating components for manufacturing equipment.

The easiest way for the home mechanic to do it is by placing the crank with connecting rod, piston and rings (don't forget the wrist pin retainer clips) between two smooth straight edges. The blades from two machinists squares are ideal.

Make a jig that hold both blades at a distance equal to the crank bearings. Plywood is fine. Use a good bubble level to get both blades a level as possible. Reverse the level so that the bubble is the same if measured either direction. (do this is in case the level is not accurate)

If you have a flat surface such as a ground steel plate, use it to make sure the two blades are parallel. If you don't have one you can do it by eye with fairly good results.

Put the crank with the rod and piston between the blades. It should not rotate no matter what angular position the crank is at. Tap the blades to see if the crank rotates. Remove a small amount of metal from the inside of the piston, or the crank counter balance as necessary.

This sounds awfully difficult, but it really quite easy.

 

[Fabian]

so what you're saying is that the connecting rod and piston should be able to hang from the crank pin as you rotate the crankshaft on the two straight edges.

If the piston/rod combo pulls the crank pin downwards, you remove a quantity of metal from the piston till it reaches equilibrium and stasis with the crankshaft counterweight.
If the crankshaft counter weight pulls the crank pin upwards, you remove a quantity of metal from the counterweight till the combination reaches equilibrium and stasis with the piston/rod combo.

Seems like an unbelievably simple method to achieve perfect primary balance.

Fabian

 

[Al.Fisherman]

The area that material needs to be removed should always be on the bottom (baring a bad bearing). Think of this as a plumb line.

 

[clay]

no. if it is balanced that way, it will be balanced until the motor fires up then will progressively become more out of balance until it shakes apart. a well balanced motor will shake at idle and will become smoother as rpm's increase.
i explained it a little here http://motoredbikes.com/showthread.php?t=12879&page=6

 

[Fabian]

Yes, clay has a good point.

In the below link, they recomend "over balancing" to reduce crank pin loads.

http://books.google.com/books?id=ie...cing#v=onepage&q=crankshaft balancing&f=false

 

[Fabian]

so, what method does the home handyman use to balance variants of Happy Time style Chinese Bicycle motors?

 

[clay]

 

[Fabian]

Okies,

After splitting the crank on my 69cc Chinese Motorised Bicycle engine to replace the big end bearing (that always fails on these Chinese engines) with a high quality item, i've been able to measure the dimensions for a replacement part.

It's an oddball size needle roller bearing: 16mm inside diameter, 21mm outside diameter and 10mm in width, also known as a timkin style bearing K16x21x10
I have been trying to find a Japanse motorbike that uses this bearing so i can order a genuine quality Japanese part.
All i've been able to find that uses this bearing is the Suzuki FZ50 and one website listed the following, though they did not give a Suzuki part number which complicates the process enormously, as the motorbike shops just want a Suzuki part number; they are not interested in the dimensions of the bearing.

JM118 JAULA DE MUNON DE BIELA - 16 x 21 x 10 HERO PUCH 65 TUBU / SUZUKI FZ50 2,13

I just need an Illustrated Parts List for the Suzuki FZ50 to work out the part number

 

[Fabian]

Ah-ha

This is looking better - a complete connecting rod kit that includes big end and small end bearings for the Suzuki FZ50.
I have no idea on the centre to centre length of the Chinese Bicycle Engine connecting rod as compared to the FZ50 but if it's the same dimensions, the quality of bearings and connecting rod would be significantly better - check out the pic

http://motorcycleproducts.co.uk/catalogue/con-rod-kit-suzuki-fz50-198086-p-133050.html