Balancing Happy Time style 70cc Crankshaft

[swifty]

In thread #10 fabians solution to engine balancing is correct in theory, but the maths is incorrect.The total weight for the reciprocating components is 137.8 not 105.5 as stated.Pic #7 piston,rings, pin,clips,s/e bearing weighing 105.5. The last pic shows the conrod small end weight as 32.3. These two weights must be added together.105.5+32.3=137.8 then multiplied by the balance factor,55% in this case.137.8 x 55%=75.79. this weight is then added to the rotating weight of 47.4(conrod bigend & big end bearing).75.79+47.4=123.19g this is the correct bob-weight for 55% over balance using the weights shown in the pics. IMO i would use 55% for engines mainly used around 6000 rpm & 58% for engines used at around 8000 rpm. Peter.

 

[FelipeCobu]

In thread #10 fabians solution to engine balancing is correct in theory, but the maths is incorrect.The total weight for the reciprocating components is 137.8 not 105.5 as stated.Pic #7 piston,rings, pin,clips,s/e bearing weighing 105.5. The last pic shows the conrod small end weight as 32.3. These two weights must be added together.105.5+32.3=137.8 then multiplied by the balance factor,55% in this case.137.8 x 55%=75.79. this weight is then added to the rotating weight of 47.4(conrod bigend & big end bearing).75.79+47.4=123.19g this is the correct bob-weight for 55% over balance using the weights shown in the pics. IMO i would use 55% for engines mainly used around 6000 rpm & 58% for engines used at around 8000 rpm. Peter.

To make things even easier:

If you have an 80 cc engine just add 44 grans to the small end of the rod (54% factor here) and make sure your big end bearing is clean and moving smoothly.

To me four 9,5mm (3/8 inch) centered 7mm from the edge of the crank (not the counter-weight) and 4 to 5 mm deep was enough, the "relived" area was near the piston pin).

 

[happycheapskate]

Before you go tearing down your engine and drilling holes in it, try installing a spring-loaded chain tensioner. My bike (stock 48cc on steel frame bike with no chain tensioner and 44t cog) vibrated so bad at 25-30mph that my feet would go numb. I thought of getting rid of it as a project bike. But I made a spring tensioner and got a better chain, and WOW was it ever better! It has some vibration. It's a $100 gasoline engine. BFD. But I can even use my mirrors now for more than just knowing if a car light is on it, and my wrists don't get numb anymore.

 

[FelipeCobu]

Before you go tearing down your engine and drilling holes in it, try installing a spring-loaded chain tensioner. My bike (stock 48cc on steel frame bike with no chain tensioner and 44t cog) vibrated so bad at 25-30mph that my feet would go numb. I thought of getting rid of it as a project bike. But I made a spring tensioner and got a better chain, and WOW was it ever better! It has some vibration. It's a $100 gasoline engine. BFD. But I can even use my mirrors now for more than just knowing if a car light is on it, and my wrists don't get numb anymore.

I had noticed in my case that some vibration comes from a wobbling sprocket, by the time I realized that if I trued the sprocket I won't be shaken when riding.

So I go for a trued sprocket and the vibrations continues.


Now I've balanced the crank in my engine, and I have the stock tensioner.
I'm having the first rides after rebuilding it, so i don't revved it more than 30 Km/H, but the feeling was the same as a regular pedal bike, except by the noise

 

[Deleted member 12676]

Why go to the trouble of modifying the flywheel when all you need to do is add or subtract weight from the piston wrist pin?
Ive done that successfully and so I know what I am talking about.
If you do it the hard way then you have more to brag about I guess. Doing stuff he hard way is just not my thing.

 

[FelipeCobu]

Why go to the trouble of modifying the flywheel when all you need to do is add or subtract weight from the piston wrist pin?
Ive done that successfully and so I know what I am talking about.
If you do it the hard way then you have more to brag about I guess. Doing stuff he hard way is just not my thing.

The real problem was that my engine locked due to a bearing failure (china bearings really sucks) anyway I should split the case and had the work done.

So I had my crank trued by a motorcycle machine shop, and done the balancing at home. Also replaced the bearings with INA bearings. Also if the crank is balanced you can replace piston/ liner as needed w/o having to balance the whole stuff again.

It isn't that hard as is seems to be. Today i noticed some vibrations I do believe is coming from the muffler (it shakes the handlebar ends - not the whole bike as it was before) now i should figure out a way do secure the muffler.

I saw some pics of your bike and it looks like a rocket, congrats!

Felipe Cobu

 

[Purple Haze]

Clay, you are completely correct.

I have stuffed up the way i look at the balance factor percentages.

Looking at my modified crankshaft, in it's current form with a 70% balance factor, it would only be achieve balance at very high rpms.

The standard crankshaft is best balanced for low rpms.
This makes sense as both the engines (the original and the replacement) started to vibrate heavilly from 3500 rpm.
Below that figure they were both very smooth in operation.

If the same crankshaft is used for the 48cc engine, it would have good balance at higher rpms - not surprisingly, people comment on the fact that the small engines are much smoother to ride.

Crikey - you've got a full 5.36 horsepower to play with - your bike would have more than enough oomph to keep you happy.

I tried a quick and easy fix, and it worked pretty well. Weigh a 49cc piston on a good gram scale, then weigh the 66cc piston, and lighten it on the underside in a drill press to the 49cc weight. (clue: may shallow holes are better than a few deep ones. more like dimples to keep the structural integrety.) Also, a slight cut on the skirt at the intake port is good, it will change the intake timing for the better, the port is restricted at tdc, opening this up really wakes up the engine at higher rpm's.