How to Balance the Engine for Less Vibration


An imbalance in the crankshaft in relation to the reciprocating weight of the upper end causes vibration and a loss of power. Making sure your engine is balanced correctly is essential, especially if you are modifying the engine to work in a different rpm range than what it was designed for. Using a lighter wrist pin can help if the vertical forces are too much.

Click here to read why I don't recommend using the balance factor method.

I had two different 55cc engines using different cylinders and pistons. One was with piston port intake and the other was reed valved. The results I got testing those, along with online calculators for upper piston assembly inertia force and the centrifugal force of the counter balance is what I based my old theory of balancing on. The piston port engine was way off in balance, and the other was perfectly balanced. Using it as a base point I developed an Excel file for calculating the rotational forces on the crankshaft which allows you to input different counter-balance hole sizes till the numbers are right.

In calculating the offsetting centrifugal force of the imbalanced flywheel we treat the weight removed by the flywheel holes as an additional weight because on the opposite side of the flywheels there exists that additional weight. For example, if you removed the flywheel and put it on two weight scales you will see that the non-holed side has weight in excess of the holed side equal to the actual weight removed when the holes were made. So we use that "missing" weight to make the centrifugal force calculations.

1st test:
Piston port intake 55cc engine (see engine details below) ported for 10,000 rpm but that achieved only 9100 since I just did the test runs with the standard exhaust pipe instead of an expansion chamber with the correct header length for 10,000 rpm. Anyway here are the details:
upper assembly weight: 105.6gm (piston, rings, bearing, wrist pin, end of conrod)
additional counter balance weight removed: 9.8gm (via 7.2mm diameter hole on each crank wheel)
The engine vibrated between 5600 and 7900 rpm and ran smooth before and after that rpm range.

specs of piston port intake engine:
55cc Grubee cylinder/head on 48cc bottom end
port durations: 185 exhaust, 119 transfers, 125 intake
transfer port walls removed for greater transfer area
stuffed crankcase
155 psi cranking pressure
18mm Mikuni
custom intake manifold
piston port intake
slant plug head with squish band .65mm from piston
Kawasaki KX65 piston and rings (adapted for use with piston port intake)
Jaguar CDI with Kawasaki KX high voltage coil
44 tooth rear sprocket
26" wheels with mountain bike tires
peak head temperature: 425F

2nd test:
My other 55cc engine (reed valve, Honda piston, torque pipe, 18mm Mikuni) with 96.6gm upper assembly with 15.8 grams removed via a 9.15mm diameter hole thru both flywheels (centered between the two balance holes) allowed my engine to go up to 9150 rpm (downhill) without any bothersome vibration.

In figuring the counter balance weight it's important to include everything that would affect it. As example: my flywheel came with two 11.5mm diameter holes through both flywheels. The stainless steel there removed adds up to 50 grams. The conrod pin added 3.3 grams after the weight of the conrod pin holes weight were subtracted from it. Its weight was only calculated using the steel weight calculator listed below, not measured. The part of the conrod that is around the bearing, and the bearing itself, weigh around 30 grams. The centrifugal force has to be figured at the distances of 19mm of the conrod pin, 36mm of the additional balance hole, and the distance of the two counter balance holes.



Here are two useful online calculators. The second one may be needed if you use two different sized drill bits in the same hole with the largest bit only drilling a portion of the full depth. You can do that if you need a certain amount of weight removed but you don't have the right size drill bit.

centrifugal force calculator  (don't enter linear speed. change m to mm, change kg to grams, change N to lbf)

steel weight calculator  (multiply kgs by 1000 to get grams)

Here is a picture of my crank assembly with an additional balancing hole just above the conrod pin. The 6 blue holes are lightening holes for better acceleration (although I wouldn't recommend any more than 4 if the bike is for street use). The blue is foam filling half the hole. The ends of each hole were later filled with JBWeld. I used foam just to reduce the amount of expensive JBWeld used. The conrod hole and two factory balance holes are already filled with JBWeld for increased crankcase compression (which isn't important unless your engine revs to 9000 or more).
 

Concerning determining the weight of the lower conrod bearing and the part of the conrod that is around the bearing: I figured that by dipping the two into a measured amount of water and seeing how many cc (ml) they raise the water level and then multiplied their ratio by the total weight. My 48cc rod end with bearing had an equivalent 30 grams.

You can drill extra balance holes with any good electric drill although it's a bit tough. Much easier to take it to a machine shop and let them put it on a drill press. Also the holes can be drilled at the TDC location of the crank wheels without even taking it out of the crank cases. Just put duct tape on the crank wheels (after cleaning them with alcohol) to keep metal shavings from going into the crankcase, and then keep the crank in correct position by using vise grips on the primary gear above and below where it meshes with the clutch gear. You can measure from halfway through the angled tip and mark the drill bit at the correct distance with black electrical tape. That way you have a visual reference while drilling.

Click here to read more about my spreadsheet which can be used to calculate the size of counter balance holes needed in any crank.