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.

When the piston goes to TDC and then is pulled back down by the crank, the force on the crankshaft is towards the cylinder head because that changing of direction creates a force of inertia upwards. Put any weight on a string and throw it away from you and then jerk it back to you and feel the outward force. Same thing. And the same thing happens at BDC. Those up and down forces create vibration. But that is countered by having holes in the crank wheels close to the conrod pin. The only problem is that the holes then allow a forward and reverse force to act on the crankshaft due to the unbalanced crank wheels. So a "balanced" engine has crank wheel holes just the right size to create equal forces in line with the cylinder and perpendicular to the cylinder. But since those forces peak every 90 degrees of crank rotation then basically the end result is a kind of triangle of forces. The graphs below are from my crank balance calculator. You can see that when the holes are too big that the predominant direction of force deviation if perpendicular to the cylinder. And when the holes are too small that the predominant force direction is in line with the cylinder.



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 or BDC 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 drill bit tip and mark the drill bit at the correct distance with black electrical tape wrapped around it. That way you have a visual reference while drilling. Here's one of my cranks (Suzuki AX100) with holes drilled 180 degrees from the crank pin:

DIY Crank Balance Test:
You can test for engine movement due to crank imbalance from mid RPM to top RPM just by holding a bicycle spoke to the cases while you rev it. Too little sized balance holes don't counteract the piston assembly weight enough and so make the spoke move in-line with the cylinder. And the opposite is true for too big holes which makes the spoke move perpendicular to the cylinder. If you weighed your pistons when changing them and the new one is heavier then usually you just need bigger holes. This happens when people put on big bore kits, but also sometimes happens when using non-OEM pistons. Of course you don't know how much of a change to make in the holes using this method. You'd need my crank balance calculator for that.

 

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

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