How to Balance the Engine for Less Vibration and More Top RPM

The fix is two-fold. The first half of the fix affects vibration all through the rpm range. The second half of the fix lessens vibration only at the top rpm range where it is caused by the standard too-advanced ignition timing.

1. After you have settled on an appropriate cylinder compression (in psi. more compression gives more power) then you can change the port timing just a bit for better high rpm power and balance the engine by removing weight from the piston and wrist pin. (Drilling holes in the flywheels only offsets the weight of the piston and wrist pin.) On bikes that self-limit their mph by vibration you can easily tell if you are making it better (more in balance) by the max velocity. The first part of the fix is to lighten the combo of piston and wrist pin. I had my wrist pin drilled out from a 5.8mm diameter hole to a 7.5mm hole. With these weak engines there is no need to worry that the pin will be too weak after drilling. The same holds true for the piston. Your machinist can use a 9/32" (7.1mm) carbide drill bit to use in his lathe on the wrist pin (available for $8 from Grainger) or you can buy a wrist pin for the 48cc piston that already has a 7.5mm hole in it, available from Treatland. Next are 11gm wrist pins with a 7mm hole (1.2mm more than stock) from pocketbikeparts.com that will work in the piston for a 55cc or 60cc Grubee engine:
Generic 36mm x 10mm wrist pin & bearing $13
Generic 36mm x 10mm wrist pin $9
They probably also work in a 69cc engine since the piston is only 2mm wider. Also available is a 37.5mm long 10mm diameter titanium wrist pin (6.3mm I.D.) which is really good since titanium has around 58% the weight of stainless steel. You can use a grinding wheel to shorten it if needed. It is 3 grams lighter than the 55cc/60cc wrist pins. But if you have a high compression head it is better to use a steel wrist pin though since the titanium pin recommended here is not treated and wears down faster under severe conditions or when muddy water is splashed onto your filter and sucked into the engine. Use high quality oil if you use the titanium pin.

You can also lighten the piston by drilling holes in it that will be no more than 5mm from the side edges of the exhaust and intake port. Drill size is 9/32" and each hole shouldn't be closer than 10mm center-to-center from the next hole. Here's a photo showing where you can drill up to 9 holes on each side. Unfortunately, if the piston wall is 1.9mm thick (as mine is), 18 holes will remove only 3.3 grams.

I have a reed valved engine and with the additional holes on the intake side mine totaled 27 holes and I have rev'd it to 37mph (8400 rpm) and it held up OK. Use an exacto knife to trim the outer edge of each hole to not be a sharp edge. I used a small bit with my dremel to grind a dent in the piston where each hole should be drilled. Then I used a small drill bit to make the pilot holes. (Otherwise the drill bit would wander off center.) Then I used the 7/32" bit to make the final size. Aluminum is very light so even 18 holes won't make a huge difference in weight but when it comes to engine balancing every little bit counts. Once you find the best balance you can change the porting a bit. With these two changes the vibration should be much less and the top rpm much more.

2. The second part of the fix is to buy the Jaguar CDI which is designed to spark later at rpm above 3600 to lessen the combustion/compression forces that the piston and flywheel have to push against. This approach is standard with 2 stroke engines. I think the CDI that comes with these engines is made for a 4 stroke since it does not have that essential change in timing at high rpm. That causes reduced power and reduced rpm, both of which are wanted by the company so that it can pass all countries regulations. But you don't want that because you want power and a bit more rpm without having to endure excess vibrations at the handgrips and seat.

Here's a quote from Crankshaft Design, Materials, Loads and Manufacturing, by EPI Inc.
"Combustion forces and piston acceleration are the main source of external vibration produced by an engine. They must be counteracted by the implementation of the crankshaft counterweights." [A counterweight essentially exists opposite the rod pivot by the removal of metal near the connecting rod pivot on the flywheels]

The graph here shows what I want to explain about engine balancing. The increase in centrifugal force from the imbalanced flywheels and the up/down upper piston assembly inertia (as it changes direction) is exponential with rpm. But the combustion/dynamic-compression force varies with rpm mostly due to spark ignition timing.The flywheel counterbalance (created by holes near the conrod pin) has to keep the total balance not too far from true at peak combustion/compression and at peak rpm when the combustion/compression is sharply dropping off due to spark retard.

 
Vibration is reported to be much more with the 69cc (80cc) engine and so this cure is even more vital for it. If not enough then you have to balance the crankshaft. Here are some reports on the motoredbike forum:

"Is the 2 cycle [69cc] motor known for a lot of vibration? I have some rubber pads between the mounts and the frame, but anything over about 15mph is just about unbearable."   post

"My seat seems to be vibrating a little too much to comfortably ride long distances at full throttle. It seems to have gotten worse recently. Any idea how I could figure out where it is coming from? The engine mounts are solid I wrapped the frame in thick leather under the mounting hardware. I'm running the Chinese 66cc 2 stroke on a mountain bike."  post

"I just finished my first motored bike build- a "Black Stallion" 66/80cc from Kings Motor Bikes. ...at the end of yesterdays ride I cranked it wide open to see what it could do. As it built RPMs, it passed a certain range and the entire bike began vibrating like the engine was totally mis-balanced! The gas tank loosened and shifted, and I had trouble keeping my hands on the handlebars! I dropped RPMs, and the vibration went completely away. I tried doing this several times, and each time I crossed that certain RPM barrier, the bike would go into wild vibration!" post

"I have a huffy panama jack bicycle, with a 80cc engine on it. I have such a bad vibration in the bike, its terrible.Ive tried rubber motor mounts, does anyone have any ideas? please help!!!!!!!!!!!!"  post

 
Crankshaft balancing (for even 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 lightens the balance area by 4.5 grams which can make a big difference in a stock 48cc. If it has higher compression and is ported for higher revs then the Jaguar CDI will be needed.

There is an old fashioned way of balancing the crank, with the peak RPM being a factor in how much weight should be removed from the counter-balance area. But in studying the subject I see that the main two forces that need to be counter balanced are changed in value of force equally as rpm increases so that rpm is not really a factor. That means that the correct flywheel counter-balance mostly depends on the upper assembly weight and dynamic cylinder pressure, not on rpm. Cylinder pressure changes non-linearly with rpm, mostly due to ignition timing. On my bike with the ignition timing curve of the Jaguar CDI it has the most cylinder pressure at around 6750 rpm.

I have two different 55cc engines using different cylinders and pistons. One is with piston port intake and the other is 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 base my 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 know I have to use 34% of the conrod weight as its contribution to the upper assembly weight. (Because the lower part of the conrod is mostly moving horizontally, not vertically, and so the lower weight does not count.) And I use the downward piston force instead of the upward force (which is greater due to more piston speed going upward but is lessened by some of that energy being used to compress the fuel/air). 

In calculating ther 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: 115.2gm
additional counter balance weight removed: 9.8gm (via 7.2mm diameter hole)
The engine vibrated between 5600 and 7900 rpm and ran smooth before and after that rpm range.

55cc high rpm 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 77.3gm 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.

Here are the force evaluations of the two engines at 4000 rpm:

Upper assembly weight (piston, wrist pin, bearing, 34% of 71.3gm conrod):
piston port engine: 115.2 grams
reed valved engine: 103.2 grams

Downward inertia force (of upper assembly weight) at 4000 rpm:
piston port engine: 67.2
reed valve engine: 60.2

Centrifugal force of counterbalance and conrod pin area*:
piston port engine: 51.9
reed valve engine: 60.4  

Centrifugal force divided by Downward force:
piston port engine: .77
reed valve engine: 1.003

*In figuring the counter balance weight you have 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, 36mm of the additional balance hole, and the distance of the two counter balance holes. (I recently realized that the correct distance to use for the calculation of centrifugal force of the side balance holes (that aren't in the crankshaft center-to-conrod-pin-line) needs to be reduced. The drawing below shows my holes at 90% of the distance of the same hole if it was in line. 90% of 36mm is 32mm so I need to use 32mm instead.)



 As an example we can calculate the needed "missing" balance weight for the piston port 55cc Grubee engine. What I first noticed is that the existing balance holes are not the same distance from the center of the crankshaft as the connecting rod pin is. That is important because the farther a weight is from the centerpoint the more centrifugal force it has for the same rpm. Using a test weight of 1kg at this site I see that at the 36mm distance it gives 1.9 times the centrifugal force as 1kg at the 19mm distance of the conrod pin. 

Upper Assembly weight and downward inertia: 34% of the 71.3 gram conrod is 24.5 grams and the piston assembly weighs 90.7 grams for a total of 115.2 grams. That weight at 4000 rpm (with 1.5" (38mm) stroke and 3.35" (85mm) conrod length) gives 67.2 pounds downward inertia force using the piston inertia calculator below.

Counter balance weight and centrifugal force: The two factory-placed holes of 11.5mm diameter equate to 50 grams of missing weight which gives 63 pound-feet of centrifugal force at 4000 rpm (at 32mm radius). 9.8 grams of missing extra centered balance hole gives 13.9 pound-feet force at 36mm. 30 grams of conrod bearing and "conrod end" added to the 3.3 grams of the extra conrod pin weight gives 33.3 grams which gives 25 pound-feet of centrifugal force at 4000 rpm (at 19mm radius). The missing hole weight forces of 63 plus 13.9 added together equal 76.9. That minus the conrod area centrifugal force of 25 equals 51.9 pound-feet centrifugal force.

Centrifugal Force to Downward Inertia Force Ratio: 51.9/67.2= .77 which is terrible since the two forces need to be close to equal. Let's try to make that ratio .97 though because the 1/1 ratio was  determined using the other engine which has 165psi compression whereas this one only has 155psi. Multiplying .97 by 67.2 we get 65.2 which is what we need to have as the final centrifugal force.

Calculating needed counter balance weight removal: So 65.2 - 51.9 = 13.3 pounds of force which (according to the centrifugal force calculator by trial and error) requires 9.4 grams weight removal at the same 36mm distance as the existing hole. 9.4 added to the existing 9.8 grams missing at the centered balance hole gives 19.2 grams needing to be missing there. A 10.1mm diameter hole drilled through both flywheels (total 31mm thick)  will result in 19.2 grams (.0192 kg) missing according to the steel weight calculator (be sure to move the decimal point of the resultant kg weight over 3 places to get grams). Usually holes drilled are not perfect and so you can add about .15mm to the size. And so using a 10mm drill bit will cause a 10.15mm hole which will remove 19.4 grams which is close enough. A good quality drill bit of an equivalent 25/64" size is available for $17.28 online from Grainger. You can drill the hole yourself with an electric drill but it is hard and slow going. Best to do it at the machine shop.

piston inertia calculator  (use the last line of "downward inertia force")

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

steel weight calculator  (use "round bar" for calculating a hole)

Since upper assembly inertia and flywheel centrifugal force stay neck and neck thru the whole rpm range then it is just the changing compression/combustion force that varies with rpm. That is influenced by cranking compression and ignition timing. My balanced engine has 165 lbs cranking compression. High compression and advanced ignition are typical of enduro bikes, not racers. Race bikes have lower compression (typically 150psi or less) and retarded ignition. So my engine, being like an enduro bike, is just right with a 1/1force ratio. A bike with near 140psi may need only .95/1 as a force ratio.

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.
 

For the "80cc" engine with a 38mm stroke and the same flywheel assembly and 85mm conrod, with an assumed piston assembly of ? grams and 11.15mm  counterbalance holes at 36mm from shaft center I figure a ?mm extra hole is needed to balance the engine. (figure forthcoming)

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. My 48cc had an equivalent 30 grams.