Crank Balancing revisited

Discussion in 'Performance Mods' started by jaguar, Jun 26, 2013.

1. jaguarWell-Known Member

Crankshaft Balancing:

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 top end by 4.5 grams which is enough to make a significant difference but not enough to completely balance the crankshaft.

50% to 85% of the top end weight (piston, rings, wrist pin, bearing, upper half of connecting rod) is recommended as the weight to be "missing" in the balance holes on the crankshaft halves close to the connecting rod pin. (Making weight disappear on one side of the halves is like making it appear additionally at the opposite side.) The conrod pin adds 3.3 grams to the balance area (over what would be if no holes were made for it). In the Tony Foale information (www.tonyfoale.com/Articles/EngineBalance/EngineBalance.pdf) he said that the balance factor corresponds to the square root of the rpm. So here I list the square roots from 5,500 to 10,000 rpm:
10,000 100
9,500 97.5
9,000 94.9
8,500 92.0
8,000 89.5
7,500 86.6
7,000 83.7
6,500 80.6
6,000 77.5
5,500 74.2

So this gives us the relation of the balance factors to each other for the different max rpm. If we assume 5,500 as needing the minimal 50% balance factor then we can derive these other balance factors with the percentages changed to fractions:
10,000 .674 (67.4%)
9,500 .657
9,000 .640
8,500 .620
8,000 .603
7,500 .583
7,000 .564
6,500 .543
6,000 .522
5,500 .500

As an example of how to calculate the needed "missing" balance weight I will use the 48cc Grubee engine. What I first notice 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 rotational velocity. Using a test weight of 1kg at http://www.calctool.org/CALC/phys/newtonian/centrifugal I see that 1kg at the 36mm distance of the balance holes gives 1.9 times the centrifugal force as 1kg at the 19mm distance of the conrod pin. So the final formula will take the upper end weight, multiply it by the balance factor, add the additional weight (3.3gm) of the conrod pin, and then divide that result by 1.9. The upper half of the conrod weighs 31 grams and the piston assembly weighs 79 grams for a total of 110 grams. 110x.50=55 +3.3=58.3 /1.9=30.7gm. The two factory-placed holes of 11mm diameter equate to 23 grams of missing weight. 30.7 minus 23 equals 7.7 grams that still needs to be removed. A single extra hole of 6.3mm diameter (1/4") drilled at the same 36mm distance will remove 7.5 grams of weight. I would also buy a 1/8" drill bit to use as a starter hole. It is hard slow going drilling a hole in that metal. (Grainger has good prices on carbide bits. On their site search "jobber drill aircraft" and the size needed.

The missing weight of any hole can be calculated at http://www.ralingroup.co.uk/weights.html but you have to multiply the resultant weight of kg by 1000 to get grams. Use half of the diameter as the wall thickness.

If the lighter weight wrist pin is used then that shaves 4.5 grams off of the piston assembly to reduce the total weight from 110 to 105.5 grams. Recalculating gives 29.5 grams needing to be counterbalanced. Subtracting 23 grams leaves 6.5 needing to be removed. A 6mm (15/64") diameter hole will remove that.

Below 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. 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.

Last edited: Jun 30, 2013

2. jaguarWell-Known Member

Test Results

The above is just theory. Next it has to be proven by real life tests. I am starting to do that. I just tested my 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: 122gm
That figures to be balanced for 5080 rpm (ie: a .48 balance factor)
The engine vibrated between 5600 and 7900 rpm and ran smooth before and after that rpm range.
So I think this test has proved the veracity of my formula so far. Next I will enlarge the extra balance hole to 15.8 gm for an rpm of 8,500 for my other 8300rpm engine and see how it runs.

Here are the square root and balance factors up to 12,000 rpm for anyone who wants them:

RPM____sqr rt
12,000__109.5
11,500__107.2
11,000__104.9
10,500__102.5

RPM____balance factor
12,000__.738
11,500__.722
11,000__.707
10,500__.691

55cc high rpm 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

Last edited: Jun 30, 2013
3. FabianWell-Known Member

A single cylinder engine cannot be balanced for a dynamic range - this subject has been rehashed so many times.

4. jaguarWell-Known Member

Any one who has studied the subject knows "balanced" in this application of the word is relative, and just means "not vibrating too much".

5. jaguarWell-Known Member

Excel calculator

I've made an Excel file for calculating the counter-balance for a 48cc Grubee flywheel. It is free to download at http://www.dragonfly75.com/motorbike/48ccCounterWt.xlsx

If anyone cares to give me the following info I can make a calculator for the 69cc:
upper assembly weight (piston/wrist-pin/bearing)
inner diameter of counter-balance holes
width of each flywheel half
distance from center of crankshaft to center of counterbalance holes

6. jaguarWell-Known Member

Second test:
My other 55cc engine (ported for 8300rpm) with lighter 77.3gm upper assembly.
the additional weight removed was raised to 15.8 grams via a 9.15mm diameter hole.
Using my formula I figure the rpm for that combo is 9380.
It allowed my engine to go up to 9150 rpm (downhill) without any bothersome vibration.
so far, so good.

7. jaguarWell-Known Member

Someone gave me the dimensions for a 69cc engine and I figured what is needed for it.
With a stock piston assembly of 107.6 grams and 11.15mm counterbalance holes at 36mm from shaft center I figure a 12.6mm diameter extra hole is needed to balance the engine.

9. jaguarWell-Known Member

The machine shop let me do the drilling as they were pre-occupied. I just drilled slow and the bits didn't get too hot. I didn't use any cutting oil either. And they still feel sharp.
I have a masonry bit and it doesn't have a sharp edge to it. Don't mean it won't work but man oh man are those flywheels made of some hard-ass steel.

10. FabianWell-Known Member

I have been through this before as i purchased a "so called" balanced and trued crankshaft (in a complete 69cc engine) from another vendor, which was tested to 9,000 rpm in pre-production development work to determine the best counter balance weight.

When receiving the package i immediately pulled off the top end to find the crankshaft counter weight holes measured in at 12mm which was up from 11mm diameter in a standard 69cc engine.

After spending time to assemble it to my SickBikeParts jackshaft system and installing the thing in my bike and giving it a test run, i was shocked as to how vicious the vibration was at normal running rpm of 3,500 to 4,500 rpm.

The vibration was completely unacceptable and many times worse than the standard engine at normal operating rpm.
Consequently engine was stripped out of my bike and returned to the vendor, upon which a standard 69cc engine was bolted in it's place.

Since that day, i have never bothered with the concept of balancing a standard engine as my research (and the maths to support it) shows that a single cylinder engine can only be balanced for a narrow rpm range at a selected rpm.
Balance the engine so that it is smoother at high rpm and it will vibrate excessively at lower rpm.

11. jaguarWell-Known Member

Enlarging those holes from 11mm to 12mm only removes 8.6 grams.
Correctly removing the needed 27 grams is done by making an extra hole 12mm in diameter.
But running the standard CDI requires a bigger hole. And running high compression also requires a bigger hole.
So that little hole enlargement they did was about 1/4th what was needed.

12. FabianWell-Known Member

so if that is true, then it should have run with 25% less vibration, but it ran with what appeared to be 500% more vibration than a standard engine.

and what would the optimal rpm be with 12mm holes as opposed to the 11mm holes?

13. FabianWell-Known Member

What would the optimal hole size for an (69cc) engine revving at between 3,500 and 4,500 rpm?

14. FabianWell-Known Member

:icon_bs:

i used to make drill bits for someone...3 inch diameter, machined lip to take carbide insert. takes some doing, sharpening a big carbide drill on a standard greenstone. they wear away really fast. make a mess.

also sharpened a few masonry drills for the cnc lathe a few times. once again, silicon carbide "greenstone". or a non segmented diamond wheel on the angle grinder makes less mess, that one. rougher finish unless you got a diamond file to hone it with. dont make carbide as "sharp" as HSS. even put a 0.2mm flat on the lip. makes it stronger.

i hate drilling hard stuff, especially if the holes already there! ruins drills.

but balancing. pffft.

best option is lightening everything. whole crankshaft to reduce rotating weight. lighten piston to reduce oscillating weight and counterweight required, which also reduces the rotary vibration.

why is a vtwin better balanced? in a single cylinder, piston goes up and down. the counterweight goes down and up. but the counterweight rotates. so it also creates a side to side vibration.

a 90 degree v twin sticks another piston here, at the side. so the side to side vibration is also countered by a piston travelling the opposite way. making for a smoother engine. next option is four cylinders, in line...

reducing total weight will reduce vibration in total.

reducing counterweights/piston weights will reduce oscillatory vibration that shouldnt really be RPM dependent at all, as the g forces increase at basically the same rate in both directions... piston goes faster? so does counterweight. it is rpm dependent though, because the piston travels linearly, and the forces are constantly changing,from dead still to full tilt, whereas the crank experiences a relatively static loading, centrifugal force only...

but even when you get this right, if the counterweights just heavy anyway, its still going to vibrate side to side no matter what you do to the piston, pin, or rod!

reducing weight will allow for faster pick up when the throttles opened. less weight to get spinning.

reducing weight will reduce bottom end torque by removing all that rotating mass!

best to leave it all alone, methinks

has anyone tried turning up acrylic/nylon/alloy weights instead of the stock steel? and looking at the crankpin earlier...theres heaps of space to machine out and press in a new crankpin at a larger offset... more stroke. more grunt

16. FabianWell-Known Member

Machine one up and i'll make a convenient Paypal purchase along with the associated shipping cost.

How much extra stroke is available in a stroker configuration?

17. jaguarWell-Known Member

A 1/2" hole is just right for most 69cc engines using the Jaguar CDI. Hole size need probably differs with a standard CDI but I need to research that with tests.
In re-evaluating what I said at the start of this thread and recalculating everything I found some unsolvable incongruities and so I have forsaken the "old" method completely. My newest method is on my site.
The old method asked you to drill bigger holes for even higher rpm. But I think the opposite is true. Let me explain. More than rpm, the important data is the amount of spark advance and combustion pressure. Engines reving high more often than not have less cranking pressure than "enduro" engines and even less spark advance. The cranking pressure adds to the combustion pressure which teams up with ignition advance (and 2 other things) to cause the only non-linear part of the "equation" that has to be balanced. Both piston inertia and flywheel centrifugal force (due to the imbalance there by the counter balance holes) change equally in response to increasing rpm. Without considering combustion pressure then these two counterbalance no matter what the rpm is. The same counterbalance works equally at 2,000 as it does at 10,000. So the only part rpm plays is that at a certain range of rpm the combustion pressure is more due mostly to spark advance (look at the graph below) and cylinder pressure. That subtracts from the upward piston assembly inertia force.
As revs increase so does dynamic pressure due to rings having less time to lose pressure as blowby past the ring ends. During the upper rpm range the expansion chamber baffle return wave supercharges the combustible mix, so there is that addition to combustion pressure. The delivery ratio decreases at top rpm so that subtracts from the total pressure. So we have a mixture of 4 things, their final curve being the dip you see in the bottom graph. And that is near impossible to predict. So all I can say is here is what worked for me given my peculiar engine setup, and knowing what I do I can predict a greater need for counterbalance with less cranking pressure.

Last edited: Jul 14, 2013

quite well explained theoretically, yes, piston and counterweight should balance out and cancel vibration IN THE PLANE OF PISTON TRAVEL. vertically. they dont. a simple air compressor proves that. ive never found a vibration free air compressor. here we have removed any influence of combustion. take the head off any single cylinder and youll find it well balanced in one plane only. which is rpm dependent to a degree.

what you do overlook is the 90 degree component caused by the rotating counterweight. im just saying. i do agree that the combustion pressure does cause some degree of imbalance. a short con rod will inrease this effect.

lightening the crankshaft as a whole will reduce the 90 degree component. but that can only truly be balanced or cancelled out by use of another piston on this 90 degree plane. be it working cylinder or a dummy. ie, balance shafts etc.

its the 90 degree part that causes the most vibration, and its the hardest to balance. the counterweight will always be pulling radially from the centre, rotating through 360 degrees. the piston weight only goes back and forth. it also "jerks" to a stop, whereas the counterweight always follows a smooth line of continuous, steady force. at 10 degrees, the weight is pulling 10 degrees to the side with just the same force as always, whereas the piston is almost stationary. its decelaration acting at 0 degrees, vertically.

a flat twin can also balance. each piston is the others counterweight. at 90 TDC, where counterweight balance would act in equal and opposite directions on the vertical plane. whilst piston forces are still cancelling on the horizontal plane. theres also little in the way of counterweight to start with. only the crank pins and rod bearings are necessary. (180 crank seperation, not a shared pin)

tis why i like this idea of plastic/ lightweight weights. use that as packer material, with the smallest counterweight possible. tungsten carbide should do the trick nice and dense, right out near the edge. then its a lot of experimenting to add weight, not remove it

FABAIAN. um.... i wish id bought that 5 tonne press the other week when it was on special? cus, im seeing over 5mm of room between conrod and crankcase. an offset of 5mm is a 10mm increase in stroke. whether the crank halves can take that much offset is another matter entirely. but i dont see its being too hard...i havent pressed one apart before? i wouldnt mind doing it to an old scrapper though to see...
whats hard is finding a new rod, cylinder and piston to suit this extra 10mm of stroke... and working out what to do with the ports?

sorry. i gotta go thrash out. bodies. drowning pool. this is not a sit still type song!

19. FabianWell-Known Member

I've gone through this discussion with Jaguar before and the requirement of a double balance shaft to properly cancel out the "Y" vector.

20. FabianWell-Known Member

This is the best method for reducing vibration in a single cylinder engine, but with the only downside being an eratic idle or an unreliable idle rpm, which is generally cured by running high idle rpm.

For this reason a good combination is a lightened crankshaft with attached flywheel weights by means of a pull start and centrifugal clutch, to give back inertia to the crankshaft for reliable idle rpm.

Even without a lightened crankshaft, it's surprising to see the improved idle quality with a pull start and centrifugal clutch attached to the engine.