Balancing Happy Time style 70cc Crankshaft

Discussion in 'Performance Mods' started by Fabian, Nov 8, 2009.

  1. Fabian

    Fabian Well-Known Member


    Just want to get peoples ideas and if possible, photos of what their balanced crankshaft looked like; the balance factor they used for the calculated rpm speed and if they used stuffers to fill the balancing holes.

    Photos are of a standard 70cc crankshaft that has travelled 700 kilometers.
    If you look closely, you can see where the connecting rod has cut into the crankshaft halves because the factory does not use thrust washers.


    Attached Files:

    Last edited: Nov 8, 2009

  2. clay

    clay Member

  3. Fabian

    Fabian Well-Known Member

    Hi Clay

    You have used a crankshaft balance factor of 53%

    Can you please explain the mathematics of deriving a balance factor for a specific rpm.
    From what i understand, the balance factor changes for the rpm range an engine will be "mostly" operated.

    The commonly used rpm range best suiting my application is 3000 - 4500 rpm - could you advise the most effective balance factor.

    For my own curiousity, what were the weight measurements of your big end bearing, the big end of the conrod, the small end of the conrod and the piston, piston pin, bearing and spring clips.

    Cheers Fabian
    Last edited: Nov 9, 2009
  4. 210061741

    210061741 Guest

    Math Behind Crankshaft Balance

    This is a very good book (and FREE ) with all the mathematics needed to figure out anything to do with ENGINES.
    A chapter in the end of the book called Balancing describes everything you need to know and at the very end even has an example of balancing a single cylinder engine.
    Simple certanly not.
    But i'm sure much more effective than guessing or following someone elses guess.
    Most likly you would need some type of engineering experience to understand the Force diagrams and have the ability to do the math.
    When i get time i will go through all the calculations and see what i come up with.

    If you go through the math please share and we will compare the results to what i calculate.
    This is gonna take some time to understand and do correctly.
    But for what you are putting into your motor i would go the extra mile.
    If this math is used to get your answers i'm positive the very best result will be achieved.

    Have fun I will.

    Link to the free book. "The theory of heat engines By William Inchley"


    By placing the crank in a stand that will allow it to turn freely.
    V-Blocks with 2 bearings on each side which the crank spins on.
    We could follow the method of balancing a grinding wheel.

    You would need some round magnets to use for weights place them on the crank to find the correct position of the weights.

    The round magnets 2 for each flywheel (4 total) should weigh more than the total weight of the unbalance. " piston conn rod pins bearings ect.

    Once you find the proper placement to achieve balance you will need to drill out the crank flywheels and place weights in there = to the weight of the magnets + the weight of the removed material.

    This would be the simpler non mathematical less accurate method of balancing.

    Let me know which way you go.
    If you get stuck in the math somewhere let me know and i'll try to help.
    I have quite an engineering background myself and know some old timer engineers that would help with the math just for fun.
    Also if you are going to school the math professor would love this one.

    Attached Files:

  5. Fabian

    Fabian Well-Known Member

    I've had a look at the book "the theory of heat engines".
    Lovely mathematics on balancing but it doesn't give an explaination for balance factor and how it relates to rpm.

    I have gained an understanding as how to set up the bob weights for a given balance factor but i haven't found any information as to how balance factor changes due to a selected rpm range.

    I can find my bob weight for 50% balance factor or 53 or 55 or 60% balance factor but what number is correct for say, 3,000 rpm or 3,500 rpm or 4,500 rpm.

    This is where i'm completely lost.
  6. 210061741

    210061741 Guest

    From my studies today it seems there is no such thing as balance in a single cylinder engine.
    As the piston speed is constantly changing.
    Balancing the crank will only produce balance at 90 degrees to the cylinder.

    So unfortunatly it seems to be a trial and error implementation.
    I don't think i will bother trying to balance my shafts.

    Rather it seems time would be best spent on designing a crank weight "the screw on plates as light as possable " trial and error" and increasing the size to take up more volume in the crankcase.

    Higher Psi in the Crank = Higher HP

    AKA (BMEP)

    But anyway let me know how it all goes when your done.
  7. tacoshell4

    tacoshell4 Member

    check out jim the manic mechanic creative engineering he has the whole scoop he uses software to figure out the sweetspots
  8. Fabian

    Fabian Well-Known Member

    Hi tacoshell4

    Can you please post a link for that information on balancing
  9. tacoshell4

    tacoshell4 Member

  10. Fabian

    Fabian Well-Known Member


    I,ve been on a quest for solution to crankshaft balance.

    From what i understand and relating the the weights of my engine components:

    1) Take the complete weight of the connecting rod big end and the big end bearing - 40.3 grams + 7.1 grams = 47.4 grams
    If your engine has thrust washers, add the thrust washer weight to "step 1"

    2) Take the complete weight of the connecting rod small end and the total weight of the piston and rings, piston pin, small end bearing and clips - totals 105.5 grams

    3) Multiply the figure 105.5, by your desired balance factor. For arguements sake, i'll use a common balance factor figure of 55% > 105.5 x 0.55 = 58.025

    4) Add the total weight of "step 1" to the balance factor figure of "step 3" - 47.4 + 58.025 = 105.425

    The required bob weight to balance the crankshaft to a balance factor of 55% will be 105.4 grams

    5) Assemble the crankshaft by lightly pressing the two halves and crankshaft pin in place, only so it will hold together and you can true the crankshaft on a set of V-blocks for the balancing proceedure.

    6) Place the trued crankshaft ends or shafts on parallel knife edged surfaces or use two reasonably sized drill bits that do not have any scratches or deep grooves - you want a perfectly smooth and horizontal surface for the crankshaft to be able to rotate on.

    7) Make some sort of hook like device to hang off the crankshaft pin that weighs exactly the same as the figure you've calculated in "step 4"

    8) You want to be able to rotate the crankshaft in any position without the crankpin falling downwards.
    Generally it seems to be accepted that a slightly heavy counterweight is desirable (the side opposite the crankpin).
    If the crankpin falls downwards, you can either remove metal from the crankpin end, or add heavy metal (like brass or malory metal) to the counterweight end, till you get the crankshaft to balance.

    9) Once the crankshaft has been balanced, take it apart and install the big end bearing and connecting rod (thrust washers also if the crankshaft used them) and press the crankshaft together and true the crankshaft.

    10) Take your crankshaft and put it in a plastic bag and place it it the freezer. After about an hour, take your crankshaft bearings (for the left and right side shafts) and remove the dust seals if they are installed.
    Now drop the bearings into a pan filled with hot oil.
    The bearings will expand in the oil and the crankshaft will shrink ever so slightly.
    Take the crankshaft out of the freezer and prop it up so the shafts are vertical.
    With a pair of smooth faced plyers, take a heated bearing out of the oil and carefully drop it on the shaft, don't delay, do the other side as well.
    The bearing should just drop straight on, no pressing or force should be needed.
    Make sure you have pressed the bearings into contact with the face of the crankshaft before the bearings cool down and the crankshaft heats up.
    If they are not seated squarely on the shaft after everything has equalised in temperature, you will have no hope of bashing them flat and straight - you'll only damage the shaft and the bearings.

    11) Before assembling the crankshaft in the case halves, along with the clutch shaft, place the assembled crankshaft with it's crankshaft bearings in the freezer and use heat to expand the cases allowing for an easy fit of the bearings into their respective recievers.
    Assembly should take place without too much force being applied on the crankshaft.
    If you need heaps of pressure to get the case halves to close up, you'll press the crankshaft out of "true" and the engine will vibrate, regardless that it has been balanced.

    12) It's best to install the crankshaft seals after the cases are clamped together.

    Although my tutorial may not be perfectly worded, it may be of use to someone wanting to balance and reinstall their crankshaft.


    Attached Files:

    Last edited: Nov 10, 2009
  11. Fabian

    Fabian Well-Known Member

    Hi Rich

    Would you be able to work out the out of balance loads on the crankshaft bearings and connecting rod and connecting rod bearings at 6000 rpm based on these component weights.

    Be interesting to see the load strain

    Cheers Fabian
  12. Fabian

    Fabian Well-Known Member


    I've assembled the crankshaft and drilled out the original inner balance holes, which were used to pilot a series of enlarging holes from 10mm through to 9/16 of an inch.

    In this particular circumstance, i put the cart before the horse, just to see if my hunch was correct and i took a few shortcuts, buy placing the crankshaft bearings on the shaft ends.
    This was just a quick test but it still worked quite effectively as the bearings are brand new.

    As it turned out i was a bit over enthusiastic and the crankshaft ended up "under balanced" for a 55% balance factor.

    To get the crankshaft into balance, i needed a bob weight of 121.1 grams instead of the required 105.4 grams.
    If you work out the balance factor for 121.1 grams the figure calculates to a 70% balance factor - far too high for industry standard practice of 50 - 58%

    It's not all bad though - the under balance can quite effectively be fixed by filling the holes with press fit aluminium stuffers to add a small amount of weight and restore integrity of a full circle crankshaft; maximising crankcase compression, or, you could simply remove a small amount of metal directly opposite the crankpin on the counterweight side.

    This test is quite informative because anyone can adjust the balance of their Happytime style crankshaft, so long as it has the same connecting rod as my engine, with the slit cut into the base (if you have a different style connecting rod, you would need to ascertain it's weight for the calculation).
    As i know know the weight of the big end bearing and the weight of the connecting rod, it's easy to use them as a partial bob weight because together they weigh 79.5 grams.
    If your crank bearings and big end bearing are in good condition, you can just balance the crank on the bearings by adding a measured amount of extra bob weight attached to the small end of the connecting rod for the total required bob weight applicable to your balance factor.

    You only need to true your crankshaft and then carefully remove metal till the crankshaft is in balance.
    This saves you disassembling and reassembling your crankshaft to get it to specification.

    Simplifies things greatly!

    Hope my explaination makes some sense

    Cheers Fabian

    Attached Files:

    Last edited: Nov 10, 2009
  13. Fabian

    Fabian Well-Known Member

    With crankshaft stuffers the balance holes i drilled would be filled with light weight, press fit aluminium slugs, looking something like the crankshaft pictured.

    Attached Files:

    Last edited: Nov 10, 2009
  14. Fabian

    Fabian Well-Known Member

    I'm definitely on the right track as to understanding the whole balance concept, verified by the crankshaft in the photo.

    More research, more research.


    This photo is of a "custom made" Happy Time style crankshaft - a professional job set up on a dynamic crankshaft balancer.

    It was spun to 8,500 rpm and the balance corrected up to that rpm.
    I have no idea as to what the balance factor is, nor the optimal rpm range.

    Just looking at it, i can see a heavier crankpin than my hollow pin crankshaft and smaller balance holes.
    It's difficult to judge but the balance holes in the photo look like they must be around 11 - 12 mm, and in a similar position to my crankshaft.
    This would lead me to conclude that it has a low balance factor, somewhere near 50%, favouring optimal balance at higher rpm.

    To get my currently under balanced crankshaft in the 55% range, i would need a heavier piston and connecting rod, or smaller balance holes, just like the crankshaft in the photo.

    SO, i think this problem, by and large, has been licked.
    One thing is for sure - if the 50cc Happy Time style engine uses the same crankshaft as the 70cc engine, it will be much better balanced.
    Another point to note is that every 70cc engine will run significantly smoother by correcting the inherent over balance set in the standard crankshaft.

    Now i just need some balancing guru to explain the logic of how balance factor changes with rpm and how to correctly dial in this factor, plotted against rpm - i'm still completely lost on this issue, like searching around in the woods on a moonless night without a torch.


    Attached Files:

    Last edited: Nov 10, 2009
  15. clay

    clay Member

    sorry it's sideways fabian.

    the main question that needs to be addressed is about the "balance factor". i was shooting for 55% when i balanced my crank, but because of the extreme increase in stroke (10mm) i had a hard time removing/adding enough material to get there. i ended up turning some new removeable crank weights out of aluminum so i could offset easier with brass slugs. anyways, the best i could do was about 53%, so i ran it. the vibration has moved up about 1000 rpms from before. i don't think i added enough weight, for my particular motor and use, i think i will try again with 56-58%. although, there is only one person that i personally know and trust enough to listen to, and he told me 50-60%, but he wasn't completely sure. i feel like i am on the right track, i just should have stuck with my original plan of 55%. it doesn't really matter though, just the fact that i stroked a chinese crank 10mm and it hasn't exploded yet blows my mind.
  16. Fabian

    Fabian Well-Known Member

    I have had the same problem with trying to find someone who knows "exactly" how balance factor interacts with rpm.

    Two days ago, i spent the whole day (it should be "wasted" the whole day) driving around to various crankshaft rebuilders trying to get a definitive answer to the question of balance factor.

    NOT ONE crankshaft rebuilder could give me an answer.
    I find this just unbelievable that so called experts have no idea on a subject that's the most important aspect of their trade.
    Like a carpenter who uses a hammer but doesn't know how it works.
    Like a surgeon who doesn't know how to use sticky tape, but knows when it all goes wrong, it keeps the bits from falling out.

    How do these people get through life - just dumbfounds me, considering some of them are earning big money.
    One crankshaft rebuilder didn't even know how balace factor worked.
    I spent 10 minutes giving him an education, finally commenting, that he should be paying me for my time - a very strange situation considering i'm the one without an education or any qualifications, failing just about every subject in school.

    Surely, there must be someone on motored bikes who is the mathematics guru; being able to give a qualified answer and explaination as how to calculate the correct balance factor for a desired rpm range.

    I'm not sleeping till the answer is found :detective:

    Last edited: Nov 11, 2009
  17. 210061741

    210061741 Guest


    At some time i may calculate all of these forces.
    But it will be difficult and timeconsuming.
    I'll have to see if i can do it with the software like JIM did.

    For now the thing i want to try is taking off the roughly machined weight disks that are on each crank arm and replacing them with the largest size aluminum disks to lighten up the crank and take up more volume.

    I have heard the cranks in theese motors are way heavier than they need to be.

    Also i believe i read on a performance tuning site that 1/3 the weight of the Piston / Pin / bearing / ect is usually used nad not the full weight.

    I'm curious to see how light i can get my crankshaft.

    For now it seems like the porting and head design with a squish band are the two aspects that the HT needs the most attention.

    But wonderfull thread you have here.
    I will ceartinly be referencing it in the future
  18. Fabian

    Fabian Well-Known Member

    Hi Rich

    You can't use a fixed figure for balancing; a "one size fits all" method doesn't work because you've got to define the most commonly used rpm range for engine operation and from there calculate the balance factor.

    This in turn will dictate the bob weight and henceforth the addition or removal of weight from the crankshaft, depending if you want greater or lesser flywheel effect.
    Cylinder porting will have the biggest effect on the useable rev range and if it's modified for power at higher rpm, a compensated balance factor will be required.

    If you just use a 1/3 method, you could be anywhere in the woods - who knows what the balance factor would be.
    1/3 the weight of a 43mm piston is different to the weight of a 47mm piston.

    From what i understand, a balance factor of 50% favours higher rpm and a 60% balance factor favours lower rpm.

    What i still don't understand is the method of finding the correct balance factor for a specific rpm range.
    It's bugging the heck out of me - i want answers.

    Last edited: Nov 11, 2009
  19. keystone

    keystone New Member

    fabian, I've been following your post and researching info. All I find is that you can't balance a single cylinder engine,only change the rpm range where it it vibrates less. everyone seems to use a 50 to 60 balance factor, even the vintage gravely L1 walk behind tractor site,rc etc. this seems like a polynomial with so many variables I wonder if there is a answer other than rule of thumb,or trial and error
  20. Fabian

    Fabian Well-Known Member

    Your assessment is spot on Keystone.

    With a single cylinder engine, you can only set up the balance for an rpm zone where the engine least vibrates.

    I am sure there's a mathematical formula allowing you to calculate balance factor for the desired rpm zone.
    I think it's important because if making changes to the power band, like cylinder porting and a tuned exhaust system that raises rpm 2000 revs from the previous optimal balance zone, engine vibration will be outside of the previous sweet spot.
    Smoothness can be restored for the new commonly used rpm range by adjusting balance factor.

    For the life of me, i can't find any information on how to do this simple task.
    I refuse to accept that it's just guesswork.
    I'll have to get in contact with an automotive engineer from one of the major car companies to sort this issue out, once and for all.

    Last edited: Nov 11, 2009