Balancing Happy Time style 70cc Crankshaft

[Fabian]

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

 

[Fabian]

Update:

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

 

[Fabian]

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

 

[Fabian]

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.

Ok,

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.

Fabian

 

[clay]

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.

 

[Fabian]

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

Fabian

 

[210061741]

Fabain

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

 

[Fabian]

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.

Fabian

 

[keystone]

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 universe.com 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

 

[Fabian]

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.

Fabian