Crank Balancing revisited

[Fabian]

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?

Even an extra 2mm crankpin offset would be beneficial because the standard 66cc or 69cc engine has the piston covering up half of the transfer ports and a good 1/3 of the exhaust port at bottom dead centre.

The extra 2mm offset would unshroud the transfer and exhaust ports and my be of greater gain than the extra 4mm stroke added to piston travel.

 

[HeadSmess]

the way i see it, where a standard lil engine usually uses the fan as the flywheel and the magneto, the HT uses the dirty big fat weights as a flywheel.

chuck an irk onto a pit bike and wacth it become useless oh, it revs up fast but has no torque anymore... they resemble the HT magneto.

so yeah, any balanced weight will increase torque and general smooth power delivery. whilst changing balance/piston weights will only alter the 90 degree or y component.

me thinks the HT has enough weight as it is.


and its not the extra stroke length that makes the difference! its the offset! the increased leverage on the crank! leverage is torque. move the point of load further from the fulcrum, with the same load, and torque is increased dramatically. yeah, theres a bit more capacity but you sorta lose that with the ports...1g/m is the same as 1 kg/cm. 1kg at 1.1cm is gunna be a lot more torque than at 1cm!

so it is a win win situation. i was contemplating only 2mm offset. and yes, it would uncover those ports more... maybe, with luck, the stock rod, piston...do some skirt chopping if necessary.. only 2mm! raise cylinder 2mm higher to compensate for piston travelling higher. piston port will change a bit, may need fiddling. may even get rings protruding at bdc... timing will change but at the moment, im not working out by how much

the extra port area will definitely help. ive noticed that with the 66 too. bad ports. terrible ports. shocking even. once again. 48cc for ever!

ill pop a 48 out 2mm

to make it easier, i was thinking of boring new holes and using the original pin, and have one 1mm more, one 2mm more, just to play with... gotta press one apart now!

 

[Deleted member 12676]

Here's an explanatory graph showing the upward and downward assembly inertia, the upward inertia modified by the combustion/compression force, being greater with more cranking pressure (greater compression ratio), and the counterbalance force (without showing their polarity). Upward inertia is around 57% more than downward inertia due to the peculiarities of the geometry.
The farther away the green or blue line is from the red one, the more the vibration. Small amounts of deviation are not bothersome.
Point A is where Fabian has his greatest imbalance. He doesn't experience the upper imbalance because his engine isn't a high rever. Less compression leads to a greater imbalance at high revs. I just proved that today by running a 135psi top end in place of the 165psi top end that ran perfectly balanced ("perfectly" being a relative term). The 135psi top end vibrated at the B area but not at the A area. My new formula is calibrated for my 165psi top end which applies more to people with aftermarket high compression heads. For lesser compression the counterbalance holes need to be bigger. I need to make some test runs with a stock CDI to see if it differs in counterbalance needs.
The second of these two graphs is more accurate. For some reason I cant delete the first one. (help!)

 

[Fabian]

My engine is not a "high revver" because i need it to be reliable for long distance haulage. A side benefit is that vibration is negligible if kept in the 3,500 rpm zone and the engine makes less noise.

 

[HeadSmess]

"peculiarities in geometry"

too true. took me a long time to realise a piston is not experiencing simple harmonic motion or plain up and down. the piston is halfway up the bore not at 90BTDC or ATDC but more like 85 degrees. so, one half stroke it travels say, 170 degrees, the other half it travels 190 degrees of crank rotation. crank is assumed to be constant velocity, so piston acceleration/deceleration is different on the upper half of the stroke compared to the bottom. as the conrod gets longer, the included angle is reduced, and the halfway points are closer to the 90 degree mark.



this should clarify. 100mm stroke, 122mm conrod length gave me 78.2 degrees!

whereas 100mm stroke, 152mm conrod is 80.6 degrees

 

[Deleted member 12676]

from http://victorylibrary.com/mopar/crank-bal-c.htm
The forces acting on the rod & crank-pin (mass inertia) are not only the reciprocating weight, but also the forces present in the cylinder and combustion chamber above the piston (cylinder compression pressure, combustion, exhaust pumping, and vacuum). When we add the dynamic effects of vacuum, compression and combustion pressure, the effects are radically altered. With a higher compression ratio the rod “sees” a slightly heavier piston than the actual weight on the compression part of the stroke, and during full throttle it also sees a heavier piston on the power part of the stroke.

 

[Deleted member 12676]

I admit that a full understanding of this subject eludes me. I have searched high and low on the internet and it seems that no one else knows. Everyone uses the 4 stroke formula and then tests and then makes additional changes. Look at my 3 tests:
55cc reed valve, torque pipe 165psi-- no vibration
48cc piston port, torque pipe 140psi-- vibration above 7300rpm
48cc piston port, torque pipe 110psi-- no vibration
pistons for the 55cc and 48cc were almost of the same weight. The 55cc was a Honda piston.
Even with a lighter wrist pin in the 140psi engine it still vibrates (although slightly less).
This is all with a 9mm counterbalance hole. (The 48cc should have a 10mm hole).
At least we have something definite we can conclude though, and that is the engine compression is of some effect in the matter of engine balance. If you drill a hole in the flywheels and still have some vibration then try a different amount of compression.

 

[bahramu]

the extra port area will definitely help. ive noticed that with the 66 too. bad ports. terrible ports. shocking even. once again. 48cc for ever!

Woah, wait a minute... My fist choice was the 66cc motor cause I thought it was better as "there is no replacement for displacement." I just searched for 48 vs 66cc topics and all I found was that 48's may run smoother (and may be more rev happy?) but 66's have more torque/power. Could you explain in more detail why you would always choose the 48? And if ports are the issue with 66, is it not something you can fix? Thanks!
I'm curious cause I'm about to start building another motor and would of chosen the 66, now having second thoughts. (I live in a very hilly area by the way)

 

[Fabian]

You always choose the largest engine as they have more usable torque.
If you live in a hilly area, then your best option is to combine a 66cc engine with a SickBikeParts shift kit for true hill climbing capability.

 

[Deleted member 12676]

If you aren't willing to take apart the engine and drill the flywheels then just get the 48cc and put a 55cc or 60cc top end on it.