Flywheel balancing?

MTRHEAD

MTRHEAD

Location
MotorCity
I posted this in a thread and it shouldn’t be tied to that thread

Curious to how the shops/people offering lightweight cut flywheels balance them? If anyone does.

I’ve seen it done at a supplier of an oem equipment manufacturer and I doubt anyone on this site has that equipment, maybe a few have access to one?

I’ve chucked flywheels up in a lathe on a perfectly indicated taper (to represent the crank snot). The outer diameter or the large stamping is defiantly not perfectly round thus cutting it down the dynamic balance that was done at the oem manufacture would no longer be valid

curious if anyone knows how these lightweight flywheels are balanced

statically on a granite surface plate?

dynamically on a balance machine?

does it matter to us on this forum? That’s probably the real point....
 
T

te4250

Location
Western NY

I haven’t actually balanced one myself but this method seems legit. I have dynamically balanced a generalized flywheel with two proximity sensors 90 degrees out and a similar compliant shaft, also had a tone ring to phase shaft angle to the prox sensor readings which made for less math. Seems like we won’t hear how #zero did it, I was hoping speedysprocket would comment in the other thread
 
kevbo

kevbo

Location
dfw
On a solid part, balance wont change as its rotated. And, in the jetski world, nobody cares. Everyone knows the name on their carburetors, no one knows the area of their exhaust port or their engines peak RPM.
 
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S

Storbeck

Location
Minnesota
kevbo said:
On a solid part, balance wont change as its rotated. And, in the jetski world, nobody cares. Everyone knows the name on their carburetors, no one knows the area of their exhaust port or their engines peak RPM.
Click to expand...

Don't be silly, it's obvious that every watercraft motor from a stock 701 to a 1200 is optimized with the same size exhaust port.
 
kevbo

kevbo

Location
dfw
An out of balance flywheel could be beneficial if its at the right place. Our cranks were originally designed for pistons much lighter than the giant slugs we all use today.
 
T

te4250

Location
Western NY
I bet static balancing would get it pretty close.

kevbo said:
An out of balance flywheel could be beneficial if its at the right place. Our cranks were originally designed for pistons much lighter than the giant slugs we all use today.
Click to expand...
That might introduce some pretty wild out of plane loads
 
MTRHEAD

MTRHEAD

Location
MotorCity
kevbo said:
An out of balance flywheel could be beneficial if its at the right place. Our cranks were originally designed for pistons much lighter than the giant slugs we all use today.
Click to expand...
When we were racing Superstock (stock cranks) we would ensure our piston assembly (85.5mm) weighed as close to oem assembly as possible. Never knew If it helped at 7800 rpm but it was piece of mind.
 
jeepcreepcj

jeepcreepcj

Location
Michigan
JC-SuperJet said:
Flywheel Ring Gear (is this what is causing the 760 Ring Gears with Lump removed and Zeelrtronic to frequently break free?)
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I suspect these 760 rings slip (If not welded) when lightened for total loss from the inside diameter of the flywheel due to reduced tension/ press fit as you are removing material from “below” the ring gear.
 
S

Storbeck

Location
Minnesota
This is probably an unpopular opinion but it seems to me like the weeble wobble of a slightly unbalanced flywheel would be trivial compared to the weeble wobble of the piston going up and down.

You can't balance an up and down with a round and round, because the round and round has an equal imbalance in every direction, cancel out the up and down and you'll end up with an equal imbalance side to side, you can only trade off between the two, the lowest overall imbalance you can get is 50/50 (cancel out 50% of the imbalance up and down and you now have the same imbalance side to side). The twin pistons going up and down opposite of each other do cancel out but they are offset, so the motor doesn't actually shake up and down, it rocks. Hold the middle of the crankshaft at a point steady, drive coupler and the flywheel bolt make circles. Changing the weight of the piston changes the up and down but not the round and round, so the balance factor changes, but there is no "balanced" there is only what percentage of up and down vs side to side, although if you make the up and down higher or lower, you could still adjust back to the 50/50 (or whatever balance factor you think is best). This is why I cringe when somebody says they have their motor "balanced" (similar to how I cringe when I hear "ported and polished"). There are reasons to choose a balance factor that is not 50%, one example would be if it is more favorable for the vibration to go in one direction or another, maybe the carbs are more affected by up and down than by side by side, or maybe you have a vehicle that transmits vibration to the rider more in one direction or another, or that the frame geometry can better handle vibration in one direction or another, or simply the crankshaft can be lighter with less mass in the counterbalance. I suspect watercraft are probably pretty close to 50%. Not so much when you stroke it 10mm and hang 9mm oversized pistons though.

There is also a second order imbalance because the rod is neither going up and down nor round and round, a little bit of both, and the proportion of up and down vs round and round changes throughout the stroke, so the proportion that is canceled out be either the up and down or the round and round changes throughout the stroke, so the amplitude and the direction changes throughout the stroke.

To me, compared to all of that, those little drill marks on the flywheel are probably not a significant amount of imbalance. But I dunno.

Experience beats theory on stuff like this, it's not intuitive.
 
kevbo

kevbo

Location
dfw
I was thinking crankwheels designed for 77mm pistons. They are much lighter than stock 84mm pistons. The engine is soft mounted so ideally it should gyrate evenly on both ends.
 
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