When a stock flywheel is lightened, does it need to be rebalanced? Or if it's done properly on a good mill or lathe, will it naturally keep it's factory balance, while just removing equal material around it?
If it does need rebalancing, how does it get balanced? I mean how do you gauge balance on a flywheel?

Cool. Thanks motorgeek. Do you think if a guy had a bad crank laying around, he could use that shaft and key? Then maybe balance it like u would a tire, only by removing weight instead of adding?

Could Crack said they balance the flywheel, might as well do  the crank at the same time.
http://crankworks.com/index.php/contact

Cool. Thanks motorgeek. Do you think if a guy had a bad crank laying around, he could use that shaft and key? Then maybe balance it like u would a tire, only by removing weight instead of adding?

No!!!!!!!!!!!!!!!!!!!!!!  You will have to use a special mandrel shaft to mount your flywheel on to dynamically balance the flywheel

 A two-stroke crank cannot be statically or dynamically balanced to eliminate all of the counter acting forces unless it has at least 4 cylinders.  There are a lot of shops that will take your money for such operations.  My opinion is they ignorant of the planes of vibration, amplitudes of the various components that contribute to the vibration and are not aware that the forces are absent necessary for dynamic balance in a less than 4 cylinder engine crankshaft assembly..........or they are crooks................. or maybe a little of both.  If a heavier than stock rod or piston is installed, weight may need to be removed from or added to the crank webs in strategic locations to return the balance factor to its original number.

Single cylinder design engineers use what is called a balance factor on single, twin and 3 cylinder crankshafts.  Since a single cylinder crankshaft cannot be dynamically balanced, we CAN control the primary plane of imbalance by changing the balance factor.  In laymen's terms: we can control the direction of the "shake".  We can make the direction of "shake" 90 degrees from the axis of the bore, inline with the bore or somewhere in between.  Design engineers test many different balance factors in a new chassis design to find the balance factor that excites the chassis the least amount and cause the least amount of discomfort to the rider.

If the vibrations are causing chassis failure or too much rider discomfort they will incorporate a balance shaft or better known as a counter-balancer to further reduce the vibrations.  Some chassis require engines with counter balancers because the natural frequency of those chassis and the plane of imbalance of a non-counterbalanced engine are not compatible. 

Engineers at Honda had discovered the need of a counter balancer in their 3 wheeler chassis on their 1981 ATC 250 R.  It took a while for Kawasaki and Yamaha to see the need on their 3 wheelers.  Suzuki was aware of the problem and incorporated counter-balancers in their first 1985 LT 250 quad.


Kawasaki basically used a KX 250 crankshaft in a three wheeler chassis.  This is the reason that the old Kawasaki Tecate  3 wheelers frames, ignition stators, and ignition flywheel failed frequently.  The vibration of a KX engine in the 3 wheeler chassis was unacceptable.  The vibrations from this combination shook the wires loose on the stator coils,  it shook the magnets loose from the flywheel shell and fatigued the steel frame to the point cranks would appear every where in the chassis in an unacceptable amount of time.  The tubing that Kawasaki used on their 3 wheel chassis was good tubing.  If the chassis was made of 4130-chrome molly it would have still failed because of the severe vibration.

A KX 500 or CR 500 engine installed in anything other than its original 2 wheel chassis shakes the chassis apart in a short time and makes the foot pegs and handlebars put the riders hands and feet “to sleep” for the same above reasons.  As the weight of the piston and connecting rod increase on a single cylinder engine, the need of a counter balancer increases. 

Counter-balancers have now become standard equipment on just about every single cylinder high quality off road engine that has a displacement of over 200cc in the last 20 years or so. 

 Motorgeek, Thanks for the in depth explanation.
@hotbutta, I've seen that video before too! I thought that was pretty neat myself.
Motorgeek, I started this post because I have an 88 lt250r motor that some kid trashed. It needs a full rebuild and the flywheel looked like it had been beaten with a hammer. My neighbor has a really nice lathe and he turned the flywheel down for me past all the dings and scratches. It looks just like the lightened flywheels you see for sale, but after reading all the replies, I'm thinking I shouldn't try to run this abused, lightened, and unbalanced flywheel that I have. Correct?

They probably beat the flywheel off because they didn't have a puller. The banging might have caused a significant loss of magnetism or even **** magnets. Like JH said, try it. But if ANY issues, get another.

Ive tryed diffrent things my bike w/ a stock flywheel ran way smoother and felt terrific but didnt put down the et's that the lightened one did.