KX250F Weld on flywheel weights

Since they dont make flywheel weights, i was going to just weld some weights on and see what happens. has anyone ever done this?


Okay, its going to be a little more technical then that. I wanted to know a few questions


1) When you buy a heavier flywheel and they advertise a +8oz, is it literally an 8oz disc? or is there some kind witchcraft math going on here to get an equivalent moment of inertia with 8oz on the perimeter?

2) has anyone ever bought a different flywheel for another bike and simply weld it on?


My plan was to get 8oz of whatever my flywheel is made of (probably aluminum) and calculate the mass moment of inertia, and then find the equivalent with perimeter weights, then TIG weld those bitches on. Balance it and grind as required to get it to be as balanced as possible.


good idea? bad idea? i am converting my 2012 KX250F to an enduro/hare scramble bike.

Idiot know nothing about this Im commenting so I can see answer lol

The "flywheels", actually the magneto rotor, on most modern MX bikes (certainly all YZ's) are steel, which somewhat simplifies the matter of welding weights on. 


The answer to question 2 is that it probably will not work due to all of the differing variables involved, such as magnet phasing, timing issues, etc.


Question one, is simpler.  Most weighted flywheels are advertised by their "weight added", not their true inertia mass.  Herein lies one of the differences between such weights as DrD or Yamaha GYT-R flywheels, which have weights turned out as steel rings that are fit to the OD of the existing flywheel and welded in place, and other weights like those made by Steahly, that are made as disks that bolt and/or adhere to the flywheel.  Because the inertia mass added by an 8 ounce ring with an ID as large as the flywheel is considerably greater than that of the same weight disk with the same OD but a much smaller ID, the two can't directly be compared solely on the basis of the weight they add.  Their overall shape and configuration must be taken into account. 


In most cases, it should be practical to machine a steel weight of the desired mass to fit onto the existing flywheel and simply weld it there.  The weight must be shaped so that it fits the available space, and does not interfere with the operation of the trigger coil that senses crank position.  As long as the weight itself is internally balanced (i.e., symmetrical), and the welds small and carefully done, the entire flywheel should remain in balance.

Thanks for the answer.


This is going to be my approach. call Dr.D and ask how much their 8oz flywheel weighs for an equivalent machine (say YZ series). Since its a disc that goes around, i will take the weight and dimensions from photographs and calculate the mass moment of inertia of the disc. Then i will recalculate the equivalent mass moment of interia with 4 pieces and back calculate how heavy these pieces need to be. because of rotational symmetry as long as all the pieces are the same width and length the only variable becomes height. the in plane direction (width and length) are mutually dependent, but the height is independent. i.e. as long as the height and width fit, i can just use the height to make up my mass.



My brother is a professional welder and his TIG skills are extremely good. Im structural engineer so moment of inertia (though we deal with area moments not mass moments) calcs are 2nd nature. So really what can go wrong?? :lol:

Edited by he man

The weight that DrD adds to their 8 ounce flywheel weighs 8 ounces. 


Using 4 pieces, a lot of things could go wrong.  An imbalance, mostly.   Build it as a single piece and very little can go wrong.

thats ideal, but i have no idea if it will or will not fit.


piece wise i would weld 4 on there. sand the height down to get the balancing right. I would spool it up and see whats up. not sure how im suppose to get it to 10,000 RPM though. im pretty sure i could send it off to a flywheel shop to get it balanced though if i end up doing it that way.

Edited by he man

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