Who has used one and what are your comments about them?
I'm still a novice/intermediate rider, but I've had a bit of experience with FWWs. I put one on my KX100. It was a high-strung bike that had to be ridden in the powerband. It had almost no low-end. It really only had power when it was in the powerband. I wanted more low-end torque, so I called Steahley and talked to him. He recommended an 8oz since the little 100 couldn't turn a heavier FWW very well. He gave me recommendations for my riding style. He said a 250 would turn a heavy FWW no problem, but a 100 would feel sluggish, especially with a heavy rider like me.
The best way I can explain the difference a FWW makes is to compare a 2t to a 4t in the way it revs up and down. A 2T is much quicker revving. You blip the throttle and it spins up quickly and spins down quickly. Some 4 strokes are super quick revving, but a lot of them take a tiny bit longer to rev than a 2T. A FWW makes the crank heavier, which means it takes a bit longer to get up and moving (still only fractions of a second), and it takes a bit longer to slow down.
Trials bikes have super heavy flywheels. It took 3-5 seconds of full-throttle in neutral to get the flywheel spinning up to max speed. And it took about 5 seconds after letting off the throttle for it to completely wind down to idle speeds.
The FWW on the kx100 was cool. It wasn't so heavy that it was slow-revving like my trials bike. It still revved quite quick. It didn't feel sliggish on the revving. It did, however, tractor on an additional 4-5 feet (on flat) before stalling. The inertia of the spinning heavier flywheel made it keep its momentum even when I wasn't giving it gas. I don't want to say it had more low-end power, it didn't, it just kept spinning longer, and was a bit harder to stall. I really liked it on that bike, and I think it made it easier to ride in the tight woods. Looking back, I wish I would have dropped the front sprocket a tooth before adding a FWW, because I think that would have been even more benefitial than a FWW.
My KDX (current bike) has a flywheel that's heavier than most MX bikes. It still revs plenty quick, but it chugs along at low idle (plus my engine was built for low-end).
I am wanting to get one as I plan on riding AFC alot and hear they really help with low rmp riding as well as smoothing out the power.
Brian and Andrew have proved that AF Canyon is PLENTY doable on a 250 2T with gearing changes. I think a FWW would help, but not as much as regearing. You don't
need one, but I think for the price, it's a great investment for that type of riding.
It will be going on a yz250 and I can not decide on what weight to get. 9oz, 11oz, or 13oz
Call Steahley. He is awesome, and really knows his product and makes great recommendations. Not that you would, but be straightforward to help him make the best recommendation for you. If you plan on riding dunes, tell him. If you're a noob, tell him. If you're more interested in quickest lap time over tractability, he'll be able to help you.
I would like enough to make a difference but not over kill, if that is possible. I hear the 13oz can be almost too much weight.... I am kind of leaning toward the 11oz weight. Not sure how much a difference 2 oz will make.
I'll bet if you were blindfolded you wouldn't be able to tell the difference between an 11 oz and 13 oz weight. 2oz is really a small difference. Pros might be able to feel the difference, and they're so in-tune with their bike that they may prefer 2 oz less. If it were me, I'd get a 13 oz. Call steahley and he'll make a good recommendation for you.
