woble at high RPM

when I spin fast, I can feel my the bike bounce a little. my first thought (and probably 90% of the problem ) is uneven pedal stroke... however, whilst servicing my bike, I spun the pedals and noticed that past a certain speed, the cranks do it on their own... then under a certain speed it calms down.

It's not a big problem, I'm just interested if there is a reason for this?

Oval chainrings?

Drive side crank arm is heavier than non drive side, so would adding car wheel weights to non drive side balance this out ?

More of a mountain biker so not used to high RPM spinning, but i do get what you mean when playing on my beater SS, a smooth cadence/spin cycle with legs and no dramas, when going faster or mashing on the pedals then i can feel a wobble when seated.
I always put it down to technique and not spinning smoothly. Curious on others take on the question.

Supermoon?

it's a sugino messenger crankset if anyone cares... I considered the chainring... but independantly wobbling despite no chain connected makes me doubt it. unevenly weighted pedals maybe? (shimano M520)

You need a inverse shim in your BB to deal with that

Drive side crank arm is heavier than non drive side, so would adding car wheel weights to non drive side balance this out ?

No. The excess mass on the drive side is more or less uniformly distributed about the rotational axis, so it's not really contributing to the wobble. The problem is that all bicycle crankshafts have a large rocking moment, the way to fix that would be with counterweights on both sides, so that the drive side and non-drive side cranks are each individually statically balanced. Obviously nobody in their right mind would bother about this, since the moment is tiny relative to the non-rotating mass.

Notwithstanding crank balance, the problem when actually riding has nothing to do with the metal bits of the system, it's an issue with the meat parts (huge rocking moment from the unbalanced legs) and the rubber parts (vibration from the leg imbalance excites resonance with the pneumatic tyres as the spring). That's why you feel "bounce" at a certain cadence, and in all probability why the bounce will vanish again if you push on to a higher cadence. The cadence value at which the system resonates will depend on the mass (bike and rider combined) and the spring rate, which is mainly a function of tyre pressure and cross section. On my RollerFort, it's somewhere around 140rpm, but it's smooth again at 160.

^this

Thats enlightened me loads.
From years of mountain bikes with big tyres and suspension you get used to pedalling induced bob from tyres and suspension bob, and linkage flex and bushing slop/bearing play.
As mdcc_tester has pointed out to me the moment is small and unlike in say a high revving motorbike engine an imbalance is unlikely to result in a massive parts failure.
From never really paying much attention to track or time trial bike set up's ( as i have never done either) i assume if the wobble was an issue it would have been looked into at a pro level and if it was of issue or a possible performance gain then the pro's would have started balancing things to reduce it, which would soon get noticed and everyone would be doing it.

:) loving the technical side of the forum

unlike in say a high revving motorbike engine

A R1 crankshaft is about the same weight as a whole entry-level road bike and turns 100 times as fast as your bicycle cranks, and it has plenty of rocking couple thanks to the cross-plane design. I don't think your bike need a balance shaft geared off your bottom bracket, but do feel free to build and test one just in case it's a marginal gain everybody else has missed :-)

Beyond my needs and ability, perhaps cannondale could have a go, they like quirky drivetrain systems for performance (in suspension action) gains.

:-)

damn son, that's technical