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do overly long or very short stems affect the handling?
Yes and no.
It affect how it feel, but doesn't affect how it ride.
Bit hard to explain, but put it this way, putting a 120mm stem on a bike may make it feel slower, but doesn't mean it'll actually be slower to turn in corner compare to a 90mm stem.
Human are very adaptable, once you get used to the short stem, it feel normal, ditto to long stem.
do overly long or very short stems affect the handling?
The distance the bars have to travel is further on a long stem than short so should be less 'twitchy '?