Originally Posted by
Onus
Is a bike significantly less stable if the front wheel is much larger than the rear, say a typical 26" in front and a 16" in the rear?
No.
Most of the stability is created by the design of the steering - the combination of head angle, wheelbase, and trail. If you take a bike designed for 26" rear wheel and put a 16" rear wheel, your head angle will become less vertical and the trail will increase, changing the handling. If you take the same bike and lower the rear dropouts 5" before mounting the 16" rear wheel, the bike should handle the same.
:)ensen.