Counterbalanced engines viabrate because they operate over a wide rpm range.
There will be an rpm, and multiples thereof, where viabrations do not exist. This is due to the primary balance factor used in the balance calculations - an engine can only be balanced for a set rpm, or multiples thereof (unless an engine, such as ducati 90 degree v twin, has perfect primary balance). Engine manufacturers decide during the R+D process where they believe the engine will spend the majority of time within the rpm range - the primary balance factor will then be derived to balance the engine at some point within the chosen band.
Anyone who believes otherwise is welcome to throw their hat into the ring on how to fit lighter pistons and conrods to a rotax V60 engine and retain primary balance via the two balancer shafts.....
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