Space Marine Bikes make excellent flanking units. They are generally assault specialists, but the unit can be given some longer ranged firepower by adding one or more Attack Bikes.
Games Workshop Models

Specialist Games Bike
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Epic 40k Bike
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Adeptus Titanicus Bike
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Adeptus Titanicus Jetbike
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Grenadier Attack Bike
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Novan Elites Lightning Skimmer
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Sisterhood Nemesis Bike
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