An Acro Bike (Japanese: ダートじてんしゃ Dart Bike) travels at the same speed as regular Bicycles, but it also allows players to perform tricks, such as wheelies and bunny hops, which let players reach special areas. It can also be used for traveling on white rails found on Route 119 and in the Hoenn Safari Zone.
Bunny hopping up and down on an Acro Bike in tall grass can cause wild Pokémon to appear, but does not count as steps towards the Safari Game or Repel.
Unlike the Mach Bike, if the player rides on Seaside Cycling Road their time won't be recorded due to the bicycle being "easy to turn" in comparison.
The Mach Bike (Japanese: マッハじてんしゃ Mach Bike) allows players to move a lot faster than regular Bicycles and the Acro Bike. It can move fast enough to ride straight up a muddy slope, or over cracked tiles without falling through (which appears in Granite Cave, Sky Pillar, and Mirage TowerE) if the player has enough momentum. If the player collides with something, all momentum gained is lost and the speed resets.
On the Seaside Cycling Road, a player's time and number of collisions while riding this Bicycle from the northern entrance to the southern exit are timed and recorded. This is part of the data sent over to other players when mixing records.
Source: Bulbapedia/ Bicycles