If Sparky is allowed to chase bikes, the person on the bike could become startled, or knocked over and hurt, Sparky could run out into traffic and be struck by a car or could get tangled up in the pedals or spokes of the bike and be injured. Nothing good is going to come of this unless Sparky's owner teaches him to stop chasing bikes!
While it may be natural for your dog to respond to a fast moving object, like a bike, by chasing, this behavior can be extremely dangerous for the people riding the bike, and for your dog. It is important to teach dogs from as young an age as possible not to chase cyclists, for their safety and the cyclists'. You will want your dog to ignore a cyclist when they pass by, behaving in a calm, quiet, manner and resisting the urge to chase after the bike. Introducing a command to 'leave' the bike or an alternative behavior to perform when a cyclist goes by are both effective ways of changing your dog's chasing behavior when presented with a moving bicycle. You can even teach your dog to run next to a bike on a leash to introduce a different way of viewing a bike, as a pack member instead of prey. This will help change how your dog views and responds to a bicycle.