That's kind of hard to answer. While they are surprisingly different, I think each requires a different approach.
Cars allow you to take in their surroundings to a greater degree. And, they also mask out some of the crappy background fodder. With bikes being smaller, fan pop-up tents and even tire barriers are much more distracting and visible.
Obviously, bikes are quicker and smaller. But, the shutter speeds are much more forgiving. You can shoot bikes (and need to) much highter.
With bikes, you have an additional subject to shoot. You can shoot the bike... and you can shoot the rider. So, you can "clip" the rear or nose (or both) of the bike and still have a stunning photo of the rider.
Color... bikes have a lot of intense color.
In the end, though... I think shooting bikes is a little bit redundant. Because of the being limited to include surroundings, a tight inside shot can look the same from one corner to the next. I think the demand for shooting on-track requires really seeking out "the moment"... a wheel lift, a spark.. really recording a slice of life.
I think if I were to shoot bikes every weekend, I'd be doing a lot of balancing my shooting with more pit and atmosphere work to go along with the on-track.
It's fun... and initially the impact of the colors, the speed, the tight crops is a refreshing change. I need to do it more often.
Thanks,
JT