define "works" Do you mean to ask if it's going to give you abs of steel? No, it won't. Do you mean to ask if it will make your abs bigger and more moticeable? No, it won't. However, your abs have to contract to support your upper body as you move on the bike.
So yeah, it works your abs. And your biceps and triceps too! Just not in a way that will do anything other than stabilization strength.
I think you have to augment ab and back workouts with cycling - but I do a lot of hills - long ones and short ones at pace - sitting and standing - and my torso (including abs) gets a workout (even trying to be as quiet as I can with upper body to save energy).
it directly works your legs, your tris, your traps, lower back and your shoulders. it's also working other muscles indirectly, so it's working quite a bit actually.
I'm not sure how you work all those muscle groups to any measurable extent if you're just riding a bike. You do almost nothing with your upper body. Your cores get about as much exercise as when you're watching TV. As for lower body, you overwork the quadriceps but in comparison ignore the hamstrings, gluteus, calves, ankles and feet
Don't get me wrong, I love cycling. But no single activity is good execise for the entire body--although walking, rowing and swimming come a lot closer than cycling, IMO.
But one good thing about cycling is that it exercises a very important muscle--the heart. And it's easier on the joints than some of the other activities.
Cycling also gets you some place, it's a form of transportation, and it can be integrated into the rest of your life better than other forms of exercise (think about commuting to work by bike, for example).
"Your cores get about as much exercise as when you're watching TV."
really? I suppose if you are going at the same pace as tv watching it could be true. but I never get that sore feeling in my torso after tv watching as I do after a good hard 2 hour ride over rolling hills.