I've been doing some crunches where you lay on your back and slightly put your feet closer to you and cross over on each side to work both sides as well. An other one I've been doing is to lift your legs up straight, lifting your waist of the ground, and slowly bringing your legs back down, almost touching the ground without having your back loosing contact with the ground. But now, I feel like I could go for harder ones. I just don't realy know which ones will be more benefitial and useful for cycling. Perhaps some of you can inform me a little more on this aspect, as I believe good abs are crucial for good endurance en force propulsed to the pedals.