That doesn't really make sense. If you can do 100 sit-ups, you should be able to support your weight with your abs.
Well, unless you're doing 100 sit-ups incorrectly, where you are relying on your back more than your abs.
Crunches are superior to sit-ups because they work solely on your abs, with a lower chance of straining your back and neck. Basically, a crunch is like a sit-up, except, instead of going all the way up, you only go up about a foot. You do them focusing solely on your abs, and it's basically the only ab workout you need. It's easy to strain your back or neck if you do them improperly(putting tension on back and neck) If your back is already super strong as you say, then that would be less of an issue, but it'd also be harder to tell if you're actually working your abs.
Try this:
Go into a push-up position on the balls of your toes, and keep your back straight, stay supporting yourself for a minute.
Another thing to try. Lie on your back, extend your legs out straight then lift them 6 inches from the ground(the lower the harder) with your abs. Can you hold that position for a minute?
If you can do either of those, then supporting your self on this wheel oughtn't be much harder.