Biking and cross-training are both good exercises and burn lots of calories but are also low impact.
Basically, the higher your heart rate goes, then the more calories you burn.
However, calories burnt doesn't nessecary equate to fat loss. Everyone has a max heart rate (calculated by taking your age away from 220, eg 220-33 = 187). In order to burn fat you should keep between 60 and 85 % (between 112 and 159 in the example above) of your max heart rate. Go above this and you still burn calories but you're increasing your cardio training ability rather than burning fat.
I found this myself when i moved from swimming to running. Noticed that i wasn't losing any weight even though i was burning loads of calories. Then bought a heart rate monitor and kept my heartrate within my fat burning zone and have noticed the difference so so much