In the future, scientists will create objects that have the properties of living organisms. This idea is a bike that loses and gains weight depending on how much it is used. The owner will have the satisfaction of getting fit themselves but also enjoy sliming down their bike as every mile passes. As the bike sits in the garage getting fatter each day the owner will feel increasingly ashamed.
Dominic Wilcox 
I really like your sketches and concepts. Let’s kiss on the mouth like friends.
All motorbikes work this way. Loosing weigh of gas while you riding.
I’ll take one, thank you.
I think the other way might be good for those who wish to lose weight while biking:
The bike starts all thin and light to entice you to exercise- you are heavy; the bike is light. The more you bike, the weight you lose is grown onto the bike, keeping you working hard to propel yourself- you are light, the bike is heavy.
Thank you for your creativity. It is quite enjoyable.
How about taking it one step further; if the bike is a living object, when it gets big enough you can then kill, slaughter and eat it, thereby erasing the evidence (in the garage, anyway) of your inactivity.
I like this idea, makes me hungry, high in iron also.