Why are bus rides so bumpy?

I ride the local city buses to and from my volunteering since I can't drive due to medical history. I have noticed how bumpy it is on the bus compared to my dad driving me in his car! Why are buses so bumpy?

Other answer:

Erin:
Busses are larger vehicles and have to have stiffer suspensions to accomodate a full load of people. They are also limited in suspension travel because otherwise the extreme ends of the bus would move up and down several feet.
DR + Mrs Bears face:
Hi harder suspension with a bus that usually carries loads of weight.
cdymlr:
the extended wheel base (distance from front wheels to back wheels) is much wider than your dads car which makes it more difficult for the suspension to absorb most of the bumps in the road. this could be why you feel a difference
john:
Buses suspension is very weak