Hmm, bumpy ride on a smooth road. Is this at all speeds or just above 30 MPH oe so?
If all speeds, try to get the wheels off the ground, 1 at a time or both. Use something such as a yardstick on a box as a pointer touching the tire tread. Turn the wheel slowly while watching the tread and pointer to make sure the rim and tire are perfectly round.
While checking the rear, also watch the chain carefully while turning the wheel 3 or 4 turns around. If the chain jerks at times, you have a binding link that needs lubing. If the chain goes tight - slack - tight - slack, it is unevenly worn and you should consider replacing it.
If the problem is only at speed (maybe worst at a particular range) chances are you have lost a balancing weight.
If it is at all speeds and goes away after 1/2 hour or so, the tires are 'flat spotting' from sitting overnight. This will get better when the weather gets warmer, but it would be a good idea to consider replacing them with a name brand tire (not Bridgestone etc) when you can.
