Originally Posted by
merlinextraligh
The "bump" is about rolling resistence. When a tire bumps upward it's costing energy that could have been driving the bike forward, and on the rear tire is interfering with the transmission of power to the road.
...and the frame and the riders body absorbs that energy -- it is lost.
Originally Posted by
merlinextraligh
A number of people maintain that the least rolling resistence in most circumstances will be in the 90-120 psi range (Zipp, Continental, Zinn for 3).
Two more for your list:
The March 2009 of Adventure Cyclist magazine had a good article on this, with a graph of optimum tire pressure vs. wheel loading and tire size. The article was written by Jan Heine, editor of Bicycle Quarterly, and reportedly based on testing. It states that the optimum "on an average road surface" corresponds to a "tire drop" of 15% -- where tire drop is the amount of deflection from the rim to the road when load is added to the bicycle. Their optimum is based on speed, not comfort. The curves of the optimum pressure were linear vs. wheel load, and steeper for thinner tires. (The optimum pressure vs. load varies more on a smaller tire than a bigger one.) A couple datapoints that may be of interest for tandem use: 28mm tire, 100lb load, 75PSI; 28mm tire, 155lb load, ~120PSI; 23mm tire, 100lb load, 105PSI; 23mm tire, 143lb load, 150PSI.
Last year I read an article in a cyclocross magazine that showed (via coasting trials on a hill) that 30PSI was faster than 70PSI on a grass surface. On a more bouncy surface, you want less bouncy tires.
To deal with pinch flats with low pressures, cyclocross racers move toward tubular tires and tubeless clinchers.