People may or may not be interested but in the on-line magazine I write for, AutoSpeed, I have posted two articles on measuring ride quality.
AutoSpeed is primarily an automotive technical magazine, but we have also covered the build-up of the three recumbent suspension trikes I have designed and built.
The two articles
Part 1 - http://www.autospeed.com/cms/A_111836/article.html
Part 2 - http://www.autospeed.com/cms/A_111837/article.html
describe how to use low cost software and an iPhone or iTouch to measure real-time ride accelerations. I measure all accelerations at the head.
In Part 2 there is data showing how a Brompton (equipped with sprung seat) compares in ride quality to a full-size bike and also one of my suspension recumbent trikes.
Esspecially in fore-aft pitch accelerations, the Brompton is very poor.
If you're interested in the ride quality of small wheel human-powered machines, it may make for relevant reading.