The Curl does not have a rear spring. It is a hardtail. The way Andrew Ritchie design the Brompton rear suspension, the spring is compressed with each pedal stroke. I've noticed that over the last ~15 years the Brompton factory & Brompton riders have moved to progressively stiffer and stiffer springs to mitigate rear squat.
.
Can you comment on this difference in ride between the Brompton and the Curl? Is the Curl more harsh due to the lack of suspension and small tires?