This post gives a detailed description, with photos and reference to a Parts Diagram, of how to replace a Dyson Slim DC18 Undercarriage. The Undercarriage would require changing if part of it broke since parts cannot be obtained individually. I changed mine when the Air Input Changeover Valve Actuator broke.
“Since I first published this article, in February 2013, I have had plenty of feedback from readers and I have gleaned information from statistical data which has encouraged me to improve it. So if you came here soon after I published it you should find better information now.” — Helpful Colin