Bumper

As noted in the Dalek Survival Guide, bumpers served to protect cars from collisions, with the guide reasoning that the same was true for the fenders situated at the very bottom of the lower sections of Dalek casings.

Another theory for the use of these Dalek bumpers was that they detected signals that warned a Dalek when other Daleks were dangerously close. It was claimed that during the Battle of Poseidon, several Dalek battalions were wiped out by their own Daleks. However, since there was no visual evidence of Daleks without these fenders, even going back to Ancient Egyptian hieroglyphic evidence, this claim was treated with a degree of skepticism. (PROSE: Dalek Survival Guide)

Other references
After psychografting herself into Rose Tyler's body, Cassandra O'Brien took great pleasure in examining her "nice rear bumper", slowly stroking her newfound posterior with one hand. (TV: New Earth)