Dalek fender

Human historians in the post-Time War universe observed that, during the Dalek Wars, most citizens of Earth secretly believed that the future of the human race was to be ground beneath a Dalek fender, (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) which the Dalek Survival Guide identified as "bumpers".

One theory for the use of these 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. The guide deemed it more likely that they served much as the bumpers of cars, protecting the Dalek casing from collisions. (PROSE: Dalek Survival Guide)

History and variance
Davros's chair, which resembled a black Dalek base unit with silver sense globes, originally had a silver fender before his apparent death in the Genesis Incident. (TV: Genesis of the Daleks) By the time he was retrieved by the Daleks, the fender was black. (TV: Destiny of the Daleks)

By 2094, a photograph of Silver Daleks with a large, blue luminosity discharger not exhibited by later models, and an enlarged Dalek fender matching that of the later Dalek Earthforce, was freely available on Earth, where it served to illustrate a news article about the recovering of a set of story-cubes about the Daleks' history. Five of the Daleks were standing in a deserted street, with other ruined casings of the same type strewn about. (PROSE: The Dalek Chronicles Found!)

The Dalek Earthforce which operated during the 2150s Dalek invasion of Earth were distinguished from the Skaro City Dalek by larger, additional fenders at the bottom of their base unit as well as solar energy collection discs. (TV: The Dalek Invasion of Earth) The Dalek Survival Guide believed that the enlarged bases were used for travel over rough terrain. (PROSE: Dalek Survival Guide)

By the time of the Mechonoid Incident, later silver Daleks dispensed with the discs and enlarged fenders and instead sported slats on their middle sections (TV: The Chase) like the grey Mark III Travel Machines presented by Davros. (TV: Genesis of the Daleks)

In their scrutiny of Dalek history, the Time Lords identified special Dalek forces who were engaged in the conflict against the Space Security Service, consisting of silver Dalek drones, a Red Dalek leader and its red-domed subordinate. These slatted Daleks all sported enlarged fenders like the Dalek Earthforce. (PROSE: Dalek Combat Training Manual)

The gold-and-black Dalek Supreme casing was distinguished from its grey Dalek subordinates by its taller fender. (TV: Planet of the Daleks)

The New Dalek Paradigm was distinguished from their predecessors by their taller fenders. (TV: Victory of the Daleks) The Time Lords believed that Paradigm casings had additional hardware in their back panels linking their weapons platforms to their fenders, allowing the Paradigm to augment their capabilities. (PROSE: )

In the rebuilt Dalek City, one older silver Dalek model sported a bronze-type fender. (TV: The Witch's Familiar)

Merchandise

 * Released by Dapol, an exclusive Davros action figure had a variant of his black chair with a gold control panel, sense globes and fender, replacing the televised silver.

Other matters

 * The Daleks seen in the theatrical films Dr. Who and the Daleks and Daleks' Invasion Earth 2150 A.D. had taller fenders than their televised counterparts. These fenders were removed for movie props which were used in The Chase, creating a new Dalek variant.
 * DWM 77's History of the Daleks uses the term "Proximity Bumpers".