The majority of rollers were of the same basic 3-roll configuration, gear-driven, with two large smooth wheels (rolls) at the back and a single wide roll at the front. (Actually, the wide roll usually comprised two narrower rolls on the same axle, to make steering easier. ) However, there was also a distinctive variant, the "tandem", which had two wide rolls, one front, one rear. Those made by Robey & Co. used their standard steam wagon engine and pistol boiler fitted in a girder frame with rolls and a chain drive to produce a quick-reversing roller suitable for modern road surfaces such as tarmacadam and bituminous asphalt. A number of Robey & Co. tandem rollers were modified to make a further variant, the tri-tandem, which was a tandem with a third roll, mounted directly behind the rear one. Robey supplied the parts, but the modification was undertaken by Goodes of Royston. Ten tandem and two tri-tandem Robey rollers survive in preservation, and one of the tri-tandems is known to have been used to construct parts of the M1 motorway.