A floating rim design senses the pressure applied by the users push and activates the motors proportionately to provide a power assist. This results in the convenience, and small size of a manual chair while providing motorised assistance for rough/uneven terrain and steep slopes that would otherwise be difficult or impossible to navigate, especially by those with limited upper-body function. As the wheels necessarily come at a weight penalty it is often possible to exchange them with standard wheels to match the capabilities of the wheelchair to the current activity. Mobility scooters share some features with powerchairs, but primarily address a different market segment, people with a limited ability to walk, but who might not otherwise consider themselves disabled. Smaller mobility scooters are typically three wheeled, with a base on which is mounted a basic seat at the rear, with a control tiller at the front. Larger scooters are frequently four-wheeled, with a much more substantial seat. Opinions are often polarized as to whether mobility scooters should be considered wheelchairs or not, and negative stereotyping of scooter users is worse than for manual or powerchair users. Some commercial organisations draw a distinction between powerchairs and scooters when making access provisions due to a lack of clarity in the law as to whether scooters fall under the same equality legislation as wheelchairs. One-arm or single arm drive enables a user to self-propel a manual wheelchair using only a single arm. The large wheel on the same side as the arm to be used is fitted with two concentric handrims, one of smaller diameter than the other. On most models the outer, smaller rim, is connected to the wheel on the opposite side by an inner concentric axle. When both handrims are grasped together, the chair may be propelled forward or backward in a straight line. When either handrim is moved independently, only a single wheel is used and the chair will turn left or right in response to the handrim used. Some wheelchairs, designed for use by hemiplegics, provide a similar function by linking both wheels rigidly together and using one of the footplates to control steering via a linkage to the front caster. Reclining or tilt-in-space wheelchairs have seating surfaces which can be tilted to various angles. The original concept was developed by an orthotist, Hugh Barclay, who worked with disabled children and observed that postural deformities such as scoliosis could be supported or partially corrected by allowing the wheelchair user to relax in a tilted position.