 There will generally also be a separate seat cushion.The larger rear wheels usually have push-rims of slightly smaller diameter projecting just beyond the tyre; these allow the user to manoeuvre the chair by pushing on them without requiring them to grasp the tyres.Manual wheelchairs generally have brakes that bear on the tyres of the rear wheels, however these are solely a parking brake and in-motion braking is provided by the user's palms bearing directly on the push-rims.