The undercarriage of the Harrier has always been one of its unique design features. Despite early attempts at a more conventional undercarriage for the P.1127, the only practicable method was the 'zero-track tricycle' (i.e. bicycle) with wing-mounted outriggers adopted. The geometry of the main units was dictated by the need to avoid interaction with the engine exhaust during jet-borne flight and to provide good ground handling. The location of the outriggers was originally intended to minimise the weight penalty they incurred, although on the Harrier II they have been moved inboard to reduce the width of the aircrafts track, easing ground and ship-based manoeuvring. From the P.1127 to the initial mark of Harrier the undercarriage underwent considerable refinement to make its handling qualities acceptable.
The single-wheel nose undercarriage unit not only supports a significant proportion of the aircraft's weight but also provides steering over a range of 45 degrees to port or starboard. It is free to castor through 179 degrees in either direction for towing. On retraction the unit's leg shortens to minimise stowage volume. The main undercarriage unit has twin wheels and is fitted with powerful brakes, retracting aft when the aircraft is airborne. The main unit leg has considerable 'give' on contact with the ground, such as to allow both outrigger wheels to achieve positive contact, although the greater part of the aircraft's weight is borne by the main unit. Each outrigger has a castoring wheel (although these were locked in the early 1980's after the loss of the tyres on several occasions), which is left exposed after the unit has retracted.
All the undercarriage wheels have low-pressure tyres to facilitate dispersed site operations from grass and other surfaces. With the undercarriage locked down the main doors are closed to reduce the risk of foreign objects entering the undercarriage bays.