A wheel unit (10) comprises a wheel rim (12), a pneumatic tyre (14) receivable on the wheel rim (12) and a runflat assembly (16) adapted to be seated on the wheel rim (12) to close off a well (22) provided in the wheel rim (12). The runflat assembly (16) includes an annular support body (24) which is radially expandable to an expanded condition and contraction means (28,30) adapted to be operated via an opening (78) formed in the wheel rim (12). The contraction means (28, 30), when activated, is operable on the support body (24) to cause the support body (24) to be contracted to a radially contracted condition. The contraction mean is accessible through the opening (78) to cause the support body (24) to be radially contracted to seat the runflat assembly (16) on the wheel rim (12) to close off the well (22). The support body (24), when in its contracted condition, has a diameter greater than that of the wheel rim (12).