Road bike and roadster: A modified roof rack for a Mazda MX-5

Every fork is different. Hub spacing, axle diameter and mounting geometry vary between manufacturers. The rack therefore uses a universal mount. For our fork, however, the aluminium block was too narrow and the existing bore too large.

Separate adapters are available for this purpose, but the correct version was difficult to obtain and its price bore little relation to the simplicity of the part.

We measured the fork and designed two precisely fitted inserts. They widen the block on both sides to match the hub spacing and reproduce the axle diameter exactly. After manufacturing, both inserts fitted without further adjustment.

It was the smaller of the two tasks, but a good example of when a purpose-built part makes more sense than sourcing the intended accessory.

The bike should sit in the centre of the car, and not only for appearance. A roof that slopes towards the sides leaves an off-centre bike tilted and loads the mount unevenly.

The three-cup version would sit in the centre of the car, exactly where the hardtop panel is flexible and has no additional support. We therefore chose the larger version with four suction cups in a row. Its outer cups rest on the stiffer edges of the roof.

This moved the problem rather than removing it. Across the width of the four cups, the roof edges sit noticeably lower than the centre. The outer cups therefore make contact first and pull the rack plate down as the vacuum builds, visibly deforming the flexible roof panel in the centre. The deformation was not permanent, but it was not an acceptable solution for us.

The objective was straightforward: all four suction cups needed to sit evenly on the roof.

Using the measured roof contour and spacing, we designed a spacer that sits between suction cup and rack plate. Its upper surface reproduces the cup's mounting geometry: the circular seat and the pockets for the locating pins that prevent rotation in the plate. Its wedge-shaped underside compensates for the angle of the roof.

The connection remains positively located, and the existing screw can be reused without additional fasteners. From the outset, the geometry was designed for 3D printing.