Sunday 1 September 2013

The MX-5 Detachable Hardtop

The 3rd generation soft top model has the option of fitting a detachable hardtop. This feature works in a similar way to previous models but the new design is a little different. The NC hardtop is made from a reinforced lightweight composite sheet moulding compound (SMC) and has a heated glass window that was designed to look more like a coupe by not having any lower trim, the edge of the glass looks as if it meets the bodywork.

The hardtop has a smooth sweeping curve shape all the way from the windscreen surround to the rear deck, it's a nice design that also features a groove on either side. The rear deck latches have been redesigned and are now slightly smaller and more curved. Inside there are three locking latch mechanisms and a black carpet headlining.

The older design hardtop could be fitted to both NA and NB generations. This is not the case with the newer roof, it will only fit NC models. Like before the roof attaches to the car via two rear deck bolts and two side latches, but there is only one centre latch at the front instead of one at either corner on the older design. Unlike the NB models which included a fitting kit on every car, the NC doesn't so to add a hardtop you had to purchase some extra pieces of trim.

These extra parts include a new rear seatbelt trim panel with hole cut out for each side, a striker plate for the hardtop to latch onto for each side, bolts to attach the striker plates to the car body, a heated window connection wire, and two rear deck locating bolts (sometimes called Frankenstein bolts) to help hold the roof in position. It's possible to save some money by using old NA/NB frankenstein bolts and cutting holes in your existing seatbelt trim panels, but the quality of finish would not be as good. The picture below shows the parts that make up a hardtop fitting kit, and the difference between the two seatbelt trim panels.
The hardtop originally cost £1599.95 so was quite expensive on top of the price of the car. When the Roadster Coupe model was launched, the price difference between a soft top with added hardtop and a coupe model was not very much. Even though the detachable hardtop looks much nicer than the compromised folding hardtop, I think the cost is why you don't see many around as many buyers opted for the extra convenience of a retractable rather than detachable hardtop.

There are several options for lightweight hardtops from aftermarket vendors. Goodwin Racing, Nopro, Garage Vary, and Treasure Coast, all have various fibreglass and carbon fibre offerings. The Jota GT4 race car, Super20, and Super25 concepts provide good examples of how the hardtop can look.
If you find your hardtop isn't quite sitting right on the car, here's how to adjust it for a better fit.
To adjust the rear deck positioning on either side:
  • Remove the screw
  • Remove the rear deck lock cover
  • Loosen the plate installation bolts
  • Install the hardtop onto the car, fastening the front latch
  • Move the plate around until it sits tight against the rear deck bolt
  • Tighten the two plate bolts to between 18 and 26 Nm
  • Install the rear deck lock cover

Adjusting the front latch is similar to the older design, you have to rotate a nut in the centre to change how much the latch pulls the roof onto the rubber seal of the windscreen surround. The gap between the front of the hardtop and the windscreen surround (labelled 'A' in the picture) should be between 4mm and 8mm. Here is how to access the nut for adjustment:
  • Remove the spring
  • Remove the snap ring
  • Remove the pin
  • Remove the screws
  • Remove the cover
  • Rotate adjustment nut
  • Install cover, screws, pin, snap ring, and spring
  • Check/readjust as needed