Saturday 17 August 2013

The MX-5 Retractable Hardtop

MX-5 Roadster Coupe
In 2006 Mazda introduced a new model to the Roadster family. The MX-5 Roadster Coupe was unveiled in London at the British International Motor Show. For the first time in the history of the MX-5, the car featured an electrically powered folding hardtop, referred to as the Power Retractable Hardtop, or simply RHT. Mazda has offered removable hardtops for the Roadster since the very early days, and even produced the NB coupe model in limited numbers, these tops offered refinement and security but where not very convenient. The folding hardtop provides that extra ease of use as well as being more secure than a fabric roof, I think it's a good compromise.

The RHT roof was made of a strong yet lightweight plastic composite with a glass heated rear window. The new roof consists of three sections with two motors on either side to operate the folding mechanism. Normally in cars with electric folding roofs you have to sacrifice some luggage space in the boot, but not so with the Roadster Coupe as Mazda managed to fit the mechanism into the space between the seats and boot. The RHT model does lose the two storage cubby boxes behind the seats, but that's not a big deal. Since the roof was made of plastic and didn't weigh much, it was able to be lowered in just 12 seconds, making it the fastest folding hardtop in the world at the time of launch.

Packaging a retractable hardtop mechanism into a small car like the MX-5 required a new rear section of the bodyshell and new bootlid. This gives the RHT a different roofline and enlarged rear end compared to the soft top models.  In case you wondered what happens if you lose power or can't operate the roof for some reason, there is a complicated procedure for operating the roof manually which involves a thin rod and lengths of cord, the cars were supplied with a toolkit for this purpose in the glovebox. Production of the hardtop's mechanism of levers and brackets was outsourced by Mazda to Japanese engineering firm Futaba Kogyo. In the pictures below you can see the additional wiring loom and the motor mechanisms on each side.

Operating the retractable roof involved parking the car, applying the handbrake, and pressing one of the buttons next to the hazard switch in the centre of the dashboard. You had to hold down the button until the operation was completed, and there were various warning beeps to remind you to lock the centre latch to the windscreen frame. There is a modification available to control the roof's electrics and override the restrictions on its use. Installing the SmartTOP module will allow you to raise and lower the roof with just one press of the button instead of holding it down, and also allows the roof to be operated while the car is moving. This means you don't have to pull over to the side of the road to raise the roof if you get caught out by the weather. It also grants you maximum low speed posing ability.

There was some debate as to the additional weight the RHT has over the soft top model. The publicity material makes a feature of the coupe's roof mechanism adding only 37 kg to the weight of the equivalent model. However there is some contradiction in the UK brochures, where the difference is published as 83 kg - quite a bit more than the 37 we were led to believe. However, after studying the figures from other markets I have a theory that the UK brochure is a misprint as a result of conversion from pounds to kilograms. Let me show you what I mean.

Published weights for 2013 2.0 6-speed manual transmission cars:

  • 2.0 RS 1120 kg
  • 2.0 RS Coupe 1160 kg
  • Difference 40 kg

  • 2.0 Miata 1139 kg (2511 lbs)
  • 2.0 Miata Coupe 1176 kg (2593 lbs)
  • Difference 37 kg (82 lbs)

It is standard practice for European "kerb weight" to add 75kg for driver and luggage, I have removed this from the UK figures for comparison with the USA and Japan.
  • 2.0 Sport Tech 1090 kg (1165 kg with driver)
  • 2.0 Sport Tech Coupe 1173 kg (1248 kg with driver)
  • Difference 83 kg

Now allowing for minor specification variations the Japanese and American difference figures are in agreement. For me it's too much of a coincidence to have the USA and UK difference in weights being 82 lbs and 83 kg. So my theory goes whoever edited the UK brochure was told the difference in weight was "about 83" and assumed the units were kilograms, when in fact they were pounds. What this means is that the UK brochure weight for a 2.0 soft top car of 1090kg is inaccurate and should be around 1130kg.