Thursday, 11 June 2015

MX-5 ND Skyactiv Transmission

Gear Ratios & Differentials
Anyone who has ever driven an MX-5 knows how important the transmission is. Along with the steering and pedal inputs, the action of the gear shift is a crucial part of how a car feels to drive. The MX-5 is renowned for having a short gear throw with a precise and positive feel that is often likened to a bolt-action rifle. They may be a little clunky from cold, but once warmed up, shifting gears was always a joy. Pressing on along a twisty backroad and taking the car up through the gears is an experience the MX-5 is famous for.

The pressure was on then to deliver a next-generation MX-5 specific transmission that could exceed the standards set by the earlier cars. Step forward Katsuaki Nobukawa, the man chosen by Mazda to lead the development of the ND's manual gearbox. Nobukawa's mentor, Takashi Matsugasako, had previously developed the transmission for the earlier MX-5s and had begun working on the next generation but sadly passed away before its completion. There is an interesting story surrounding the new transmission and how Nobukawa dedicated it to his former boss.

"“I would like to take this transmission to the grave of Matsugasako, who had to leave us before the job was done, and show him what we have made. Matsugasako always told us that MX-5 fans across the world were waiting; that we had to build a transmission that was worthy of the MX-5 name. I want to tell my mentor Matsugasako that we finally built it." - Katsuaki Nobukawa

The original Roadster's 5 speed gearbox for comparison

The new Roadster has a choice of a 6-speed manual Skyactiv-MT or 6-speed automatic Skyactiv-Drive transmission (built by Aisin). The automatic gearbox uses the same ratios as the NC auto did and may well be the same unit carried over. Automatic cars also have paddle shift, an automatic throttle blip function on downshifts, and a 'Sport' setting which increases acceleration response.

The manual transmission uses a variety of synchro types and makes use of carbon coating material for durability. The initial gears uses a triple cone synchro to reduce the amount of force needed to shift these gears, this is especially useful when the gearbox is cold. It's also interesting to note from Mazda's published technical info that the new Skyactiv drivetrain weighs in at 82kg, just 1kg heavier than the original. The gear knob itself weighs around 300g.

Limited Slip Differential

Special & Leather Package manual transmission models for the Japanese market are again fitted with a torque sensing Super-LSD manufactured by GKN Driveline. The differential has a new, more compact casing. The Japanese base model has a regular open differential. For the UK, as with the NC, only the 2.0 manual models feature the LSD as standard equipment.

2.0 differential case has cooling fins

Table of MX-5 Gear Ratios

1st 5.087 3.538 5.087 - 5.087 3.538
2nd 2.991 2.060 2.991 - 2.991 2.060
3rd 2.035 1.404 2.035 - 2.035 1.404
4th 1.594 1.000 1.594 - 1.594 1.000
5th 1.286 0.713 1.286 - 1.286 0.713
6th 1.000 0.582 1.000 - 1.000 0.582
Rev 4.696 3.168 4.696 - 4.696 3.168
Final 2.866 4.100 2.866 - 2.866 3.454
LSD Yes No Yes (2.0) - Yes (Club MT) No

From the above table of ratios, we can see that 6th gear is a 1:1 ratio and therefore a direct drive with the input and output shafts connected. This is a change from the NC gearbox where 4th in the 5-speed was 1:1, and 5th in the 6-speed was the direct drive gear. The reason for this change is the 1:1 ratio is the most efficient in terms of fuel use, and in the major markets such as the USA, most of the mileage is spent in top gear on highways. The change was purely for fuel economy gains.

Using a simple mathematical gearing calculator such as that on allows us to look at the gearing in more detail. The chart below for the JDM 1.5 shows what happens when the shift point is set at 7000rpm (max power), and how the revs drop back into the power band, the engine's most efficient rpm range. With the redline for the Japanese spec 1.5 engine set at 7500rpm, theoretically the 6MT car is geared for over 180mph. However in the real world where drag matters, with just 131PS top speed is likely around 120mph.


The 6-speed manual transmission will take 2.0 litres of 75W-90 (GL4) gear oil. For the differential, Mazda recommends 0.6 litres of their own long life hypoid SG1 gear oil.