Aug 3, 2011 update to the Mazda MX-5 Miata project car

Diff’rent Strokes

Project STR waits its turn at the Windy City Miata Club shakedown event.
Here's the OS Giken diff installed in the spare diff housing.
The ring gear is removed from the OEM differential as it awaits the new OS Giken LSD.

As we look through our comments in the logbook, one that stands out is rear wheelspin that occurs when we try to accelerate to the next corner. We’ve worked on the suspension in an attempt to increase rear-end grip, and we even tried different tires. While this work has helped, at most event sites we can still slide the rear when exiting medium- to high-speed corners, resulting in sideways motion without forward progress. A look at the data stream in our onboard data logger shows that the OE Mazda diff still allows some wheelspin on a single wheel, and we’re getting inside-rear wheelspin—not just sliding both rears.

So we turned to the guys at OS Giken, makers of some tricky driveline parts and the OS Super Lock LSD. This tunable limited-slip diff allows the rear end to lock smoothly with power application. Plus, it can be tuned to have different lock rates during acceleration and deceleration.

To simplify our installation and give ourselves the opportunity to try some back-to-back testing, we sourced a separate diff carrier in which to preload the OS Giken unit; it took a little over an hour to swap over the differential. The separate diff carrier allowed us to swap out the preloaded diff housing in about 2.5 hours. We also had the help of a co-worker’s lift—some things just shouldn’t be done on the garage floor.

After the car racked up a few break-in miles, Project STR driver Chris Harvey joined the fun. He took the car to the Windy City Miata Club outing to get an initial read on how the freshly installed OS Giken limited-slip differential works. The competitors enjoyed great courses and lots of runs as they battled against the clock throughout the day. Chris piloted the MX-5 to the quickest time of the day for all Mazdas entered and second overall for the event, just a few tenths behind a well-prepped and -driven BMW M3 running on ultra-grippy Hoosier autocross tires.

With just a month to go before the Solo Nationals, our MX-5’s next event will be the SCCA Great Lakes Division Championship event held August 6-7 at Grissom Air Reserve Base in Peru, Indiana.

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