Martijn’s M135i

Ever since getting this car, well, actually since I knew I wanted to buy one, I knew a limited slip differential would be an essential upgrade. A co-worker / mate has an F20 that was totally transformed by this upgrade. Back in March when I got it the worn original PSS’s and the colder/wetter roads helped a little, but fresh PS4’s and dry road really made this issue more apparent. Biggest issue to me is the unpredictable reaction you get, sometimes the car may go sideways, but upon hitting a bump mid-corner it unweights the inner wheel and snaps back with just one wheel spinning. 

I know there are lots of options by now (MPerformance, Drexler directly, Quaife, Wavetrac, MFactory) but I went for an OSGiken. Fully explaining the workings of the Superlock is out of both my technical and English skills, but it’s a great solution. In short it’s kinda like plated clutch pack held together by springs, enabling it to be daily friendly and still lock up to 100%.

From OSGiken’s FAQ:
Under racing conditions, the function of the LSD is to allow the drive wheels to apply equal traction under acceleration and deceleration. Our OS Giken Super Lock LSD is engineered to be extremely responsive to throttle input to give the driver full control over differential action. Our units are a negative preload design which allows for minimal lock when power is not delivered (turn in) and progressive maximum lock 100% on corner exit. Our unit is capable of controlling when 100% lock happens dependant on estimated power delivery and grip available (tires).

Through its innovative and patented design, up to twice as many friction plates are able to fit into the housing when compared to our competitors’ products. This enables the OS Giken Super Lock LSD to achieve true 100% locking capability. Our LSD is also customizable by adjusting the spring inside the pressure ring to achieve progressive lock and allow high powered vehicles controllable lockup. Additionally, its superior design allows for excellent smoothness in regards to lock, virtually eliminating the “chatter” associated with other clutch type LSD brands.

Each component of every OS Giken Super Lock LSD uses only the highest grade materials and manufacturing processes. They are constructed and finished to extremely tight tolerances to eliminate “break-in” period prior to racing conditions. Expect longer life out of LSD performance compared to its competition. Rebuilds are rarely needed and is recommended to have fluids changed periodically to prolong the life of the unit.

Dutch BMW specialist Hardeman Motorsport imports and sells these units, so I made an appointment with them. As the final drive is one piece with the stock diff, they machine these to fit the OSGiken diff and mount this into a overhauled diff housing with uprated bearings. That way the swap itself only takes an hour or so, but they do keep your original diff to build for the next swap:

Car raised, exhaust removed:

Remove diff and let it drop, install in reversed order:

Filled with fresh 80W250 (no typo!), they advise to change it after some 1000 km and sent you off with a couple of cans of this:

They also recommended to change the gear box oil. Didn’t have any issues with it, but I’m always open to suggestions:

Have to say that this purple fluid did make a difference, shifts quite a bit smoother now:

The drive home was a tad boring, with mainly motorways and lots of traffic. A couple of noises from the rear on the first few corners (as expected) but totally silent after that, even on parking an manoeuvring around. But it only takes a little bit of spirited driving to feel the difference, you can really feel the rear outside wheel work to get you out of corners.

The next day I set off to Northern Italy, to start on our annual road trip with a couple friends to Croatia. Hours and hours of German Autobahn, so again not a whole lot of limited slip action, apart from a small demonstration at an ‘Autohof’ to show its workings to my significant other. 

During the next week I really got to enjoy both the AST suspension and the LSD on lots of twisty mountain roads. It really transformed this car, feeling so much more balanced. Cornering speeds are higher, without feeling twitchy. And if you do cross the line on grip, it stays much more predictable. It also made Sport+ into a very usable setting, enabling you to play with the cars balance while still having an invisible hand keep you in check should you seem to overdo it.