[Edslittleworld]

I am tempted to upgrade my stock 2017 M240i with the BMW M Performance LSD. Cost is a little over $3K. What bothers me is that the M Performance LSD is rated at only 30% lockup on acceleration and 9% lockup on deceleration. That sure doesn't sound like much to me. On the other hand, doesn't the M2 have a 100% lockup LSD since it's electronically controlled? If so, the M Perf diff doesn't seem a worthy upgrade. Am I wrong?

[6speed_M2]

Shouldn't it be the same diff off the M2? Is the 240 with the electronic assisted steering vs hydraulic?

[SoftShoe]

[EDIT - Yup.

Others gave better (if much harder to understand ) answers in the thread below.]

From ecs:
“ What exactly does an LSD do you ask? well without an LSD your car is actually 1 wheel drive, in that it can't supply torque to both wheels at the same time and will only supply torque to the wheel with less grip, which is what the shortfall is to an open differential (what most vehicles have).
An LSD only lets the wheels spin at different speeds by a limited amount, which is typically a percentage difference, which is when the differential locks and both wheels get power. This is not how locked differentials work such as in off road trucks which supply equal torque and rotational speed to both rear wheels at the same time, which can cause issues with handling.”

[aerobod]

The 30% lock on the M-Performance plate-style Drexler LSD refers to the torque bias possible between the two wheels, effectively allowing up to 65% usable torque on one wheel and 35% on the other compared with 50% on each wheel for an open differential and 100% and 0% for an axle without a differential or one that can be fully locked to equal that scenario. This is usable torque though, not available torque, which means the torque that can be handled by the lowest grip tyre will determine the total torque transmitted by both tyres.

Looking at the M240i manual RWD in a high grip scenario, say at a drag strip in a straight line with R-comp tyres. Let’s say in this scenario the total torque transmitted via the tyres can be equal to the maximum torque available in 1st gear, which will be engine torque minus driveline losses times the gear ratio (gear x final drive):

Maximum torque = 480Nm
Driveline loss (approximate) = 15%
Gear ratio = 4.11 x 3.077 = 12.65
Tyre kinetic friction is 0.7 of static friction (i.e spinning tyre is giving 70% of the grip of non-spinning tyre, this can vary a lot, but assume 0.7 for this scenario)

Total torque transmitted for both tyres not spinning (for a 100% lock) would be 480 x 0.85 x 12.65 = 5,161Nm total or 2,580Nm per tyre

In the case of an open differential, assume one wheel loses grip, it’s torque transmission capability is now reduced to 0.7 of what it was, i.e. 0.7 x 2,580 = 1,806Nm. The other tyre can only receive this torque, even if it still has grip for a total of 3,612Nm.

In the case of a 30% locking differential the spinning tyre will still receive 1,806Nm, but the torque bias to the non spinning wheel can be 0.65 to 0.35 up to the tyre maximum grip, which would be 1,806 x 0.65/0.35 or 2,580, whichever is less. In this case it would be 2,580 for a total of 4,386Nm.

Things get a bit more interesting when cornering, let’s say that in cornering the inner tyre has 1.5 times the torque capacity of the outer tyre. Using a grip scenario where 50% of the tyre grip is available for acceleration and 50% for cornering, for a spinning tyre that would give 1,806 x 0.4 (722Nm) for an inner tyre and 1,806 x 0.6 for an outer tyre (1,084Nm). A non-spinning tyre would be 1,032Nm and 1,548Nm respectively (this is simplistic, as the cornering grip would also change due to a spinning tyre).

With the 100% lock, the maximum torque transmitted would be 1,032 + 1,548 = 2,580Nm total before wheel spin and 1,806Nm after wheel spin.

For the open diff it would be 1,032 x 2 = 2,064Nm before wheel spin and 722 x 2 = 1,444Nm total with one wheel spinning.

For the 30% lock diff it would be 1032 + 1,548 = 2,580Nm before wheel spin (as 0.65/0.35 x 1,032 is more than 1,548), 722 + 0.65/0.35 x 722 = 2,063Nm total with one wheel spinning and 1,806Nm with both wheels spinning.

In the case of a mechanical plate diff it is difficult to achieve 100% lock in a reliable manner without causing other handling or tyre wear issues as wheel speed differential due to cornering and wheel spin can’t be separated. With an electronically controlled diff such as on the M2, wheel spin and cornering effects can be separated due to input from the steering and wheel speed sensors, allowing 100% lock only when needed to prevent handling issues when wheel spin isn’t present.

[XutvJet]

The MP LSD is absolutely the best mod I've done to my M235. It essentially unlocks the chassis. The amount of extra grip on rear axle is immense. Like I've said a number of times of this forum, it feels like there's an extra 500lbs over the rear axle. You can launch far harder and apply WAY more throttle in the lower gears from a roll and coming out of the apex of a turn. When hammering the throttle out of a turn, instead of the backend just sliding wide and a loss of momentum, the LSD allows you to steer with the rear WHILE still accelerating through the turn. The open differential with the E-LSD in operation (only works in DSC Off) cannot do this.

With respect to the 30% lock on acceleration and 9% on deceleration for the MP LSD, the %'s are not what you think. Aerobod's detailed description is exactly what happens. The available lock-up is ideal for the street and light track driving. It's aggressive, but not overly so. I wouldn't want any more lock up for a car that spends a majority of it's time on the street.

Yes, the M LSD found on the M2/3/4 can be a full locker in certain situations and it's amazing, but in most situations, it performance similar to the MP LSD.

The LSD is built for BMW Drexler. Drexler is one of the best LSD makers in the business. The only downside I see to the MP LSD is the price. It's unfortunate that we have to pay so much to add something that should have come stock. I spent about $2,700 to buy it and have it installed by my local BMW dealer.

[ggggbmw]

I would say that the MPerf LSD is certainly 'good enough'. (And I have one on my car.)

You could argue the relative merits of a Torsen style diff. The lack of clutch wear would be a big positive, but you lose braking lockup. And you can get different diffs with higher lockup ratios. And a full electronic lock diff like the M2 has is certainly cool. But outside of a track, you probably wouldn't see/feel any difference.

And all of these will cost more than the MPerf LSD, for very limited improvement if any. Or just aren't available, like the M2 electronic diff. So, with some economic input, the Mperf is a very good choice.

[Ugly Kar]

The best LSD was made in the 60's by a dude named Bear Stanley. He made LSD for Jerry Garcia.

[dradernh]

Originally Posted by Ugly Kar

The best LSD was made in the 60's by a dude named Bear Stanley. He made LSD for Jerry Garcia.

Was it better than Owsley's “White Lightning”?

[cristo]

I hear the brown stuff was really special.

[Paulvin]

Definitely worth it. This should sway you;

https://www.drive.com.au/reviews/2015-bmw-m235i-review/

Brings me to a question for the experts: Long flowing 270deg perfectly cambered section of road leading onto the highway where I live. Pull away from the lights, steadily accelerating, lateral G coming on, reaching the exit apex (~100kph), and the back starts to give a little requiring about 30 degrees counter steer on exit. Heaven but......what happens once the LSD has given all it can vs the laws of physics and a bit of spin comes into play? Does the eLSD now kick in and the software start mucking with brakes to bring DTC into play?

[Pork Rind]

Originally Posted by dradernh

Was it better than Owsley's “White Lightning”?

Owsley is Bear.