Quote:
Originally Posted by The Cthulhu
Does a mechanical helical LSD have any advantages over the electronic LSD aside from placebo effect and innaccurate assumptions?
The electronic LSD can't be used in conjunction with traction and stability control. A mechanical LSD is not designed to be used in conjunction with traction and stability control because these systems were designed for an open helical diff.
The electronic LSD brakes the inside wheel if it starts to spin, diverting power through the open differential to the outside wheel (don't argue that power doesn't get diverted, it's illogical and incorrect). A mechanical LSD directs power to both the inside and outside wheels at a ratio, often allowing for some inside wheel spin.
If the electronic LSD clamps down on the inside wheels rotor in hard jitters, it would be pretty efficient.
My real questions are as follows:
1. When BMW says "it brakes the spinning wheel" does it mean that all wheel spin will be directed to the outside wheel if a driver mashes the throttle hard enough in a turn?
2. Does it mean that wheel spin will be adjusted from side to side at some interval so that they will spin together if pushed hard?
3. If #2 is correct, and the interval is kind of long, would this be that much less efficient than a mechanical LSD due to force juggling (I assume the left and right rear brakes are not allowed to clamp simultaneously during electronic LSD activation)?
Please think before you post... none of these "Brakes make your car slower!" epiphanies. If anyone has good lap data on both systems for several runs, please post. I have a feeling that this thread is going to get illogical...
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Heeeeere we go....
I'll start by correcting your assumptions:
- Yes, a mechanical LSD is a WORLD of difference from an open diff with traction control. There is a reason why virtually every performance/racing car has one.
- The "E-LSD"
IS traction control. That's exactly what it is; nothing more. Calling it an "E-LSD" is BMW marketing nonsense.
- Traction Control and Electronic Stability Control have nothing to do with whether or not the car has an open diff. A mechanical LSD will supplement those programs, but does not replace them. On most cars, the TC/ESC are standard "off-the-shelf" programs made by Bosch and are the same on cars with and without a mechanical LSD.
- The "E-LSD" (call it traction control because that's what it is) brakes the slipping wheel in pulses, resulting in an ON-OFF-ON-OFF power application, sending the vehicle into a bucking motion. You are correct, during the time that the slipping wheel is being braked, power is sent to the outside wheel; but again, it's in an ON-OFF-ON-OFF manner. A mechanical LSD
DOES NOT send power to one wheel at one specific ratio; it sends power at a variable ratio that depends totally on the instantaneous grip available to both rear tires. No mechanical LSD splits power at a specific set ratio.... they all vary based on the instantaneous grip available.
- "If the electronic LSD clamps down on the inside wheels rotor in hard jitters, it would be pretty efficient." - NO, it wouldn't. It would be very inefficient. This is why, again, virtually EVERY racing car on the planet has a mechanical LSD. A mechanical LSD acts instantaneously and constantly. An E-LSD
reacts to the loss of grip that already happened. On a track, that = time lost.
On to your questions:
1) No. BMW's "E-LSD" allows a massive amount of inside rear wheel spin. I'll post pics later tonight.
2) No. The E-LSD has a very low threshold where it simply stops engaging and allows all-out wheelspin.
3) Every traction control system I've tracked has been massively inefficient and in no way a substitute for a proper mechanical LSD.
Hope that helps clear things up...