Sorry for the book report. For those with a short attention span, the TL;DR version is: The brake based limited slip solution in our Q50's may be a pretty good solution after all.

I am trying to comprehend what exactly my RS400 has in terms of a limited slip capability. It seems there has been much discussion on these forums over the years of the relative merits of a LSD in the various vehicles Nissan/Infiniti has produced. Having read the various comments and rants, I became convinced that the open differential in my Q50 RS400 is a vastly inferior solution to an LSD by applying power one wheel when one wheel has less friction than another.

So, I looked to understand the basics. I found a series of short instructional videos from http://www.learnengineering.org/search?q=differential discussing the open, clutch pack LSD, and Torsen LSD. They helped quite a bit. I also went read the wiki on LSD’s https://en.wikipedia.org/wiki/Limited-slip_differential to try and understand more. (another useful link: http://www.trak-life.com/basic-introduc ... tials-lsd/

This is what I think I have learned. An open differential will always supply the same torque (force) to both wheels. When one wheel breaks loose due to relatively lower friction, that limits the amount of torque that gets to the wheel with good traction. The “dead wheel” is not really “dead”, it’s just the low friction wheel essentially limits the torque that gets passed to the high friction wheel. The faster the wheel spins the lower the coefficient of friction, the lower the torque applied to the non spinning wheel. All limited slip arrangements seek to limit the relative motion between the two wheels in a RWD or FWD vehicle thereby increasing the net amount of torque that can be applied to both wheels.

The limited slip works to transfer the available torque to the wheel that has the best traction by mechanically moving the open diff toward a solid axle. So Infiniti has had viscous LSD in the G. This a less efficient solution (think Automatic vs manual transmission) than a gear controlled solution, but it is less noisy in our luxury car. All purely mechanical LSD’s have the advantage of putting more of the rubber to work during acceleration. They have the advantage/disadvantage of creating a power-on oversteer situation.

So the Q50’s have an open diff with no mechanical LSD option. “We only get one wheel spinning so we cannot put the power to the ground. WHY WOULD THEY DO SUCH A THING!!!!!” Well, it seems a brake actuated limited slip solution is just another way to transfer more torque to the low friction wheel. Applying the brake to the low friction (i.e. faster spinning) wheel is similar to what a clutch pack LSD does in that it reduces the relative velocity of the two halves of the axle by “locking” them together. This raises the amount of torque the open diff is able to pass to the low friction wheel thereby raising the torque the diff can pass to the high friction wheel.

Fundamentally, the biggest “loss” from the brake actuated limited slip is that Infiniti’s nanny does not want us to get the car sideways and the brake solution tends to keep us novice drivers from backing it into the fence. Another loss is that the rear brakes will get quite a lot of wear if I insist on trying to boil down my tires. My research has moved my thinking toward seeing the brake actuated limited slip as a cousin to launch control off the line and a further development of the VDC when I am hustling the car. If the VDC keeps the back tires in incipient slip during hard acceleration, that theoretically will give the lowest ET. If the sport+ mode applies just the right amount of braking to allow for a little throttle on oversteer, then I am impressed by their programming. I am not nearly as ready to run out and get me a quaife (which I had installed in my just traded G37 sedan).

I think I understand what is going on. I am sure there are those who will take issue with some or all of my description. I welcome corrections. Please help me comprehend what is truly going on if I have missed it somehow.

Nice writeup!

Where I have run into trouble is when the traction control/brake "torquing" conflict with each other. An example of this is when you're traversing a median and have an uneven surface and need to quickly cut through traffic - a loss of traction is severe enough to override the brake-diff and basically shut down throttle. This creates a potentially dangerous situation that could be avoided with a mechanical limited slip. Even the viscuous setup in my 370Z was superior in this particular situation. Optimally, you'd never have to encounter this as it's dangerous to cut things close, but any of us that have driven in rush hour and have to take a left across traffic know that sometimes this is required if you don't want to sit in a turn lane for an hour.

In a straight line, I have noticed the rear can step out on a hard start, but it corrects itself adequately, though not as quickly as I have noticed on other LSD setups.

I'm hitting a maintenance milestone right now - I have brakes, tires (non run-flats and WHEELS?!?! ... keep an eye out for another post this afternoon), diff fluid, brake fluid, SS lines, and a couple other goodies to come. I'm going to see how fresh brake gear and tires help the traction as my pads in the rear are toast and the tires aren't far behind (I got nearly 40k out of a stock set of summer run-flats so I'm not terribly upset about that).

If I'm able, I'll have another tail of the dragon run with the car at NICOFest TOTD on labor day weekend.

There is a company that makes a helical diff that is an improvement on the quaife LSD. Specifically it deals with the one wheel off the ground situation. In a zero traction situation, where all other solutions tend toward zero compensation for the flying wheel, this diff gets t produced he power to the ground. The company is called wavetrac based on the design of the limited slip mechanism They have diff's for the 370z.

I have communicated with Chris at wavetrac.net. They do not have a design that will fit the Q50. However if there was sufficient interest (25 orders) and a dealer who would take those orders. I think if there were enough interest, Z1motorsports might be such a dealer, but there would have to be genuine interest from the site. I think this would be an excellent solution that would transform the off the line behavior and power on oversteer coming out of a corner.

I saw on another thread someone wanted to get a group to commit to a quaife with similar interest needed. This is likely in the 2k range on cost. If anyone is interested. Indicate so on this thread. If there are several serious about getting such a diff in their car, I would be glad to contact some potential dealers.

I say again, if anyone is seriously interested indicate so on this thread. We'll see how badly others want a mechanical limiited slip.

Regarding wavetrac, would it cost more or less than the Quaife diff?

Cost? Wavetrac diffs seem to be competitive with Quaife. (i.e. < $1300 for the pumpkin) However, the Wavetrac solution for the Z/G required the replacement of the axle flanges. (~$200 ea.) The Quaife I had installed on my 2010 G37 sedan cost around $1800 all in (pumpkin was $1250).

If we could get a wavetrack, I estimate the parts cost at about $1700 if the axle flanges need to be replaced. Install would bump that up.