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Discussion Starter · #1 ·
I’ve been snooping around the forum for a while, and it seems like one of the most common and highly recommended modifications is a limited slip differential of some type (plates for heavy track use, torque biasing for more street focused drivers).

I’ve got a few questions regarding the advice and some thoughts of my own. Try not to get offended if this goes against the grain, I’m just trying to fill in my understanding of the subject. If these questions have been answered before, please point me to the right thread. I’ve searched and didn’t find what I was looking for.

First, I’ve been fortunate enough to drive several cars with all sorts of differentials. I’ve had a 1993 Toyota MR2 Turbo (plate type limited slip), a 2002 Corvette Z06 (also a plate type), a 2004 Honda S2000 (torque biasing), and a 2006 Cayman S with the stock differential.

From my experience, all three cars seem to drive about the same. The few times I’ve had them on ice and snow, all three cars had a lot of wheelspin with an eventual crawling pace. Not surprising since they all have summer only tires. Also, under power, they all behave about the same. The biggest similarity is between the Cayman and Z06. Both cars have enough power to spin their tires in first gear on a cool day or around a corner. Both of them will squirm under acceleration as the power is apportioned from side to side. They also both put power down well in tight corners, and slide progressively if you get a little too aggressive with the throttle.

The only appreciable difference I’ve noticed in any of the cars is in tire wear. Both the Corvette and MR2 with the plate type differentials seem to wear much more on one drive wheel versus the other. The S2000 with the torque sensing differential and the Cayman have even wear across both rear tires. On the Corvette, the difference in torque to the wheels is so great that they were known for frequently breaking one axle shaft when used with slicks at a track.

I’ve also seen some evidence that the brake differential works well from magazine test results. First, I haven’t seen many complaints from writers about the lack of a limited slip in the case of the Cayman. Second, track testing shows the car performs well. Zero to sixty is in the 4.7-4.8 range, which is exactly what you’d expect for a car with its power and weight. Also, Motor Trend seems to get great numbers from the car in their figure 8 test, which is like a very tight racetrack. In addition to all the testing, both McLaren and Ferrari are moving to electronic differentials in their new cars. These cars are expensive enough to use any kind of differential they want, yet they’ve chosen to move from mechanical designs to electronic differentials using the brake system to prevent spin.

Finally, a mechanical limited slip can only send so much power to one wheel or the other. For example, Porsche says that their limited slip in the new 911 and Cayman models can lock up to 22%. Unless I’ve misunderstood, that means that the car can send up to 22% of the torque applied to the spinning wheel over to the wheel with grip. Granted, there are higher degrees of lock available (I’ve seen BMW claim as much as 90% from the M differential fitted to the M3), but this seems to be related to an increasing tendency for understeer as the percentage of lock goes up. I also understand that a limited slip can provide some lock on overrun, but I believe this is also met with increased understeer.

So my question is, what makes a mechanical limited slip better than an electronic differential? It seems like an electronic differential that can split power in any percentage to either wheel would be the best solution. Magazine test numbers, my own experiences, and the choices of high end manufacturers seem to agree that the electronic differentials work just as well, if not better, than mechanical differentials.

Thanks,

Mike
 

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I think you are confusing an electronic differential with electronic control of braking in order to try and get power to the wheel with grip. These are two completely different things, Porsche has active braking, but relies on a mechanical differential. When you are talking electric differential you are talking about the following:

In conventional road vehicles an electronic differential is usually a mechanical differential that uses an onboard computer to directly control and adjust the torque split and difference in wheel speed across the axle. This is acheived by electronic actuators engaing clutch packs and locking mechanisms to acheive the desired traction condition. It is esentially a computer controlled LSD and differential lock in one. This means the computer can calculate the optimal torque split and actively vary it on the fly.

In electric vehicles with separate motors driving each wheel, an electronic differential is simply a computer that controls the wheel speed of the inside and outside wheels based on the steering wheel angle, to acheive the same effect as a normal mechanical differential.

The classical automobile drive train is composed by a single motor providing torque to one or more driving wheels. The most common solution is to use a mechanical device to distribute torque to the wheels. This device, called mechanical differential, is responsible to allow different wheel speed when cornering. With the emerging of electric vehicles new drive train configurations are possible. Multi-drive systems become easy to implement due to the large power density of electric motors. These driving schemes with one motor per driving wheel need an additional top level controller which is responsible to perform the same task as the mechanical differential. Named electronic differential, it is capable of substituting its mechanical counterpart with significant advantages.

The ED scheme substitutes the mechanical differential with several advantages:

simplicity, it avoids additional mechanical parts like gearboxes or clutch;
independent torque for each wheel allows additional capabilities (e.g., traction control, stability control);
reconfigurable, it is reprogrammable in order to include new features or tuned according to the driver’s preferences;
allows distributed regenerative braking;
no mechanical differential limitation where the torque is limited by the wheel with least traction.
Moreover, a multi-drive traction system with distributed electric motors have advantages over the traditional solutions[1]:

faster response times;
accurate knowledge of traction torque per wheel.


So the answer is YES an electronic differential where truly separate electric motors are controlling the torque delivered to the wheels is a superior setup to a mechanical limited slip differential. I imagine they are also more expensive. I know the new Jaguars have them on some models and they are getting rave reviews. I suspect there are also motor differences and programming/learning differences between various manufacturers and their implementations.

Last, but not least, I do beg to differ with your statements about Cayman reviews (we have many here in our Articles section) and journalists not mentioned the open diff in the Cayman or the need for a limited slip (LSD / ATB) rear end. Most did mention it and most did so because the Cayman can really spin the driven wheel in a tight corner causing a loss of traction. My lap times improved significantly at the track once I had my Quaife unit installed because I was able to put the power down faster/sooner, as well as the car was more predictable and stable overall. I think the Cayman requires one and it is one of the "must have" mods! :)
 

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I think Porsche programs the PSM for street safety by reducing throttle when wheel spin is detected. I wish they would modify the PSM programming in the PASM Sport setting to use the brake pads to limit the slip on the spinning tire and let the open diff apply torque to the loaded wheel. Such a mechanization could eliminate the understeer characteristic of an LSD and actually could provide an oversteer moment. The response time of the system would have to be very fast in order to effect control of the speed of the wheel in a lightly loaded scenario. Maybe our system doesn't have the dynamic range to handle it.
 

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Being a Land Rover fanatic I have learned a thing or two about LSDs and Lockers.

First, the electronic LSD that Jaguar is using was developed for the Range Rover and then shared with Jaguar.

About you question concerning percentage of power transfer in LSDs...

A locker locks the axles together and gives you 100% power to both rear wheels. Great for rock crawling and drag racing but for cornering, not so much.

Enter the LSD which mechanically senses wheel slip and then transfers some of the torque from the non slipping axle to the spinning one. A limited slip cannot transfer 100% of the power, I have a Tru-Track for offroading and it will transfer about 60% of the power of the non slipping axle to the slipping one.

Depending on their design, LSDs will transfer differing amounts of torque to the slipping wheel depending on their puropse.

For high speed cornering you do not really want to be fully locked or you will break an axle because the outside wheel must travel further than the inside one. I have a Detroit Locker in one of my trucks which is normally locked, the tires will chirp if you try to turn to tight on pavement at slow speeds like in a parking garage.

For cornering you want a progressive transfer of torque to the slipping axle. If the transfer is too quick then you will destabilize the car.

The reason I know this is because I have my LSD on the front of my truck and the Detroit on the back. The LSD will pull left and right just from getting on the gas or off it when cornering.

The Tru Trac is made to be aggressive in transferring torque because it is suited for offraod applications.

Quaife and other manufacturers make LSDs for track use on pavement and I would not get too hung up on the transfer percentage because it will lead you in the wrong direction.
 

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Another point that is understood but not spoken of much is that the electronic aids that manufacturers tout as replacing locking differentials to accomplish the same thing, i.e. ebd and psm, stop the loss of torque by either cutting the torque supply(psm) or damping its exertion(ebd), these methods SLOW the car. Yes it stops the inside wheel from spinning but it does so by effectively slowing the vehicle's acceleration. Whereas a mechanical differentials redistribute the torque for use allowing the vehicle to accelerate more effectively.
 

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Discussion Starter · #6 ·
Kenkg - The traction control portion of the PSM stability program does in fact cut power to the rear tires. However, Porsche has another feature on the car called Active Brake Differential. This system does not cut the power, uses the brakes to slow the spinning wheel and redirect torque to the wheel with more grip. This feature, to my knowledge, is not something that can be turned off as is the case with the stability control. So it is actually doing what you suggested, but if PSM is left on, it will also cut the power or apply a single brake to try and keep the car on its intended course. How fast the system works, I'm not sure.

RingSport - When the brake is applied to the spinning wheel on an open differential, it will not absorb much of the power, if any. Rather, the open differential will redirect the power to the other wheel that is spinning faster. This really isn't so different from a mechanical limited slip differential that uses friction to redirect power from one wheel to the other. In both cases, some of the power is turned into heat, either in the brake disc for the electronically controlled differential, or in the clutch pack or gearing for a mechanical limited slip. The brake is slowing the wheel and that in turn sends the torque to the other wheel, so it is not lost, just redirected.

McLaren is using a system that is similar to the one on the Cayman: a conventional open differential that uses the braking system to send power from one side to the other. This is also the way that the torque vectoring system on the new 911 Turbos works. Once the inside wheel is being slowed, an open differential will no longer send power to that wheel, but rather will direct it to the outside wheel that has grip. This isn't a matter of slowing the vehicle or stopping the flow of power, but of proportioning the power from side to side as grip levels change.

Mike
 

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In theory yes but those who've added a tbd or lsd can attest a mechanical diff does a much better job at redistributing while minimizing the torque loss. I've wondered what if anything mechanical is changed when the new torque vectoring option box is checked. Is it a matter of programming or is there a mechanical difference or is it just that EBD is applied to all four wheels?

Having driven the CS with an LSD at the track back to back with my CS with open diff and EBD there is a noticeable difference. You are always aware of the weakness in the system whether you're on a wet road or at the track. It is a very poor substitute imo for a locking diff. YMMV
 

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Not an expert but from reading this months magazine reviews I believe this electronic brake interference has been developed by Porsche also and can be found on the latest 911 Turbo as "Porsche Torque Vectoring".

The review in Evo magazine was inconclusive which seems to read that although it was effective in some circumstances you really have to push hard to see its benefits.
 

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Kenkg - The traction control portion of the PSM stability program does in fact cut power to the rear tires. However, Porsche has another feature on the car called Active Brake Differential. This system does not cut the power, uses the brakes to slow the spinning wheel and redirect torque to the wheel with more grip. This feature, to my knowledge, is not something that can be turned off as is the case with the stability control. So it is actually doing what you suggested, but if PSM is left on, it will also cut the power or apply a single brake to try and keep the car on its intended course. How fast the system works, I'm not sure.



So this should mean that with PCM off the car acts as though it had a limited slip differential?
 

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Kenkg - The traction control portion of the PSM stability program does in fact cut power to the rear tires. However, Porsche has another feature on the car called Active Brake Differential. This system does not cut the power, uses the brakes to slow the spinning wheel and redirect torque to the wheel with more grip. This feature, to my knowledge, is not something that can be turned off as is the case with the stability control. So it is actually doing what you suggested, but if PSM is left on, it will also cut the power or apply a single brake to try and keep the car on its intended course. How fast the system works, I'm not sure.



So this should mean that with PCM off the car acts as though it had a limited slip differential?
No. The car will send power in a way to account for the difference between the distance the inside wheel needs to cover compared to the outside wheel.

This explains nothing, but is a classic Cayman on Youtube video...:hilarious:

 

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Discussion Starter · #11 ·
AJK - Yes, that's the idea behind the brake differential. When one of the wheels starts to spin appreciably faster than the other, the car should apply that brake, which will direct the power to the other wheel.

I imagine this is how most of the car magazines that perform instrumented testing get their numbers (Car and Driver, Road and Track, Motor Trend).

The stability management system should cut power and independently brake one wheel (a front wheel for oversteer, or a rear wheel for understeer) to try and put the car on what it thinks is the driver's intended path. That system is similar in how it works as it will apply the brakes, but is more focused on preventing an accident by steering the car than getting power to the pavement. The stability management and traction control are what will cut the power from the engine in an extreme situation. If you've never had an opportunity to experience it, it's actually amazing how well they can work. I've never had it come in on the Cayman, but it did several times with the Corvette. On that car, it was VERY forceful and decisive in the way it righted the car. I wouldn't recommend trying to make it work, especially on a public road. The systems aren't perfect, but they are the best thing out there if you lose control.

Dave - Great video! Glad I'm not paying for his tires!

Mike
 

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Kenkg - The traction control portion of the PSM stability program does in fact cut power to the rear tires. However, Porsche has another feature on the car called Active Brake Differential. This system does not cut the power, uses the brakes to slow the spinning wheel and redirect torque to the wheel with more grip. This feature, to my knowledge, is not something that can be turned off as is the case with the stability control. So it is actually doing what you suggested, but if PSM is left on, it will also cut the power or apply a single brake to try and keep the car on its intended course. How fast the system works, I'm not sure.

RingSport - When the brake is applied to the spinning wheel on an open differential, it will not absorb much of the power, if any. Rather, the open differential will redirect the power to the other wheel that is spinning faster. This really isn't so different from a mechanical limited slip differential that uses friction to redirect power from one wheel to the other. In both cases, some of the power is turned into heat, either in the brake disc for the electronically controlled differential, or in the clutch pack or gearing for a mechanical limited slip. The brake is slowing the wheel and that in turn sends the torque to the other wheel, so it is not lost, just redirected.

McLaren is using a system that is similar to the one on the Cayman: a conventional open differential that uses the braking system to send power from one side to the other. This is also the way that the torque vectoring system on the new 911 Turbos works. Once the inside wheel is being slowed, an open differential will no longer send power to that wheel, but rather will direct it to the outside wheel that has grip. This isn't a matter of slowing the vehicle or stopping the flow of power, but of proportioning the power from side to side as grip levels change.

Mike
Thanks, Mike. I was aware that that PSM would use individual wheel braking to stabilize the car, but did not know that that it would limit wheel spin. I'll file that away for the day I am good enough (and brave enough) to turn PSM off.
 

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No. The car will send power in a way to account for the difference between the distance the inside wheel needs to cover compared to the outside wheel.

This explains nothing, but is a classic Cayman on Youtube video...:hilarious:

Anyone know where that video takes place? I found it by accident. I had entered the rotary and I had this really wierd feeling that I had been there before, but I knew that was impossible. Then on my way back, as I was approaching the rotary for the second time it dawned on me that it was the rotary in this video. Who knows where this is?
 
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