Not really. If you look at a FF which has more grip than any road legal car they don't need the LSD. Some of that is they are also lower and have a much lower ratio of vertical CG to track and wheel base.

At first I also thought a brake based system was dumb because you were burning up power vs transferring it to the other wheel. Of course the other issue with a brake based system is does it actually work well? A LSD is flawed in how it distributes torque but it can be made to operate very smoothly. However, just about any time (I haven't proven to my self that the real answer is ANYTIME) you have enough power to cause one wheel to slip you have more power than you can deal with (ie more than the tires will put to the road at this moment). That means the power you are putting into the brakes would otherwise be wasted (or not requested by reducing throttle). Even when your power to weight ratio is modest I believe a GOOD brake based system will offer better performance. I do not believe it will be cheaper or easier to implement.

 

Besides the effective power loss with the brake-based "electronic LSD" systems, the most serious problem with such systems is that they are terrible for prolonged performance driving because they cause and exacerbate brake overheating.

Mini Cooper Ss are no longer available with the mechanical LSD option. The LSD used to be a $500 a la carte build option that was pretty much the best bargain on the Mini options list. In 2011s model year, the LSD was discontinued and a new brake-based "Electronic LSD" replaced it. Strangely, the new "Electronic LSD", even though it was nothing more than software code in the ECU, also costs $500. Minis with the LSD are fantastic track day cars because they are lightweight cars and come with phenomenal brakes from the factory. Overheating brakes are rarely a serious issue with a Mini on a track day. The owners of 2011+ Minis with the Electronic LSD report that, regardless of what brake pads or fluid they use, the brakes overheat and turn to mush within 2 or 3 laps due to all the extra braking generated by the Electronic LSD.

Andrew
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I believe the loss of power argument is a non-issue as at the time you are using the system you have an excess of power (hence the wheel slip). I don't disagree with the brake heating issues. This is why I have to qualify the claims with well designed and I also make no claims as to service costs etc. I wouldn't claim the brake based system is the best compromise, only that it does a better job of managing the torque to the wheels.

Mini's pricing to the consumer isn't a good data point. Mini charged $240 for cruise control in the cars. Since the car is throttle by wire cruise control is the cost of the switches in the dash. I have no idea what changes were necessary to implement the current ELSD in the car. I wouldn't assume it to be trivial but I also don't know what base hardware is already in the car. Finally, as I have said, compromise. I never said the ELSD was going to be the best compromise, only that if done well it would have better torque management.

 

I'm certain this was not a cost saver for McLaren. I'm also certain that the company was worried about the negative impact of LSDs on the handling of a car. The ELSD addresses that issue. So long as they accounted for the drawbacks of the ELSD during design I don't see any drawback other than cost and higher service costs. Perhaps due to autocross days the LSD is seen as a required performance part yet it often isn't. The SCCA F1000 class allows LSDs but last year's national champion used an open diff. When they had trouble with wheel spin it was addressed through chassis/suspension changes, not a LSD. They didn't want to add the mass of a LSD nor accept the handling tradeoffs. In the end they didn't have to.

As I've said before, many cars are faster with the LSD. I have one in my car. I'm aware of both the pros and cons. I wrote the original post because it was clear in discussions in another thread that people didn't understand how the diffs worked or even that they could have drawbacks. Even if we are willing to accept those drawbacks, I have explained why they fundamentally exist. Some people don't believe they even exist.

 

I just wanted to say that, speaking as someone with almost no knowledge of these technical issues, this is a fantastic thread. Seriously.

 

For road cars, yeah, really. I didn't need an LSD back when my 240Z was only making ~170rwhp and I was on street tires, but as soon as I put Hoosiers on it I got inside wheelspin for days.

FF is of course a totally different story.

Dont' follow your logic here. Clearly while cornering you can put a LOT more torque to the road on the outside tire than the inside. Ability to transfer power from the inside wheel to the outside wheel is a HUGE advantage. Throwing excess torque which would overpower the inside wheel it into the brakes throws away some engine power, whereas with clutch-lsd you're transferring it back over to the other driven wheel.

No it won't, particularly if power/weight is modest. If you still have enough power left over to overcome traction at the outside tire, yeah, you're not losing anything except fuel mileage (which is an issue for races of longish duration!). If you don't have any hp to spare, you lose thrust/acceleration.

Of course it's cheaper/easier to implement the virtual brake-system LSD. If the car has 4-channel ABS, you have all you need in terms of hardware, it's just software to implement it. Whereas a *real* limited slip requires additional hardware.

 

But if you had a proper LSD, it would mechanically transfer the power from the wheel with less grip ("an excess of power") to the wheel with more grip (not enough power) to help accelerate the car out of each corner faster... With the "electronic LSD", that engine power that could otherwise be partially transferred to a wheel with more grip to help accelerate the car is instead turned into heat and noise by the brakes.

I just thought that Mini's option pricing scheme was more amusing than anything else.

Sticking to the Mini example - It's well known that a Mini Cooper S with the mechanical LSD has significantly more torque steer than one with an open diff. And this certainly supports your earlier point that from a feel and drivability perspective, an LSD is sometimes not ideal. However from a performance perspective, the Mini with the LSD is always going to offer superior performance to an identical Mini with the open diff.

Andrew

 

For road cars, yeah, really. I didn't need an LSD back when my 240Z was only making ~170rwhp and I was on street tires, but as soon as I put Hoosiers on it I got inside wheelspin for days.

FF is of course a totally different story.[/quote]
No, the FF is simply at extreme of a continuum. Whether or not a LSD helps and how much it helps depends on many things. I have a friend who used to track his Miata all the time. He used summer only street tires but nothing as aggressive as R tire. He didn't have any issues with inside tire slip thus a LSD wasn't necessary for his car as he was driving it. With a different setup it would be helpful. The point is don't automatically assume the LSD will make you faster and certainly don't assume it will make the car handle better. Of the latter point it almost certainly will be neutral to negative even if it lowers lap times.

If the torque available to the diff is fixed then yes, it would be better to transfer from inside to outside (which only happens when the inside wheel is spinning faster than the outside). If you have excess torque available from the engine you can increase torque to the outside wheel by sending more total torque to the diff. Let's assume the outside wheel can handle 100 lb-ft and the inside can handle 50. Well ideally I would sent 150 lb-ft to the diff and let it sort out which wheel gets what. Unfortunately that doesn't really happen. The only way I can get 100 to the outside and 50 to the inside is if the inside wheel is slipping and spinning FASTER than the diff. At that point my inside tire has a reduced traction circle and isn't great at keeping the rear of the car in line. A better plan is to send 200 lb-ft to the rear diff and then let the inside brake burn off 50 lb-ft. The outside gets 100, the inside gets 100-50.

This is one you need to prove via math. One of the reasons why I created my original post was to explain the math that justifies the idea that a LSD will send the most torque to the inside wheel (ie the slower wheel) so long as we don't have slippage. I showed that via numbers. I haven't tried to figure out this part yet but simply stating "no" is not proof by any means.

Again, where is your proof? My ABS works by temporarily unloading the brakes. Now I need a system that will reload them. Even more importantly, I need a system that can vary how much pressure is applied. Making a lousy e-LSD system may be cheap in a car that already has a brake based stability control feature. However, that doesn't mean it performs at an optimal level. Think of the early traction control systems. They weren't able to keep the tires at their optimal slip rates. They simply brought power well under the threshold needed to cause wheel spin. Net result, you didn't accelerate faster using TC. The same is true here. To really make an E-LSD system work the hardware will need to be better than the stuff that is sufficient for stability control. It really is very illogical to assume that McLaren would skip a LSD that might cost them $500-1000 in favor of the cost of developing and implementing an E-LSD system.

 

No, that is not true. This myth is one of the reasons why I created the post and it doesn't hold up when you do the math. In the case of the torque sensing LSD torque is transferred to the SLOWER wheel. SOMETIMES the wheel is slower because it has more grip. This is true if we have a wheel on sand and a wheel on asphalt. This is NOT true when cornering. When we corner the car leans to the outside thus transfers weight to the outside tires. Those tires have more grip because they have more normal load. However, they also turn faster. That means we transfer torque to the inside tire! Thus we didn't transfer torque to the wheels with more grip. That added torque to the unloaded wheels means they will slip SOONER than with an open diff. With the open diff we at least have equal left to right torque vs more torque going to the unloaded, inside tire. Again, if you disagree with that statement show it in the math or at least show why my math is wrong.

Also, remember the other issue, to get the torque transfer to the outside tire I have to let the inside tire spin faster than the diff. Ideally I want the inside tire to spin at some speed that maximizes it's grip. That speed is going to be just slightly faster than the road speed. If it spins faster than that I lose grip. Thus I am better off with a system that let's me control torque AND wheel speed independently. An e-LSD system does that. I control torque to the outside wheel via the engine. The inside wheel has excess torque AND speed so I use the brakes to apply enough slowing torque so that the wheel speed is "ideal". Because I have two control inputs I have a more controllable system.

With an LSD I can only control total torque into the system. So I push the engine so the outside tire is on the verge of slipping and can't accept any more torque from the engine. Now the inside tire is spinning faster than the road. It's spinning faster than I want and thus doesn't grip as well as a tire spinning at just the right speed. The only way to slow the inside tire is cut torque to both. That slows me down because I no longer get to send the ideal amount of torque to the outside tire. I have a less controllable system because I have only one control input.


I agree. BMW definitely prices based on "value" not "cost"!

That I would completely agree with. What you just said is my other main point! Many people who aren't going to track or drive the cars at 10/10ths think a LSD is a must have option. I was one of them! However, if you care more about how a car FEELS vs lap times sometimes the LSD hurts the feel because it fights your steering when the tires aren't slipping.

 

For road cars, yeah, really. I didn't need an LSD back when my 240Z was only making ~170rwhp and I was on street tires, but as soon as I put Hoosiers on it I got inside wheelspin for days.

FF is of course a totally different story.[/quote]
No, the FF is simply at extreme of a continuum.[/quote]

With a c.g. height on the order of 1/2 that of my similar power/weight 240Z with similar track width and wheelbase, and with a significantly greater %age of its static weight on the drive wheels, a FF *is* a totally different story as far as the need for a limited slip is concerned. Even allowing for its greater cornering g's (probably ~1.2x that of the Z on Hoosier A6s).

 

For road cars, yeah, really. I didn't need an LSD back when my 240Z was only making ~170rwhp and I was on street tires, but as soon as I put Hoosiers on it I got inside wheelspin for days.

FF is of course a totally different story.[/quote]
No, the FF is simply at extreme of a continuum.[/quote]

With a c.g. height on the order of 1/2 that of my similar power/weight 240Z with similar track width and wheelbase, and with a significantly greater %age of its static weight on the drive wheels, a FF *is* a totally different story as far as the need for a limited slip is concerned. Even allowing for its greater cornering g's (probably ~1.2x that of the Z on Hoosier A6s).
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The same rules of vehicle dynamics apply thus a continuum.