Hi, guys. Great discussion you've got going here. Lots of knowledgable people here 
I read this thread last night (quite awhile ago) so I probably won't remember to hit all the points that are being discussed here.
The original poster's friend said he was setting up his car to deliberately lock the wheels in an effort to regain control of the car. What he didn't say is if he was doing this to lock all four wheels or just two of them, and he didn't say if "regaining control" meant trying to recover from a spin or avoid plowing (understeering at/over the limit) off the track. When I think about regaining control it's all about getting out of a slide I didn't want to be in, so I'll assume he's talking about the same thing.
1) Locking the fronts: This should indeed be a quick way to recover from a slide because in doing so you pretty much kill off all the side force the front tires are producing (not really, it depends on the angle of slide, but it will generally reduce the side force a whole lot very quickly). Depending on what exactly is going on, this could be a very quick way to straighten up the car and might even be quicker than countersteering. So yes, it should indeed work to some extent. However, as many of you pointed out, this isn't the best way to go about things as you can overheat and flat spot the tires. If the purpose is to recover from a spin then the fact that the grip drops off only goes to reducing that side force even more, which would straighten up the car faster. Note that this is different from trying to stop the car as quickly as possible. Here we're interested in recovering from a spin, not stopping ASAP.
2) Locking the rears: I think we all know what happens when you do that. Around you go in a real hurry. The only time I can see this being a useful thing is in a car that is understeering through a slow corner and about to go off the outside of the track. A quick jab of the brakes could get the car more sideways and keep you from going off. A setup like that could of course make for hairy moments during all normal braking maneuvers, so I doubt that's generally a wise thing to do. I sure wouldn't want it, but there could be times when you might want that to recover from a boo-boo.
3) If he's locking all four tires in an attempt to recover from a spin, well, if the balance and so on is just so then this might help too, but it's a bit unlikely. This one probably is not a good idea.
I'm betting that #1 above is what the OP's friend was describing. Indeed, this could very well save you from some nasty moments by nearly killing off all sideforce at the front tires which should indeed straighten up the car in a real hurry, albeit at the expense of the tires. A setup like that will have less braking force at the rear than you could get away with normally (otherwise they'd probably lock too when you emergency-jabbed the brakes to straighten the car), so to have this crutch you're most likely giving up a fair amount of your regular braking performance. He probably brakes earlier going into the corners than somebody with a more typical setup, and probably does not trail brake at all. All braking is done in a straight line.
Bottom line there really is he's most likely giving up performance in many areas of the track in order to have a sort of "oh crap" button at the bottom of the brake pedal for emergency spin recovery. I used to do that in GPL too, but haven't found it necessary in LFS due to my superb driving skills
Some thoughts on ABS: I am not familiar with the current ABS technology in production cars. However, theoretically it should be possible to make a system that under straight line braking outperforms most drivers. The paper one of you posted shows that for the nine or so vehicles they tested this indeed is the case, so perhaps the technology has been to that level for some time now. On the other end of the argument I must agree that perhaps Mario Andretti or the likes might outperform the ABS systems in certain maneuvers, so this is a case where both sides might really be right. ABS technology development is essentially about trying to control the slip ratios better than a human can, so it's more or less a game between the programmers/hardware developers and real drivers. Some may be better than others at it (both on the dev side and the real drivers' side).
Locking wheels and stopping distances: There may be a bit more here than initially meets the eye. First, as most of you have pointed out already, a locked tire produces less grip than a rotating one. The 11% slip ratio "rule" that Wikipedia posted is in the ballpark, but it actually varies quite a bit from one tire to the next. Anyway, that's not important. What's important is that there is some slip ratio where the braking force will be highest, and if you go any higher than that then the braking force reduces (as does the road reaction torque on the wheel). If the brakes aren't released a little bit at that point, the wheel will lock up. If you do this on all four tires then you could expect straight line braking distance to increase.
Another possible scenario is this: The rears might be at the 11% (or whatever) slip where the maximum braking force is. If the fronts are too then this is where you're getting the shortest straight line braking distance. If you then lock up either the front or the rears then the distance will increase due to the grip dropping off after the peak. Keep in mind that the grip when the tires are locked is changing quite a lot throughout time due to the temperature changing, so it's not quite as clean and neat as the force graphs show in the case of a tire that has actually locked. I've seen some old braking test data where an accelerometer was put in a couple of cars to measure the deceleration rate under a panic stop situation. It peaked before the tires locked, then dropped somewhat gradually to a constant (but lower) deceleration rate. So the tires indeed lost grip. This was done with no rear wheel braking at all if I recall correctly, so it was purely the fronts doing all the work.
What's important to understand is that most production cars are built more for safety than performance. For the most part this means directional stability (building in enough understeer so if you suddenly swerve to avoid something at typical highway speeds you don't spin out), but perhaps more importantly, the panic stop/swerve situation. I bet that most sim racers that are really into this wonderful hobby will react very differently to an emergency than your average joe on the street would. Nonetheless, production cars aren't designed to handle the way you and I want, but rather to keep Mom and Pop and their teenage maniac newly-licensed kids safe.
Probably the most important emergency situation that a vehicle engineer is likely to be considering is the panic stop/swerve, as mentioned. I.e., you're puttering along and suddenly there's something stopped in front of you. Average Joe does up to three things in this situation, in this order:
1) He slams the brake pedal to the floor.
2) He steers the wheel wildly in one direction.
3) Upon finding that the front wheels have locked and his car isn't turning at all, he steers the wheel all the way to full lock. Nothing new happens there.
Then, usually there's a crash of some sort
Ok, from the "engineering for safety" standpoint, you want minimum stopping distances. However, you don't want Average Joe to slam the brake pedal to the floor, steer wildly to one side, only to find himself sliding sideways into goodness knows what. One sure fire way to avoid that is to design the braking system so the front wheels lock and the rear wheels do NOT. Ideally, the rear wheels would hit their peak 11% or whatever and at the same time, the fronts lock up.
The purpose of this is to do keep the car straight, which is exactly what the original poster's friend is trying to do. What he's essentially done is designed into his LFS car the same safety feature that's built into our road cars. Does this produce the best stopping distance? Probably not, but it actually might do so indeed depending on how the braking system is set up.
Here's the exception: If the brake pedal on the above car is pushed down to the point where the front wheels are at their 11% slip, the rears will be well below that. So the fronts are making their peak braking force, but there's still plenty more left at the rear. If you now push the brake to the floor, you lock up the fronts, which reduces front grip, but in turn you increase the rear braking. Which one wins? Well... It depends on the car. The point is that from one production car to the next I wouldn't be surprised at all to find some cars that stop better with the front wheels locked and others that stop better with a "best effort" of feathering/pumping the brake.
Ideally really you don't want Joe to spin out or be forced to go straight either. You want the car to react the way Joe wants it to. However, if you have to pick one of the options, it's generally going to be better if he just plows straight ahead into whatever he was trying to avoid while slowing down as much as possible at the same time. Imagine spinning sideways into oncoming traffic or off the road into a tree. It's usually much better just to rearend somebody. Fender benders really don't hurt as much as people think as long as they have their seatbelts on, but people absolutely freak out when they're about to even rear end somebody at 10-20 mph. I saw a video of a bunch of cars sliding one after the other on slick ice into a line of cars. More than half of the people were literally jumping out of their cars to avoid the impact. Bumper cars hurt more than that, so stay in the car!
Enter ABS and other systems. As many of you pointed out, the main reason for ABS and stability control on production cars is to keep the fronts from locking, which sends Joe straight into whatever he was trying to steer around. At the same time, in a straight line, a regular braking system assisted with ABS is able to go ahead and try to keep all the slip ratios at 11%. When he turns the wheel to avoid the big bad object, without the ABS the wheels would then suddenly lock. ABS will relieve some of that brake pressure so the slip ratios stay at 11% (or whatever the ABS designer wants it too; the algos are probably much more complex than that), so he's able to get plenty of braking but also is able to steer around the big bad obstacle (BBO), while at the same time not spin out during his frantic, panic stricken effort to avoid smashing into the BBO.
This is a neat discussion largely because nearly all people on all sides of the debate here are really correct, even though some of the points seem to contradict each other
I read this thread last night (quite awhile ago) so I probably won't remember to hit all the points that are being discussed here.
The original poster's friend said he was setting up his car to deliberately lock the wheels in an effort to regain control of the car. What he didn't say is if he was doing this to lock all four wheels or just two of them, and he didn't say if "regaining control" meant trying to recover from a spin or avoid plowing (understeering at/over the limit) off the track. When I think about regaining control it's all about getting out of a slide I didn't want to be in, so I'll assume he's talking about the same thing.
1) Locking the fronts: This should indeed be a quick way to recover from a slide because in doing so you pretty much kill off all the side force the front tires are producing (not really, it depends on the angle of slide, but it will generally reduce the side force a whole lot very quickly). Depending on what exactly is going on, this could be a very quick way to straighten up the car and might even be quicker than countersteering. So yes, it should indeed work to some extent. However, as many of you pointed out, this isn't the best way to go about things as you can overheat and flat spot the tires. If the purpose is to recover from a spin then the fact that the grip drops off only goes to reducing that side force even more, which would straighten up the car faster. Note that this is different from trying to stop the car as quickly as possible. Here we're interested in recovering from a spin, not stopping ASAP.
2) Locking the rears: I think we all know what happens when you do that.
Around you go in a real hurry. The only time I can see this being a useful thing is in a car that is understeering through a slow corner and about to go off the outside of the track. A quick jab of the brakes could get the car more sideways and keep you from going off. A setup like that could of course make for hairy moments during all normal braking maneuvers, so I doubt that's generally a wise thing to do. I sure wouldn't want it, but there could be times when you might want that to recover from a boo-boo. 3) If he's locking all four tires in an attempt to recover from a spin, well, if the balance and so on is just so then this might help too, but it's a bit unlikely. This one probably is not a good idea.
I'm betting that #1 above is what the OP's friend was describing. Indeed, this could very well save you from some nasty moments by nearly killing off all sideforce at the front tires which should indeed straighten up the car in a real hurry, albeit at the expense of the tires. A setup like that will have less braking force at the rear than you could get away with normally (otherwise they'd probably lock too when you emergency-jabbed the brakes to straighten the car), so to have this crutch you're most likely giving up a fair amount of your regular braking performance. He probably brakes earlier going into the corners than somebody with a more typical setup, and probably does not trail brake at all. All braking is done in a straight line.
Bottom line there really is he's most likely giving up performance in many areas of the track in order to have a sort of "oh crap" button at the bottom of the brake pedal for emergency spin recovery. I used to do that in GPL too, but haven't found it necessary in LFS due to my superb driving skills
Some thoughts on ABS: I am not familiar with the current ABS technology in production cars. However, theoretically it should be possible to make a system that under straight line braking outperforms most drivers. The paper one of you posted shows that for the nine or so vehicles they tested this indeed is the case, so perhaps the technology has been to that level for some time now. On the other end of the argument I must agree that perhaps Mario Andretti or the likes might outperform the ABS systems in certain maneuvers, so this is a case where both sides might really be right. ABS technology development is essentially about trying to control the slip ratios better than a human can, so it's more or less a game between the programmers/hardware developers and real drivers. Some may be better than others at it (both on the dev side and the real drivers' side).
Locking wheels and stopping distances: There may be a bit more here than initially meets the eye. First, as most of you have pointed out already, a locked tire produces less grip than a rotating one. The 11% slip ratio "rule" that Wikipedia posted is in the ballpark, but it actually varies quite a bit from one tire to the next. Anyway, that's not important. What's important is that there is some slip ratio where the braking force will be highest, and if you go any higher than that then the braking force reduces (as does the road reaction torque on the wheel). If the brakes aren't released a little bit at that point, the wheel will lock up. If you do this on all four tires then you could expect straight line braking distance to increase.
Another possible scenario is this: The rears might be at the 11% (or whatever) slip where the maximum braking force is. If the fronts are too then this is where you're getting the shortest straight line braking distance. If you then lock up either the front or the rears then the distance will increase due to the grip dropping off after the peak. Keep in mind that the grip when the tires are locked is changing quite a lot throughout time due to the temperature changing, so it's not quite as clean and neat as the force graphs show in the case of a tire that has actually locked. I've seen some old braking test data where an accelerometer was put in a couple of cars to measure the deceleration rate under a panic stop situation. It peaked before the tires locked, then dropped somewhat gradually to a constant (but lower) deceleration rate. So the tires indeed lost grip. This was done with no rear wheel braking at all if I recall correctly, so it was purely the fronts doing all the work.
What's important to understand is that most production cars are built more for safety than performance. For the most part this means directional stability (building in enough understeer so if you suddenly swerve to avoid something at typical highway speeds you don't spin out), but perhaps more importantly, the panic stop/swerve situation. I bet that most sim racers that are really into this wonderful hobby will react very differently to an emergency than your average joe on the street would. Nonetheless, production cars aren't designed to handle the way you and I want, but rather to keep Mom and Pop and their teenage maniac newly-licensed kids safe.
Probably the most important emergency situation that a vehicle engineer is likely to be considering is the panic stop/swerve, as mentioned. I.e., you're puttering along and suddenly there's something stopped in front of you. Average Joe does up to three things in this situation, in this order:
1) He slams the brake pedal to the floor.
2) He steers the wheel wildly in one direction.
3) Upon finding that the front wheels have locked and his car isn't turning at all, he steers the wheel all the way to full lock. Nothing new happens there.
Then, usually there's a crash of some sort
Ok, from the "engineering for safety" standpoint, you want minimum stopping distances. However, you don't want Average Joe to slam the brake pedal to the floor, steer wildly to one side, only to find himself sliding sideways into goodness knows what. One sure fire way to avoid that is to design the braking system so the front wheels lock and the rear wheels do NOT. Ideally, the rear wheels would hit their peak 11% or whatever and at the same time, the fronts lock up.
The purpose of this is to do keep the car straight, which is exactly what the original poster's friend is trying to do. What he's essentially done is designed into his LFS car the same safety feature that's built into our road cars. Does this produce the best stopping distance? Probably not, but it actually might do so indeed depending on how the braking system is set up.
Here's the exception: If the brake pedal on the above car is pushed down to the point where the front wheels are at their 11% slip, the rears will be well below that. So the fronts are making their peak braking force, but there's still plenty more left at the rear. If you now push the brake to the floor, you lock up the fronts, which reduces front grip, but in turn you increase the rear braking. Which one wins? Well... It depends on the car. The point is that from one production car to the next I wouldn't be surprised at all to find some cars that stop better with the front wheels locked and others that stop better with a "best effort" of feathering/pumping the brake.
Ideally really you don't want Joe to spin out or be forced to go straight either. You want the car to react the way Joe wants it to. However, if you have to pick one of the options, it's generally going to be better if he just plows straight ahead into whatever he was trying to avoid while slowing down as much as possible at the same time. Imagine spinning sideways into oncoming traffic or off the road into a tree. It's usually much better just to rearend somebody. Fender benders really don't hurt as much as people think as long as they have their seatbelts on, but people absolutely freak out when they're about to even rear end somebody at 10-20 mph. I saw a video of a bunch of cars sliding one after the other on slick ice into a line of cars. More than half of the people were literally jumping out of their cars to avoid the impact. Bumper cars hurt more than that, so stay in the car!
Enter ABS and other systems. As many of you pointed out, the main reason for ABS and stability control on production cars is to keep the fronts from locking, which sends Joe straight into whatever he was trying to steer around. At the same time, in a straight line, a regular braking system assisted with ABS is able to go ahead and try to keep all the slip ratios at 11%. When he turns the wheel to avoid the big bad object, without the ABS the wheels would then suddenly lock. ABS will relieve some of that brake pressure so the slip ratios stay at 11% (or whatever the ABS designer wants it too; the algos are probably much more complex than that), so he's able to get plenty of braking but also is able to steer around the big bad obstacle (BBO), while at the same time not spin out during his frantic, panic stricken effort to avoid smashing into the BBO.
This is a neat discussion largely because nearly all people on all sides of the debate here are really correct, even though some of the points seem to contradict each other
just too late and was typing too fast. i'm also dyslexic which dont help any 
thanks for trying