OK, I will show my age and ask if LSD is the real name for Positraction? In the muscle car days it was for the crazies that wanted to leave two burnout tire lines. I was out of that scene and into my Bugeye racing around pylons in parking lots on the weekends. I want to get back into that type of sport again with my new MCS.
I won't be able to add LSD later, so I better figure out If I want it NOW! I am not an SCCA quality driver and need to drive to work in the car on Monday. Can someone help with some questions I can ask myself about this option or opinions on spending the $$ for what I want to do?

 

Positraction is a brand name(GM?) for a form of Limited Slip Differential. If you will be "racing around pylons in parking lots on the weekends" with the MINI you will want the LSD. An LSD will keep your inside wheel from spinning faster than the outside one on tighter turns when accelerating. It probably helps poor drivers more than great ones.

 

If you are getting a MCS i would definetley get a LSD, in fact not getting it crazy. The DCS off is the only way to get the car to slide and with that off the torque just opens up the diff and makes the car a one wheel wonder.

On the MC i have no idea if that is the case.

 

Not to pick on you, personally, Bhatch, but I've got to address something that bugs me every time I see it. A regular non-4wd car without a LSD is NOT a "one wheel drive". I understand what people are getting at when they say this, but it's misleading to people who might not be familiar with how differentials work. An open differential still supplies power to both drive wheels as long as they both have traction (and even, to a lesser extent, if one has partial traction). If you've got the tires to handle it, you can launch and/or accelerate an open diff car as hard as a LSD car, and both wheels will be pushing (or in our case, pulling).

Sorry, I just get irked when I see people referring to "one wheel drive" cars, which don't exist among four-wheeled vehicles.


That said, I too would recommend getting the LSD, especially if you're going to be autocrossing or tracking. I wish ALL of our cars had LSD, and it's on my list of future mods for my '03. If I were ordering an '07, it would be a must-have.

 

So, then, why does the differential suddenly completely disengage from a wheel that has no traction???

 

" deleted passage by maxshrike0" replaced with " This site can probably explain everything a little better: http://auto.howstuffworks.com/differential.htm" from Pop!

I definitely opted for LSD... It's a cheap option that at some point or another makes for better vehicle control and more power to the pavement...

 

A regular (open) differential applies almost all torque to a slipping wheel. An LSD applies less than all the torque to the slipping wheel. No differential (AKA locker or spool) forces both wheels to always be at the same RPM until an axle breaks. Differential means the wheels are allowed to turn at different RPMs.

In cars without power to ever spin one wheel, even in tight turns, an LSD is not needed.

 

Yes, and as I have seen a non-lsd differential open and it has 3 gears, the central trapazoidal one, and a match on each axle, how can it allow different rotation of the wheels other than by only controlling one wheel at a time??? Or is it because the gear is angles, the movement to or away from the larger part will change the wheel rotation? In any case, obviously both wheels cannot be held to the same rpm or the axle would break every time you turned.

 

I opted for no LSD on my MCS, as there is some talk on the forums that the LSD increases torque steer. The MCS demo at my dealer has PLENTY of torque steer with no LSD, and I thought it drove just fine other than that, so I decided I didn't want it.

 

Eeeehhh.... sorry, but you have it backward.

A differential actually allows the driven wheels to turn at different rpms around turns, and powers them at the same rpm in a straight line. When the car turns, the outside wheel is drawing a larger radius arc than the inside wheel, and since the wheels are the same size, the outside wheel has to turn faster to "keep up". If no differential is used, and you just have a simple solid axle arrangement, when going around a turn the wheels are going to want to turn at different speeds, but they can't because they are on a common axle. Because weight shifts to the outside wheel in a turn, the outside wheel will have more traction and the inside wheel will slip - essentially "peel out" because it's trying to go around a smaller arc at the same faster speed as the outside wheel. On cars with old-type clutch disc LSDs (GM's Positraction, for example), you'll see - or more likely hear - this happen on turns sometimes. A clutch type LSD basically creates a solid axle arrangement at all times, unless/until the different wheel speeds create enough force to overcome the gripping power of the clutch disks, at which point the differential acts as it should. Torsen type LSDs are more complicated in operation, and are what the MINI has.

This site can probably explain everything a little better: http://auto.howstuffworks.com/differential.htm

The "one wheel burnout" is a result of the open differential doing its job at the extreme. Let's imagine a rear-wheel-drive car with a 1:1 final drive ratio. When the car is going straight, the axle(s) is turning at the same speed as the driveshaft. When the car turns, the differential allows the inside wheel to turn at a slighly slower speed than the driveshaft, and the outside wheel to turn at a proportionately higher speed than the driveshaft. Say 9/10 of driveshaft speed for the inside wheel and 11/10 of driveshaft speed for the outside wheel. Now imagine that the car is turning so tightly that the inside wheel is stationary and the outside wheel is doing all the rolling. In that case, the inside wheel would be spinning at 0/10 of driveshaft speed while the outside wheel would be going 20/10. A one wheel burnout is basically that scenario happening in a straight line. The driveshaft is turning, the "inside" wheel isn't going anywhere, and the "outside" wheel is spinning twice as fast as the driveshaft.

Note that the cars doing one wheel burnouts do move forward, because the differential is supplying power to both wheels. It's just that because one wheel has less traction it's allowed to use more of that power and spin faster. Because that wheel is spinning faster than necessary (say 15/10), the other wheel, mathematically, has to turn slower than necessary (5/10). If the spinning wheel has *some* traction, as on dry asphalt, it will apply some of the power toward moving the car forward, and as it does this the "slow" wheel will eventually gain momentum. As the car gains speed, the *differential* in wheel rotation speed lessens until both are turning the same speed. The problem arises when one wheel has no traction, such as on ice or slick mud, and cannot therefor apply any of the power toward moving the car forward. In that case, the wheel with traction just sits and the wheel without just spins. Note that on a frozen lake, an open differential will spin BOTH wheels just like a LSD, because nothing is stopping them from both spinning at the same speed. ALSO NOTE that a Torsen differential (which MINIs have) will act like an open differential if one wheel has no traction, like on ice. A Torsen (Torque Sensing) is designed to function with both wheels having at least some traction and therefor having an opposing force to work against. If your LSD MINI is parked on the side of the road with one wheel on dry asphalt and the other in slippery mud, the LSD is likely not going to help you get moving much more than an open diff would. This situation is where a clutch-type LSD has the advantage, because it essentially eliminates the differential and supplies power as though the axle was solid from one side to the other.

 

See above ^^^. It doesn't disengage - in fact, quite the opposite. It FULLY engages the power to the slipping wheel, and saps it from the wheel with traction.

 

If this is true, then what is the purpose of the torsen LSD?

 

The side gears - the ones parallel to the wheels - turn with the axles/wheels when the car is moving. In fact, the power supplied to those side gears is what rotates the axles/wheels and moves the car. The perpendicular gears which span between the side gears do not rotate at all if the car is going straight. They just remain meshed with the teeth of the side gears and revolve with the rest of the assembly. Because the perpendicular gears are attached by short axles to the differential case, the force of the case turning transfers to the side gears because of their meshing with the perpendicular gears. When the car turns, the side gears will want to turn at different speeds. This is when the perpendicular gears will start to turn slowly, rolling along the side gear for the inside wheel and pushing the side gear of the outside wheel. This allows power to be supplied to both axles' side gears while allowing them to turn at different speeds for turning.

This is all MUCH easier to explain and demonstrate with a live model.

 

The theory of the Torsen is that it "multiplies" the torque of the wheel with less traction (the inside wheel in a turn) and applies it to the wheel with more traction (the outside wheel in a turn). It operates with gears and centrifugal force rather than with friction (clutches). Because it "multiplies", it requires the inside wheel to have some traction. If that wheel has zero traction, then any multiple of that will be zero. 0/10 times 20/10 equals 0/10.

I read a really detailed description of how Torsens are constructed and operate, and while I got some of it, I still don't feel totally comfortable with how they operate.... or with the math theory behind them, lol.

Do a Google search for "Torsen", and you'll get several sites with more info.

Oh, and the "purpose" of the Torsen in our cars is to keep the inside wheel from spinning during acceleration out of turns, and to instead apply that otherwise wasted power to the outside wheel which has more traction due to the inherent weight transfer when turning.

 

So then what does the driveshaft go to?

 

As to the torsen, would that not lessen, rather than increase, the effects of torque steer?

 

The basic operation is: driveshaft turns the (small) pinion gear; pinion gear turns the (larger) ring gear (the difference in size between the ring and pinion is what gives the "final drive ratio" i.e. 3.83:1 or 4.11:1); the ring gear is bolted to the differential case, which turns with the ring gear; the case is attached to the perpendicular gears; the perpendicular gears are meshed with the side gears which are attached to the axles.

The above example is imagining a front engine/rear wheel drive car, with a traditional driveshaft going back from the transmission to the rear end. In a front wheel drive car where the differential is in the transmission case, there is surely a slightly different arrangement of parts, but the concepts are the same, and the differential will have the same operation.

 

If I'm not mistaken, the "torque steer" feeling with the LSD is caused because the two tires are in essence fighting to see which one can break traction (under normal light acceleration, you shouldn't feel anything). You're accelerating hard. One wheel starts to slip a little (probably imperceptively). The Torsen says, "whoa! send more power to the non-slipping wheel". It sends more power to the other side. Now the side with more power starts to slip a little, and the process is repeated, going back and forth from side to side. As this happens, the steering wheel will be pulled back and forth depending on which wheel is getting more power. It won't be a violent jerking of the wheel, but more like a tugging.

 

Yes, which is why I would say it counters torque steer, which generally seeks to pull the car to one side....

 

I think people confuse "torque induced steering wheel sensation/pull" with torque steer.

 

I think most people call it torque steer because the car is being steered other than where pointed due to torque. It is not merely a sensation.

 

I'd go along with that. And for the record, I haven't driven a front wheel drive car with LSD, so I couldn't say how strong the pull from side to side is.

 

I do and I don't really notice side to side pull, but I do notice a great deal of pull to the left on hard acceleration.

 

Hmm. Interesting. I wonder if the process of power transferring through the gears causes some weight transfer to one side, throwing the perceived traction and therefor power distribution to be slightly favored to one side.

That's the reason that the "one wheel burnout" cars usually left a black mark from the right tire and not the left.

 

Of all the definitions given this is the first one I read that's correct.