Positraction?

[mknox]

Anyone know if the Model S transfer case has the equivalent of Positraction? (transfer of power from a slipping wheel to one with grip). I know it has traction control, but am unsure about this.

 

[ElSupreme]

I don't think the Model S has a limited slip differential (Positraction is a GM brand name). It isn't listed anywhere on the spec sheets, and it isn't something that would be left out.

And thinking about the gearbox/linkage to the drive shaft I really don't see where they would put a differential.

I am just speculating, I do not know for sure.

 

[TEG]

I think a lot of modern vehicles forego the mechanical limited slip differentials, and instead use electronic traction/stability controls that can use the ABS system to apply braking to any drive wheel that starts to spin too much faster than the other.

 

[dhrivnak]

If it is like the Roadster there is no positraction but I say the traction control is better. My Avalanche, and several other cars have had positraction and I have been impressed with what Tesla has done. So no need to worry you will not be stuck.

 

[mknox]

I think a lot of modern vehicles forego the mechanical limited slip differentials, and instead use electronic traction/stability controls that can use the ABS system to apply braking to any drive wheel that starts to spin too much faster than the other.

My current Cadillac CTS has torque vectoring AWD, Positraction rear end, Traction Control and Multi-Mode Stability Control. Just words, I guess, but I have had some hairy winter experiences that the car dealt with quite nicely.

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And thinking about the gearbox/linkage to the drive shaft I really don't see where they would put a differential.

There would have to be some sort of differential mechanism to deal with the differing rotational speeds of the wheels as you go around corners...

 

[ggr]

Yes, Teslas use an open differential, with the equivalent of limited slip being provided by braking on the slipping wheel, which automatically transfers power to the other wheel.

 

[jerry33]

Posi-traction is somewhat analogous to traction control. Both limit wheel slip but they use different mechanisms to do so. Stability control is different and shouldn't really be in the same sentence. It's about keeping the front end and back end from swapping places. It also keeps the back aligned with the front when the front starts to slide out. From the manual:

The following are two examples that can be considered as circumstances in which the tires exceed their lateral grip limit. The Enhanced VSC system is designed to help control the vehicle behavior by controlling the motive force and the brakes at each wheel when the vehicle is under one of the conditions indicated below.

• When the front wheels lose grip in relation to the rear wheels (front wheel skid tendency).
• When the rear wheels lose grip in relation to the front wheels (rear wheel skid tendency).

Method for Determining the Vehicle Condition

To determine the condition of the vehicle, sensors detect the steering angle, vehicle speed, vehicle’s yaw rate, and the vehicle’s lateral acceleration, which are then calculated by the skid control ECU.

1) Determining Front Wheel Skid

Whether or not the vehicle is in the state of front wheel skid is determined by the difference between the target yaw rate and the vehicle’s actual yaw rate.

When the vehicle’s actual yaw rate is smaller than the yaw rate (a target yaw rate that is determined by the vehicle speed and steering angle) that should be rightfully generated when the driver operates the steering wheel, it means the vehicle is making a turn at a greater angle than the locus of travel.

Thus, the skid control ECU determines that there is a large tendency to front wheel skid.

2) Determining Rear Wheel Skid

Whether or not the vehicle is in the state of rear wheel skid is determined by the values of the vehicle’s slip angle and the vehicle’s slip angular velocity (time-dependent changes in the vehicle’s slip angle). When the vehicle’s slip angle is large, and the slip angular velocity is also large, the skid control ECU determines that the vehicle has a large rear wheel skid tendency.

Method for Enhanced VSC Operation

When the skid control ECU determines that the vehicle exhibits a tendency to front wheel skid or rear wheel skid, it decreases the engine output and applies the brake of a front or rear wheel to control the vehicle’s yaw moment. The basic operation of the Enhanced VSC is described below. However, the control method differs depending on the vehicle’s characteristics and driving conditions.

1) Dampening a Front Wheel Skid

When the skid control ECU determines that there is a large front wheel skid tendency, it counteracts in accordance with the extent of that tendency. The skid control ECU controls the motive power output and applies the brakes of the front wheel of the outer circle in the turns and rear wheels in order to restrain the front wheel skid tendency.

2) Dampening a Rear Wheel Skid

When the skid control ECU determines that there is a large rear wheel skid tendency, it counteracts in accordance with the extent of that tendency. It applies the brakes of the front wheel of the outer circle of the turn, and generates an outward moment of inertia in the vehicle, in order to restrain the rear wheel skid tendency. Along with the reduction in the vehicle speed caused by the braking force, the excellent vehicle’s stability is ensured.

In some cases, the skid control ECU applies the brake of the rear wheels, as necessary.

 

[William13]

Before adding air to one rear tire, my Tesla's tail wiggled when floored. It did not have any spinning/slipping on the road surface. After filling the tires properly this decreased. This result works well for this inexperienced driver going very quickly to 60 mph.

 

[TEG]

...There would have to be some sort of differential mechanism to deal with the differing rotational speeds of the wheels as you go around corners...

Yes, but an "open diff" (which unchecked will send more power to the wheel slipping the most) is smaller/lighter/cheaper than limited slip types.
Some diagrams of differential innards can be found here:
http://members.rennlist.com/951_racerx/ps84gleason.html

 

[Cottonwood]

Yes, but an "open diff" (which unchecked will send more power to the wheel slipping the most) is smaller/lighter/cheaper than limited slip types.

I agree. The easiest way to think of an "open diff" is that it allows the two wheels to go a different speed, but applies the same torque to each wheel. The problem is that if one wheel is slipping (zero torque) then the other wheel also gets zero torque, and you go nowhere. Good traction control applies some brake action to the slipping wheel, creating some back torque and that torque is reflected to the wheel with traction. The big reason that this is not used more is that by applying the brake to the slipping wheel, you heat up that brake. Good traction control vehicles often need larger heat dissipation in the the brakes to work well.

The ultimate in traction are the ARB air lockers that I have in my Jeep Cherokee. In 4 wheel low, the center transfer case is locked up and front and back diffs are fed the same speed input. With pneumatic control, I can change the front and rear open diffs to locked diffs and go into bulldoser mode, all wheels going the same speed no mater what. This is not good for tire ware on pavement, but in loose conditions, if one wheel has traction, the Jeep moves. :biggrin: