Welcome to the forum, @ashleysilva334.

Hi Norman,

Good question, and I'm not sure if you can get a definitive answer. I have not been able to find any published torque numbers that is applicable to any specific wheel.

Here's my conjecture based on some facts.

Subaru developed the Solterra/bZ4X's AWD system and proudly uses its trademark of Symmetrical AWD. To that end, front and rear motors are matched and I presume are ON all of the time vs. the typical AWD EV that can shut OFF one of the front or rear motors to save energy as well as often having different front/rear motors. The Solterra's cousin, the Lexus RZ450e, went its own route with asymmetric front/rear motors and the ability to shut one down for energy savings.

The torque vectoring is apparently based on how many cars implement vehicle dynamic control, i.e. via selective application of braking to specific wheels to achieve the desired goal. The Solterra/bZ4X system applies braking to the inner wheels in a turn to enhance handling with less understeer. This differs from mechanical Torsen differentials but the end result is the same. The opposite extreme is an open differential where 100% of the torque goes to the wheel with the LEAST traction.

I hope this helps a little, though it is a bit of hand waving.

 

Subaru developed the Solterra/bZ4X's AWD system and proudly uses its trademark of Symmetrical AWD. To that end, front and rear motors are matched and I presume are ON all of the time vs. the typical AWD EV that can shut OFF one of the front or rear motors to save energy as well as often having different front/rear motors.

I can’t explain it better than that. The Subaru philosophy is like that of a part-time 4WD operator who engages it as soon as he gets off-road so he has it before he needs it—preventing wheel spin in the first place. That’s opposite of one who doesn’t engage it until he’s in trouble with 2WD—in response to wheel spin.

 

there is nothing special about how subaru is able to "send" torque to individual wheels. its simply marketing.

they use the traction control and ABS hcu to apply the brakes to the wheel that is slipping the most forcing torque to the other wheel. this is how traction control in nearly every modern vehicle operates.

 

See my reply to Bill for context.

 

I sometimes turn off traction control when I am driving through several inches of soft snow. Several times in my old outback, when I did not turn off the traction control, the car just stopped as the brakes were applied when all four wheels started spinning.

When I turn off traction control, does that mean the car gives all the power to a wheel that is spinning?

 

there is nothing special about how subaru is able to "send" torque to individual wheels. its simply they use the traction control and ABS hcu to apply the brakes to the wheel that is slipping the most forcing torque to the other wheel. this is how traction control in nearly every modern vehicle operates.

This is true. However, traction control is part of electronic stability control and it doesn’t operate standalone. It is almost always used with throttle reduction and the amount of that varies widely. Some vehicles can’t even apply enough power to move out of the shallow dip (or off a roller) when it engages.

I sometimes turn off traction control when I am driving through several inches of soft snow. Several times in my old outback, when I did not turn off the traction control, the car just stopped as the brakes were applied when all four wheels started spinning.

When I turn off traction control, does that mean the car gives all the power to a wheel that is spinning?

First, the button you pushed to do that has the icon of a fishtailing car—electronic stability control. You’re disabling the vehicle’s ability to control yaw in addition to allowing the wheels to spin.

There’s no need to turn off ESC to disable traction control on any modern Subaru with X-Mode. In the deep snow/mud setting, it does that for you (among other things) and the purpose is to apply power to wheels that are spinning to allow them to sling snow or mud out of the tire tread grooves. If you don’t need to do that, try the snow/dirt X-Mode setting.

 

This is true. However, traction control is part of electronic stability control and it doesn’t operate standalone. It is almost always used with throttle reduction and the amount of that varies widely. Some vehicles can’t even apply enough power to move out of the shallow dip (or off a roller) when it engages.

Correct. pre ETB systems could not cut throttle so they pulled ignition timing and/or fuel. my 03 cobra would stop dead in the water the second it started spinning. this was extremely dangerous if trying to hop out into traffic.

 

Solterra owners who have driven on ice and snow have said how well the system works. In addition, there is apparently some finesse in the suspension such that the ride over articulated bumps and ruts are as comfortable as it gets, compared to 4x4s and even other Subarus.

 

The Solterra has a few benefits over, say a Model Y:

1) Ground clearance

2) Front/Rear motor are always engaged (This is good for bad weather, but bad for efficiency. The Model Y is RWD most of the time, unless you slip or engage Off Road Mode).

3) Software tuning. VDC, traction control and the amount of slip allowed at each wheel is specific to Subaru’s inputs. X-Mode 1 and 2 further enhance the amount of slip and clamping applied under certain scenarios. (Tesla’s Off Road Mode offers a similar idea).

So, is the Solterra superior in low traction settings to other EVs? We really don’t know. No one has done a comparison test of the Solterra vs the competition. The assumption is that Subaru customers expect greater overland/winter traction, so it should be the same or better.

Torque is controlled at each wheel by using traction control and VDC to slow a spinning wheel, thus sending more torque to the other wheel on the axle with grip. That’s how all modern cars work, EV or not.

Symmetrical AWD for the Solterra, is really just branding. Symmetrical AWD in a gas powered Subaru refers to the Longitudinal engine block sitting in the middle of the front wheels and being in-line with the front differential and transmission (unlike a transverse engine requiring 90 degree bends and a transaxle to route torque which adds energy losses). That’s why it is called Symmetrical. The Solterra has two transverse eMotors that power each axle, but they are situated in the middle of the axle (just like every other AWD EV). So, again, it’s branding, since they are all basically the same mechanically now.

 

Thanks for the info.

You have answered my original question which was other dual SUV EVs are symmetrical in layout so why does Subaru say the Soltera is the only symmetrical AWD EV? That is not true and you have confirmed that. Thank you.

 

I think they are one of the only dual motor EV’s that never disengages the front motor. Which is bad for efficiency but great for traction.

 

Thanks. Good point.

I’d rather have traction than efficiency.

 

I can't tell you how or why exactly, but do know that the Solterra does better in steep, slippery off road conditions. We have one really steep part on our cabin road, and when it is wet (like after a rain) there can be considerable slippage making it up that hill. It is also full of deep ruts (from trucks spinning out) which you have to maneuver through. My old Crosstrek (2018) handled it OK in X-mode, but would struggle sometimes as it switched torque as required. Sometimes it would really hesitate as it dropped power, and almost felt like I had to turn off TCS to keep moving. But it always persisted and made it up that section without having to back down (unlike the trucks).

My Solterra on the other hand goes up that same hill in the same conditions without any hesitation. I just use the first level X-mode, and no need to go to the super sensitive grip control. So there is no question, for whatever reason, the Solterra AWD works better. It may be that with electric the torque vectoring is more instantaneous. Don't exactly know, just that is works better.

 

 

Thanks.

Thats encouraging - nothing like personal experience.

Also the bigger tires will make a difference.

Appreciate your feedback.

 

Just to play the devil’s advocate, the Solterra AWD is the only EV I’m aware of that is symmetrical front to rear—same motors operating full-time.

While most AWD EVs are RWD based with FWD added as needed, there are some such as the 10 module Ultium platform, that are FWD based with RWD added as needed. Their 12/24 module Ultium platforms are RWD based.

 

My original observation was that there were other electric SUVs that had symmetrical AWD and not just Subaru as they claimed. The layout of the motors - not the capability of the vehicle.

Having all wheels powered all the time is a Subaru plus and that’s why I have bought eight Subarus over the years but that wasn’t what I was talking about originally.

 

My interpretation of Subaru of Canada's statement quoted above ("The torque is automatically shuttled to the wheels with the most traction available AND the brake based torque vectoring VCS system helps cornering") is to split it in two halves:

1. "The torque is automatically shuttled to the wheels with the most traction available" This is just referring to front axle wheels vs rear axle wheels (not individual wheels).

2. "the brake based VCS ...", can brake the inside-of-turn front wheel, so not both both wheels on an axle.

Now Subaru uses the term "Symmetrical AWD" for at least 4 different ICE AWD systems, and the Solterra's system could be thought of as just a fifth one. In particular the most popular Subarus in the last ten years use a multiplate center clutch with a default 60/40 front/rear bias but changes dynamically from 90/10 to 50/50. The Solterra AWD can, I think, in theory have a front/rear bias of anywhere from 100/0 to 0/100 but my guess is they keep it somewhere in between these extremes all the time (with software dynamically controlling the power sent to each motor second by second). This allows more flexibility in the dual motor set up compared to the multiplate clutch setup. So there is a sense that for front-to-rear symmetry, the Solterra is more symmetrical that the multiplate clutch ICE Subarus (which also use the brake based VCS). Of course other dual motor EVs can have the same basic setup but there is an infinite variation in the software control, for example some have default FWD most of the time, whereas, I assume, Subaru designed the software more for performance and safety than efficiency, to keep with their tradition.

 

... To follow up, it occurs to me that the three drive modes eco, normal, power could also be changing the front-rear motor bias (when driving on a flat non-slippery road). I had previously assumed these three modes just changed the peak power level. Anyone know?

 

I think that drive mode only changes the ”throttle” mapping.

 

Correct. Just like old Subaru Si-Drive, the modes are only for throttle mapping. Eco is muted, Normal is 1:1 (but it still feels muted), and Power is multiplied (but it feels normal, unmuted, my everyday mode).