The all-new Holden Commodore VXR has torque vectoring, all-wheel drive, a nine-speed auto and no rear differential…we explain how it all works and what makes the set up more clever than you think…
NONE OF THE stuff in the Commodore VXR is groundbreaking, but it’s interesting to look at the whole package in one car. There’s a cool torque vectoring system, an example of the new wave of adaptive gearboxes, driving modes and adaptive dampers. Read on…starting with why this car has no rear differential.
Torque Vectoring with Twinster
The humble differential has been an integral part of almost every car built for over a hundred years, often spoken about but not so often understood. So if you realise the Commodore VXR doesn’t have a rear differential at all…you’d be interested, but maybe wondering why and how.
Let’s back up a bit and discuss why differentials need to exist, and then what the VXR does and why will become clear.
When a vehicle drives around a corner the inside wheel takes a shorter arc than the outside, and therefore the inside wheel turns slower than the outside wheel. A differential is a clever bit of gearing which allows both wheels on an axle to be driven, yet at different speeds. This means the car can turn the corner with both wheels driving, and not force both wheels to turn at the same speed which would scuff the tyres.
So differentials are great, but suffer from a a huge problem – they are ‘lazy’. If one wheel on an axle is easier to turn then the other, the differential will spin that easy-to-turn-wheel and not send turning force, or torque, to the other wheel. Technically, a differential always equalises torque between two wheels on an axle, but it’s easier to think of it as just being lazy and turning the wheel that’s easiest to turn, not the wheel that it should turn to keep the car moving.
The unfortunate side effect of this differential problem is that when a sports car corners at speed the car’s weight shifts to the outside, which means less weight and grip on the inside wheel, which is prone to spin. And as a result, the outside wheel gets less of that torque turning-force. So you don’t go as fast as you could.
You can see this effect in certain suburbs most Friday nights as owners of older Commodores attempt circle work but succeed only in converting one rear tyre to smoke while the other one does nothing at all. And this differential problem is also the same reason that limited-slip differentials and various other traction devices exist, and why some cars don’t have a differential at all, such as drift cars, V8 Supercars and even my little old Pulsar racecar.
The VXR Commodore solves this problem, on the rear axle anyway, with what General Motors call a Twinster drivetrain.
Think of it this way; instead of a differential, there’s just a plain old bevel gear, and a fixed axle with two computer-controlled clutches on either end, one for the left wheel, and one for the right. The computers can vary the torque on the left and right wheels, so you get the differential effect of driving both wheels at different speeds around a corner, but you also avoid the lazy-differential problem.
Here’s what it looks like in reality: 
To go back to our example of a sportscar cornering hard, and the inside wheel lifting – with a normal differential that inside wheel would spin or slip, which reduces torque to the outside wheel, and the car would slow down. A limited-slip differential would help, but it’d actually restrict the turning ability of the car, hence the name “limited slip” which means a limited differential action.
Another solution is brake traction control, where the computer applies the brake to only that inside, spinning wheel which has the effect of increasing torque on the outside wheel. That is effective, but takes energy out of the car which isn’t ideal, and it’s reactive, needing to wait for the slip to happen.
The Twinster system is the best solution of all. This is because it starts to work even before there’s a problem with an inside wheel spinning. As the car turns, the Twinster system starts to proactively send more torque to the outside wheel and less to the inside wheel, which helps the car turn. A normal differential keeps the torque exactly equal, and just lets the speed vary as the cars turns. The difference is that the Twinster system actively helps turn the car, whereas a normal differential doesn’t.
If a wheel does spin, the Twinster system can simply reduce torque to that wheel, and/or increase torque to the other wheel. In contrast, brake traction control has the effect of *increasing* the torque to the spinning wheel, but stopping it spinning by applying the brake to that wheel, an inefficient process. Also, because the Twinster system is ahead of the game, already figuring out how much torque each of the rear wheels should get, then you’re much less likely to have inside-wheel wheelspin in the first place.
How much torque each wheel on the rear axle gets is calculated by the car’s computers, and that’s done by gathering data from all sorts of sensors; speed of each wheel, throttle position, yaw (rate of turn), pitch (nose up and down), roll, steering wheel angle and much more. Those computers also work out the car will turn before it actually starts turning, so Twinster is helping initiate the turn, not just react to it.
Here’s a way to understand torque vectoring with the Twinster – think of a rowing boat with one person on the oars, and another steering the boat via a rudder. If you wanted to turn the boat you could just get the rower to keep rowing with both oars equally, and have the guy with the rudder turn the boat. That’s kind of like a car driven by a normal differential. But it would be much better if the guy with the oars pulled a bit harder on the outside oar and a bit less on the inside oar as well as the rudder. Well, that’s how Twinster works, and you can think of the rudder as the front wheel steering.
You can think of brake traction control as another guy grabbing the rower’s inside arm and making it harder for him to row with that arm, he still puts in the same amount of energy to both oars, but most of that energy for the inside oar is now spent overcoming the resistance of the guy grabbing his arm, which means the outside oar gets more energy and there’s a rotational effect.
From the driver’s seat, the effect of the Twinster system becomes more noticeable the harder and faster you drive. At slow suburban speeds, there’s no difference to a normal differential – and there’s no negative effects such as tyre scuff as you’d find with some limited-slip differentials. But the harder and faster you drive, the more the Twinster’s torque-vectoring system becomes apparent. The VXR turns in with an eagerness that surprises, and sustains mid-corner speed that has you searching for superlatives. On exit, you apply the throttle and the car arcs forward on your chosen line, not running wide with understeer.
The Twinster system also works when no throttle is applied, so for example when braking into a corner it will help rotate the vehicle into the turn.
Basically, Twinster means the VXR is a much, much faster machine into, through and out of the corners than would otherwise be the case if had a conventional differential. And as a bonus, the Twinster system is lighter than a differential too.
Hiper Suspension
You’ve probably heard of a MacPherson Strut, a popular suspension design because it’s light and cheap. It works by having the shock absorber (damper) also be the part of the car that helps locate the wheel relative to the chassis. In the case of the front suspension, there’s an unwanted side effect; when the front wheel moves up and down there’s also some undesirable change in the front wheel geometry, for example the camber angle, and there can be other issues such as torque steer which is where front drive cars swing to one side under power (more on that here).
On a modern car the wheels don’t point precisely ahead, they’re either slanted slightly inwards or outwards (known as ‘toe’ in or out), and they don’t quite sit vertically either, known as ‘camber’. You can see a big camber effect on the front wheels of V8 Supercars. Basically, the exact geometry of especially the front wheels on a car has a big effect on handling, braking, tyre wear and safety, so it’s important to get it right.
The Hiper suspension is a variant of the MacPherson strut that is a bit heavier and more expensive but has a major advantage – it entirely separates the up-and-down movement of the suspension from any secondary effect on the front-end geometry of the wheel such as camber. The result is a cleaner handling car, and no torque steer although in the case of the all-wheel-drive VXR that’s not really a major concern anyway.
Here’s a demonstrator rig with a conventional MacPherson strut on the left and the Hiper suspension on the right:
You can see how it works in this video below:
Electric LSD
On the front axle the Commodore VXR has something that the Holden marketing people called an “Electric LSD”. This is just plain old brake traction control; as explained above in the Twinster description, when one wheel on the front axle starts to spin then the brakes are applied to that wheel only, which has the effect of increasing torque on the opposite wheel. Given the effectiveness of the Twinster system on the rear axle there’s less need for brake traction control on the front axle.
Adaptive All-Wheel Drive
At one extreme, we have a front wheel drive only vehicle that never drives the rear wheels. At the other, we have an all-wheel-drive vehicle that drives all four wheels, all the time, and the exemplar of the latter is Subaru.
Then there is on-demand all wheel drive, where usually the front axle is driven all the time, and when traction loss is detected, the rear axle is driven too. Many softroaders run this configuration, for example the Honda CR-V. The reason for on-demand is to save fuel by not driving the rear axle when there’s no need. However, the problem with on-demand is needing to wait for a slip to occur before the rear axle comes in to play.
Holden’s approach with the Commodore is adaptive all-wheel-drive, which is basically a smarter version of on-demand. Instead of waiting for wheel slip to engage the rear axle, the car figures out in advance when all-wheel-drive is required and engages the rear axle ahead of time. For example, when powering out of a tight corner, or pulling away from a standing start. When the car is sure the rear axle isn’t needed, for example on a freeway cruise, then it’ll disengage.
In practice, our testing found this delivered the benefits of permanent all-wheel-drive without actually driving all four wheels all the time.
Adaptive nine-speed automatic transmission
The VXR runs a nine-speed automatic gearbox, and how that works varies between its Normal, Sports and VXR (kind of a super-sports) mode.
The shift points are the same throughout, but the change is when gears are selected. When the car detects it is being driven hard, based on throttle, yaw, braking and so on – then in Normal mode nothing will happen. But in Sports mode it will start to delay upshifts, downshift earlier, and hold gears. The same is true of the VXR mode except the threshold for entering the revised transmission pattern is lower, and the pattern is more aggressive.
This is change from how older adaptive transmissions worked. In their sports modes, you’d instantly notice a difference as generally the gearbox dropped down a gear or two, and revs were held. However, in the Sports and VXR modes that’s not the case. If you’re cruising at say 100km/h and switch from Normal to Sports or VXR nothing happens…until you start to drive faster and then you unlock the more sporting gearbox mode.
The advantage of this design is that you can stay in say VXR mode all the time, and it only activates when you need it, and you’re not needing to put up with low gears when you’re just freeway cruising. The disadvantage is that the car needs a short while to realise you want it in beast mode. And if you’re wondering how a 9-speed auto compares to a 6-speed, then look at this graph:
Interesting points: – top gear ratio is about the same, which means the 9-speed doesn’t reduce the revs much at 100 to 110km/h. – second gear in the VF is about equivalent to 4th gear in the ZB – third gear in the VF is about equivalent to 6th in the ZB – fourth gear in the VF is about equivalent to 7th in the ZB – The ZB packs three gear ratios in more or less the same space as the ZF’s 5th-to-6th gap. The gearbox is smooth shifting, so you don’t notice all these gears. It does mean the engine is pretty much always in its best power or torque band.
What modes do what?
The VXR has three modes, Normal, Sport and VXR. There’s also a stability control button. Here’s what effect those controls have on the car:
- Electric Power Steering – as the modes go from Normal to VXR the weight of the steering increases, which is the effort required to turn the steering wheel a given amount.
- All Wheel Drive system – in the Sports and VXR modes there is a bit more torque sent to the outside rear wheel on a turn to help rotate the car, making it feel a bit more rear-drive.
- Adaptive Cruise Control – in Normal mode the system is a little bit extra cautious compared to VXR or Sports.
- Stability control – as the modes go from Normal to VXR the point at which the computers intervene, and the extent to which they intervene are changed to allow more slip.
- Sound – the engine note is augmented via the speakers to a greater degree in VXR and Sports modes.
- Suspension – there are three settings, one each for Normal, VXR and Sports. As you’d expect, the latter two are the stiffest. The damping is not instantly computer-controlled, as in stiffening the outside dampers when cornering – the three settings are presets.
- Transmission – explained above.
The stability control button has two effects; in any mode a single press disables engine traction control, and second press engages what Holden call Competitive Mode, which is where stability control is still engaged, but very much de-sensitised. A press of over 5 seconds deactivates both stability control and engine traction control.
Engine traction control is when the car decides that too much power is being applied to be useful, and restricts the throttle accordingly. Stability control is the system that detects understeer – running wide – and oversteer, which is when the back end steps out, and then takes corrective action such as braking individual wheels to bring the vehicle back into the desired line. In practice, simply selecting the VXR mode should disable stability control enough for pretty much any purpose.
What does this all mean?
So what does all this mean for the driver? There’s not a whole lot for the driver to adjust to based on this knowledge as the car does all the work for you, but it’s useful to know how it functions. For faster driving you’d want VXR mode, as the difference between it and Sport aren’t massive and you may as well go straight to the maximum. Even in that mode the car is very easy to drive and predictable, and the electronic safety systems are still there. There’s no real need to use the stability control button, with the possible exception of a racetrack but the VXR isn’t really a track car. It’s just a fast roadcar, thanks to a lot of modern tech.
