RE: VW's VAQ 'diff' explained

RE: VW's VAQ 'diff' explained

Wednesday 30th July 2014

VW's VAQ 'diff' explained

Is it an 'e-diff', a limited-slip diff or even a diff at all? Chapter and verse on VAQ here...



People seem to have got very confused about VW's 'Vorderachsquersperre' or VAQ system as fitted to the Golf GTI Performance and SEAT Leon Cupra. It acts like an electronically controlled locking differential but is actually entirely independent of the diff cage and, rather, distributes torque across the front axle using a Haldex style electronically controlled clutch system. For reference then here is the official VW description, lifted directly from the Golf GTI press pack. For further clarification on different types of limited-slip and locking differentials see our previous story here.



[Verbatim reproduction of Volkswagen press pack]
Front differential lock
A newly engineered electronic front differential lock is being used exclusively in the Golf GTI Performance. To date, Volkswagen is the only carmaker to utilise an electronically controlled differential lock in a front-wheel drive production model. Compared to purely mechanical locks, the front differential lock integrated in the Golf GTI Performance offers advantages such as a variable degree of locking and comprehensive interfaces to the ESC, EDS and XDS+ functions. This makes it possible to completely avoid negative effects on steering response and steering precision that otherwise occur with mechanical locks. As a result, the system realises the full potential and maximum performance of a differential lock with regard to vehicle dynamics, because comfort is not impaired under any circumstances.

Leon Cupra also benefits from the VAQ system
Leon Cupra also benefits from the VAQ system
Functionality of the front differential lock. The front differential lock operates without any power losses here, so that the power produced by the engine is transferred 100 per cent to the road and is fully available to propel the GTI.

The electronic front differential lock utilises a multi-plate unit located between the right side driveshaft and the differential case. The hydraulic pressure needed to actuate the plates is generated by an electric motor driven piston pump. The locking moment that is generated here is proportional to the hydraulic pressure. The hydraulic pressure is controlled by the pump speed that is prescribed by a control module. This control module takes numerous parameter inputs - such as wheel speed, vehicle speed, yaw rate and transverse acceleration - and computes the ideal locking moment.

1,600 Nm maximum locking moment. If the control module detects wheel slip at one of the front wheels, the plates are actuated to redistribute the drive torque from the wheel with the lower grip level to the wheel with the higher level. The maximum locking moment is 1,600 Nm, so that if necessary all of the drive torque can be directed to just one front wheel; that corresponds to a locking value of 100 per cent. This produces maximum traction for a front-wheel drive vehicle, even under difficult roadway conditions and in turning situations.

So it's not a diff but it is very clever - clear?
So it's not a diff but it is very clever - clear?
Torque vectoring effect. When accelerating out of a bend, the drive torque is increased at the wheel on the outside of the bend. This produces an asymmetrical drive torque distribution that matches the dynamic wheel load distribution. This is known as a "torque vectoring effect" which reduces acceleration-related understeer. As a result, the Golf GTI Performance handles neutrally and precisely tracks along the ideal line. The existing grip level is optimally exploited. This lets the driver apply much greater force to the accelerator pedal at the apex of a bend, which in turn results in significantly higher speed exit speeds of the Golf GTI Performance out of bends.

ESC intervenes more gently and with delay. In highly dynamic situations, such as in fast driving through alternating curves, unexpected avoidance manoeuvres or load changes, the front differential lock is used to dampen yaw movements. When oversteer occurs, the front differential lock generates a stabilising yaw moment; this means that ESC interventions can be made gentler and later or might even be avoided altogether. So, the controlled front differential lock stands for even more driving fun and better performance.

ESC Sport
In the new Golf GTI, Volkswagen is offering the "ESC Sport" function for very experienced drivers. The system is activated by a two-stage switch on the centre console. If the driver pushes the button once briefly, it deactivates the ASR function (traction control). When the button is pressed longer than three seconds, Electronic Stability Control (ESC) switches to the "ESC Sport" mode. In very fast driving with lots of bends - such as on a race course - the ESC system reacts with a delay, which enables even greater agile handling properties. As an alternative to activation by the pushbutton on the centre console, ESC can now also be activated or deactivated by settings in the CAR menu.

[Source: Volkswagen]

Author
Discussion

biscione

Original Poster:

275 posts

195 months

Wednesday 30th July 2014
quotequote all
I'm confused

r11co

6,244 posts

229 months

Wednesday 30th July 2014
quotequote all
I'm not.

To sum up - clever, but exists only to minimise the effect of another compromise.

TooMany2cvs

29,008 posts

125 months

Wednesday 30th July 2014
quotequote all
biscione said:
I'm confused
Basically, AIUI, there's no diff at all in the gearbox - just a single output shaft. Then there's this gizmo between that output shaft and one driveshaft, with an output each side, driving each of the two front wheels. Normally, they're free to rotate separately, as per a normal diff. But, when the computer feels like it, they're locked to each other with varying amounts of force, so that the torque sent to each wheel can be varied, effectively giving a smooth electronicly controlled range of lock, between "fully open" and "fully closed".

anonymous-user

53 months

Wednesday 30th July 2014
quotequote all
That has an odd machine translated feel to it

EricE

1,945 posts

128 months

Wednesday 30th July 2014
quotequote all
Can you please post the german version too or maybe a link to it?

KTF

9,788 posts

149 months

Wednesday 30th July 2014
quotequote all
This sounds complicated and expensive to fix should it go wrong.

Vee12V

1,329 posts

159 months

Wednesday 30th July 2014
quotequote all
"This makes it possible to completely avoid negative effects on steering response and steering precision that otherwise occur with mechanical locks."

[megane rs]What negatives?[/megane rs] ;-)

RacerMike

4,192 posts

210 months

Wednesday 30th July 2014
quotequote all
There is interestingly little info about how they're achieving the torque vectoring effect. Without additive torque from an additional motor or some very complicated gearing I can only assume they're using torque vectoring by braking the inside wheel, thus transferring the torque across.

JDMDrifter

4,039 posts

164 months

Wednesday 30th July 2014
quotequote all
Very clever but wont it numb the feel of a fwd? A good old fashioned locking diff gives a more aggressive raw feel does it not?

Hooli

32,278 posts

199 months

Wednesday 30th July 2014
quotequote all
r11co said:
I'm not.

To sum up - clever, but exists only to minimise the effect of another compromise.
Yup.

Millions spent to try and cure an overweight car of it's basic FWD design.

Edited by Hooli on Wednesday 30th July 12:09

A Scotsman

1,000 posts

198 months

Wednesday 30th July 2014
quotequote all
But why? It adds cost and complexity which if it breaks is going to be a real pain in the wallet to fix.

Pixelpeep7r

8,600 posts

141 months

Wednesday 30th July 2014
quotequote all
isn't this the same as porsche trying to engineer out the issue of an engine in the back? they got it right eventually smile

zeppelin101

724 posts

191 months

Wednesday 30th July 2014
quotequote all
RacerMike said:
There is interestingly little info about how they're achieving the torque vectoring effect. Without additive torque from an additional motor or some very complicated gearing I can only assume they're using torque vectoring by braking the inside wheel, thus transferring the torque across.
It reads to me as the diff can act independently, i.e. if they want to "torque vector" then they just run 100% of the power to the outside wheel which is controlled externally by the pump.

It's not really true torque vectoring (applying a negative torque), which is why they call it a torque vectoring effect in the blurb.

Clivey

5,108 posts

203 months

Wednesday 30th July 2014
quotequote all
Hooli said:
Yup.

Millions spent to try and cure an overweight car if it's basic FWD design.
hehe I'm saying nothing...

...but of course, I am.wink

Pixelpeep7r said:
isn't this the same as porsche trying to engineer out the issue of an engine in the back? they got it right eventually smile
In the case of the 911, I think they've gone too far (I like character).

Dan Trent

1,866 posts

167 months

Wednesday 30th July 2014
quotequote all
And here's the reason I set this story live this morning!

Cheers,

Dan

anonymous-user

53 months

Wednesday 30th July 2014
quotequote all
With out the ability to "Overspeed" a wheel this system is not true "torque vectoring". it can only modify the torque split between the front wheels within the limits set by the wheel speeds as a result of the wheel paths (difference in radii of the driven wheels when cornering).

It's main benefit is actually yaw stability (to lock the front wheels together) which is a very useful attribute in a road car.

Beyond that, all it does is prevent the lightly loaded inside wheel from "spinning away" drive torque (in a normal open differential system, the wheel torque must always be of equal magnitude).

The advantage of adding electronic control is that the compromises of a normal mechanical locking diff can be avoided to a large degree. ie clonking / oscilations when using lots of lock at low speed, having to set your positive torque lock thresholds in conjunction with your negative torque ones and being effectively "invisible" to the vehicle dynamics (and driver) until required etc

The technology used is pretty std, and GKN have been supply this into RWD applications for years now. In fact, the most surprising fact is that the production costs have been brought down enough to fit this to £30k hot hatches, rather than £60 performance saloons where it is a lot easier to justify the on-cost of the system.

Atmospheric

5,305 posts

207 months

Wednesday 30th July 2014
quotequote all
r11co said:
I'm not.

To sum up - clever, but exists only to minimise the effect of another compromise.
This 1000%.

voicey

2,453 posts

186 months

Wednesday 30th July 2014
quotequote all
It sounds similar to the E-Diff fitted to the F430 - lovely bit of kit.

EricE

1,945 posts

128 months

Wednesday 30th July 2014
quotequote all
So what about the Golf R? Does it have a VAQ up front? LSD? None of these and just two open diffs with simulated locking via brakes? Or maybe a combination of some of these? confused


Hooli

32,278 posts

199 months

Wednesday 30th July 2014
quotequote all
Max_Torque said:
With out the ability to "Overspeed" a wheel this system is not true "torque vectoring". it can only modify the torque split between the front wheels within the limits set by the wheel speeds as a result of the wheel paths (difference in radii of the driven wheels when cornering).

It's main benefit is actually yaw stability (to lock the front wheels together) which is a very useful attribute in a road car.

Beyond that, all it does is prevent the lightly loaded inside wheel from "spinning away" drive torque (in a normal open differential system, the wheel torque must always be of equal magnitude).

The advantage of adding electronic control is that the compromises of a normal mechanical locking diff can be avoided to a large degree. ie clonking / oscilations when using lots of lock at low speed, having to set your positive torque lock thresholds in conjunction with your negative torque ones and being effectively "invisible" to the vehicle dynamics (and driver) until required etc

The technology used is pretty std, and GKN have been supply this into RWD applications for years now. In fact, the most surprising fact is that the production costs have been brought down enough to fit this to £30k hot hatches, rather than £60 performance saloons where it is a lot easier to justify the on-cost of the system.
Doesn't sound much more interesting than the Tru-Trac diff I had in the front of a Discovery years ago. Probably does away with the second or so delay from wheelspin to the diff reacting though.