Sporty \ Rally 4x4 Drive Bias

[SystemParanoia]

So I was browsing some old youtube vids, and came across this one by Chris Harris

http://youtu.be/4_tTsJCcBlc

skip to 5:14 to 5:40

So the current crop of rally drivers like a front bias to their 4x4 delivery over the rear bias of the Mcrae era drivers.

on the weight of this single interview, does this mean I should really re-evaluate my opinion on front bias 4x4 cars instead of casually dismissing them as irrelevant. actually get some miles under my belt with it and see what I've been ignoring all this time?


I speak mainly about the

VAG Haldex
Maxda 6 MPS
Volvo's

and many others

What are PH'ers opinions about this.. considering we're all driving on public roads so will never be getting the major slip angles of these driving hero's .. does it actually matter?


Vag slo-mo

http://youtu.be/nWjYBJMBE9Q

Not-at-all standard Impreza slo-mo

http://youtu.be/C8zAX6-CHoA

Edited by SystemParanoia on Sunday 27th January 00:41

[GravelBen]

I don't Hirvonen was saying its an actual front drive bias, more that it feels a bit front biased in order to drive it straighter/faster instead of sliding so much. But unless I heard wrong he still thinks a rear bias is more fun despite the extra sliding being a bit slower.

I don't know how much you can compare a full WRC drivetrain with various road systems either, they're not really in the same league.


ETA: Whether or not you get big slip angles on public roads depends a lot on where you live, and what sort of public roads you have to play on. Plenty of fun to be had where I am.

Edited by GravelBen on Sunday 27th January 10:30

[interloper]

As far as I'm aware VAG haldex is fwd until it senses a loss of grip, then punts power to the rear. So it doesn't behave at all like any proper WRC transmission set up which may be slight more front than rear biased.

The thing with rallying its about who can go the fastest, unfortunately if the fastest set up is with more front bias, then that is what they will go for. You don't get points for being a stylish loser!

As for how you want your 4wd road car set up, well that depends on how you drive, were you drive (B roads, track days possibly?) and what your preferences are. Not simply trying to copy what Leob does, because it doesn't have much relevance in the real world.

[thinfourth2]

faster does not equal more fun

[SystemParanoia]

All very true.

Does this mean that to get the most from the VAG system you have to drive it like a hamfisted moron and get the fronts spinning as often as possible to ensure you're operating the rear diff?

or is it designed more like a safety net to help you out if you understeer by providing a sudden dose of oversteer?

I agree completely that copying the best doesn't guarantee results for yourself..
but if you're going to copy anyone.. surely its a good starting place

Rear bias will always be more "fun" and isnt that the main reason we buy these expensive to run and insure things in the first place?

[davepoth]

SystemParanoia said:

All very true.

Does this mean that to get the most from the VAG system you have to drive it like a hamfisted moron and get the fronts spinning as often as possible to ensure you're operating the rear diff?

or is it designed more like a safety net to help you out if you understeer by providing a sudden dose of oversteer?

I agree completely that copying the best doesn't guarantee results for yourself..
but if you're going to copy anyone.. surely its a good starting place

Rear bias will always be more "fun" and isnt that the main reason we buy these expensive to run and insure things in the first place?

The idea of the VAG system is that as soon as there's an indication of front grip loss it starts feeding the torque out to the back. Rather than waiting for a slide to start, it works to stop one.


As far as the rallying goes, the ideal is going to be somewhere around 50/50 due to the extra work the fronts have to do with throttle on (steering). An equal split will feel a bit understeery.

And as it happens, that's exactly what the spec gearbox does.

http://www.xtrac.com/sectors/motorsport/rally-offr...

[SystemParanoia]

Thank you for that.. i always wondered what the WRC/IRC guys actually ran

[Caulkhead]

The Mazda 6 MPS system is different from the haldex one in that it always has some drive (upto 50%) going to the rear LSD except when the system detects the handbrake applied or reverse gear selected so no slip detection is necessary and no delay is noticeable.

It makes this kind of thing easier :

http://www.youtube.com/watch?v=mXpa7v5MQs0

[SystemParanoia]

Informative gold!!

Ive been searching on and off about various 4x4 systems for quite a while, but i never manage to find this stuff out.
dealersip glossys and salesmen are of absolutely no use what-so-ever!

[ArsE92]

Caulkhead said:

The Mazda 6 MPS system is different from the haldex one in that it always has some drive (upto 50%) going to the rear LSD except when the system detects the handbrake applied or reverse gear selected so no slip detection is necessary and no delay is noticeable.

It makes this kind of thing easier :

http://www.youtube.com/watch?v=mXpa7v5MQs0

<heads off to classifieds>

[Caulkhead]

SystemParanoia said:

Informative gold!!

Ive been searching on and off about various 4x4 systems for quite a while, but i never manage to find this stuff out.
dealersip glossys and salesmen are of absolutely no use what-so-ever!

Here you go, this is the technical training manual sent out to dealerships and has all the technical drivetrain detail you crave!

http://car2tw.com/Documents/Mazdaspeed%206%20Techn...

Page 34:

Front/rear wheel torque varies between 100/0 and 50/50 through an electronic
torque coupling mounted in front of the rear differential

100/0 is only used when the hand brake is applied or at low speeds during
tight turns (parking lot maneuvers)

Many AWD systems in FWD cars require front wheel slip (viscous couplings)
before power is transferred to the rear, MAZDASPEED6 has a strong rear
wheel bias in most driving conditions

Almost constant torque to the rear requires a water cooled transfer unit

[SystemParanoia]

nom nom!

[Disco You]

SystemParanoia said:

So I was browsing some old youtube vids, and came across this one by Chris Harris

http://youtu.be/4_tTsJCcBlc

skip to 5:14 to 5:40

So the current crop of rally drivers like a front bias to their 4x4 delivery over the rear bias of the Mcrae era drivers.

on the weight of this single interview, does this mean I should really re-evaluate my opinion on front bias 4x4 cars instead of casually dismissing them as irrelevant. actually get some miles under my belt with it and see what I've been ignoring all this time?


I speak mainly about the

VAG Haldex
Maxda 6 MPS
Volvo's

and many others

What are PH'ers opinions about this.. considering we're all driving on public roads so will never be getting the major slip angles of these driving hero's .. does it actually matter?


Vag slo-mo

http://youtu.be/nWjYBJMBE9Q

Not-at-all standard Impreza slo-mo

http://youtu.be/C8zAX6-CHoA

Edited by SystemParanoia on Sunday 27th January 00:41

Maybe you should drive some cars, rather than regurgitating others opinions?

[SystemParanoia]

i did actually say in my post that maybe i should get some miles under my belt using these various systems.

but at the same time.. i dont want to buy one thats absolute dogst either

[micawrx]

davepoth said:

As far as the rallying goes, the ideal is going to be somewhere around 50/50 due to the extra work the fronts have to do with throttle on

The regulations have decreed that the torque split is fixed, 'ideal' would be a centre differential that split the torque dynamically front to back (as they used to do before the FIA changed the rules)
The centre torque split was determined by wheel, clutch, propshaft and laser groundspeed sensors... the 50/50 was a playing field leveler.

[Twincharged]

He actually says that it doesn't pull more from the front, but that's the feeling you get. (5:36 onwards)

Besides which, a WRC car is a very different beast to a road car weight distribution-wise. If you open the bonnet on a road car, you've got the radiator at the front, and the engine behind that, whereas if you open the bonnet on a WRC car, you see a radiator mounted low down, in a plane parallel to that of the bonnet aperture, and vertical axis fans pulling air through it. The engine is mounted further down and further back than in a road car. For a given torque bias front:rear, it's therefore going to feel a lot different than the road car.

[micawrx]

Weight distribution is done by making the fundamental components as light as possible with some light repositioning ... The remaining weight required to meet the minimum allowable is created by using useful ballast... usually getting it as low as possible.
Christian Loriaux was the pioneer of this with the 2003 Ford Focus.. (cage thickness was a major discussion point with FIA)
...Not relevant in a road car... the radio and battery would be on the floor!

Tarmac Set Up Rally car feel without active diffs? Anything that responds to throttle input through the handling..
If you want the understeery look of the current WRC cars.. any Audi

[anonymous-user]

The point of an all wheel driven vehicle is to provide maximum tractive effort at all times. Hence, the starting point is to have the basic torque split matching the vehicles static longitudinal mass distribution. In this way, the front and rear tyres get the same ratio of normal load to longitudunal force at the tyre contact patch. However once the car starts to actually accelerate, the effective mass distribution changes, with weight moving rearwards. To optimise traction at that point you also need to bias drive torque backwards to match.

The fully active WRC cars (banned) used wet clutch pack actuated epicyclic overspeeding centre differentials to carry out this torque split modulation. In order to cut costs, the FIA banned active diffs, and current cars are now passive (some even have spool centre diffs, i.e. no diff!). This means the engineers have to make a choice between setting the car up for maximum static traction and biasing the torque rearwards for "handling" effects (usually driver confidence).

But, the fastest stage time on a gravel event is still all about traction out of slow corners (the speed you gain early out of a corner is carried for the full length on the next "straight", and hence has the biggest effect in increases average speed over the closed course) And as a result, the passive split will be very near that which matches the cars static mass distribution. These passive cars as a result tend not to be quite so capable across all conditions and surfaces as the old active cars, and the drivers need to adapt their driving style to suit the cars, rather than vise versa!


For road cars, this is all somewhat irrelevant! The basic balance of a road car is set by it's mass distribution (often poor) and the basic chassis wheel rates and lateral stiffness/roll stiffness. Something like a 4x4 golf handles like it does because the people who engineered it wanted a safe, reasuring car that would respond slowly and repeatidly. The vast majority of road drivers have no experience or skills at operating out of the "non linear" control region, in which rally drivers pretty much operate all the time.

The more chassis "attitude" you engineer into the suspension, the closer to optimum traction condition you can make your torque split. A WRC driver will be quite happy at say 100mph to have to put on half a turn of opposite handwheel angle as they brake for a corner, if the reward is a car that cleanly accelerates out of a tight slippery hairpin bend. The average road car drive would just crash at that point ;-)

Edited by anonymous-user on Sunday 27th January 13:58

[SystemParanoia]

wheels do seem to "spin up" at the same time in the Mazda 6

http://youtu.be/3U_GeDBaejY

[GravelBen]

I still like my simple old Subaru system - 50:50 static split with centre and rear LSD. All mechanical, none of this electronic business.