jgriff

Haldex responds with 15* of spin (about 2.5 inches of travel). So inducing wheel spin to promote understeer is not going to happen easily. Instead the torque will be put to use accelerating the car (using rear wheels if front wheel spin is detected). Understeer comes into play when the limits of lateral adhesion are reached (steering input and speed)

smallTTs

and if it's forward (front tire drive torque) it gives up lateral (turning) force. Now if rear starts helping drive forward, it relieves some of the front's drive and allows more lateral.
Of course, as rear's start driving, they can't generate as much lateral. The result should be more lateral force at front and less at rear, which decreases understeer or increases oversteer tendency.


In practice, I've found the system to be almost seamless, and it doesn't require any non-intuitive control inputs.

My $.02

MATTTT

On the one hand you're saying the Haldex will prevent front wheel spin by sending torque to the rear. Ok, good, I like it so far.

Then on the other hand you seem to be saying you can't lose traction at the front without exceeding lateral adhesion.

If the latter statement is true why bother sending any torque to the rear in the first statement?

jgriff

Torque is only transfered when there is differential wheel spin. Read smallTTs post above for more info on how it works.

It certainly is seamless as noted above. I understood your question as "can you induce POWER oversteer?" the answer is no. If your question is "can you induce oversteer?" the answer is yes. If your question is "can I get all 4 wheels to spin?" then the answer is yes again.

It really is not complicated, I am apparently not doing a good job of explaining it though.

To resond to your last question:

"Why bother sending any torque to the rear if you can only lose traction at the front through lateral acceleration?"

You send torque to the rear, to prevent torquefrom spinning the wheels. But haldex can't do anything about lateral slippage, that is why you now have ESP.

MATTTT

Actually my original question is more like "does the Haldex Quattro reduce power-understeer?".

As for smallTTs post, I thought he was agreeing with me

jgriff

Haldex will reduce power understeer, that is the beauty of quattro and why all wheel drive systems are so good competitively despite the wieght disadvantage to FWD or RWD systems.

smallTTs

but the explanation has to include an understanding of how tires work. Basically we're all sorta agreeing here. AWD gives better turning performance than FWD in a front-heavy car, at least for most folks in most conditions.

Maybe a good way to demonstrate would be on a slippery surface like snow or a very wet parking lot. If you've driven FWD in these conditions, and you tried to achieve max turning performance and then you added power, you got "terminal push"(TP) because you were asking front tires to do more than they are capable of, but you weren't asking rears to do any more lateral work..in fact, as turn radius inreased due to the pushing (understeering) fronts, rears had less to do, and stuck better, which made the TP worse.

You don't even have to be on the power to get "TP" as the 60/40 weight distribution says the fronts have to work harder than the rears to keep the car turning. This can be tweaked by roll couple distribution, etc., but when you finally get the front tires to their limit, rears should still have something left. Does that sound like we should have larger fronts for optimum turning? You bet! (I'm wearing my Nomex undies.)

By adding AWD with Haldex, you can reduce that terminal push, as per my earlier post.

Marius

The best simple way to describe the Haldex system in the TT is to start with a regular FWD drivetrain, and then add an additional rear drivetrain that feeds off the front drivetrain IN PROPORTION TO how much power or engine braking you apply. The application (and continuous adjustment) of the power transfer to the rear is instantaneous in comparison to human perception and reactions.
The Haldex takes input from the ECU (and ESP), so throttle movements will influence it, but there's no "learning function", so you can't get it to understand that you enjoy power oversteer. ;-)

The front drivetrain is always 100 % connected to the transmission, while the rear connection may vary between 0 and 100%. Braking will not disengage the rear, but the hand-brake OR ABS braking OR ESP interference will.

The slip that is always brought up CAN be the split-second front wheel slip that is noticable in a hard launch from a stand-still, but usually it is just the un-noticeable gradual "give" that will always take place in a driving (or braking) wheel. If, at speed, we suddenly floor the pedal or lift off, the Haldex Quattro will react fast enough to equal the power distribution of regular torsen Quattro.

Audiboy

The haldex is located on the rear axle. There is only one drive shaft from the front to back, so a torque split is not sent by the haldex from back to front The tranny is directly connected to front wheels

john s

Full pressure is achieved with 40° of pump displacement. The TT has a 17/27 bevel box so this equates to 25° of differential tire rotation, about 5 ½ inches of differential travel. You can kick the back end out with the throttle (ESP on). If you go into the corner understeering ESP will penalize you. Grip is not exactly a vector sum, a little torque to the front helps. Don't know what you'll find with ESP off. To really get the benefit of quattro stop approaching corners in an understeering mode. Use the weight transfer of braking & the torque transfer of throttle to your advantage. The car responds beautifully.