Marius

I wrote a rough version of this to a thread way back in the pages, I might as well offer it here since it goes to the heart of the discussion whether our Haldex quattro is "real" AWD.

The difference between slip and sliding IS a source of confusion. Slip is happening all the time as a consequence of horizontal forces between road surface and tyres. Sliding is what happens when you spin your driving wheels, lock the brakes or "break traction" in more dramatic fashion.

The driving wheels on a 2WD car will normally rotate faster than the trailing wheels. Vice versa, the trailing wheels turn a bit slower. If brakes are applied, the wheels will turn slower than car speed would indicate. All this is traction slip or braking slip, usually measured by slip ratios.

Side wind will affect the path of the vehicle, even if it’s still pointing straight ahead. Driving in circles on a skid pad will have the wheels track to the outside of the direction they’re actually pointing. This is called lateral slip, usually measured by slip angle.

In real life slip will be a mixture of traction and lateral slip, and there will always be some slip present. Minimum slip occurs when you’re coasting in a straight line.

Transition between slipping and sliding is usually pretty gradual, as racing drivers know, but in contradiction to what many others believe.

Slip will be different between the separate wheels according to car dynamics, and the difference can be detected by the ECU via the ABS sensors on each wheel. This begins to explain how the Haldex system can get input data to control torque transfer in ordinary driving, without a hint of sliding.

Jeff M

Here are some bits and pieces of info people posted that do a good job of explaining why Haldex _DOES_ work. I started that thread to make a collection of pro-Haldex facts, and that's what I got.

Q: Haldex doesn't activate until 45 degrees of slip?
A: No, haldex can't apply 100% of torque to the rear wheels until 45 degrees of slip occurr. And that 45 degrees is measured by relative front/rear shaft rotation. You can bet your pants Haldex is kicking in at much less slip, and with enough gumption to matter. John S said 10 to 15 degrees can give up 1000 Nm of torque. PLUS, apparently all of this "degrees of slip" stuff applies only to launching from a stand-still. If you're already moving, then haldex is probably already doing something, if only a little. (right?)

Q: EDL/ASP/ABS must activate and fail before haldex does anything.
A: Nope. The haldex system is activated on differences in the input and output shaft speeds, not what EDL/ASP/ABS is doing.

Q: Just driving along down the road doesn't incurr enough slip to make haldex do anything.
A: Read the post to which mine is a reply. (From Marius)

Jeff M

Yet I still have two questions unanswered. Can anyone complete my arsenal? (Questions Follow)

Jeff M

I think the Haldex box can withstand far more than 300 HP because I read somewhere that it can handle some incredibly high torque rating. Like... REALLY high numbers. So if torque and HP are somehow related, I don't understand why the Haldex box wouldn't be able to handle it.

YET, I don't know enough about the relationship between torque and HP or how they affect mechanics to really feel comfortable claiming otherwise.

Jeff M

I've seen this argument somewhere in all of my searchings. I think it relates to another statement I read somewhere that goes something like "The Haldex system can apply up to 100% of torque to the rear axle for short periods of time".

So what happens if you use a Haldex system in a rear-engined car with a weight distribution of 40/60? I would suspect that the front wheels would slip much more frequently. Would this create a reliability problem for the haldex box?

In other words, is the Haldex box designed to withstand a 300 mile road trip with a 50/50 torque split?

Some would argue that the haldex box wouldn't be used in such a car so it doesn't really matter. Still I've seen people post claiming that you can alter the static torque split through software. Some people say constantly activated haldex would chew itself up. I tend to think otherwise especially since Marius started this thread... if Haldex is always doing SOMETHING, then it's probably designed for continuous use, right?

Marius

I’ve been wondering about the lack of cooling ribs on the Haldex casing since it was first presented. If pressed for a theory, I’d say that even if large amounts of torque is transferred more or less continuously, the rotational speed difference in-out will be next to nothing compared to normal shaft speeds or to the main clutch, which must handle up to a 6500 rpm difference (even if it’s just for less than a second) in a launch. The heat that needs to be dissipated is proportional to the product of torque and rotational speed difference, and if the latter is close to zero there’s really no problem.

Marius

I'm sure there's a substantial margin in our cars. When hotter models come they'll probably reinforce the whole driveline.

Jeff M

I don't understand. Don't be afraid to talk down to me, I know nothing abou this stuff.

Marius

... by letting go at pre-set level, but you don't want the driveline letting go too often. The biggest loads come down the driveline when tranny is in first gear and driver revs the engine before stepping off the clutch. In that moment, clutch strength and/or momentum of engine + flywheel, times gear ratio determines the torque loading further down the driveline. Horsepower doesn't enter the issue directly.

CoreyL

I seem to remember that the AWD system Porsche uses is also based on the Haldex, and it seems to be able to handle the massive torque of even the new turbo. The TT's unit may not be okay with 400 lbs-ft, but if Audi wanted a 420HP TT, they could use a Haldex on it.

Besides, like anything else, they will only put in the car what is needed. If Audi wants a 300HP TT then they will modify the Haldex to handle it.