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Old 05-03-2006, 12:21 PM   #1
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Default quattro vs x-drive

I saw this video on Google Video. I cant believe that the A8 is unable to get off the rollers without assistance.
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Old 05-03-2006, 12:38 PM   #2
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Default Thats a little shady....I'll tell you of something really stupid I did, but Audi Quattro saved me.

I had my A8 on the car carrier on my move to OH last year. Upon arriving at my new home, I went to unload my car. Stupid me overlooked the fact that I had to pull the wheel ramps from the car trailer to the ground. It was a heart stopping moment when I felt the car bottom out on the trailer! I didn't know what to do, so I put it in drive and drove forward so the rear wheels came back onto the trailer. No wheelspin, no drama, it just pulled the rear wheels back up. The front at first dragged the car forward a few inches, then the rear wheels kinda grabbed on. This was with my 19" wheels so the tires weren't so giving either on the corner of the trailer. I dunno why the A8 in that video didn't transfer the power to the front. They might have tweaked it.
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Old 05-03-2006, 12:39 PM   #3
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Default What happens to nose-heavy cars with no rear-biased quattro?

It's surprising, though, and doesn't seem to make sense. Shouldn't quattro pick-up rear-wheel spin and transfer to the front wheels? A problem with the limited-slip differential on the Audi? Who produced this video?
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Old 05-03-2006, 01:31 PM   #4
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Default The video correctly shows what to expect.

The center differential in the Audi is a Torsen without the locking feature. Normally torque is split equally front-to-back. Under low friction conditions, torque can be split up to 2:1 or 1:2 front to back. In the situation shown in the video, the maximum torque that can be utilized in the rear is zero. Since two times zero is zero, zero torque gets transferred to the front, and the car is stuck.

Note the "Total Tractive Effort" at the lower left of the chart. The Torsen behaves exactly like an an open in this situation, zero front and zero back when surface friction coefficient is zero.



The BMW must have a locking differential, either mechanical locked based on wheelspin, or a viscous coupling (as used in many Range Rovers.
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Old 05-03-2006, 01:36 PM   #5
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Default Re: Rear Differential

The traction control system will use the ABS system to brake a spinning wheel when it detects a speed difference across the two rear (or two front) wheels. In this case, however, both rear wheels are happily spinning at the same speed, and the traction control system has nothing to do.
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Old 05-03-2006, 02:31 PM   #6
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Default the same thing happens to allroads if one wheel leaves the ground

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Old 05-03-2006, 08:41 PM   #7
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Default I don't get is. My center diff might be different, but the more gas I give the more it locks up.

If I turn slowly on the parking lot, everything is smooth, but if I step on it the whole car shakes and all four tires spin at the same speed. I've tried it with two wheels on ice. With little gas the two wheels on the ice was spinning. With full throttle it locked all the wheels and went forward without feeling the ice at all.
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Old 05-03-2006, 10:54 PM   #8
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Default Lame. No one would just lay on the gas in this situation. Wouldn't

applying pressure to the brakes while accelerating fix this?
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Old 05-03-2006, 11:10 PM   #9
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Default Finally a reason for heavy wheels

Spinning them up to a specific rpm requires more torque, or torque for a longer time, resulting in the same being delivered to the other end of the car.

Seriously, can't ASR apply brakes to two spinning wheels, albeit while cutting engine power, thereby giving Torsen something to react against? I thought EDL was a separate side-to-side program, differing from ASR in not being switchable and operating over different speed ranges.

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Old 05-04-2006, 12:49 AM   #10
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Default the torsen is "locking"

it locks until it hits the bias ratio (torque split across the diff), where it unlocks and allows differentiation in the same manner as a limited slip differential. this is what is shown in the graph you reference.

edl is also used to control the torque bias shift across the centre differential to limit the biasing effect, but the primary determinate is the bias ratio - relatively low (3:1) on our torsens as they are mainly designed for good traction. high bias torsens (5:1) are available and have been used by some to limit understeer (e.g. statis racing).

fwiw, the centre differential in the latest quattros is entirely different from our cars, it has a combination of an epicyclic diff with a torsen, in a similar manner to a viscous coupling (as opposed to a viscous diff). it is called the torsen-c.
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Old 05-04-2006, 12:49 AM
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