Audi A8 Towing Update
12-18-2002, 02:34 PM
#11
Torsen Differential: How, Where, and When Not To Tow (long)
Paul:
Nice cutaway picture!
The artist took some liberties, but great diagram nevertheless.
The Torsen differential is clearly illustrated: it is comprized of the ganged gearsets at right angles to the rear drive output shaft.
BTW, a Torsen differential is one of those mechanisms ingenious in its simplicity. Basically, it works on the principal of a worm and pinion. The spinning worm gear can easily drive the pinion, but backdriving the worm gear via the pinion requires a lot of torque. And, as you attempt to backdrive the worm faster, exponentially more torque is required. Think of a clockwork mechanism to envision the worm and pinion arrangement.
The amount of torque required to backdrive a Torsen (Torque Sensing) differential is determined by the pitch, diameter, and number of worm and pinion gearsets. And, like any torque control mechanism, heat is generated when it is used.
When the relative difference in speed between Quattro front and rear driveshafts is small (eg, rounding a corner), little offset torque is created in the Torsen center differential. When the difference is great (eg, front axle on ice, rear on pavement), a tremendous amount of backdrive torque - and heat - is instantaneously generated.
The Torsen differential works well in low duty cycle applications - short duration duties with lots of tiime to cool down in between.
Towing an A8 with the rear axle on the ground is backdriving the Torsen differential. OK for short trips at low speeds, but potentially damaging otherwise. I like the "tow it only on a flatbed" suggestion.
The Torsen differential oil drain plug is located on the transmission final drive housing, just in front of the rear driveline output flange.
If the lube is contaminated (looks like metallic lacquer when draining), the diff is shot.
Also BTW, towing any auto trans car with a drive axle turning will eventually ruin the transmission, due to lack of lubrication (pump is not turning).
Any manual transmission design with a countershaft which only rotates when the engine is running will suffer the same fate when towed.
Audi was one of the first to adopt the Torsen differential. It works great in an all wheel drive center differential role. Due to the ability of modern wheel velocity sensors (eg, ABS) to selectively apply braking force to a single spinning wheel, "limited slip" axle differentials are used less and less in high end cars.
The "Hummer" is about as stone axe as they come. I guess that is where the appeal lies. Having said that, our stone axe Miata R has a Torsen rear diff.
Nice cutaway picture!
The artist took some liberties, but great diagram nevertheless.
The Torsen differential is clearly illustrated: it is comprized of the ganged gearsets at right angles to the rear drive output shaft.
BTW, a Torsen differential is one of those mechanisms ingenious in its simplicity. Basically, it works on the principal of a worm and pinion. The spinning worm gear can easily drive the pinion, but backdriving the worm gear via the pinion requires a lot of torque. And, as you attempt to backdrive the worm faster, exponentially more torque is required. Think of a clockwork mechanism to envision the worm and pinion arrangement.
The amount of torque required to backdrive a Torsen (Torque Sensing) differential is determined by the pitch, diameter, and number of worm and pinion gearsets. And, like any torque control mechanism, heat is generated when it is used.
When the relative difference in speed between Quattro front and rear driveshafts is small (eg, rounding a corner), little offset torque is created in the Torsen center differential. When the difference is great (eg, front axle on ice, rear on pavement), a tremendous amount of backdrive torque - and heat - is instantaneously generated.
The Torsen differential works well in low duty cycle applications - short duration duties with lots of tiime to cool down in between.
Towing an A8 with the rear axle on the ground is backdriving the Torsen differential. OK for short trips at low speeds, but potentially damaging otherwise. I like the "tow it only on a flatbed" suggestion.
The Torsen differential oil drain plug is located on the transmission final drive housing, just in front of the rear driveline output flange.
If the lube is contaminated (looks like metallic lacquer when draining), the diff is shot.
Also BTW, towing any auto trans car with a drive axle turning will eventually ruin the transmission, due to lack of lubrication (pump is not turning).
Any manual transmission design with a countershaft which only rotates when the engine is running will suffer the same fate when towed.
Audi was one of the first to adopt the Torsen differential. It works great in an all wheel drive center differential role. Due to the ability of modern wheel velocity sensors (eg, ABS) to selectively apply braking force to a single spinning wheel, "limited slip" axle differentials are used less and less in high end cars.
The "Hummer" is about as stone axe as they come. I guess that is where the appeal lies. Having said that, our stone axe Miata R has a Torsen rear diff.
12-18-2002, 06:08 PM
#12
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Re: The Artist tool some liberties ...
.
Looking at this illustration from Torsen.com and the one in Paul's post, is that huge lower "distributor" gear on the tail end of the transmission the equivalent of the ring gear in a normal differential?
In Paul's illustration, a shaft is visible a little bit forward of the lower distributor gear and coming from it that looks like it might be driving the front drive shaft with another set of gears. If so, then the front drive shaft must be driven off a shaft that passes thru the distributor gear, and my description of what's happening with the Torsen in my "BTW: I'm not sure ..." post is completely wrong.
Sorry :-(
and thanks for the great write-up.
<img src="http://www.torsen.com/images/T1_wpl.jpg">
<img src="http://www.autoreview.ru/new_site/year2002/n18/audi/800/A8020096.jpg">
Looking at this illustration from Torsen.com and the one in Paul's post, is that huge lower "distributor" gear on the tail end of the transmission the equivalent of the ring gear in a normal differential?
In Paul's illustration, a shaft is visible a little bit forward of the lower distributor gear and coming from it that looks like it might be driving the front drive shaft with another set of gears. If so, then the front drive shaft must be driven off a shaft that passes thru the distributor gear, and my description of what's happening with the Torsen in my "BTW: I'm not sure ..." post is completely wrong.
Sorry :-(
and thanks for the great write-up.
<img src="http://www.torsen.com/images/T1_wpl.jpg">
<img src="http://www.autoreview.ru/new_site/year2002/n18/audi/800/A8020096.jpg">
12-18-2002, 06:09 PM
#13
AudiWorld Super User
Great explanation, I understand now *light goes on*
Just so everybody knows, where I thought the torsen was and where it actually is, is two different places. It's the set of a bunch of gears at the back end of the tranny, not where I said it was towards the front of that shaft.
pw
pw
12-19-2002, 08:36 AM
#14
Quattro Torsen Power Flow
Randy:
I like the Torsen diff diagram you posted.
Using it as an illustration, here's the answer to your question:
- Power flow is from the auto trans ouput shaft (top "distributor" gear) to the Torsen differential case (eg, the case is driven by a ring gear in your Torsen example diagram).
- The PW illustration artist cut away the Torsen outer case to show the worm and pinion pairs inside
- The lower distributor gear dives the Torsen differential case
- The Torsen case output shafts drive the front and rear drive shafts (just like the Torsen case drives the left and right axle shafts in your example diagram)
- What you can't see in the PW diagram is that the front drive layshaft runs through the center of the lower distribtion gear
- As you noted, the layshaft in turn drives the front drive shaft through a gearset. I guess it is offset in this fashion to clear the torque converter housing
- The front driveshaft is totally enclosed in the Quattro transaxle housing
- BTW, PW's illustration clerly shows the front driveshaft driving the front axle pinion - ring gear - front differential case
- Note the A8 front axle diff is a conventional open differential with typical side gears and case pinions. The rear axle diff looks the same inside
Hope this helps
I like the Torsen diff diagram you posted.
Using it as an illustration, here's the answer to your question:
- Power flow is from the auto trans ouput shaft (top "distributor" gear) to the Torsen differential case (eg, the case is driven by a ring gear in your Torsen example diagram).
- The PW illustration artist cut away the Torsen outer case to show the worm and pinion pairs inside
- The lower distributor gear dives the Torsen differential case
- The Torsen case output shafts drive the front and rear drive shafts (just like the Torsen case drives the left and right axle shafts in your example diagram)
- What you can't see in the PW diagram is that the front drive layshaft runs through the center of the lower distribtion gear
- As you noted, the layshaft in turn drives the front drive shaft through a gearset. I guess it is offset in this fashion to clear the torque converter housing
- The front driveshaft is totally enclosed in the Quattro transaxle housing
- BTW, PW's illustration clerly shows the front driveshaft driving the front axle pinion - ring gear - front differential case
- Note the A8 front axle diff is a conventional open differential with typical side gears and case pinions. The rear axle diff looks the same inside
Hope this helps
12-19-2002, 12:38 PM
#18
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In very slippery conditions when you're going VERY slow, ...
.
lightly apply the brakes at the same time as you increase throttle.
I don't know if the '96 has an ECU override that cuts the throttle in this situation. If not, braking the end of the car that's spinning a wheel transfers torque to the other end of the car. This makes the Torsen act like a poor man's limited slip differential when one wheel has zero grip, and it's a "trick" I've used when driving offroad in a Humvee, which has two Torsens.
lightly apply the brakes at the same time as you increase throttle.
I don't know if the '96 has an ECU override that cuts the throttle in this situation. If not, braking the end of the car that's spinning a wheel transfers torque to the other end of the car. This makes the Torsen act like a poor man's limited slip differential when one wheel has zero grip, and it's a "trick" I've used when driving offroad in a Humvee, which has two Torsens.