Thu Nov 20 12:01:46 EST 2003
INTER-AXLE DIFFERENTIAL: OPERATING MODES (torsen)
An interaxle center differential operates in the following 4 basic modes show
in fig 10. In reverse, DRIVE becomes COAST and vise versa.
In DRIVE mode, a Torque Sensing differential will distribute the higher
driving torque to the axle that tends to turn slower than the other one.
In COAST mode the higher braking torque will be distributed to the axle which
tends to turn faster than the other one."
That indicates to me that in reverse, more torque goes to the axle spinning
faster. The torque split (TBR) doesn't change, just the allocation
properties of reversing the input torque. IOW, engine braking is the same as
driving in reverse. Under engine braking the effects of it in terms of
tractive force is nil. Under engine torque (reverse), the effects of the
drive mode greatly affect tractive force.
Let's not confuse this with TBR in forward or reverse axle rotation. It's
CONCLUSION: A torsen Center audi quatro will have better traction in forward
>Its difficult to use, because torsen, despite its cute name, IS sensitive to
>axle speed difference. And not just alittle, but notoriously.
>But yes, if you look for hint, then this hints indirectly that torsen tested
>there was indeed indifferent to direction of torque..
ONLY in terms of TBR. Not in terms of which axle gets "more" torque.
>Engine has some finite rotational acceleration. Rotational acceleration of
>wheel is called only to compensate for that rpm difference. When engine rpm
>is limited, torque goes to 0. Torque=Inertia*acceleration. Its quite easy
>to derive max torque from rotational inertia if we know max rotational
>acceleration of engine, divided by final drive.
Keith is thinking oh no. I was here at one point too Andre. Torque at the
ring gear doesn't change, hence torque at the torsen = engine torque.
Tractive torque (supported by wheels on ground) + inertial torque (slip) +
frictional loss = torque at the torsen/engine torque. Add more engine torque
beyond what's supported by wheels on the ground, that torque goes to slip.
>but no, this is not the case. Reality is that if you engage full engine
>then 90% of it will go to accelerate low-mu wheel, 2% on traction from it
>and 8% to the gripping one. Or more correctly, you really can't engage all
>of the engine torque, only 10% of it. Too quick wheel acceleration will
>prohibit any further torque increase.
Tractive force vs wheel spin. Add them up, you get Teng = Tring gear
>Take a look at it slightly differently. torsen has locking ability. Locking
>axles takes force. This force comes from jamming wormgears between engine >
>and traction forces. When there is traction, there is jamming force. When
>is no traction, there is also no jamming force, no locking. Peculiarity of
>torsen is in that it takes least tractive force, and multiplies it via
>and uses that as locking force. It has linear dependance on least tractive
>force, and multiplier being ca bias ratio. When both axles have equal
>balance comes at 50/50, when different, after reaching 80/20, locking force
>not enough to keep axles locked, and they start slipping. But ratio of
>force stays same, 80/20. Thats why even when differentiating, spinning, etc
>there is more force on best axle. But not more than least tracking axle
Almost perfect. Except a torsen allocates based on turning ratio as well.
>Take open diff. We could say it has bias ratio 1.0:1, 50/50 split at most.
>Could we say that with one axle in air, we can apply 50% of engine torque
>onto gripping wheel?
An open diff allocates 50/50 ALWAYS T1=T2. If torque is 1 then .5 goes
forward, .5 goes back. If one axle is in the air, T1 = 0, then T2=0. No
engine torque can go to gripping wheel.
>right, VERY small amounts of time. But there is another trick to make
>poor-man EDL ;) Revv engine to good revs, letting slipping wheel spinning
>and gaining rotational momentum. Then, pulsate brakes shortly and sharply.
>Rotational momentum will be partly grounded (slipping wheel), but partly
>also transferred to other axle (flywheel). After spinning wheel slows down,
>it acts again as rotational inertia support force. Perfect job for abs.
No disagreement. In fact, that's what EDL systems do. A BETTER way would be
to have EDL AND a rear torsen. Some of the S car boys are doing that. An
even BETTER way would be the ability to reduce engine output as well.
>>So I'll restate it..
>>"Torsen is AT MOST capable of transmitting 80% of current engine torque
output to the axle with most traction"
>>Does that work?
>+ nope. ;) and yep ;) depends on what you want to "see" in that sentence.
>OK.. this then..
>"Torsen is AT MOST capable of transmitting 80% of current engine torque
output to the
>axle with most traction and current torque output is limited to 5 times
>capacity of the axle with the least traction or limited by the drivers
desire not to
>spin the low traction wheels at very high speeds.."
>Yes ;) thats correct. If one wants to nag further, it must be noted that
>5 times traction of worst axle cannot exceed sum of traction of worst and
>best axles. Or, in other words, this applies only after 80/20 max split
>is reached, and does not apply to highgrip torque shifts due to turn radius.
Sure it does. Remember a Torsen isn't really sensing torque. A torsen
senses axle speed diferentiation at a very low angle, that's all. You could
build more slop into it so that it's more like a VC, but then you'd have
abrupt allocation and lose many of the straight line advantages.
The torsen doesn't change it's TBR based on drive or coast modes, nor does it
change it's TBR less or more so based on either actual traction or turning
It's just too dumb to know the differences between these actual events.
Scott "torsen boy" Justusson
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In a message dated 3/20/2003 7:01:34 AM Central Standard Time, andre at online.=
I believe the answer can be found below. To understand it, think of re=
versing input direction (engine braking torque force =3D reverse drive mode)=
More information about the Torsen