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Thu Nov 20 12:01:46 EST 2003
"1.1<BR>
INTER-AXLE DIFFERENTIAL: OPERATING MODES (torsen)<BR>
An interaxle center differential operates in the following 4 basic modes sho=
w in fig 10. In reverse, DRIVE becomes COAST and vise versa.<BR>
<BR>
In DRIVE mode, a Torque Sensing differential will distribute the higher driv=
ing torque to the axle that tends to turn slower than the other one.<BR>
<BR>
In COAST mode the higher braking torque will be distributed to the axle whic=
h tends to turn faster than the other one."<BR>
<BR>
That indicates to me that in reverse, more torque goes to the axle spinning=20=
faster. The torque split (TBR) doesn't change, just the allocation pro=
perties of reversing the input torque. IOW, engine braking is the same=
as driving in reverse. Under engine braking the effects of it in term=
s of tractive force is nil. Under engine torque (reverse), the effects=
of the drive mode greatly affect tractive force. <BR>
<BR>
Let's not confuse this with TBR in forward or reverse axle rotation. I=
t's still 75/25/75. <BR>
<BR>
CONCLUSION: A torsen Center audi quatro will have better traction in f=
orward gears<BR>
<BR>
<BR>
>Its difficult to use, because torsen, despite its cute name, IS sensitiv=
e to<BR>
>axle speed difference. And not just alittle, but notoriously.<BR>
>But yes, if you look for hint, then this hints indirectly that torsen te=
sted<BR>
>there was indeed indifferent to direction of torque..<BR>
<BR>
ONLY in terms of TBR. Not in terms of which axle gets "more" torque.<B=
R>
<BR>
<BR>
>Engine has some finite rotational acceleration. Rotational acceleration=20=
of<BR>
>wheel is called only to compensate for that rpm difference. When engine=20=
rpm<BR>
>is limited, torque goes to 0. Torque=3DInertia*acceleration. Its quite e=
asy<BR>
>to derive max torque from rotational inertia if we know max rotational<B=
R>
>acceleration of engine, divided by final drive.<BR>
<BR>
Keith is thinking oh no. I was here at one point too Andre. Torq=
ue at the ring gear doesn't change, hence torque at the torsen =3D engine to=
rque. Tractive torque (supported by wheels on ground) + inertial torqu=
e (slip) + frictional loss =3D torque at the torsen/engine torque. Add=
more engine torque beyond what's supported by wheels on the ground, that to=
rque goes to slip.<BR>
<BR>
<BR>
>but no, this is not the case. Reality is that if you engage full engine=20=
torque,<BR>
>then 90% of it will go to accelerate low-mu wheel, 2% on traction from i=
t<BR>
>and 8% to the gripping one. Or more correctly, you really can't engage a=
ll<BR>
>of the engine torque, only 10% of it. Too quick wheel acceleration will<=
BR>
>prohibit any further torque increase.<BR>
<BR>
Tractive force vs wheel spin. Add them up, you get Teng =3D Tring gear=
<BR>
<BR>
>Take a look at it slightly differently. torsen has locking ability. Lock=
ing of<BR>
>axles takes force. This force comes from jamming wormgears between engin=
e >torque,<BR>
>and traction forces. When there is traction, there is jamming force. Whe=
n there<BR>
>is no traction, there is also no jamming force, no locking. Peculiarity=20=
of<BR>
>torsen is in that it takes least tractive force, and multiplies it via w=
orms,<BR>
>and uses that as locking force. It has linear dependance on least tracti=
ve<BR>
>force, and multiplier being ca bias ratio. When both axles have equal tr=
action,<BR>
>balance comes at 50/50, when different, after reaching 80/20, locking fo=
rce is<BR>
>not enough to keep axles locked, and they start slipping. But ratio of l=
ocking<BR>
>force stays same, 80/20. Thats why even when differentiating, spinning,=20=
etc<BR>
>there is more force on best axle. But not more than least tracking axle=20=
times<BR>
>bias ratio.<BR>
<BR>
Almost perfect. Except a torsen allocates based on turning ratio as we=
ll.<BR>
<BR>
>Take open diff. We could say it has bias ratio 1.0:1, 50/50 split at mos=
t.<BR>
>Could we say that with one axle in air, we can apply 50% of engine torqu=
e<BR>
>onto gripping wheel?<BR>
<BR>
An open diff allocates 50/50 ALWAYS T1=3DT2. If torque is 1 then .5 go=
es forward, .5 goes back. If one axle is in the air, T1 =3D 0, then T2=
=3D0. No engine torque can go to gripping wheel.<BR>
<BR>
<BR>
>right, VERY small amounts of time. But there is another trick to make<BR=
>
>poor-man EDL ;) Revv engine to good revs, letting slipping wheel spinnin=
g<BR>
>and gaining rotational momentum. Then, pulsate brakes shortly and sharpl=
y.<BR>
>Rotational momentum will be partly grounded (slipping wheel), but partly=
<BR>
>also transferred to other axle (flywheel). After spinning wheel slows do=
wn,<BR>
>it acts again as rotational inertia support force. Perfect job for abs.<=
BR>
<BR>
No disagreement. In fact, that's what EDL systems do. A BETTER w=
ay would be to have EDL AND a rear torsen. Some of the S car boys are=20=
doing that. An even BETTER way would be the ability to reduce engine o=
utput as well.<BR>
<BR>
>>So I'll restate it.. <BR>
>>"Torsen is AT MOST capable of transmitting 80% of current engine tor=
que output to the axle with most traction"<BR>
>>Does that work?<BR>
><BR>
>+ nope. ;) and yep ;) depends on what you want to "see" in that sentence=
.<BR>
><BR>
>OK.. this then..<BR>
><BR>
>"Torsen is AT MOST capable of transmitting 80% of current engine torque=20=
output to the<BR>
>axle with most traction and current torque output is limited to 5 times=20=
traction<BR>
>capacity of the axle with the least traction or limited by the drivers d=
esire not to<BR>
>spin the low traction wheels at very high speeds.." <BR>
><BR>
>:-)<BR>
<BR>
>Yes ;) thats correct. If one wants to nag further, it must be noted that=
<BR>
>5 times traction of worst axle cannot exceed sum of traction of worst an=
d<BR>
>best axles. Or, in other words, this applies only after 80/20 max split<=
BR>
>is reached, and does not apply to highgrip torque shifts due to turn rad=
ius.<BR>
<BR>
Sure it does. Remember a Torsen isn't really sensing torque. A t=
orsen 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 yo=
u'd have abrupt allocation and lose many of the straight line advantages.<BR=
>
<BR>
The torsen doesn't change it's TBR based on drive or coast modes, nor does i=
t change it's TBR less or more so based on either actual traction or turning=
radius.<BR>
<BR>
It's just too dumb to know the differences between these actual events.<BR>
<BR>
Scott "torsen boy" Justusson<BR>
<BR>
<BR>
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