Locking Torsen

[Jens Rösner]

Hi Lawrence!

> >> is full time all wheel drive, which 
> >> means that approximately 50% of the torque is directed to one front
> >axle,
> >> the other 50% is directed to one rear axle. 
> >As long as there is no serious slippage on the axles.
> True - that's why the approximately. It's only certain to be 50/50% when
> quattro1's center diffy is locked

...but then with no slippage. As you know, a slipping wheel cannot
transmit the same torque. So if you have a locked center diff and one axle on ice,
the torque will drop on the icy axle, remaining high on the other one. So,
there is no 50/50 for the torque, but a 1:1 for revolutions per minute.
If you always have a 50/50 for the torque, you have an open diff.
A locked diff has 50/50 only with identical slippage on the axles.
I having identical dynamic wheel loads on the axles, you can replace
"slippage" with "µ" in that sentence.


> >> Any non-limited slip front or
> >> rear drive car has approximately 100% of it's torque directed to one 
> >of
> >> the two driven axles 
[snip]
> An open diff RWD car will direct approximately 100% of the engine's
> torque to one rear wheel, especially in a slippage situation.
> Unfortunately, the wheel that gets that 100% is the wheel that is (or
> will be) slipping. 

So, the last "axles" in your first sentence of that paragraphe was
actually a "wheels"? Then I understand everything what you mean, I think.
_But_: With an open diff, both wheels always get the same torque! (That is
the meaning of differential: equally dividing torque)
And the determining wheel is the wheel on ice!
With a 100% locked diff, you get an uneven torque distribution (nearly
100:0), but even rpm ratio of 1:1.
UC, front2back is basically the same as left2right, as you already know.

> >And where did you get that from?
> Standard operating procedures for an open diffy.
Ok, misunderstood the sentence I think (s.a.)

> Mercedes and BMW (and now Acura) use the brakes to overcome this
> difficulty, as the brakes serve to lock the wheels across and along the
> car body.
> I don't know what the
> allroad does for this, but I'd bet it's similar.  

AFAIK, the allroad has Torsen center diff with EDS (elektronische
Differentialsperre= electronic diff lock) on the wheels.
But this EDS is slow, as it is a very difficult task for the brakes. It is
not useful for real offroaders, as the brakes really have to suffer.
The guys at our Departement for Vehicle Technology explained that ASR
(Antriebsschlupf Regelung=control of driving slip; basically the same as EDS) is
the hardest task for electronic systems and brakes. Yes, harder then ESP
(electronics stability programme).

> And if all four are on ice, well...
> >(wet ice has about µ=0.1 and below, down to 0.05, dry asphalt has up 
> >to
> >1.2, motorcycle tires are capable of reaching 1.4)
> >...you will start moving slowly, very slowly.
> Only if you can seriously control your torque application via clutch
> and/or throttle. Here's where I'd bet an auto may actually be a bit
> better, i.e. creep. 

Hm, think about that torque multiplying of the hydrodynamic Trilok thing.
Might get you just the torque impuls that lets the wheels spin. If your
automatic trans is clever, it has a snow setting (or recognizes it by itself)
and starts to move in a high gear: 2, 3 or even more, to lessen that torque
impulse. But you can do it as well with a manual, just shift to 2nd, 3rd or
something and "play" with the clutch. Of course, here the driver has to be the
controller, so you are right about the potential problems (not to forget
the termal stress for the clutch!).
But honestly, without a smart ASR, you are lost.


> You can't afford wheel spin, remember m for sliding
> friction is lower than static friction. That's why locked wheels don't
> stop as well as threshold braked (or ABS braked) wheels. 
True with that µ, but there is a µ, only very low, so, if you are lucky,
there is no hill and the ice is not too wet, you will start moving.

CU
Jens

http://www.AudiStory.com
Member of the AFOSAT
(Association for open speech about Torsen)

-- 
Sent through GMX FreeMail - http://www.gmx.net