re: Re: Questions on AWD, ABS, and locking the rear differentia

[AMorris@brwncald.com]

>> 
>>    o  What is meant by "locking the rear differential"?  Is this
>>       required for AWD, or is this simply one way of using AWD?
>>       Is it done manually or automatically?

>This implies that you have what is referred to as an "open" differential. 
> 
>Open differentials are OK for most situations, but when you come upon 
>compromised traction situations (e.g. mud, snow, ice) an open diff will 
>tend to allow the wheel with less traction to receive power to the point 
>where one wheel will spin and very little power will be sent to the other 
>wheels.  Locking the differential guarantees that power is sent to both 
>output shafts ... in fact both *have* to turn at the same rate.  Older 
>quattros have open, lockable differentials as well, the newer cars have 
>Torque-sensing ("Torsen") differentials that allow power to be sent in 
>both directions even if one is slipping, and in fact tend to send power 
>to the place where there is the most traction.
>> 
>>    o  Thanks :-)
>> 
>You're welcome ... hope it helped.

>Steve Buchholz
>s_buchho@kla.com
>San Jose, CA (USA)




Steve is correct, but one thing to remember about open differentials:  
They transmit the *same* amount of torque to both wheels.  The problem is 
that if one is on ice, and the other on dry pavement, the one on ice 
slips, and the torque applied to the dry pavement is not sufficient to 
move the vehicle.  

This is why four wheel drive trucks, suvs, etc.  will often become stuck 
on "easy" ground:  They spin one front and one rear wheel. . .Where an 
Audi (with either locking or torsen differentials) will spin three (both 
back wheels, and one front wheel.  Three wheels are normally enough to get 
you moving on glare ice. 

-------------------------------
Allan Morris
Phoenix, Arizona
(602) 222-4465
Home:  allanm@primenet.com
Work:  AMorris@brwncald.com
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