torque and diffs

["Matt Martinsen" <mattmartinsen@hotmail.com>]

Scott wrote:
>With a
>locked diff on the ground 50% of the torque is on each axle, by 
> >definition.

Take the example of a four wheel drive pickup with the front locking hubs.  
It has essentially a locked center diff.  The front drive shafts are always 
spinning, it is in locking the hubs that torque is transfered to the 
wheel/ground.  With hubs free wheeling, all available torque goes to the 
rear, the front driveshafts will spin at the same speed as the rear.  With 
the front hubs locked to the front driveshafts, front and rear still spin at 
the same speed, but torque is now going to the front wheels also.  The 
center diff remained locked the entire time, the only thing that changed is 
the torque required to spin front and rear wheels at the same speed.  Taking 
the above quotation would mean that a system as described above, in front 
hubs freewheeling mode, only half of the available torque would be moving 
the vehicle which is obviously not the case.  In other words, if a locked 
diff split torque 50%, then the same truck on a rear dyno with front hubs 
freewheeling might show 100 lb ft (or Nm, units don't matter) now lock the 
front hubs, put it on a four wheel dyno and it would show 200 lb ft? It 
would still show 100 lb ft of total torque. Wrong.  Open diffs evenly 
distribute torque, individual axle speed can vary, locked diffs evenly 
distribute axle speed, torque can vary.  If there is no (or very little) 
resistance on any given wheel, with a locked diff causing the wheels to spin 
at the same speed, no (or very little) torque will be at the wheel without 
traction, hence the torque must go to the other wheel.  Try to put 50 lb ft 
of torque on a stripped bolt, can't do it, in the same manner a wheel with 
no traction can't receive any torque.
-Matt Martinsen
Seattle, WA



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