use brakes to help gain traction? - Page 2 -
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post #16 of 18 Old 03-19-2013, 03:55 PM
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1990 YJ Wrangler 
Join Date: Nov 2009
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You are fixated on the t-case splitting torque, when the limiting factor in this situation is the open differential. You are failing to understand the mechanics of an open differential. You are confusing an open differential with a locking differential. Just because you have traction at the rear and are moving forward, doesn't mean that an open front diff magically turns into a spool locker.

I'll try explaining it again:

For an open differential to be able to generate torque and drive a vehicle forward, BOTH wheels need to be turning and meet with resistance such that they grab the ground (gain traction). If one wheel loses traction and spins freely without resistance, there is no way on God's green Earth that the non-spinning wheel is going to get torque until the free spinning wheel gets traction and can transfer that power through the spider gears to the other wheel.

A locked differential only needs one tire to have traction in order to be able to generate torque. Both axle halves are locked together and spin at the same rate no matter what. This produces a true 50/50 split at all times.

Open differentials however are not locked together to always spin at the same rate. They are linked together by a spider gear that allows for differential rates. A free spinning wheel gets all the power from the engine but it has no traction, this lack of traction (resistance to the spinning) also produces no torque, thus the free spinning wheel cannot send any torque across the spider gears to the tire that could actually make use of it. This is simply the mechanics of how spider gears work - they allow one axle half to spin faster than the other. No traction on one wheel = no torque for the entire axle.

Again, a free spinning wheel in an open differential PREVENTS power from being sent to the tire that is not free spinning.

Neither a locker in the rear axle, nor the power split at the t-case, have any influence on the open differential front axle getting torque. For an open differential, only traction, when present at both wheels, will allow torque to be generated and actually move the vehicle forward.

That's all I can do for you to explain why it doesn't work like you think it works.

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post #17 of 18 Old 03-19-2013, 10:25 PM
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I figure it out from doin some research. The following quote in from an article sent to my by Quaife engineering. The open differential spllts Torque 50/50 all the time. It doesnt however split POWER 50/50. Power as u may know is torque times rpm or wheel speed. So when one wheel starts to spin and the friction limit is met the Power is limited or Torque if u divide out the wheel speed.

This is why braking works. Applying the break increases friction so in order for the wheel to continue to spin at the same speed more torque is needed sending more torque to the other wheel.

Same as with the Transfer Case which I was wondering about. A locked rear will increase the overall demand for torque, limited by the power the locked rear can transfer to the ground, from the engine that is split 50/50. More Torque will go forwards with a real locker, however you are still limited to the Power the wheels can put to the ground.

Illustration 1A and 1B show the common open differential. The driving torque, taken from the ring gear passes through the differential housing to a crosspin, which passes the torque to the spider (or planet) gears, thence to the output, or side gear. The open differential always splits the torque 50% to each wheel. The even torque split gives the most predictable handling and is easiest to set up, tunable with just springs, swaybars and dampers and caster. Handling is consistent through a wide range of power application.

The limitation of the open differential is that the thrust available is limited to the maximum thrust (acceleration grip) of the wheel with the poorest grip. Again, with a 50-50 split of torque, both tires have exactly the same thrust, limited by the tire with weaker grip.
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post #18 of 18 Old 03-20-2013, 07:19 AM
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I use to have to do this all the time for my old job. weather beaten dirt roads in winter make this technique a must for an open diff vehicle. I simply couldnt get to some job sites without it. it wont work for rock crawling but really helps for slippery hill climbs. i caught myself doing it with lockers in my jeep because i was so use to doing it. hahaha

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