Torque split math

[RETHomer]

I sometimes have problems with the term "torque split" when discussing a 4WD driveline. Consider this theoretical scenario on a very simple 4WD vehicle, discounting losses due to friction, etc. In fact, think of a the transfer case as a simple drop case where the driven gear is mated to both front and rear driveshaft yokes (no separate chain drive for the front axle). The engine is producing 200 lb-ft torque. First gear on the transmission is 3.0:1 and the transfer-case ratio is 2.5:1. That equates to 1,500 lb-ft torque. If you measured the torque at the each driveshaft, would the gauges read 750 lb-ft or 1,500? Remember, the driveshafts have a solid connection through the driven gear in the transfer case. If there is a 4.3:1 ratio in each axle (again let's use a solid connection like a spool instead of a differential), would each axle at the wheel hub get 6,450, 3,225 or 1,612.5 lb-ft of torque? Once I get this math down, I can tackle tougher scenarios involving traction control, viscious coupling, etc. where the torque split is manipulated. Thank you.

 

[Bribyk]

Each wheel hub "could" see 6,450 ft-lbs but you don't really know what each one is getting unless you're measuring in real time. Due to differences in tire rolling radius and gear/spline backlash you don't actually know what each wheel is seeing for torque even when driving in a straight line. If three wheels were not in contact with the ground the one remaining wheel that was would get the full 6,450 ft-lbs (neglecting driveline friction).

 

[BobM3]

Depends on the traction. Slipping tires won't see much torque. The gears just force them to turn at constant speed ratios.

 

[pontiacjack]

In an "ideal world", your example could apply 1,612.5 lb.ft. of torque simultaneously to each of the four axles.

 

[GregLocock]

Or 6450 lbft to any one tire. Or anything in between. Or, a negative torque on one wheel and then more to compenstae elsewhere.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

If you are considering locked differentials, the torque distribution will be analogous to a straight shaft with multiple brakes down it's length. With simple differentials the torque splits 50-50. large trucks with three driven axles have a differential that gives a 1-2 torque split. The "1" side drives one axle. The "2" side drives a 50-50 interaxle differential to power the other two axles. You end up with the torque split equally between three drive axles.

Bill
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