Human Nature and Mechanics

Have you ever noticed that in winter driving conditions there appears to be a disproportionate number of four wheel drive vehicles in the ditch?

I’m pretty sure there’s a lesson there.

Now there’s a technical issue that I never considered until I got a 4WD pickup and experienced it myself.  When the old-fashioned 4WD is engaged, the front and rear drive lines are locked together– they cannot rotate at different rates.  That’s pretty much common knowledge (you can’t turn a corner without one or more tires slipping) but it has another implication:  When you hit the brakes it is impossible for only one wheel to lock up.  Since the two drivelines cannot rotate differently, you either lock up one wheel on each axle or you lock up all four, i.e. your braking ability on ice is better with 4WD than it is with 2WD and no ABS.


3 thoughts on “Human Nature and Mechanics

  1. Not quite true. Conventional four wheel drive does lock the front and rear drive shafts together in the transfer case. However, the front and rear differentials allow the wheels on each axle to spin independently. In order to truly lock all four wheels together, you must also have locking differentials.

  2. It is 100% true. Maybe you live in warmer country. I’m talking about driving (breaking actually) on ice with two standard open differentials in a run-of-the-mill ’94 F150 4×4. Read the post more carefully, especially the last two sentances, and learn something new and interesting.

  3. I still disagree. Take an extreme example. Three wheels are on dry pavement, one is on ice. If you lightly apply braking, the wheel on ice will stop rotating, but the others will continue to rotate. The front and rear differential carriers are required to rotate at the same rate due to the connection through the drive shafts and the transfer case. However, the individual axle shafts are still allowed to rotate or not rotate relative to each other due to the spider gears inside the carrier. As to warmer country, the four feet of snow in my front yard might suggest otherwise.

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