I was talking to a friend about rwd and fwd cars. My friend told me that only one of the wheels was spinning when he lifted his civic up. Another car was a Dodge duster and we were amaze that only one rear wheel is spinning.

My question is, does only one wheel actually spins? Or is it just old technology or just I never learned about it.

Sorry if this is a dumb question.

 

Its called cars with a open differential. Both wheels spin once it makes contact with a hard surface, like pavement or a dyno roller.

 

Yup. And that is why LSD or limited slip differentials come in handy.

 

Or welded diffs if youre a cheapo and don't care much about low speed turning radius!

 

An "open" diff enables the driven wheels to rotate at different speeds - handy when making a moderately tight turn as the inside and outside wheels describe arcs of a bit different lengths. Unfortunately, this simple differential also "favors" the wheel with the least traction, directing the majority of the torque to the unloaded (inside) wheel, causing it to spin. When both driven wheels on a single axle are off the ground, the engine running and the transmission in gear, the wheel connected to the axle with the ring gear (usually) will spin - unless you do something stupid like grab it - in which case, the opposite wheel will start to spin (and you may find yourself in a "jackass" video).

A "Positraction", "Tork-Lok", "Torsen", or otherwise "limited slip" differential uses one of a number of mechanisms to just the opposite. While permitting opposite wheels to turn at different speeds, it directs the power to the wheel with the greater traction. This is extremely useful in marginal traction or high-performance applications, especially in "streetable" racers and drag cars.

Locked or "solid" differentials are extremely simple devices used primarily on dirt track cars, rock crawlers, and dragsters. With the two wheels sharing a solid common axle, each wheel receives exactly the same amount of torque - but the inside wheel will have to slip when the vehicle turns a corner. This is a benefit on a dragster because it tends to make the car run straighter - and on a dirt track, the wheels are spinning in relationship to the track most of the time anyway. When strength and reliability are needed, a "locker" can't be beat . . . except you will have a great deal of difficulty turning the car on a high traction surface.

Some high-end 4X4s use a "lockable" differential - where the driver operates a vacuum or electrical actuator to engage a clutch that locks the two wheels together for maximum traction, or releases them for street use. Best of both worlds, but expensive and a possible maintenance problem.

BTW - Next time you have the drive wheels off the ground, engine off, with the transmission in park, or the manual tranny in gear, try rotating one of the drive wheels manually . . . see what the opposite wheel does! Figure that one out!