Differential and Final Drive Questions and Answers

Answer: C

Two or four planets carried on a cross pin mesh with the sun gears splined to the half shafts, and any speed difference makes the planets rotate on their pin. In a straight line they are carried round without turning. A symptom that appears under one condition and not another is what narrows the diagnosis quickly. The shared behaviour explains why these belong together and the differences explain why both exist. Most errors here come from carrying across a result from a neighbouring case where it does not hold.

Answer: D

The differential constrains the sum of the two output speeds to twice the cage speed, so if one wheel is held stationary the other turns at twice the cage speed. This is the fundamental kinematic relation and the origin of the runaway wheelspin problem. The order in which the effects act is what decides the outcome, so it is worth establishing first. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: A

An open differential always delivers equal torque, so the wheel with grip is limited to whatever the slipping wheel can react, which may be almost nothing. The vehicle is then immobile despite one wheel having full grip. Limited slip and locking devices exist to address exactly this. Checking which component actually carries the load settles most questions of this kind at once. Reading the definition literally, rather than recalling a summary, avoids the usual confusion.

Answer: B

A limited slip unit uses clutches, gears or a viscous coupling to resist the difference, so torque can be biased towards the wheel with grip. Locking differentials go further and connect the shafts rigidly. Electronic brake based systems mimic the effect. Tracing the requirement back to the physical constraint that produces it makes the answer follow. Most errors here come from carrying across a result from a neighbouring case where it does not hold.

Answer: C

The torque bias ratio is the maximum ratio of the torque delivered to the gripping wheel against that delivered to the slipping wheel, typically between two and five to one. A higher ratio transfers more. An open differential has a ratio of one. The shared behaviour explains why these belong together and the differences explain why both exist. Naming what changes and what stays the same is the reliable route through questions of this shape.

Answer: D

Stacks of friction plates between the sun gears and the cage are preloaded by springs, and the separating force of the differential gears under torque adds further load. Torque sensitivity is therefore built in. The plates wear and the unit requires periodic rebuilding. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision. Working the distinction through on a single concrete case fixes it more firmly than any rule of thumb.

Answer: A

The worm or helical gears within a Torsen unit generate friction proportional to the torque passing through them, so the bias rises with the load. It is entirely mechanical and instant in response. Its weakness is that with zero torque at one wheel it can transfer nothing. Reading the definition literally, rather than recalling a summary, avoids the usual confusion. A symptom that appears under one condition and not another is what narrows the diagnosis quickly.

Answer: B

Because the bias is a ratio, multiplying a torque of nearly zero by even a large ratio still gives nearly zero. Manufacturers add a small preload or use the brakes to provide a reaction. It is the fundamental limitation of a torque sensing design. Most errors here come from carrying across a result from a neighbouring case where it does not hold. The order in which the effects act is what decides the outcome, so it is worth establishing first.

Answer: C

Applying the brake to the spinning wheel gives the torque sensing mechanism something to work against and restores the bias. The combination of a mechanical differential with brake intervention is now common. A parallel viscous unit is a genuine alternative used on some designs. Naming what changes and what stays the same is the reliable route through questions of this shape. Checking which component actually carries the load settles most questions of this kind at once.

Answer: D

Silicone fluid between closely spaced perforated plates shears as the two halves rotate at different speeds, transmitting torque that rises steeply with the difference. The unit is sealed and cannot be serviced. Magnetorheological fluids are used in dampers rather than couplings. Working the distinction through on a single concrete case fixes it more firmly than any rule of thumb. Tracing the requirement back to the physical constraint that produces it makes the answer follow.