Differential and Final Drive Questions and Answers

Answer: A

The final drive provides the last reduction between the gearbox output and the road wheels, and in a longitudinal layout it also turns the drive through ninety degrees. The differential housed within it divides the torque. Both reduction and direction change are performed by the same pair of gears. A symptom that appears under one condition and not another is what narrows the diagnosis quickly. Tracing the requirement back to the physical constraint that produces it makes the answer follow.

Answer: B

The hypoid set turns the drive through a right angle and provides the reduction, with the pinion offset below the crown wheel centre. That offset lowers the propeller shaft and the transmission tunnel. It also introduces sliding contact requiring extreme pressure lubrication. The order in which the effects act is what decides the outcome, so it is worth establishing first. The shared behaviour explains why these belong together and the differences explain why both exist.

Answer: C

Offsetting the pinion below the crown wheel axis lowers the shaft and allows a larger pinion with more teeth in contact, which is stronger and quieter. Efficiency falls slightly because of the sliding. The lubrication requirement is the price paid. Checking which component actually carries the load settles most questions of this kind at once. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

The sliding along the tooth demands sulphur phosphorus additives that form a sacrificial layer on the steel surface. Without them the teeth scuff and fail rapidly. The same additives attack brass and must not be used in a synchromesh gearbox. Tracing the requirement back to the physical constraint that produces it makes the answer follow. Reading the definition literally, rather than recalling a summary, avoids the usual confusion.

Answer: A

With the engine across the vehicle the drive already runs in the right direction, so a simple pair of helical gears provides the reduction. No direction change is needed. The arrangement is more efficient than a hypoid set and needs no extreme pressure oil. 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: B

Car final drives lie in the region of three to four and a half to one, with the gearbox supplying the remaining reduction. Commercial vehicles use higher ratios. The overall reduction in first gear is usually twelve to fifteen to one. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision. Naming what changes and what stays the same is the reliable route through questions of this shape.

Answer: C

A higher numerical ratio multiplies the torque more but raises the engine speed for a given road speed, so acceleration improves at the cost of cruising economy and top speed. The choice reflects the intended use of the vehicle. Reading the definition literally, rather than recalling a summary, avoids the usual confusion. Working the distinction through on a single concrete case fixes it more firmly than any rule of thumb.

Answer: D

Multiplying the two ratios gives the reduction between the engine and the wheels, which together with the rolling radius determines the road speed for a given engine speed. A change of final drive affects every gear equally. Most errors here come from carrying across a result from a neighbouring case where it does not hold. A symptom that appears under one condition and not another is what narrows the diagnosis quickly.

Answer: A

Road speed is the wheel rotational speed multiplied by the circumference derived from the rolling radius. Fitting a larger tyre raises the road speed for a given engine speed and makes the speedometer under read. Width affects grip rather than distance. Naming what changes and what stays the same is the reliable route through questions of this shape. The order in which the effects act is what decides the outcome, so it is worth establishing first.

Answer: B

Through a corner the outer wheel travels further than the inner, so a differential is essential if the tyres are not to be scrubbed. An open differential divides torque equally regardless of grip. The reduction is provided by the crown wheel and pinion. Working the distinction through on a single concrete case fixes it more firmly than any rule of thumb. Checking which component actually carries the load settles most questions of this kind at once.