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Into high gear

Have you ever wondered why we use gears? I don’t mean manual gearboxes.

india Updated: Aug 27, 2009 16:04 IST
Grease Monkey

Have you ever wondered why we use gears? I don’t mean manual gearboxes. Even automatic gearboxes (except the Continuously Variable Transmissions, or ‘CVTs’) have gears in them. Why can’t we do away with the hassle of a clutch and shifting gears and have cars that don’t need gears at all? We do, actually. Electric cars have no gears, because they don’t need them.

Why we need them
Your normal everyday internal-combustion-driven car, on the other hand, needs them, which is why they have them.

A gearbox can have as little as two gears, and as many as eight. In our country, the Santro automatic’s gearbox utilises three speeds (or ‘gear ratios,’ what we call ‘gears’) and premium cars usually possess automatic gearboxes with six speeds. To understand why we need gearboxes, we’ll have to go off on a tangent to the engine. An internal combustion engine (what is under your car’s hood, the nuclear ones haven’t been invented yet) produces useable force in the form of rotation.

The amount of turning force it produces is called ‘torque’, and ‘power’ is merely a function of torque. This twisting force isn’t spread evenly over the engine’s rev range — there will always be a ‘zone’ (that isn’t a technical term) in which the engine will seem happy at operating. Step on the gas too low in the rev range, and it will strain with not much acceleration, and try the same thing high in the rev range and it might just sound like a bolt will pop out at any moment.

Power and torque curves are plotted by putting the value of the torque (or power) generated at a specific engine rpm, down on a graph. The torque peak is where the engine makes the most twisting force. Electric motors don’t have a torque curve, it’s more of a torque line, since they generate maximum torque right from zero rpm to whatever their maximum rpm may be.

Driving in the happy zone
Internal combustion engines need to be kept in their ‘happy zone’, where they generate enough torque to pull the vehicle along, and yet don’t strain themselves like they usually do near the redline. While moving off from the rest, a vehicle needs a lot of shove to gather momentum, so first gear makes the driven wheels rotate once while the engine rotates many times, just like if you switched to lower gear in a bicycle.

Your legs will be a blur and you’ll huff and puff, but you’ll be going at 5 kph. You won’t meet much resistance at the pedals, though. On the other hand, switch to a high gear, and you’ll fly with very few rotations of the pedals, but going up an incline might just give you a hernia. A car or bike’s gears work in the same way, which is why acceleration is best in first gear and you get the most speed out of top gear.

Some gear math
Gears are ‘torque multipliers’ — that is, first gear multiplies the force from the engine to the wheels, which makes it easy for the car to start from rest or go up an incline. Overdrive gears like fifth or sixth make the wheels move faster than the engine, which helps fuel economy, but try overtaking in overdrive, and you’ll feel the lack of pulling power.

We’ll discuss gear ratios and gearboxes in a little more detail next week. Until then, keep your engine in the ‘happy zone’ — it will give you the best compromise between fuel economy and wear and tear on moving parts.