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Get into gear

Take a good look at a bicycle, specifically at how the chain transmits power from the pedals to the rear wheel.

india Updated: Aug 27, 2009 10:39 IST
Grease Monkey

Take a good look at a bicycle, specifically at how the chain transmits power from the pedals to the rear wheel. If you already have and remember how it works, you’ll remember the wheels with teeth that help the chain transmits power. These are what gears look like.

Cars need gearboxes for a simple reason: the engine operates efficiently within a particular rev range. For example, if you’ve ever floored the throttle in first and second, you’ll notice that even the smallest cars will manage to reach about 40 kph in first and 80 kph in second, which is more than enough for the city and highway.

Yet automobile manufacturers fit in five gears, because revving the engine to the redline wears it out really quickly, it’s really noisy (and our streets are noisy enough already) and it wastes fuel unnecessarily.

Since the engine operates comfortably only from idling speed to the redline, it cannot come to a halt. Therefore, we have the clutch, which in simple terms engages and disengages the engine from the gearbox. This makes it easy to stop the vehicle, and change gears. It is possible to change gears without the clutch — we’ll come to that in a bit.

Gears are wheels with teeth on their outer edge. A vehicle’s gearbox consists of a number of gears of different sizes, just like the different-sized sprockets on a mountain-terrain bicycle’s rear wheel’s hub. The lower gears like first and second are larger in size (that is, diameter) compared to high gears like fourth and fifth.

The larger diameter makes the wheels turn only once for many turns of the engine, which means that if the engine turns over 2,000 times for a single rotation of the wheel, that much energy is transmitted to the wheel. Fourth gear is usually small in diameter, and in fourth, the speed of the wheel’s rotation roughly equals that of the engine.

That means that for every rotation of the engine, there is a rotation of the wheel as well. This lends a lot of speed to the vehicle, but not as much power.

This is also why gears are called ‘gear ratios’ — they multiply the force that goes to the wheel. This is proved every time you see a car chugging up a slope in a low gear. Fifth and sixth gears are usually ‘overdrive’ gears, which means the wheels turn faster than the engine does. This helps fuel economy.

The teeth of the gears in a gearbox aren’t cut straight across like you might imagine, they’re cut at an angle. This helps keep the gearbox quiet. Reverse gear is ‘straight-cut’ because this type of gear is easy to engage. Increase speed in reverse, and an unfamiliar whine will accompany the usual sounds. This is the reverse gear’s straight-cut teeth engaging with another gear.

The size of the forward gears is also an exact science; if random sizes were chosen, it would be really hard to engage gears all the time, and gears would be recalcitrant, like old trucks or really old cars. This is what a ‘synchromesh’ gearbox does — no matter what the conditions, it makes sure the gear slots in smoothly.

Automatic gearboxes shift gears on their own, depending on the conditions. The old ones weren’t very intelligent and quite unloved by enthusiasts, but dual-clutch gearboxes like Volkswagen’s DSG have turned that impression around. There’s even an auto gearbox without gears! The Honda City and most gearless scooters use a Continuously Variable Transmission (CVT), which involves a belt and a cone.

Gearboxes are integral part of a car’s transmission, and we’ll revisit them soon. Till then, do not attempt changing gears without the clutch!

If you have questions or comments for Grease Monkey, email him at carsnbikes@
hindustantimes.com with Auto Tech 101 in the subject line