Most cars have four or five gears, but big trucks often have 12 or more... Why? Doesn’t that make their gearboxes unduly large, heavy, expensive and less robust?
The main reason is that trucks have large and very low-revving engines – for the ‘torque’ power to pull very heavy loads, even up steep hills. For that, they need very closely spaced gear steps.
Most truck engines give peak power at about 2,000 revs, so if they stayed within their optimum rev band, five closely spaced gears would unduly limit their top speed to about 30-40 kph.
But they also need to cruise on highways, certainly at 80 kph and on motorways often 100 kph - on the flat and sometimes downhill, when less torque is needed and swifter progress is desired.
The greater number of gears in trucks does not add much to the size, weight or cost of the gearbox. Because just one extra cog can ‘double’ the number of ratios available between the lowest and the highest.
The six main gears can be and are spread to enable speeds from a standstill up to 100 kph, all at 1,000 to 2,000 revs, but the ‘gaps’ between them have to be very large.
What the extra cog does is turn a sequence of 1,2,3,4,5,6 (5 steps) into 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5,5, 5.5, 6 (10 steps that halve the gap between each stage).
Cars don’t need that, because their engine revs range is 1,000 to 5,000 (or more). Power and speed can be balanced, delivered and sustained much more flexibly.
For those not at ease with cog-rev ratios, a more visible example can be readily seen on bicycles. Their gearbox is the cog between the pedals. Their propshaft is the chain.
If that goes to just one cog on the back wheel, they have only one “gear”. But if there are five cogs of different sizes on the back hub (the chain can be selectively moved from one to another), they have 5 gears.
But to make the bike a 10-speed requires only one more cog… at the front between the pedals, so the rider can select 1,2,3,4 or 5 with the cable that operates the back, 0.5, 1.5, 2.5, 3.5, 4.5 with the cable that alternates the front cogs.
A ten-speed bicycle. Add just one more cog at the back and one more at the front, and you’ve got an 18-speed bike. (six options at the back and three at the front).
The same basic principle applies to 4WD vehicles fitted with what is known as a “low ratio transfer box” which does the same job as the second cog at the front between the pedals. In one move, it lowers the ratio of all the main gears.
Cars and trucks, and 4WDs have two more “fixed” ratio elements – the “final drive” ratios (that’s the gearing in the differential) and the diameter of their wheels and tyres. Bicycles have the wheel size…and the length of their pedal arms.
