A person who has only driven a passenger car with a manual or automatic transmission his whole life may be seriously surprised to find himself in the cab of a truck. One of the first things that is guaranteed to attract the driver’s attention is the presence of not 5 or 6, but at least 12 gears in a truck. Let’s try to figure out why the big cars need so many gears and why we can’t do with the usual number.
The truck rarely goes faster than 120 km/h, so why does it need 12 or more gears? Actually, the speed in this case is of indirect importance. If we do not take into account the size of the gearboxes, then in general the truck transmission is not so much different from the box of a passenger car. That said, despite all the design similarities, trucks do have more gears. Twelve is not much more. It is quite normal for a large truck to have 24 gears!
The thing is that most of the trucks engines have a small operating range (as a rule 300-900 rpm) and high torque of 1,200 Nm and more. Not every passenger car can boast such characteristics. However, in the case of trucks, torque for the most part “goes” into the greater mass of the vehicle and the even greater mass of the load. An 18-ton truck hauling another 20 tons of cargo is commonplace. So the performance of the truck engine is predictably highly dependent on the degree to which the truck is loaded. It should be obvious enough…
But the fact that the performance of a truck engine, depending on the load, tends to change radically is not so obvious! Having become 1.5, 2 or even 3 times heavier, a truck even with a very powerful engine can no longer simply pick up speed. The attempt to do it will result either in power train breakage, or transmission breakage. And the last is much more probable. To avoid this, it is necessary to increase speed and to change gears even more smoothly, than in a passenger car. This is why you need such quantity of “gears”.
Why in quotes? Because the truck actually has the same six gears. However, in the transmission design the so-called dividers and demultipliers are used. They are used to upshift and downshift, respectively. The manual transmission with a splitter turns from a 6-speed into a 12-speed, and the box with a splitter demultiplier increases the number of gears to 24, not counting the reverse. This large number of gears is needed to maintain the optimum engine rpm range under sharply increased load.