Why do trucks have so many transmission gears?
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Ever ride a multi-speed bike? See how it’s much harder to get started and accelerate in top gear than it is in low gear? Now, imagine you’re pedaling to push 100000kg instead of 70.
It takes a certain amount of force to move an object at a certain speed. Through various means, you can move more weight with the same force, just the same amount more slowly. One of those methods is gear reduction, which is the “gears” in a transmission. Gears take the input force from one gear to spin another gear more slowly, so the output gear spins more slowly but with more force. The force produced by an engine is a twisting force called torque. A semi-tractor engine produces around 2000 lb-ft of torque. Fully loaded, a semi truck and trailer can weigh up to 80,000 lbs. On flat ground, that means it can take around 30,000 lbs of force to move that much weight. If the engine were directly acting on the wheel with no gear reduction, it could produce about 2000 lbs of force. No where nearly enough to move the truck right? This is where gear reduction comes in. First gear in a transmission is the most speed reduction producing the most force. Typical first gear ratios in a class 8 semi will be something like 15 to 1. That means that the force from the engine has been multiplied by 15 times but is spinning 15 times more slowly. This is further reduced by the axles, which have their own reduction, called the final drive ratio. This is typically around 3 to 1. And these ratios multiply. So in our typical case, in first gear the 2000 lbs of force is now multiplied by 45, but is moving 45 times more slowly. So we’re now making 90,000 lbs of force, more than enough to move the truck! But. A typical class 8 engine spins at a maximum of about 2000 rpm. Reduce that by 45 times, multiply that by the circumference of the tire and first gear gets you a maximum speed of… about 3.5 mph. But, now that it’s in motion, it takes much less force to keep it in motion, because inertia says an object in motion wants to stay in motion, and moving friction is less than static friction. So you go to second gear, which doesn’t increase the force by as much but also doesn’t reduce the speed by as much, letting you speed up. The heavier the load to move, the more reduction it takes to get it moving, the more gears you need to get the load up to speed. So a typical car makes around 200 lb-ft and weighs about 4000 lbs. A typical class 8 truck makes around 2000 lb-ft, 10 times as much, right? But it weighs typically around 60,000 lbs, 30 times as much. And while a typical car engine can spin around a minimum of 6000 rpm, a big diesel engine can only spin around 2000 rpm easily. So, it requires many more gears to get up to a usable speed.
In theory, an infinite step gear box is the best one. The theory is that as the vehicle moves, the gears are shifted to keep the engine in the best operating speed range.
If the gears are wide apart, when the gear is shifted, the drop in engine speed will fall below the max torque speed and will cause engine stall.
If I was trying to explain this to _my_ five year old..
Engines can only spin up to a highest point. The engines in trucks are very strong but don’t spin very fast so they need a bunch of little gears to break up the workload.
In a more technical approach, the optimal point of torque and horsepower is lower than that of gasoline engines so revving higher in diesel trucks is inefficient. Torque drops off drastically and that torque is what gets the load moving.
But this time like you’re actually 5:
Imagine that accelerating up to highway speed is like climbing a ladder, and the rungs of the ladder are like the gears of the transmission. Each one allows you to reset your feet and take another step up. For a truck which is very, very heavy, the rungs of the ladder need to be very close together, so that the truck doesn’t need to take long upward steps. It can take tiny little steps at a time, and eventually, after lots of those steps, get all the way to the top of the ladder without pulling a muscle (blowing a head gasket).
trucks are really big and heavy, so they need lots of small gears for getting started and staying at low speed. on level ground with a light load, most trucks start in third gear. that means if they’re *really really* heavy, or if they’re starting uphill, they have the option to start in a lower gear.
now the low end of the gearbox is crowded. so if they stopped at 6 gears like a normal car, they wouldn’t be able to get up to highway speeds.
trucks are really big and heavy, so they need lots of small gears for getting started and staying at low speed. on level ground with a light load, most trucks start in third gear. that means if they’re *really really* heavy, or if they’re starting uphill, they have the option to start in a lower gear.
now the low end of the gearbox is crowded. so if they stopped at 6 gears like a normal car, they wouldn’t be able to get up to highway speeds.
So much misinformation and overcomplication here…
Trucks have lots of gears to keep the engine close to peak horsepower as the wheel speed increases.
You can make unlimited torque with enough gearing. You can’t make unlimited horsepower.
Because they need to. They are carrying much more weight than a car is carrying.
When the truck is empty, often the lowest few gears can be skipped plus in the rest of the shift sequence you might be able to upshift 2 gears at a time. With a full load, the truck needs all of those gears to start moving as well as to reach the highest gear.
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