A tractor-trailer fully loaded can weigh up to 80,000 pounds. That’s 40 tons. In fact, that is the legal weight limit in the U.S. The weight includes the truck and trailer as well as all the cargo carried by the unit. When you weigh that much, it takes a long time to slow down and come to a complete halt.
For this reason, manufacturers give Big Rigs multiple gears to shift through to make slowing easier and faster for the trucker. The average truck has a total of 10 gears. However, some manufacturers load their trucks with more. There have been as many as 18 gears in a trucker’s crankshaft. These gears give the truck extra power to get up to speed faster. More gears mean more energy for pulling all that weight. They also are helpful in slowing down, because if you’re coming around the mountain in a state like Tennessee where there are steep downward slopes, and you are pulling 40 tons of dead weight, you need those gears.
You learn how to do this right in truck driving school. One of the things that we’ll teach you is how to downshift from that top gear to the lower gears easily and efficiently so you can slow your truck down and stop it when you need to. There are times when an unexpected halt in the traffic can cause you to slow your truck down rapidly and stop in the midst of heavy traffic. It could be an accident or a police checkpoint. Either way, you need to stop.
A good truck driver can bring his truck to a dead stop quickly using all his truck’s gears–whether 10 or 18–and do it without causing panic or hurting anyone. We’d like you to be that truck driver.
Why Do Semi Trucks Have So Many Gears?
Most commonly, semi-trucks have ten forward gears and two reverse gears. However, these big rigs can have 9, 10, 13, 15, or 18 gears.
It is quite different driving a truck compared to a standard transmission car, and the trucks have different techniques for shifting gears.
Size of the Engine and Vehicle
The size of the engine and the overall size of the vehicle determines the number of gears in the vehicle’s transmission. Bigger engines need more gears. Also, the type of vehicle determines the number of gears. Usually, there is a difference in the number of gears if the transmission is manual or automatic. Manual transmissions have more gears than automatic transmissions.
Particularly in manual transmissions, the driver needs to know when to engage a particular gear. Reverse gear is present in all automobiles. There are also forward gears that vary with the transmission type and the engine size.
Typical sedans have four or five forward gears. We call these four or five-speed manual transmission vehicles. Sports cars often have more gears. They may have six-speed or seven-speed transmissions. Trucks can have 18-speed transmissions with more than one reverse gear option.
Power of the Engine
The engine power and the amount of torque an engine has can also determine the number of gears. Trucks have massive engines that can produce an enormous amount of acceleration and torque. Their brakes and clutch are stiffer as well. That is why these vehicles are more difficult to drive.
For manual transmissions, each gear has a particular range and fuel distribution. The gear ratios differ with the engine type and the number of gears. A normal sedan is started at the first gear, then allowed to gain a certain amount of speed before engaging the second gear.
You are always advised not to drive in one gear for too long and not to drive in the low gears for too long. As the car’s speed increases, you have to switch gears to the one above.
For a sedan, starting in the second gear puts a lot of pressure on the clutch cable and plate and may also damage the pressure plates. Thus, to get optimal performance, you need to start a car in the first gear.
However, this is not the case for trucks. Trucks have huge engines. These engines produce massive acceleration and torque. Thus, truckers usually avoid starting the truck in the first gear to avoid losing control.
If you start a truck in the first gear, it gains higher speeds quickly because the first gear has a higher fuel distribution. Other gear mechanics also contribute to the truck gaining higher speeds.
It is, therefore, harder to control the truck in the first gear, and there is a higher possibility to lose control of the truck and cause it to spin. A spinning truck is a great disaster because it can crash into anything or even topple over.
Truckers are often advised to start the truck in the second gear to reduce the shoot-up effect and to make the take-off slower and more controllable. Thus, one reason for trucks having many gears is to compensate the gear ratios for not having to start from the first gear to gain stability.
Upshifting and Downshifting
18-speed transmissions are laid out in an ‘H’ pattern. The driver shifts from the second to the fifth gear as you would for a normal sedan then split up the gear into the high range. Then, shifts again from the sixth to the ninth gear. From the ninth gear, the driver splits up to overdrive for the top gear.
There are lots of gears in between since all the main gears have a high and low range with different RPMs. However, their power band is very narrow, and the difference in revolutions between the gears is often less than 750 RPM. So, as the driver, you can split these gears as necessary. As the driver practices and gains more experience driving semis, he will realize that the gear being used may not be the right one for where he wants the RPMs to be.
Here, he will shift up or down as needed to match the RPMs to the sweet spot of the engine. This is another reason semi-trucks have so many gears; to select the right RPM depending on the speed of the truck.
However, since trucks produce a lot of torque, it becomes more difficult to bring them to a stop using the brakes only. Thus, you need to downshift the truck to lower its momentum before finally bringing it to a stop.
That is why you need to reduce its speed by downshifting to a comfortable speed before applying the brakes. Otherwise, you will destroy the brake pads, clutch, and many other components in your truck.
Types Of Semi-Truck Gear Transmissions
At one time, all heavy-duty trucks employed transmissions derived from tractors. But this was only sensible when heavy-haulier drivers had an agricultural background and had lots of practice operating manual transmission machinery.
The transport industry has learned to accommodate folks from all walks of life, and this has had many surprising benefits.
Many people mistakenly assume that the transmission in heavy trucks is an enlarged version of those found in cars. Although their principle of operation is the same, big rig transmissions often lack the synchronizers that simplify shifting in cars.
These slider gears are placed between the transmission’s gear sets and enable the driver to engage gears without perfectly matching the RPMs. To use these transmissions, you need a great deal of practice.
Manual truck transmissions also have a separate inbuilt, pneumatic-controlled dual-speed transmission that controls the low and high range. Most of these transmissions will require the driver to start on the low range, shift through all transmission gears before engaging the high range.
Others, however, require the driver to use the range control between the gear-shifts. Their shift pattern goes something like: first low gear, first high gear, second low gear, a second high gear, etc.
An automated manual, which some drivers mistakenly refer to as “automatic,” is internally similar to a standard manual transmission, although it uses a series of computer-operated servos to waive the need for manual shifting.
To easily understand how an automated manual works, imagine a robot seated in the passenger seat doing all the shifting for you.
Since they always shift at the precise RPM and don’t grind the gears, an automated manual transmission has many advantages over the standard manual. They give you increased acceleration, increased ease of use, better fuel economy, and longer transmission life.
When most people hear the word “automatic,” this is what they think of, and it is used in most cars. Planetary-Gear Automatic (PGA) transmissions are not so common in heavy-vehicle applications. The reason is that PGAs depend on several hydraulically controlled clutches for power transfer.
These clutches are prone to slippage and do not provide a positive engagement like a standard transmission. This is critical for trucks since they depend on engine back-pressure to descend safely on hilly surfaces.
If the internal clutches in the transmission slip while the truck is descending, they may cause an uncontrollable acceleration because of the force of gravity; a dangerous condition known as “runaway,” which contributes to hundreds of deaths every year.
Common Signs And Troubleshooting Of Transmission Problems That Require Immediate Attention
When hauling important cargo while trying to beat a deadline, the worst thing that can happen to you is your rig developing problems. Problems with your transmission can have some of the most costly ramifications.
The good news is that with a little insight and keenness, big rig drivers can recognize warning signs indicating transmission problems early.
As the truck driver, he needs to shift the gears frequently to let the engine run at the most efficient speed. The gears help to divide the speed into semis: the more gear, the narrow powerband.
Ie. In this way, truck drivers could shift to the right gear depending on different terrains and load weight, avoiding the shortage of power and waste of fuel.
Optimize the Dynamic Performance
For manual transmission trucks, every gear has a specified range and fuel distribution. Most trucks are diesel vehicles, with engine speed range 1000-2500 rpm, which is narrower than gasoline cars. More gears help to make full use of engine torque within the truck speed range, optimizing the engine efficiency as well. Drivers normally shift the fittest gear all the way depending on road and load situation. For example, if the truck is in empty load, high-speed gear works better to save fuel and time. When the truck is in full load, the truck could get more power in low-speed gear.
This is another main reason for the design of many gears. The more gears ensure the power engine is always working in the economic speed range. When starting or speeding up the truck, low-speed gears could reduce the engine speed, move in gear shifts and save fuel at the same time.
Multi-gear is also more fuel saving when the truck is running on mountain roads. In the long slope road, 3 gears waste too much while the engine power is insufficient in 4 gears. Also, the truck is fuel-guzzling and not in smooth running in 5 gears. That’s why the trucks add more gears to avoid the disadvantage. More gears help to expand the truck speed range as well.
Smooth gear shifting
The more gears, the narrower speed range, the more smooth shifting between 2 neighbouring gears. Truck drivers could select the best gear depending on exact speed, load and vehicle weight. And it’s easier to control the engine working speed and power as well.
More gears benefit the truck’s dynamic performance, fuel efficiency and driving operability well. It makes the drivers manage the business in the more safe, productive and comfort way.
Drivers won’t use all the gears all of the time; they’ll ‘block shift’ or skip gears to reduce wear and tear on the gearbox. A truck driver with 12 gears is never going to need first gear unless they are heavily loaded and starting uphill, or they need very fine control for manoeuvring. They then are unlikely to use all remaining 11 gears and certainly not while changing down while slowing to a stop. In a six-speed truck, it’s more normal to start in second gear; in a 12-speed truck, you would start in third or fourth if you are empty and perhaps second or third if you are pulling a load.
Starting in a higher gear is much smoother, and it reduces the chance that the wheels will spin as there is a less rotational force (torque) able to be exerted on the tyres.
Unless it’s an automatic gearbox, the driver has to control the gears by listening to the revs and feeling whether the truck is labouring (i.e. will lose speed and eventually stall), or whether the rev range is too high and there’s a risk of engine damage.