A semi-trailer truck, commonly known as a semi-truck, big rig, or semi, refers to a truck that comprises a tractor and a semi-trailer. These monsters have a lot of gears and complex transmission systems.
Why do semi-trucks have so many gears? The reason is that the big engines fitted into semis have a very narrow power band, and as a driver, you need to shift the gears frequently to keep the engine running at the most efficient speed. Sometimes these gears have less than 750 revolutions per minute (RPM) between them, and so the trucks need many more gears to operate optimally while carrying heavy loads in many terrains.
A semi-trailer is half supported by its own wheels that support only the rear part of the trailer, while the front of the trailer is supported by the rear wheels of the tractor. Most people often wonder why semis have very many gears, or whether they are necessary. Read on to find out why semi-trucks have many gears.
Why Do Semi Trucks Have So Many Gears?
Most commonly, semi-trucks have 10 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 determine 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. A 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 splits 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-hauler 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.
Manual Transmissions
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.
Automated Manual
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.
Planetary-Gear Automatic
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.
When troubleshooting your semi truck repair needs, keep these symptoms of semi-truck transmission problems in mind.
● Unusual Noises: Although semis are loud machines, it is the responsibility of the driver to distinguish between the sound of power and the sound of a problem. If you hear noises such as grinding, screeching, buzzing, or clicking, you need to have your truck checked by a specialist.
Usually, these sounds show that major components of the transmission system are failing, and these include the clutch release and the pilot bearing. If these parts get damaged completely before you get your rig checked, you may incur additional damage and expenses.
● Inconsistent Shifting: Whether your semi has an automatic or manual transmission, if you detect any strange behavior when shifting gears, you may be in for trouble. Some common signs of inconsistent shifting include slipping gears, inability to get into gear, revving between shifts, and taking too long to shift.
Sticky shifting can result from various problems and may include problems with computer cables or sensors or faulty oxygen sensors.
● Leaking Transmission Fluid: The transmission fluid lubricates the transmission system and helps in preventing heat from building up, which can cause significant problems. Low fluid levels mean there may be insufficient lubrication to allow gear shifting. If your truck has inadequate or dirty transmission fluid, expect considerable rough gear shifting.
However, the greatest problem arises when the heat builds up in the transmission system, causing the gears to go kaput. When the gears expand and contract from extreme heat, they can develop cracks, or the teeth can break. Insufficient fluid can also cause them to break because of the friction between the metallic parts.
A semi-truck is a costly vehicle that critically depends on its extensive gear system to travel over long distances and haul heavy loads over all kinds of terrains. Thus, it is important to understand their transmission systems and why they need so many gears.
Equally important is knowing how to troubleshoot the rig for any common indicators of transmission problems to prevent expensive damage to the transmission system. This information will help you understand much more about semis and their nuances.