In the previous article we looked at the various types of transmissions, but there’s a fair bit more to your drivetrain than just the transmission - here we will have a look at differentials, transfer cases, hubs and transaxles.
Diffs - what is a differential?
When your car turns, the outer wheels actually travel further than the inner wheels, and so need to travel faster. A differential, or simply “diff” facilitates this, by allowing the drive wheels to rotate at different speeds. In rear wheel drive cars, they also change the direction of the rotational force provided by the transmission and driveshaft and send it out to power the wheels. Diffs also act as a final gear ratio reduction before the power passed on by the transmission reaches the wheels, slowing the rotational speed and adding a little more torque one final time. There are a few different types of differential used in cars with each offering benefits and drawbacks, but they can be broken down into two categories - Open, and LSD.
What is an open diff?
The differential described above, is what’s called an open differential, and one of the side effects of its design is that it applies the same amount of torque to each wheel. The drawback to this is that the amount of torque that can be sent to each wheel is limited by both the vehicle’s power output and the traction that the wheel has. In situations where the tyres have a lot of grip, then the amount of torque that can be applied to the ground is only limited by the engine, transmission and the gearing of the car; whereas in a situation where there is limited traction - such as in the wet, or on sand - the most torque that can be applied is limited to the amount that will cause the tyres to lose grip and simply spin. The same principle applies when one of the drive wheels leaves the ground - if one of the wheels is in the air, it will just spin around, and so the other wheel will provide no torque as well, since they always apply the same amount.
The solution to these issues can be to lock the differential, which is a common solution for dedicated off-road vehicles. Some vehicles even come equipped with driver-activated diff locks that essentially force the wheels to move at the same speed no matter what. Locking a diff is advantageous when trying to drive over surfaces that offer very little traction, however the major drawback is that it causes the inner wheel to slip when cornering, therefore making ordinarily driving both unpleasant and dangerous.
Alternatively, both of these problems can be avoided with the use of a special type of diff called an LSD.
What is an LSD?
An LSD is a Limited Slip Differential, and refers to the ability of a differential to reduce the amount of slippage encountered, by providing uneven amounts of torque to each wheel. There are a few different types of LSD, that solve the aforementioned issues encountered with open diffs in a variety of ways - Clutch-type, Viscous and Torsen diffs, as well as complex electronic options have all been designed to do the job. Each type has its benefits and drawbacks, and each is used fairly extensively in different applications.
Clutch type LSD - This is the most common type of LSD and consists of an open diff that has an added set of clutches and a special spring pack that applies force to the gears inside the diff. The basic premise is that when something happens to make one wheel spin faster than the other (such as loss of traction in the wet) then the clutches will try to force both wheels to spin at the same rate. The amount of torque that can be transmitted by the wheel that isn’t slipping is then limited by the friction of the clutches and the strength of the spring pack inside the diff, and the wheel that has no traction can only slip if it can overpower the clutches. These LSDs are the cheapest to produce, since they are entirely mechanical, though the drawback is that the clutch packs and springs can and do wear out. They are also
Viscous - Viscous diffs use two sets of plates that are each connected to one output shaft. Between the plates, there is a thick fluid that usually spins at the same rate as the plates as you drive along. When one set of plates starts to spin faster than the other (if its connected wheel is slipping) then the viscous fluid will try to catch up to the rate that it is spinning at, and as a result, will speed up the other side as well, which will transfer more torque to the wheel that isn’t slipping. The downside of this system is that the viscous coupling will only really pass torque to the wheel that needs it once the other one has already begun to slip to a point that it can spin up the fluid. The solution to this, is to have some sort of way to detect the beginnings of slippage and traction loss and adjust torque preemptively.
Torsen - Enter the Torsen diff. Torsen is short for Torque-Sensing, and as the name implies, is equipped to sense the initial slippage due to the amount of torque that a wheel is applying. Torsen diffs don’t actually use electronics or sensors to do this however. In fact, they are entirely mechanical and feature complex gearing that binds up at specific predetermined ratios in order to provide torque to the wheel that requires it as traction loss begins to occur. The drawback of Torsen diffs is that in situations where traction is completely lost, they are unable to actually provide any torque to the other wheel. This is because Torsen diffs multiply the traction of the slipping wheel, and any multiplication of zero is still zero.
TVDs and ELSDs - TVDs are Torque Vectoring Differentials. They are sophisticated diffs that use complex arrangements of gears and clutches to control torque distribution smoothly and actively. They are a type of ELSD which stands for Electronic Limited Slip Diff. All types of ELSD will use sensors and computer controls to adjust their performance on the fly, and react better to traction loss than simple mechanical diffs can. For this reason, any type of ELSD will usually cost quite a bit more than its mechanical or viscous counterparts.
What is a transfer case?
The transfer case is an integral part of any four wheel or all wheel drive vehicle. It’s job is to transfer the power provided by the transmission to both the front and the rear axles. Some transfer cases allow you to select between two and four wheel drive, as well as locking the axles to provide a similar function to a locked diff, but between the front and rear wheels. Most transfer cases either operate using gears, or are chain driven, with the former being generally more robust and the latter, lighter and quieter.
Gear ranges - What is the difference between 4 high and 4 low? Usually, a transfer case will also be home to an additional set of gears, that provide a low range. This decreases the rotation speed, but vastly increases the available torque and so allows better off road performance. When driving in the higher range, your transfer case will usually keep the ratios provided by the transmission, and apply the same rotation speed and torque to your wheels at the front and rear. Some vehicles, that operate in full-time four wheel drive will have this as their default setting, and will only offer low or high ranges, whilst others will allow you to select between low and high four wheel drive, or simply two wheel drive. Whatever the setup, high range is best for road travel, as it allows for higher speeds, without over-straining the engine and transmission. In low range, the transfer case engages its own set of gears, that reduce the speed that the wheels can turn, and instead provide far more torque to them. This ratio is designed to be useful for intensive off-road driving, though is terrible for normal onroad driving as it greatly reduces the maximum wheel speed that any engine rev range will allow.
What are Locking hubs / Free-wheeling hubs?
Some 4x4s, particularly older ones, come equipped with “free-wheeling” or “locking hubs”. These take the form of a little dial on the front hubs, that simply allow the hubs to be locked to the axles, or to be unlocked and “free-wheel” instead. The purpose of this is that even when four wheel drive is disengaged, the actual axles that connect the wheel hubs to the front differential will still spin as the wheels turn, so unlocking the hubs can reduce wear by eliminating this axle and diff movement. The drawback to this system is that if you wish to go offroad and engage four wheel drive, you’ll need to exit your vehicle and lock the hubs each time, and also remember to always lock both hubs in order to avoid potential damage. Then, once you’re done, you need to remember to unlock them. For this reason, most modern 4x4s either did away with locking hubs, or implemented electronically controlled versions.
Transaxles - What are transaxles?
A transaxle is a component that combines the functions of your transmission, differential, and drive axles all in one. It is primarily used in vehicles that have the engine mounted at the same end of the car as the drive wheels - almost all front wheel drive cars follow this pattern, as do many rear, or mid-engined vehicles. Since it is pretty much a combination of the parts we have previously looked at, it shares a lot of the same features and serves the same purposes. One defining factor is that because of the centralised location of almost all drivetrain components, it is generally cheaper to build and so is favoured for economy vehicles - hence why most manufacturers offer front wheel drive vehicles as their budget option.
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