A drivetrain is needed in automobiles to propel the vehicle; otherwise, it would just be a horse-drawn buggy. Several main components make up the drivetrain, one of these being the automatic transmission, which is our usual topic of discussion. Each of these components affects the driveability and performance of the transmission in some way. We will discuss the component that distributes power to all the drive wheels.
Tying into our article discussing overall tire size, we will cover 4WD systems and how they could affect vehicle operation. But first, we must understand how these systems work.
Vehicles today use various four-wheel-drive (4WD) and all-wheel-drive (AWD) systems to enhance traction, particularly in off-road, snowy, or slippery conditions. These systems are categorized based on their operation and the driver’s input level.
PART-TIME FOUR-WHEEL DRIVE (4WD) SYSTEMS
These are commonly used in trucks and off-road vehicles because of their rugged and reliable construction. This system is best suited for offroad or low-traction surfaces because power is distributed equally between the front and rear axles through the transfer case. This system often offers a selectable two-speed transfer case with high range (direct ratio) and low range (gear reduction) for a mechanical advantage when certain conditions demand it.
The drawbacks to this system are:
- They require input from the driver. Usually, this is done by moving a shift lever or electronic switch to lock the transfer case (Figure 1).
- Some systems require exiting the vehicle to engage locking hubs at the front wheels in addition to switching the 4WD on (Figure 2).
- Their rugged construction often comes from using heavier less efficient rotating components that increase fuel consumption.
- Equal power distribution when engaged causes binding while turning on dry pavement. This binding often causes driveline damage if prolonged.
FULL-TIME 4WD/ALL-WHEEL DRIVE (AWD)
Common in crossover SUVs and compact cars, AWD systems are designed primarily for on-road use and offer enhanced stability in inclement weather or light off-road situations. It distributes power to all four wheels with minimal driver input.
Power is continuously sent to all four wheels through a viscous or electronically controlled coupler that allows the front and rear axles to rotate at different speeds. This enables smoother handling on dry pavement while delivering power to all drive wheels in slippery conditions.
A variant of full-time AWD is on-demand AWD, which operates primarily in two-wheel drive, favoring the front or rear depending on the design. Using speed sensors along with various other yaw, and g-force sensors, the system detects wheel slippage and sideways vehicle motion, then engages the other drive wheels to distribute power as needed.
Performance-oriented AWD found in sports cars and high-performance SUVs offer dynamic handling by varying power distribution between individual wheels in conjunction with the braking system, often using torque vectoring technology. Depending on the driver’s steering input, this helps the vehicle turn or straighten itself in all conditions.
The drawback to most AWD systems is that they are constructed with small, lightweight components and are not built to withstand heavy towing or off-road rigors.
ADVANCED 4WD SYSTEMS
Many modern vehicles incorporate electronic controls that allow automatic engagement of 4WD based on traction needs. This blends the benefits of both part-time 4WD and full-time 4WD systems mentioned above. Features like the ability to select 2WD and low-range gearing allow the vehicle to climb steep inclines while still conserving fuel when 4WD is not in use. It also allows the driver to select 4WD auto and allow the vehicle to power all four wheels as needed in low traction situations without binding when turning.
This mode is especially helpful in snowy climates because the system adapts to road conditions that can change from dry pavement to snow and ice rapidly. The switch in Figure 3 shows all the options the diver may select with this system to suit their needs best. Each system has distinct advantages tailored to specific driving environments and purposes. Understanding their functionality helps drivers choose the right vehicle for their needs.
Key differences
- 4WD is better for rugged terrain and heavy-duty applications, offering driver-selectable modes and more durability.
- AWD provides seamless operation optimized for varying road conditions and is user-friendly for everyday driving.
CASE STUDY
While working on a 2004 Jeep Grand Cherokee with Quadra-Drive, an “all-time 4WD,” a vibration was observed while driving at highway speed. The vibration felt almost like a clutch shudder in the transmission. However, the shudder did not go away or change in nature when disabling transmission lock-up or selecting other gears.
The vehicle was brought back into the shop and placed on a lift for inspection. Having prior experience with the system, the fluids were inspected and found to have a burnt smell, but otherwise looked good. Looking at the tire size, everything looked okay. They were all the same size. However, when looking at the tire brand, it was different from front to rear, and there was a distinct difference in the tread design.
A call was placed to the customer, and they confirmed the two front tires had been replaced because they were worn and had failed inspection, but the rears still had enough tread to pass, so they were not replaced.
A quick rollout measurement found that the tire circumference front to rear varied over 1.25”. This was an extreme amount of variance; most AWD systems allow up to 1/4” variance. What happened was the customer was trying to save money by using an off-brand tire to replace the worn front tires, which were a little taller than the stock OE tires.
Compounding this was that the rear tires were worn, making the size differential even greater. The fix for this was replacing the rear tires with the same size and brand as the front tires and replacing the transfer case fluid for good measure.
How do we measure rollout (circumference)?
You will need:
- 4 Plumb bobs on a string ($2.99ea at your favorite cheap tool store, Figure 4)
- Tape measure
- Masking tape
- Sharpie
Start with the vehicle wheels pointed straight, on a flat, even surface. Make sure the surface is long enough to roll the vehicle forward far enough for the tire to rotate at least once (Figure 5).
For the most accurate results, make sure the tire’s air pressure is set to manufacturer specifications before taking these measurements.
*Note for steps 1-4: the vehicle must remain stationary.
- Place a piece of tape on the floor at the bottom-most section of all four tires.
- Place a piece of tape on the outside diameter of each tire.
- Draw a line on the tape on the floor and the tape on the tire that lines up with each other.
- Hang one plumb bob string at each of the four wheels with string length so the plumb bob reaches just above the ground. The string must hang across the wheel center like in Figure 6. Tape may be used to hang the string from the fender and achieve this alignment.
- Now, roll the vehicle forward so the tire has completed one full revolution and the line marked on the tire lines up with the plumb bob.
- Place a piece of tape on the floor at each plumb bob and mark where the plumb bob points (Figure 6)
- Now, measure between the two marks you made for each tire. This will give you the distance each tire traveled in one revolution. This measurement is the circumference of the tire. All four tires should be within 1/4.” of each other for most AWD/4WD systems.
An alternative to measuring rollout would be using a tire stagger gauge. It is slightly less precise but an acceptable method. It can be done with the vehicle on a lift and doesn’t require as much setup to make the measurement. You are measuring the diameter of each tire by simply placing the tool over the outside diameter. Now, you compare the readings, looking for a difference higher than .” in circumference. The tool likely comes with markings or a chart to convert diameter into circumference, but if it doesn’t, think back to geometry class (dia. x 3.14 (pi) = circumference).
SUMMARY
Not all 4WD/AWD systems are created equal. With a better understanding of how they work, we can properly diagnose and distinguish when one is operating correctly or when it needs servicing. We also stress the importance of properly sized tires for the driveline to work smoothly and how to measure them correctly.
