What is 4WD binding (windup)?
To understand the 4WD binding better, let's consider an example of a typical 4WD pickup truck without a center differential. Traditionally, a 4WD system based on
2007 Dodge RAM with Part-Time 4WD (NV243 transfer case).
a rear-wheel drive with a simple locking two-speed transfer case as in this truck is called Part-Time 4WD
. Society of Automotive Engineers or SAE defines this 4WD system as Part-time AWD
. According to SAE definition, a Part-time AWD requires driver intervention to rigidly couple and decouple primary and secondary axles. SAE is a global organization that develops standards for automotive engineers. To avoid a confusion, we will use the traditional term Part-time 4WD, but Part-time AWD means the same thing.
An old-school part-time 4WD system typically has a 4WD High and 4WD Low modes. The 4WD Low means low range.
Part-time 4WD binding.
When 4WD is engaged in either of these modes, a transfer case 3 (see the diagram) rigidly locks the front (1) and rear (2) driveshafts (prop shafts), so both front and rear axles can only rotate at the same speed.
When the vehicle is turning, the front and rear wheels rotate at different speeds. The size of a blue arrow at each wheel indicates the speed of the wheel in the diagram. Why does this happen? In turn, front tires follow a larger turning radius and therefore have to cover the greater distance. In the case of our Part-time 4WD vehicle, this means that the front axle, as well as the front driveshaft 1 (in the diagram) will want to rotate faster than the rear axle and the rear driveshaft 2. Because the transfer case 3 is locked, this will create tension in the 4WD system, as front and rear axles are essentially working against each other.
It's not a problem when driving off-road, on a dirt trail or snow, because the wheels can spin easily on a slippery surface relieving the tension in the 4WD system.
However, if the transfer case is locked when turning on a dry pavement, it puts a lot of stress on 4WD components, causing driveline shudder, popping cracking, possibly wheel screeching and other noises. This is what's called 4WD binding
or driveline windup
or "crow hop". The 4WD binding can cause increased wear of the 4WD system components (transfer case chain, diveshafts, U-joints, etc.) and tires.
If the tire pressure varies between tires, the 4WD binding with a locked transfer case (or differential) can happen even when driving straight. This happens for the same reason: a tire with a lower pressure has a smaller diameter and therefore rotates faster than the tire with higher pressure.
Adjusting tire pressure
Using tires of different size or mixing new and completely worn-out tires will cause the same effect. That's why experts recommend using the same size tires and keeping the tire pressure up to specs in any 4WD vehicle. The difference in tire pressure or tire size can cause various problems in many vehicles when driving in 4WD Auto mode. For example, in our friend's BMW X3 AWD we fixed the annoying jolt when coming to a stop by adjusting the tire pressure up to specs. In this vehicle, the specs for front and rear tire pressure were different.
For the reasons discussed above, the part-time 4WD system should only be locked on slippery surfaces. The 4LO or 4HI modes should be disengaged when driving on a dry pavement.
For example, the owner's manual for the 2007 Dodge RAM (NV243 transfer case, Part-Time 4WD) says: The 4HI and 4LO positions are
designed for loose, slippery road surfaces only.
Driving in the 4HI and 4LO positions on dry hard surfaced roads may cause increased tire wear and damage to the driveline components
Turning in a Full-time AWD vehicle with a center differential.
4WD Binding in Full-Time AWD vehicles.
By SAE definition, a Full-Time AWD
vehicle is the one where front and rear axles are driven at all times though a center differential
. Essentially, if it has a center differential, it's a full-time AWD vehicle. A center differential (CD in the diagram) can transfer torque to front and rear axles, while allowing the front (1) and rear (2) driveshafts to rotate at different speeds.
Does 4WD binding happen in AWD vehicles with a center differential? Yes, the driveline can still bind if a center differential is locked
when driving on a dry pavement. For example, the owner's manual for the 2019 Toyota Land Cruiser that has a locking center differential says: For normal driving on dry and hard surface roads, unlock the center differential.
So, in this type of a full-time AWD vehicle, you can drive it on a dry pavement as long as the center differential as well as front and read differentials are not locked.
Binding with a locked rear or front differential.
Can driving with a locked rear (or front) differential cause binding? Yes, for the same reason: in turns the left and right wheels of the same axle turn at different speeds, so driving with a locked front or rear differential on a dry pavement will cause binding of that axle. The owner's manual of the 2010 Toyota FJ Cruiser
, for example, recommends using the rear differential lock only when wheel spinning occurs in a ditch or on a slippery or rugged surface
Binding with a 4WD turned off.
In some vehicles a problem with a transfer case components or control circuit could cause the 4WD system to stick in 4WD mode or to engage it by itself. Symptoms of this problem are similar to 4WD binding. If this happens, have your vehicle checked out. If the 4WD mechanism simply stuck, it might help to drive slowly in reverse and then forward to release it. Adjusting the tire pressure to the specs also might help in some cases.
In which 4WD modes can you drive on a dry pavement?
There is no simple answer to this question because there are 3 types of AWD systems according to SAE definitions plus there is a variety of modes and options in each type.
On top of that, manufacturers often use marketing names instead of engineering terms. What is a Locking Center Differential in one car, in another vehicle could be called Intelligent Rugged Terrain Handling Control System or something like.
The best way is to check the owner's manual. For example, which mode can you use on a dry pavement in the same Dodge RAM truck?
Driving in a 4WD Auto mode.
The 2007 Dodge RAM we mentioned above, comes with an optional on-demand active transfer case (NV246) that allows driving in 4WD AUTO mode on dry pavement. The system automatically "pre-engages" the front axle when switched into 4WD Auto mode, but sends the torque to the front axle only when the slip is detected, using an electric motor actuator and a clutch pack. The owner's manual for this truck says that the Electronically Shifted transfer case is designed to be driven in 2WD or 4WD AUTO for normal street and highway conditions (dry hard surfaced roads). However, in an 4WD AUTO mode, the fuel economy will be lower.