Can You Switch to 4-Wheel Drive While Driving: CAUTION!
Most modern 4WDs allow you to safely switch to 4-wheel drive while driving at speeds below 60mph. Alternatively, when switching from 4H to 4-Lo (low-range), it is recommended to slow down to 5mph without the gas pedal depressed and the transmission in the (N) Neutral position. Older 4WDs without automatic locking hubs require you to stop the vehicle completely, exit the vehicle and manually engage the front hubs. Once done can you then engage 4H from inside the cabin?
Let’s now look at when it’s safe to switch to 4WD while driving and which surface conditions are compulsory to do so for a safer more controlled driving experience.
So before we can explain if it’s possible to switch to 4-Wheel Drive while driving and more importantly why it’s so crucial to avoid switching to 4WD in certain driving conditions, we need to take some time, to first, explain what happens to the vehicles drivetrain, transmission, differentials and steering when you switch to 4WD.
It’s important for a 4WD owner to understand his 4×4 system
Let’s look at what happens to the drivetrain first.
The 4×4 Drivetrain
When using a part-time 4WD, driving in 2H mode is suitable for high-traction surfaces like city roads and highways to prevent drive-train issues. In this mode, power is evenly distributed between the rear wheels using an open rear differential.
Switching to 4H connects the front and rear driveshafts, locking the drivetrain with a 50/50 torque split, restricting different speeds between driveshafts.
In contrast, permanent 4WD remains in 4H mode but employs a component like a viscous coupling in the center differential, allowing varied speeds between the front and rear driveshafts during turns. This prevents drivetrain issues and enables safe turning on high-traction surfaces.
Tires Fighting for Grip
Engaging 4H on high-traction surfaces like tarmac or cement pavement causes excessive tire wear due to equal grip demand on front and rear tires. The locked-up drivetrain generates high turning resistance, accelerating tread wear, especially on the outer wheels during turns.
However, on slippery surfaces, using 4H poses no risk as all wheels can slip, preventing potential drivetrain binding. Turning on such surfaces creates a wider turning circle for the front wheels, causing different rotational speeds between the front and rear wheels and consequently affecting the front and rear propeller shafts’ speeds.
Gearbox/Transmission
When powerful torque from the engine travels through the clutch, gearbox, and transmission to the wheels on a surface with good traction, it can cause stiffness and jamming in the gears due to the lack of slip.
Reversing ‘bind-up’ can be achieved when two wheels are on a high-traction surface while the other two are on a slippery one, allowing varied rotation and releasing tension in the transmission. This process restores proper gearbox and transmission functionality.
Differentials
Your truck’s 4WD differential is crucial as it enables wheels on the same axle to rotate at different speeds while receiving power from the engine via the gearbox and transfer case. When turning, the outer wheels need to rotate faster than the inner wheels. The differential’s gear system permits safe, independent rotation of left and right wheels at varying speeds without slippage.
Steering
Selecting 4WD while driving on high-traction surfaces might cause slight understeer, indicating potential drivetrain windup. This condition leads to jammed gears and unpleasant, jerky steering due to the front wheels conflicting with the rotational force from the front driveshaft.
Engaging 4WD on such surfaces risks severe damage to drive-train components and difficulty in returning to 2WD mode.
When Is It 100% SAFE to Switch From 2H to 4WD
The following road surfaces are completely safe to engage 4-wheel-drive.
| Surface | Description |
|---|---|
| Slippery Wet roads | Roads with wet surfaces, prone to slipping |
| Snowy and icy road surfaces | Surfaces covered with snow and ice |
| Loose Sandy tracks | Tracks with loose, sandy surfaces |
| Beach sand dunes | Sand dunes found along beach areas |
| Boggy mud trails | Trails with muddy and boggy conditions |
Let’s look at each condition separately to identify what to expect and if it’s completely safe to engage your 4WD while driving on these road surfaces.
Wet Roads
Living in a region with heavy and frequent rain or a wet winter season might favor owning a full-time 4WD due to its advantages. Among these benefits is the ability to drive in 4H in treacherous conditions, enhancing traction on wet roads, which commonly contribute to numerous accidents annually.
Wet roads, with shallow pools causing aquaplaning, pose risks, especially during bends or sharp corners.
The continuous push and pull of all four wheels in 4WD significantly improve vehicle stability and traction, reducing the chance of losing control.
Snowy Icy Roads
Engaging 4H on low-traction snowy roads optimizes handling and stability by locking front and rear driveshafts, leveraging the surfaces’ sufficient slippage. Despite the enhanced stability, drivers should not disregard physics; it’s not a cue to speed up in a 4WD vehicle. Instead, adjust driving style by lowering speed and adapting driving techniques for safer navigation on snowy roads.
Sand tracks
This setting is ideal for activating your 4WD mode. Not engaging 4H while driving on sand may limit your progress significantly. Loose sand demands maximum traction from your 4WD, and activating it while driving in sand is entirely safe.
While navigating sand, utilizing momentum helps traverse deep, soft sections, and staying alert to changes in sand dune shapes and sizes is crucial.
When driving on soft sand or attempting steep dune climbs, it’s essential to adjust tire pressures before engaging 4WD. Deflating tire pressures is among the primary steps to take before initiating 4WD.
Beach driving
Beach driving shares similarities with sand driving, but extra caution is crucial, particularly due to potential increased vehicle traffic during peak holiday seasons. Vigilance is essential for marine life, beach campers, and swimmers often present on beaches.
Using 4WD on the beach allows staying on firmer, moister sand closer to the shoreline, enabling easier tire movement without sinking. However, it’s vital to maintain distance from the shore and remain vigilant about high tides. Negligence and lack of awareness have caused unsuspecting 4×4 drivers to lose vehicles on beaches. Keeping a tide table as a reference is advisable for safety.
Muddy Tracks
Another perfect driving surface to engage 4WD. You could even switch between 4hi and 4Lo when in these situations. Deep mud is a high-resistance surface and places tremendous strain on your clutch and drivetrain system. With a modern 4WD, you can safely switch from 4Lo to 4Hi while driving, however, you will always need to stop before engaging 4Lo from 4Hi.
Similar to sand, you want to drive in 4H since it offers you stability and you can build momentum as you track through the soft boggy mud. Mud holes can be dangerous and you need to exercise caution when doing so.
Locking The Center Diff With A Permanent 4WD
Locking the center differential of a permanent 4WD while driving on a high traction surface will have the same effect as engaging 4H on a non-permanent 4WD.
If the surface is not slippery you risk causing major damage to your drivetrain and gearbox components.
Always refer to your owner’s manual for instructions on how and when it is appropriate to engage 4WD while driving.
Switching From 4Hi to 4Lo While Driving
Again, consult with your owner’s manual first, but for the sake of this article, we need to advise you never to attempt this. Switching from 4hi to 4Lo while driving is a huge no-no. The “lo” in 4Lo represents the low ratios in the gearbox, that being, the short ratios designed for low speed. They are not designed to be engaged if you are driving at speeds over 10mph.
If you somehow manage to engage, expect a violent reaction that could be very unpleasant and even cause injury to yourself or the occupants.
The low-ratio gears are designed for serious rock crawling situations and challenging driving conditions that require high power outputs at very low controlled speeds. Most manufacturers of modern sophisticated 4WD have incorporated a safety mechanism to prevent drivers from engaging the 4Lo functionality while driving.
In fact, in most 4WD’s you are required to bring the vehicle to a complete stop first, then engage 4Lo, then proceed.
Permanent 4WD vs Part-time 4WD Pros and Cons
A full-time 4WD minimizes concerns about road traction compared to a part-time 4WD system. The latter essentially operates like a 2WD on city roads and highways. When driving on a moderately wet road with medium traction, deciding whether to engage 4WD is a choice you must carefully consider.
Having a full-time 4WD provides the flexibility to navigate various surfaces without the constant need to activate 4WD, as the vehicle remains in 4H mode.
This consistent functionality eliminates worries about potential drivetrain damage. Offering the advantages of both AWD and the ground clearance of a part-time 4WD, a full-time 4WD system strikes a perfect balance between safety and versatility.
Final Thoughts
The initial query raised the safety of shifting to 4WD while driving. The answer hinges on several factors we’ve discussed. For a part-time 4WD, it’s safe to engage 4WD solely on low-traction, slippery surfaces or while driving below 60MPH, as specified in the owner’s manual.
In cases with manual locking hubs, the process involves stopping, exiting the vehicle, locking the front hubs, and then engaging 4H. Fortunately, technological advancements allow skipping this intricate process by simply engaging a transfer box gear lever to 4H or using a straightforward dial rotation.
