Understanding when it is acceptable to drive fast in 4WD (4H), as well as the maximum speed you should drive in 4H, is of utmost importance to your safety as well as the mechanical preservation of your 4WD. There are certain situations and driving conditions that allow you the freedom to drive at certain speeds in 4H. Then there are other situations where you need to exercise extreme caution. This article will discuss the dangers of driving fast while in 4WD in certain conditions, also the function of 4H and 4-lo, and how it affects the driving dynamics of a 4WD.
How fast can you drive in 4WD high-range? It is not recommended to drive faster than 55MPH in 4H on low traction surfaces. Driving in 4WD-Lo should not exceed speeds of 10mph. If you can safely drive faster than 10mph in 4WD-Lo, it is advisable to switch to 4WD-High.
So now that we know the maximum recommended speed to drive in 4WD (4H) is 55MPH and 4-low is 10mph, let’s now look at a few other factors, for example, what happens to the drivetrain of the 4WD when we engage 4H and when driving fast in 4WD is not always a good idea.
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Driving Fast in 4WD-High
To reiterate, when we engage 4WD we should always be on a low traction surface. This is very important to remember since there is always the risk of drivetrain “binding” if we don’t adhere to this rule. With that being said, a low-traction surface could be a dirt road, sand, grass, a muddy track, or a slippery snow highway. These are all perfect driving conditions for 4H. 55MPH and below is suitable under most of the above-mentioned conditions.
There will, however, be certain road surfaces and environments, such as wide dirt roads that we might feel comfortable driving in excess of 55mph. Always exercise caution since there are always factors that are outside our control that we need to be aware of. For example:
- wild animals running in front of the vehicle
- Tire burst
- Sudden bends in the road
- Uneven road surfaces
- Deep holes camouflaged by snow, leaves, or other debris
By driving over 55MPH on these low traction surfaces with the above-mentioned risks, we should ask ourselves, will we have sufficient time to react and stop the vehicle or slow down enough to avoid any possible obstacles or collisions?
Let’s now look at what driving surfaces warrant engaging 4WD-4H.
Where Should You Drive in 4H
When we venture outdoors on our 4WD trips, we are still required to cover large distances on city highways before we reach the beginning of our adventure. If you are driving a part-time 4WD you will no doubt be in 2H mode on a tarmac or cement driving surface with good traction. This is essential to prevent drive-train binding.
Read More About Drivetrain Binding Here
When we reach the dirt roads and off-road mountain tracks we immediately engage 4H. Unless you have a permanent 4WD, then you simply proceed as normal since you already are in 4H all the time. Alternatively, if your vehicle is equipped with a mode modern 4WD system that has 4H-Auto then this would be a good time to engage 4A.
When your vehicle is in High-range 4H, the engine torque is sent to both the front and rear axles. The torque is, in turn, split 50/50 between each wheel on that axle, which means each wheel receives 25% of the engine’s total power. This makes the vehicle handle more stable and grip a lot better since more traction is gained by all 4 wheels propelling the vehicle forward. You gotta love 4WD!!!!
Examples of low-traction surfaces include:
- Slippery icy roads
- Snowy conditions
- Sand tracks
- Muddy tracks
- Farm roads
- Wet rocky surfaces
- Grassy surfaces
How Driving Fast in 4H Affects Braking
When driving on a slippery low traction surface, the braking distance on a vehicle is drastically increased. This means you need to seriously adjust your driving style to compensate for the extended stopping time. The stopping distance on a low traction surface can increase by as much as double (2x) that of regular braking distances, such as when driving on a tarmac surface.
The 3 key elements that make up the braking distance are the following:
- Reaction time
- Stopping time
So when driving on a low traction surface such as dirt, ice, or snow your reaction time combined with the stopping time is increased by ten. If that’s not a sobering fact, then I don’t know what is!
How Driving Fast in 4H Affects Cornering
When cornering on a low traction surface we need to consider the dynamic of the vehicle and the changes it undergoes in these conditions. Elements like cornering, braking, and acceleration are all negatively affected and we need to be aware of and prepared for that.
When cornering in 4WD on a slippery surface we want to avoid sudden braking and aggressive cornering. The vehicle can easily lose traction and veer off the road, resulting in an accident. Also, practice progressive braking where you gradually apply lighter pressure from further back to slow the vehicle down before the bend. Avoid braking in the bend is also a key factor since the weight will be shifted and traction is lost easier.
Keep the steering motion smooth and controlled and avoid sudden jerky motions.
How Driving Fast in 4H Affects the Handling
If you are driving fast on a low traction surface the handling of a 4WD or any vehicle is drastically reduced. The steering is no longer crisp and direct. If you feel the vehicle not responding well while turning, In most cases it means traction is low, hence the understeering effect.
A 4WD can easily drift in a corner when surface traction is low, and if the 4WD is not in responsible hands, an accident is likely to happen.
Again, avoid sudden jerky motions on slippery roads since this will negatively affect handling. Progressive braking combined with slower turning and easier throttle pressure is all safer driving practices.
The Function of 4WD Low-Range
The “Lo” on 4-Lo represents the low-range gearing that has been engaged. This means the ratios of the gearbox combined with the transfer case are extremely reduced and only designed to be used in extreme driving conditions.
Most off-road 4×4 vehicles have a 2H, 4H, and 4-Lo setting. When 4-Lo is engaged it means the 4WD power and torque are more readily available in the sense that, the revs will almost always be in the powerband. Stalling is virtually impossible and many modern 4WDs are equipped with anti-stall devices which allow you to almost stop in gear without clutch control on a manual 4WD.
When in 4-Lo mode the 4WD can more easily drive on challenging terrain without placing strain on the gearbox, clutch, and engine. Challenging terrain could be:
- Loose deep beach sand
- Thick mud
- Rock crawling
- Steep uneven low-traction Inclines
- Long technical declines
- Deep River crossings.
The function of 4-Lo is to provide maximum power and traction under very challenging driving conditions. This is only to be used when driving conditions do not allow you to drive over 10MPH. If you fear damaging your 4WD, rather engage 4WD and take it slow and steady. If you are doing any sort of rock crawling where you are required to negotiate each obstacle precisely and choose a driving line, will be an ideal time for 4-Lo. Driving through thick loose beach sand with a load will require 4-Lo to prevent damage to your vehicle’s clutch or to decrease the risk of overheating the gearbox if you are driving an automatic 4WD vehicle.
As a 4WD driver who drives challenging terrain, it’s always important to know when to engage 4H and 4Lo. Knowing how fast to drive in 4WD is essential to preventing serious damage to your truck, your wallet, and your ego. Do not attempt to drive over 55-60mph when in 4WD mode irrespective of the driving conditions. Remember, the vehicle dynamics are very different on low traction surfaces so adjust your driving style.
Stopping distance, cornering dynamics and acceleration are all affected when driving low traction surfaces, so make sure you give yourself enough time to react.