If you would like to know when it is applicable to engage 4-wheel-drive and when it’s not, this article is for you. Also, we will discuss the risks involved in switching from 4H to 4Lo without stopping. This article also covers how to safely engage 4WD and what happens to the vehicle’s drivetrain and other components when you switch to 4H mode.
Most modern non-permanent 4WD’s allow you to safely switch from 2H to 4H while driving below 60mph. Each manufacturer has a specific sequence to follow so it’s beneficial to first read the owner’s manual for the correct instructions and procedure on how to engage your 4WD’s 4H mode. More sophisticated 4WD vehicles are safe to engage 4H while driving whereas older, more traditional 4WD’s without automatic locking hubs involves a more specific procedure that requires 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 absolutely safe to engage your 4WD and which surface conditions are compulsory to do so for a safer more controlled driving experience. Treacherous road surfaces like snowy roads, wet and icy surfaces, as well as soft loose sand amongst others, will be discussed in more detail.
What happens when you switch to 4WD while driving
So before we can explain if its possible to switch to 4WD 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 firstly, explain what actually happens to the vehicles drivetrain, gearbox, differentials and steering when you switch to 4WD.
I think it’s imperative for a 4WD owner to understand the mechanics of his truck to give him a clearer understanding of what occurs when engaging his 4WD system.
Let’s look at what happens to the drivetrain first.
When you drive a part-time 4WD you are always driving in 2H mode. This will mainly be when driving in the city on tarmac highways and cement pavement surfaces. Basically, any high traction surface should be driven in 2H to avoid drive-train windup or “binding”. This setting allows the rear diff to split the power evenly between the two rear wheels by means of gears inside the differential that shifts power accordingly when cornering. This allows the outer wheel to rotate at a faster speed than the inner wheel to prevent “binding”
For the purposes of this article, we are going to assume you are driving a part-time 4WD like a Navara, Ranger or a Tacoma. These type of trucks should always be driven in 2H on the tarmac. When you engage 4WD by selecting 4H from the dial or the short ratio gear lever you are connecting the front and rear driveshafts. Usually, it’s only the rear drive-shaft that is activated in 2H and not the front. The front wheels simply coast along. The front side-shafts are not even actively engaged.
So, in essence, when you select 4H, you are connecting the front and rear drive-shafts together by means of the transmission box, to rotate together as a single unit. The drive-shafts cannot rotate at dissimilar speeds due to the absence of a viscous coupling and power is split 50/50 between the front and rear axles. You have now locked the drivetrain in 4H.
By way of comparison, a permanent 4WD is always in 4H mode but can function safely on high traction surfaces because it has a viscous coupling in the center differential which allows the front and rear driveshafts to rotate at varying speeds when necessary. This component allows the vehicle to turn safely and prevents drive-train “binding”.
When you switch to 4H your front and rear tires are both equally fighting for grip. If you engage 4H on a high traction surface like tarmac or cement pavement, huge pieces of your tires will chip and break off. Your tires’ tread will wear off very quickly due to the high turning resistance generated by the locked-up drive-train. This is because when turning in 4H the 2 inner wheels need to rotate slower than the outer 2 wheels and since it’s driving on a high traction surface, usually the weakest component will take the punishment first, and in this instance, it’s your tires. When engaging 4H on a slippery surface there is no risk since all wheels can slip and release any windup in the process.
When turning, the swiveling of the front wheels creates a wider turning circle than the rear wheels and this causes a difference in the rotational speeds of the front and rear wheels. This, in turn, causes a difference in speeds of the front and rear propeller shafts (Driveshafts)
Since there is a difference of rotational speed between the inner and outer wheels, both axle shafts also need to rotate at varying speeds, to compensate for the difference in speed and distance covered. It’s this variation in speed that causes uneven tire wear between the front and rear wheels if driven on high-traction surfaces. The situation can be aggravated further by varying tire pressure levels between the front and rear wheels.
Moral of the story; if you want to preserve your drivetrain and tires, avoid engaging 4H while driving on a high traction surface, in a part-time 4WD.
So all the power and torque is generated by the engine and sent to the gearbox via a clutch system mounted between the two. Massive amounts of torque is sent through the gearbox gears and through the transmission and finally towards the axles and wheels via the front and rear driveshafts.
If the surface you are traveling on has no “give”, meaning it’s not slippery enough and traction is good enough, the massive amounts of torque generated will be transferred back up the drive train and into the gearbox and transmission case. This will cause your gears to feel very stiff and eventually get jammed. No amount of pressure can be applied from inside the vehicle cabin to remove that gear.
Reversing “bind-up” can be done by pulling your vehicle to the side of the road with two wheels firmly on the tarmac and the other 2 wheels on a slippery surface like grass, snow or sand. You want to create a scenario that allows the two left wheels to rotate at varying speeds to the two right-side wheels. This reversal will release the wind-up in the transmission box. Once the wind-up has been released you will then be able to use your gearbox/transmission properly again.
Your 4WD “diff” is one of the most important components of your truck. The power generated by the engine is sent to the differentials via the gearbox, transfer case and then the drive shafts. The purpose of the “diff” is to allow both wheels on the same axle to turn at varying rotational speeds when receiving power.
As previously explained, when cornering the wheels on the outside needs to rotate at a higher speed to the wheels on the inside of the bend. If the wheels were connected by a solid driveshaft then the wheels will have to slip in order to successfully turn. The clever gear design inside the diff allows the left and right wheels to safely turn at dissimilar speeds.
Another effect caused by selecting 4WD while driving is a slight change in steering feel. The driver could experience slight understeer if the driving surface is not a low-traction surface like tarmac or pavement. This is a sign that you could possibly be setting yourself up for drivetrain windup. Always make sure the surface is slippery enough to warrant 4H.
Avoid engaging 4WD on a high traction surface at all costs
This effect causes gears to get jammed and makes steering unpleasant and even jerky. This sensation is caused by the front wheels clashing with the rotational force transferred from the front driveshaft as it attempts to slow down the front wheels. This phenomenon is what causes the massive under-steer effect.
I’ve said it before and I will say it again, you should avoid engaging 4WD on a high traction surface at all costs. The longer you drive in that mode, you risk serious damage to your drive-train components and you will find it increasingly difficult to remove it from 4WD mode and switch back to 2WD mode.
When is it 100% SAFE to switch from 2H to 4 wheel drive
The following road surfaces are completely safe to engage 4-wheel-drive.
- Slippery Wet roads
- Snowy and icy road surfaces
- Loos Sandy tracks
- Beach sand dunes
- Boggy mud trails
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.
If you live in a part of the world that experiences heavy rain on a regular basis or your winter season is very wet then you most likely would be better off owning a full-time 4WD. There are so many advantages of driving a 4WD such as having the ability to engage 4H when road surfaces become treacherous and traction is reduced due to wet weather conditions. Wet roads pose a serious threat to drivers and are often the cause of thousands of accidents annually. Actually, it’s not so much the wet roads, but rather people’s unsafe driving habits that cause the accidents.
A wet road often has shallow pools which cause the vehicle to aqua-plane, especially if it is located on just the right spot, such as a bend or on a sharp corner. If you are driving in 2H at pace and you unknowingly drive through a shallow puddle on a bend, you stand a pretty good chance of losing control of your vehicle. The beauty about 4WD is all 4 wheels are propelling the vehicle forward and constantly pushing and pulling the vehicle at any given time which dramatically improves the traction and stability of the vehicle.
So, in the event of you finding yourself driving on a very wet road while it’s still raining and surface traction is low, it would be an ideal time to engage your 4wheel drive. But ONLY if traction is very low like when the surface is covered in water and the tires are still able to slip a bit, it will be ok to engage 4H with a semi-permanent 4WD for a while.
However, if its light rain and you drive a semi-permanent 4WD you might want to re-think this option since traction might still be relatively good. You could risk “drive-train windup” and “drive-shaft binding” if traction is still acceptable, not allowing the tires to rotate is different speeds, especially when cornering. You as the driver need to make that call by reading the road surface conditions and deciding if it’s really that necessary to engage 4H with a part-time 4WD.
The driver really needs to exercise caution in this instance and you really need to learn to read road surface conditions when driving a semi-permanent 4WD. A Full-time or permanent 4WD is ideal for this type of driving conditions since it’s always in 4H, thanks to a smart component called a viscous coupling. This component allows the front and rear drive-shafts to rotate varied speeds when needed while sending power to both front and rear axles. I’m starting to like a full-time 4WD more and more.
(I wrote a more comprehensive article about drive-train binding – which you can read here)
Snowy Icy roads
Driving on your 4WD on low-traction snow roads is the ideal opportunity to switch your 4H functionality. This is the perfect opportunity to lock your front and rear drive-shafts for improved handling. Snowy and icy road surfaces offer enough slippage, unlike a wet road that might be slippery at certain spots but not consistently slippery.
When you engage 4H while driving on snowy roads you will feel the vehicle has a more stable feel and almost appears to defy the laws of physics. This should not be an invitation for you to increase your speed since the laws of physics still apply even when driving a 4WD. Adapt your driving style by adjusting your speed accordingly.
When engaging 4WD while driving on snow-filled roads almost gives you the impression the vehicle could fly up any incline with ease. This is when you want to exercise extreme caution by not powering up slippery hills since this is when you could easily break traction causing your wheels to spin out. It’s best to build up some momentum before you hit the hill and allow the inertia to assist you and carry you over. Make sure you time your speed perfectly and before you hit the top allow the inertia to reach the crest of the hill and continue to proceed down slowly.
Most people are under the impression that engaging 4WD improves braking abilities. This is completely false and you always want to increase your following distance when driving in snow or icy roads to five or six seconds. This increased margin of safety will give you a longer stopping distance in the event of an emergency stop.
This is an excellent environment to engage your 4WD mode. In fact, failing to engage 4H while driving on sand could result in you not getting very far. Loose sand requires maximum traction from your 4WD and engaging it while driving in sand is perfectly safe.
When on the sand you want to always use momentum to propel you through deep, soft sand and scan the surface ahead for any major changes in the shape and sizes of sand dunes.
When you are 4-wheel-driving on soft sand or attempting to climb out a steep sand dune, you always want to ensure your tire pressures are deflated accordingly. This is one of the first things you should do before you even engage 4WD.
Beach driving is very similar to the above-mentioned sand driving and most of the rules still apply, however you need to exercise even more caution on beaches since there is always a possibility of more vehicle traffic on the beach during peak holiday seasons. Always be on the lookout for marine animals like turtles, beach campers and swimmers on the beach.
The advantage of driving in 4WD on the beach is you can stick to the firmer sand which is the more moist sand closer to the shoreline. The wet or moist sand is more compact and allows your tires to glide over with ease without digging into the sand.
A word of caution, do not drive too close to the shore and be aware of the high tides. Many unsuspecting 4×4 drivers have lost their vehicles on the beach due to negligence, and lack of awareness. Have a tide table handy as a reference.
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.
(I wrote a complete mud driving how to guide – you can read it here)
When you should NEVER switch to 4WD
As mentioned previously, any high traction surface, for example, cement, tarmac, and concrete are complete no-no’s to engage 4WD. Engaging 4WD while driving on a high traction surface will cause drivetrain binding so exercise extreme caution when doing so. Learn to read driving surfaces well.
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 accomplish this you will experience a violent reaction from your 4WD which could cause injury to vehicle occupants and damage to your drivetrain components.
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.
The beauty of 4A & Super Select
Mitsubishi’s Super select gearbox is exactly that… super! I owned a Pajero a few years ago which had a super-select box and it worked fantastically. There are four modes: 2H – 4H – 4H LC – 4L LC. The best part of it all is, you can switch 4WD modes between 2H and 4H and while driving without any risks of speeds below 60mph/100km/h.
You could drive it in 2H with only the rear wheels propelling the vehicle forward or when traction gets a bit “iffy” you just pop it into 4H – no problem at all. Mitsubishi’s 4H functions like a full-time or permanent 4WD. In fact, you could just leave it in 4H all the time with no risk at all, like a permanent 4WD SUV.
When the going gets a bit rough you simply engage 4H LC which means it’s still in 4Hi mode but the center diff is locked, (Locked Centre) so then it behaves like a part-time 4WD drive that has 4H engaged, it’s amazing!! Just avoid engaging 4H LC on high-traction surfaces like tarmac.
For the serious stuff, you switch to 4L LC which means it’s now in low range, with the center diff locked. What a brilliant system! All 4WD should be built this way in my opinion. There is literally no surface it can’t handle and the risk if drivetrain binding is minimal if even any.
Jeep Wrangler 4A Mode
4A on a Jeep Wrangler works very similarly to the super-select since you can engage 4WD 4A while driving without any risk. The Wrangler has 2H – 4H – 4A – 4Lo. 4A stands for 4WD mode Auto. Meaning the vehicle is now in 4H mode and will take care of things automatically, you can just leave it in 4A mode even on high traction surfaces, no problem… Brilliant! It takes some of the guesswork out for the driver and the risk of damage is minimized considerably. It can function like a 4-wheel-drive and an AWD (All Wheel Drive)
The beauty of permanent 4WD
I’m starting to think Full-time 4WD is one level up from semi-permanent 4WD.
Why do I say so? Well, you get all the benefits of 4WD…. ALL THE TIME, irrespective of the driving surface, low traction or tarmac, no big deal. It just deals with it in its stride. You have the advantage of all four wheels always propelling you forward at any given time. Wet, dry, slippery, good traction, none of these surfaces pose a major threat.
The only area where a full-time 4WD falls short could be in the fuel consumption department, and even that is negligible with modern 4-wheel-drives. I think I’m sold on this concept!
Permanent 4WD vs part-time 4WD pros and cons
With a full-time 4WD, you never have to worry about road surface traction as much as a part-time 4WD. Reality is, a part-time 4WD basically functions like a 2WD in the city and on the freeway. Do you as the driver necessarily want to take the risk of engaging your 4WD while driving on a say, a wet road, with medium traction? You need to make that decision if it’s warranted.
A full-time 4WD affords you that luxury to drive on a mixed variety of surfaces. You seldom need to concern yourself with engaging 4WD on a wet road surface since your vehicle is always in 4H mode. It’s quite advantages to have this functionality always active and never have to worry about causing costly damage to your drivetrain system. A full-time 4WD functions like an AWD but also has the ground clearance of a part-time 4WD. It’s the perfect balance between safety and versatility.
So the original question was, is it safe to switch to 4WD while driving. The actual answer is… well, it depends, however, we’ve covered a few variables in this article to help us derive to a conclusion, which is: If you drive a part-time 4WD it is only safe to engage 4WD if the road surface is low traction and slippery enough. It’s also safe to engage 4WD if you are traveling below 60MPH or as specified by your vehicle manufacturer in the owner’s manual.
If you have manual locking hubs, you will be required to stop, exit the vehicle, lock the front hubs, and then finally engage 4H. Fortunately, technology has afforded us to now skip this tedious procedure by just engaging either a transfer box gear lever to 4H or even easier, rotating a small dial.
No stopping required!