Permanent 4WD vs AWD on Dry Pavement

This article discusses the effects of driving on a high traction surface like a dry pavement or a highway while 4-wheel drive mode is engaged. We will be focusing mainly on part-time 4-wheel drive trucks since full-time 4-wheel drive vehicles can safely drive on low and high traction surfaces such as concrete pavements and tarmac roads and highways without causing any drivetrain problems, due to its drivetrain system design. Driving on dry pavement or highways with a part-time 4-wheel drive while the center diff-lock is engaged should be AVOIDED AT ALL COSTS. The reason for this rule is, the drivetrain design of a part-time 4-wheel drive is not intended for high-traction surfaces such as dry pavements but rather for slippery, low traction off-road terrain where traction is limited. Drivetrain binding will result if a part-time 4WD is driven for many miles on twisty highways. Transmission windup occurs due to the front and rear driveshafts not being designed to rotate at dissimilar speeds while the vehicle is turning. Full-time 4WD’s and AWD’s that incorporate viscous couplings or multi-plate clutch systems are able to safely drive on dry pavements and highways. So what happens to your drivetrain when you engage 4WD or 4H on dry pavement, or you forget to disengage 4H after you leave the dirt roads? How long will it take for binding to occur? This is a common occurrence and can easily happen to even the most experienced 4-wheel driver. The next part of this article will go into more detail about the cause and effects of Drivetrain binding as well as how the viscous coupling of a permanent 4-wheel drive functions. We will also discuss what happens when we reverse in 4WD mode on dry pavement. https://4wheeldriveguide.com/biggest-tire-on-stock-toyota-tacoma-with-without-lift/ Driving in 4Hi on Dry Pavement Dry concrete pavement is porous and very grippy So we’ve established that a part-time 4-Wheel drive is not designed to drive on dry pavements or highways for extended periods of time since drivetrain binding will eventually occur. That we understand. So what happens when this phenomenon occurs to your 4WD? Drivetrain binding can result in all sorts of expensive damage from damage to u-joints, yoke failure, driveshaft twisting, and transmission torque build-up causing major damage to the internal gears. Now the explanation might sound contradictory? Simply because, on the one hand, you need to engage 4-wheel drive for improved traction, but on the other hand you need the wheels to be able to lose traction for it to work safely? WHAT? It's for this reason why 4WD's and dry pavements or highways don't gel! This is since the 4-Wheel Drive mode requires the wheels to slip to a small degree, especially while turning, and the elements of concrete pavements are designed to offer optimal grip and traction under most conditions. These driving conditions include steep inclines, declines, wet roads, bends, and more. Concrete is made up of three basic components: water, aggregate (rock, sand, or gravel), and cement. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates. Most 4-Wheel drives are equipped with AT (All-Terrain) tires. The type if Rubber compound used in AT tires is designed to grip well on a variety of surfaces, including highway tarmac and dry concrete. That’s bad news for your 4WD when neither one wants to concede. All-Terrain Tires offer acceptable grip on most surfaces, including wet highways Dry pavement is a surface that is very “grippy”, which is ideal for 2-wheel drives in the form of front and rear wheel propelled vehicles. The concrete compound is even designed to deliver sufficient grip when wet and snowy conditions. This is possible since the concrete surface is not 100 percent smooth but rather coarse and porous, allowing soft compound tires to grip for reasonable levels of traction, even on rainy days. So armed with the above information, it’s reasonable to conclude that a locked center differential of a part-time 4-wheel drive and a high traction dry pavement or highway are not a good combination. Next, let’s look at the effects of driving on the highway in 4 wheel drive. https://4wheeldriveguide.com/biggest-tire-on-stock-toyota-tacoma-with-without-lift/ Driving in 4 Wheel Drive on Highway Driving my Toyota Hilux in 2H on the Highway When driving on the highway, your part-time four-wheel drive should always be in 2H. This means the front driveshaft is disengaged and the front wheels are simply coasting along. Great! No risk of drivetrain binding there. In 2H mode, all the power is being exclusively sent to the rear axle via the rear driveshaft. The transfer box of a part-time 4WD is also not engaged in 2H mode. A part-time 4-wheel drive is designed to be driven in 2H mode on a high traction surface such as a concrete pavement or a tarmac highway. This will return decent mpg figures since the 4WD will now function similar to a rear-wheel-drive vehicle. Where only the rear wheels are being propelled. It's also easier on your tires. Highway Tarmac is designed to offer decent levels of grip under all conditions What compound is the highway tarmac made of? Tarmac is a material that covers the surface of the road or any outdoor area. Tarmac primarily consists of broken stones and tar. It was first patented by English inventor Edgar Purnell Hooley in 1902. Most countries around the world’s highways are covered in tarmac since it offers high traction in a variety of conditions. So when driving on tarmac, the same as when on dry pavement, make sure you are in 2H mode. When is it safe to drive in 4-wheel drive on the highway? There are certain low traction road conditions such as snow-covered tarmac or icy road surfaces that could warrant driving in 4-wheel drive mode on pavement or highway. This decision is purely up to the drivers' discretion since he will need to physically engage 4H. The driver needs to be 100 % sure the surface is slippery enough. Again, if the tarmac is not slick enough there is always the risk of drivetrain binding. As explained before, this is because a part-time 4WD does not have a viscous coupling, like an AWD or a permanent 4WD, that allows varied rotational speeds between the front and rear driveshafts. https://4wheeldriveguide.com/can-you-switch-to-4-wheel-drive-while-driving-best-advice/ AWD Functionality Below is an extract from one of our articles that briefly explains the functionality of an AWD drivetrain. This system is permanently in 4H mode and cannot be changed to 2H where only the rear wheels are propelled. AWD’s do not have Low Range (4Lo) functionality. The AWD drivetrain is usually managed by a computerized system which controls the power split ratios. Power is fed to each wheel and certain sophisticated AWD systems permit power splits to be adjusted on the fly by the driver from inside the cabin i.e. 50/50% or 70/30% split etc.Other AWD systems function automatically and adjust the power splits once the computer senses traction has been lost on either front or rear axles. Manufacturers make use of various torque splitting methods such as torsion splits, viscous coupling or electronic clutch mechanisms, etc. Each has its advantages and disadvantages.extract taken from article: AWD vs 4WD If you want to read more about 4WD vs AWD you can read that article here https://4wheeldriveguide.com/difference-between-4-wheel-drive-and-all-wheel-drive-explained/ Next, let’s look at what physically happens to the drivetrain components when binding occurs. 4-Wheel Drive Drivetrain Binding So let me tell you a short story first. So the exact phenomenon occurred to me when I purchased my first 4-wheel drive. It was one of the most exciting moments of my life. I couldn’t wait to test out my vehicle’s 4-wheel drive system and headed off to the nearest forest track in my area. Everything went fine and I was so impressed with the capabilities of my new 4-wheeler. I tested every single function on the super-select gearbox, and boy does it have options. There’s literally a gearbox selection for every driving scenario, it was such an amazing design. You could even engage the rear diff-locker when the 4H was engaged. How cool is that! Anyway, so I tested it off-road and did my thing, after which I headed back home, driving on the tarmac highway. Everything was going fine, however, I happened to notice the vehicle handling just seemed slightly “tauter”, more “compressed” than before. After taking a few turns, I thought I heard some weird noises emanating from my rear tires. It had chunky BF AT’s fitted. I slowed down a bit and poked my head out the window to try and see what’s up, only to realize, I’d been driving in 4H all the way on the highway. Now this being my first 4-wheel drive and me being caught up in the 4x4 hype, I forgot to disengage the 4H mode. My heart almost sank to my toes. I immediately stopped the vehicle, jumped out, and had a look around. I checked to see if I’ve damaged any drivetrain components or my tires. Strangely enough, everything seemed fine. I disengaged and put it in 2H mode and proceeded to drive off at a very slow speed. The vehicle felt fine, it just felt slightly “tighter” when I drove in 4H. After having a read in my owner’s manual, it was then I realized the super-select box allows for 4H driving under most conditions, and it shouldn’t cause any damage, even when driving on the highway. What a relief! Had it been any other vehicle, I would have ripped a new one in my wallet. Don’t let this happen to you! Remember to disengage 4H when you leave the dirt roads and head back onto the highway. Here is a brief explanation about drivetrain binding 4 Wheel Drive Locks Up When TurningThis effect causes your vehicle to under-steer heavily, gears to get jammed and makes steering very difficult and even jerky. This phenomenon is caused by the front wheels battling the rotational force coming from the front drive shaft as it tries to slow down the front wheels, causing the massive under-steer effect. 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.4WD Binding – How To Fix Transmission Wind-UpIf you have forgotten to take your 4WD out of 4H after heading back on to the tarmac you will definitely experience transmission wind-up after a while. One way to identify if you have transmission windup is, your vehicle being stuck in a gear. You can yank and hang on the gear lever but nothing will release that gear due to the immense forces and torque built up inside the transmission. So how do you fix transmission wind-up? The easiest way to fix bind-up is by pulling over to the side of the road with two wheels firmly on the tarmac and the other 2 wheels on a slippery surface like grass, mud, snow or sand. This allows the wheels to rotate at altered speeds releasing 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.The other option is to reverse in the same direction you were driving. So if you were driving forward in a left direction, reverse in a left direction and allow the wind-up in the transmission to reverse itself naturally.Extract taken from the article: Why Your 4 Wheel Drive Jerks and Feels Hard to turn in 4WD https://4wheeldriveguide.com/why-it-jerks-and-is-hard-to-turn-in-4-wheel-drive-mode/ Permanent 4WD vs AWD on Dry Pavement So we know a permanent 4WD and an AWD can safely drive on dry pavement since they have a special component called a viscous coupling or a multi-plate clutch system. These clever designs allow for varied rotational speeds between front and rear drive-shafts, allowing them to drive on a variety of road surfaces. The main difference between the AWD and a permanent 4WD is the following: The AWD does not have a center differential which can be engaged or left open when needed. The AWD also does not have low-range in the form of a transfer box. The driver also cannot switch off the traction control systems completely, limiting its use off-road. When the AWD is equipped with an electrical multi-plate clutch system, there are clever electronics and wheel sensors that govern and manage the traction of all four wheels. In most cases, this cannot be disabled. A permanent 4-wheel drive, on the other hand, is a more basic design with rugged off-road capabilities. It has low-range gearing and in most cases diff lockers in the rear axles for challenging off-road driving. When driving under normal conditions like a highway or dry pavements the center diff is left open. Both AWD and permanent 4WD can safely be driven on a Dry pavement without risking damage. Can you drive a Jeep Wrangler 4WD on dry Pavements? Jeep Wrangler fitted with All-Terrain tires So you might live in a country that experiences heavy snowfall. Under these conditions, it is highly recommended to engage 4WD on your Jeep for better traction on those slippery roads. However, what if you park your Jeep in 4WD for a few days and forget to disengage 4H the next time you drive it. You’ve now driven your Jeep on the highway, taken a few turns, and drove some more for a few miles with the 4-wheel drive engaged. Should you immediately sell your Jeep to the highest bidder? Don’t panic! Your Jeep should be fine. These things are built tough. The drivetrain components and transmission are not so fragile that they will cause major problems after just a few short miles on dry pavement or a highway. In fact, driving in a straight line should cause minimal impact, but rather, it’s the continued driving in 4H and turning that will slowly build up torque transfer inside the transfer box, and eventually something in your drivetrain will fail. Transmission Binding effects Usually, it’s the front drive-shaft that packs up first, however, you will notice a drastic difference in the handling and especially turning when drivetrain binding has occurred. The Jeep will feel extra clunky, especially when taking tight corners, you will feel the hesitance and the resistance on the steering wheel. The sharper you turn, the worse it gets. Your Jeep will talk to you and give you signals that it’s not happy driving in 4WD on any high traction surfaces. You should hear a few strange noises at first, followed by the inability to turn. Wheel hop is another phenomenon caused by drivetrain binding. It’s all about mechanical stress on the system, wear, and eventually breakage. The bottom line, get to know the behavior of your Jeep well. In the event that you accidentally forget your Jeep in 4WD and drive on the highway or back-up out of your driveway in 4H, you’ll immediately notice a difference in its behavior. Also, It helps to have a quick peek at the dash from time to time to check for any illuminating 4WD lights. Can you back up in 4WD on dry Pavements? When you have a part-time 4WD you need to be aware of the dangers and risks of engaging 4H anywhere other than when driving off-road. So the rules apply exactly the same when backing up in 4WD. You want to avoid backing up or reversing in 4WD on pavement if the ground surface is dry and traction is good. You will experience strain, in the sense that, the vehicle will feel very constricted and hesitate to turn. Steering will feel very taut and understeer will be obvious. You’ll hear tires screeching as they lose grip on the pavement due to the drivetrain's inability to compensate for the differences in rotational speeds between front and rear drive-shafts. These are all signs that the driving surface is not slippery enough and that you probably should disengage 4H. https://4wheeldriveguide.com/reverse-in-4-wheel-drive-without-doing-damage/ Conclusion So we’ve established driving on the highway with a non-permanent 4-wheel drive is not recommended. Full-time 4WD and AWD’s are safe. A few miles on a straight road shouldn’t do much, but don’t make it a habit of leaving your vehicle in 4H on the highway, irrespective of what all the internet “gurus” say. Get to know the behavior of your 4WD so you can quickly identify if anything feels out of the ordinary with its handling and steering. We’ve also established that 4WD mode should be engaged when driving conditions are bad and surface traction is low. These include muddy trails, icy roads, snow roads, sand, and dirt roads. All other driving on highways and concrete surfaces is best done in 2H.