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What is the “Middle Lock” and Its Importance in Off-Road Vehicles?

Our topic today is “Off Road Intermediate Lock”

From the comments made to my article in which I examined the relationship between off-road and differential , I realized that there is a conceptual confusion about the differential middle lock . I will try to make this matter as clear as possible.

4×4 systems are not systems that send equal power to all tires simultaneously . Motor power is like electric current. That is, there is a tendency to flow in the direction of the least resistance. The part that produces the power is the engine (engine) , the part that transfers it is the transmission , the part that distributes it in the 1st stage is the off road transmission case (transfer case), the off road accessory that distributes it in the 2nd stage is the differentials .

Let us note that there is an order based on producing – transferring – distributing . Off-road transmission and differentials are the most important elements in the vehicle’s walking or staying power.

If there is no limiting factor for the division of power at these power distribution points, it will determine how much of the power goes to which side and how much resistance is on which side. Then let’s first determine what is the element that creates the resistance, which will also answer the question of what is off-road . The main factor that creates resistance is the tire’s road holding ability. If the tire can hold the road firmly, it will show high resistance. If the tire cannot hold the road, it will show low resistance or even no resistance. That differential will also absorb all the power coming on the land gearbox to which it is connected with the shaft. This means that in a vehicle that is running without any lock activated, the loss of traction of even one wheel will make it impossible for the vehicle to move. This is a 4×1 situation and all the power can be transferred to only one wheel, and the vehicle cannot walk because that wheel cannot hold the road. In order to eliminate this situation, mechanisms that limit the division of power at the power distribution points (terrain transmission and differentials), which we call “locks”, have been developed. in the power distribution stage, that is, in the field gearbox “middle lock” refers to the one in the second power distribution stage, i.e. the differential “differential lock” When the middle lock is engaged, 50% of the engine power will be transferred to the front and 50% to the rear automatically. In this case, if the front two wheels or the rear two wheels can provide traction, the vehicle will be able to continue walking. In other words, we can roughly call this situation a 4×2 situation. However, if each of the front and rear wheels lose traction, this time the front wheel that loses 50% of all the power that comes to the front, the rear wheel that loses all of the 50% of the power that comes to the rear will absorb 100% of the engine power. it will still flow to the wheels that have lost traction. This is what we usually refer to as crossover. It rarely occurs on tires on the same side. When this situation occurs, the rear lock will be activated. When the rear lock is engaged, each of the rear wheels will equally share 25% of the engine power. And if the traction provided by the wheel is sufficient to drive the vehicle, the vehicle will walk. We can roughly call this situation the 4×3 situation. The next stage is when the front lock is also activated and the vehicle is a real 4×4. Since all locks are engaged, 25% of the engine power will be transferred to all wheels simultaneously. And if even one wheel is in a position to hold the road, the vehicle will be able to walk. We call this situation roughly being a real 4×4. The center lock function can be performed using different methods from brand to brand or model to model. Detailed information is available for different applications on the internet. Again, I will make my own classification independent of the literature.

1-) Power distribution systems with a differential mechanism in the middle: In these systems, power is distributed to the front shaft and rear shaft with a structure similar to the classical differential we know. In the open position, power is transferred to the front and rear shafts, while differential rotation of the shafts is not limited. When locked, this power distribution takes place with a fixed shaft rotation to the front and back. As far as I know, there is this type of transmission in Defender. See. picture 1

2-) The systems that transmit constant power to the land transmission: In these systems, the land transmission is in itself; 2H: High speed two wheel drive4H: High speed four wheel drive unlocked4HLc : High speed four wheel locked 4LLc : Low speed four wheel locked mode. Not all of these modes are available on all terrain transmissions. Since they are systems that provide a fully mechanical power distribution and transmission, both equal power and equal turns are provided to the front and rear shaft in 4HLc and 4LLc modes. This is one of the main differences between AWD and 4×4 vehicles. Philosophically, I will talk about the difference between AWD and 4×4 in a future article. See. Picture 2,3

Picture 2
Picture 3

3-) Cross-country transmission systems with variable power transmission: These systems are systems that can do more than just uniform power transmission with open or locked logic. They are systems that can decide how much power will be transferred to which shaft by processing the information (ABS, speed, steering angle, traction force, lateral acceleration rate, etc.) It can even transfer almost all of the power coming from the engine to a single tire, with systems that provide similar service in differentials. These systems are generally seen with different names in AWD vehicles rather than the real 4×4’s we are used to seeing in the field. See. Picture 4.5

  • Picture 4
  • Picture 5

All of the 2H, 4H, 4HLc, 4LLc traction modes that I mentioned above are not available in all terrain transmissions. For example, Jeep Cherokee KJ has all 2H, 4H, 4HLc, 4LLc modes. (This vehicle can be used 4×2 on asphalt, 4×4 can be used both on asphalt and off-road Jeep Grand Cherokee WJ has only 4H, 4LLc modes. 4LLc modes are available. is that it causes it to spin in different rounds. This is not the case. In normal road conditions, although a cross country transmission transmits 60% power to the rear shaft and 40% to the front shaft, the shafts will rotate in the same revolution. The ratio of the transmitted power and the amount of rotation of the shaft are different concepts . When the middle lock is on, the cause of the squeezing when cornering the vehicle is not the power transferred to the front and the rear at a different rate, but the inability to meet the different lap tendencies of the tires due to the lock. This may lead to premature wear and even breakage. Therefore, the basic rule is ; middle lock should only be activated in off-road conditions and on slippery surfaces.One of the factors that may affect the vehicle selection decision is the situation in which the middle lock of the off-road vehicles can be activated. For example, if the middle lock can only be activated when the middle lock is activated at low level, inadequacies may occur in certain conditions on the field. The most important parameter of a successful off-road driving is not only the middle lock and differential locks. Driving techniques depending on the track conditions are also important. For example, on tracks where the ground is very slippery with a short transition, if the slippery area is not entered quickly enough, the vehicle may remain despite all the locks. A vehicle without any locks can come and go at the appropriate speed. Or, if we put the vehicle on Low to activate the middle lock on a track where we can walk and pass by pressing the gas, no matter how much we press the gas, the tires will not be able to reach the speed to get us out of where we are. Conclusion : A vehicle that can be activated with the middle lock function in 4HLc, ie high speed 4×4, will be more desirable in the field.

Best Regards, 03/03/2020 Süleyman AtılganContact: Facebook & Instagram

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