# That’s what metrics are the most important for SUV

## How jeep manages to be passable?

**Allowing the vehicle to resist and to crawl forward on the steep slopes, why drivers pereobuvatsya their car to increase the transmission power on the ground during traction with the road, what is the coefficient of the scan and what it means to SUV?**

The purpose of enthusiasts to create a car with maximum maneuverability. The most favorite term today, to the point and not used by the manufacturers of the Jeep is the notorious “Ratio scan”. Any advertising of the car says that the new jeep has a percentage threshold 73:1, and that is why it is the best SUV since army Willys MB.

But in fact, the bypass ratio has a minimum value when overcoming obstacles or driving on mountain passes. More importantly for off-road qualities of the car is how much the tire tread pressure on the ground surface.

For example, off-road “talent” Jeep Wrangler Rubicon are defined in the first gear ratio of low range in the transfer case. Only in these jeeps it is increased to 4.0, while the serial model, this figure rests at 2.7. Such transmission plus a powerful axle Dana 44 eliminates any question of patency of “Rubicon”. The motor will pull the wheels even in the cooling concrete.

Of course, high rates of bypass will help to increase the strength of the car, but there is also the tyre size, the speed and torque of the engine. All this allows the jeep to climb rocks and easy to move in swamps. So, what does any SUV.

## 1. “The coefficient of accelerated motion”

The so-called “coefficient of accelerated motion” is the ratio of torque to the wheels to the same setting on the engine flywheel. Practically, this means, how many times will increase the revolution of the wheel depending on KMD. There are three car node where the rotation of the wheel will increase — these units allow one to determine the ratio of slow-motion transmission, gearbox and differential.

Engine power goes through the transmission, transfer case and differential. That is why the differential is often called the “final drive”. Power from motor is constant parameter. The driver can obtain from the motor more energy is just rearranging the transmission, but can provide more torque.

As soon as the power of “moves” on the main road nodes, angular velocity, torque changing. Every second, when the power of the motor passes through the reduction gearbox, and differential razdatki, it turns out that the output speed is lower than the input, but the output torque of the above. How to determine how much more KM on the wheels than at the flywheel? KMD multiplying the numbers in three gear — this is the ratio of accelerated motion.

Jeep Cherokee with automatic gearbox (4-speed AW) with the transmission parameters of 2.80, 1.53 m, of 1.00 and 0.75. It completed razdatkoy NP231 with a reduction in “low range” of 2.72:1 and a 3.55 axle.

To calculate the “coefficient of accelerated motion”, you should multiply the number of the first transmission ratio low range transmission and the attitude axis. The resulting number in this case 27.

Latest new Wrangler Rubicon with manual control, a gear ratio of 4.46 razdatkoy 4:1, with 4.10 axle ratio gets accelerated motion 73.

### The value of the time idling

But the odds for the indicators of overcoming obstacles and speed parameters of the machine means very little. You need to understand what is actually torque multiplied by a factor of accelerated motion to find out how the machine will overcome the boulders and other obstacles.

27 coefficient, which was calculated for Jeep Cherokee XJ, it is quite acceptable, since the engine 4 liter gives enough torque at idle (200 lb-ft). These parameters allow the car to slowly but surely crawl over the rocks. If the car has a 2.5 – liter engine, the ratio of “creep” would lead to the possible overturning of the machine and the impossibility to overcome the mountain passes. That is why all the four cylinder XJ’s come with a 4.10 differential gears.

So torque is important for high permeability of the car, but that’s not all. Tires also play a significant role.

## 2. Tires

The transition from “skinny” 30-inch knobby to 37-inch tires helped to create the perfect balance between ground clearance and traction, which gives the jeep an optimum acceleration. Newton’s law, applying to modern cars is that the acceleration of the car equals the force acting on the vehicle divided by its mass.

Roughly speaking, the reason why the car is accelerating is a “pusher” power of the soil, resulting from the force with which the tire presses on the ground. The reaction force of the ground is always equal to the force acting on it.

To determine the force that the tyre exerts on the road is enough to recall that KM (torque) equal to the product of force into distance. The force that the wheel transmits to the ground equals the force of the torque to the wheel radius. Practically, this means that the larger the tire, the less its impact on the earth. Therefore, the acceleration of the car is greatly reduced. This is the answer to the question of drivers who wonder why all of a sudden the car stopped accelerating after “pereobuvki” for wider tires.

This is a rough description of how tires affect off-road qualities of the car. Here are not taken into account other, not less important laws of physics that act on the car: the force of inertia, which is more effect on large tires requires more torque, etc.

## 3. Optimal control

A huge advantage right gear is the ability to overcome obstacles, moving them slowly. This allows the driver better control of the car, and this is the main importance of a large ratio of accelerated motion.

Engines create maximum power in your rpm range. To overcome steep slopes, should ensure that the car high torque at low speeds. In other words, it is necessary that the engine gave maximum power, and the wheel was turned slowly. This jeep must be fully controlled by the driver.

The most complete control of the vehicle is provided with a high digit ratio accelerated motion — it not only allows you to multiply the engine torque, but also to maintain it in the optimal range of revolutions at low speeds.

If the motor vehicle has enough power to slowly overcome obstacles is one thing, but if the car has enough power at idle, this allows better control of the car.

In cars with the “mechanics”, where the wheels are rigidly connected with the power unit, it is important to have the limitation of low speeds. High digit ratio ensure the minimum wheel speed. Wheel will be spinning much slower than the crankshaft — this will allow the car to have a low speed on steep slopes.

So, if jeep installed huge tires, the minimum speed at which you want to overcome obstacles, is high enough, the coefficient of accelerated motion is subject to correction. The drivers know a lot of techniques that you can reduce the speed, e.g. braking using shifting. You have to use the starter to move, to allow the tires to slip and use the grip to the maximum. It’s not really easy drive, especially if the front and sides of the towering 50-meter high cliff.

If the car is equipped with automatic transmission, the minimum speed is reached a lot easier. You can move as slow as you need without stopping the motor due to the correction of the torque Converter. It will cause the engine to run at idle, even with fixed wheels. However, there will be a significant waste of oil and warming up the transmission, but that’s all.

## 4. The jeep on the highway

You should carefully see what speed the car is perfectly overcoming all obstacles of a mountain landscape, show on the road. It is important to remember that the short transmission increases KM on wheels, will provide acceleration, but will reduce the settings of the maximum speed.

High ratio of accelerated motion means that the wheel will give a little rpm, while the motor will run at full. In this case, should “shoed” vehicle to properly distribute the force. So always jeep with a huge factor of accelerated motion and larger tires can not show the track at its maximum speed.

High speed transmission always provides the driver with fuel economy, but increases the minimum speed bypass. Larger tires always reduce cravings and lowers the transfer, that does not save fuel.

The coefficient of accelerated motion does not save the situation, if you do not understand how to interact with each other tire sizes and transmission, the car is moved on the road and provides a comfortable ride on the highway.

Author: **S. Vasilenko**