When buying a truck, dump truck drivers often ask, is it better to buy a truck with a larger or smaller rear axle speed ratio? In fact, both are good. The key is to be suitable. To put it simply, many truck drivers know that a small rear axle speed ratio means small climbing force, fast speed and low fuel consumption; a large rear axle speed ratio means strong climbing force, slow speed and high fuel consumption.

But why? We need to know not only the facts but also the reasons behind them. Today, let’s talk with driver friends about the speed ratio of the rear axle of trucks!

The rear axle speed ratio is just a common name. The academic name is the main reduction ratio, which is the gear ratio of the main reducer in the car drive axle. It can reduce the speed on the drive shaft and increase the torque. For example, if the rear axle speed ratio of a truck is 3.727, then if the drive shaft speed is 3.727 r/s (revolutions per second), it will be reduced to 1r/s (revolutions per second).

When we say that a car with a larger rear axle speed ratio is more powerful, or a car with a smaller rear axle speed ratio is faster, we must be comparing the same models. If they are different models, it is meaningless to simply compare the size of the rear axle speed ratios, and it is easy to draw wrong conclusions.

Because the rear axle is used in conjunction with the gearbox, the speed ratios of different gears in the gearbox are also different, and the total speed ratio of the car is the result of multiplying the speed ratio of the gearbox and the speed ratio of the rear axle.

Why do trucks with smaller rear axle speed ratio run faster?

Without considering external factors such as load, wind resistance, uphill resistance, etc., and only considering the transmission ratio, we can deduce the vehicle speed through a formula:

Vehicle speed = 0.377 × (engine output speed × tire rolling radius) / (gearbox gear ratio × rear axle speed ratio)

Among them, 0.377 is a fixed coefficient.

For example, if the same model of light trucks are light truck A and light truck B, they are equipped with 7.50R16 radial tires, Wanliyang WLY6T120 manual transmission, with 6 forward gears and one reverse gear, the highest speed is overdrive, the gear ratio is 0.78, the rear axle speed ratio of light truck A is 3.727, and the rear axle speed ratio of light truck B is 4.33.

Then when the gearbox is in the highest gear and the engine speed is 2000rpm, according to the above formula, we calculate the speed of light truck A and light truck B respectively. The rolling radius of the 7.50R16 tire is about 0.3822 meters (the rolling radius of tires of different specifications can also be derived according to the tire parameters. To simplify the results directly quoted here, this rolling radius has an error range.

Speed of light truck A = 0.377 × (2000 × 0.3822) / (0.78 × 3.727) = 99.13 (km/h);

Light truck B speed = 0.377 × (2000 × 0.3822) / (0.78 × 4.33) = 85.33 (km/h);

For the same model of vehicle, when the engine speed is 2000rpm, it is theoretically deduced that the speed of light truck A with a small rear axle speed ratio reaches 99.13km/h, and the speed of light truck B with a large rear axle speed ratio is 85.33km/h. Therefore, the vehicle with a small rear axle speed ratio runs faster and is more fuel-efficient.

Why do trucks with a large rear axle speed ratio have strong climbing ability?

Strong climbing ability means that the truck has strong driving force. The theoretical calculation formula for truck driving force is:

Driving force = (engine output torque × gear ratio × final reducer ratio × mechanical transmission efficiency) / wheel radius

For the light truck A and light truck B above, the wheel radius of the 7.50R16 tire is about 0.3937m (the radius of tires of different specifications can also be derived based on tire parameters. For simplicity, the results are directly quoted here.

If you are interested, we will introduce it in detail later). If light truck A and light truck B are in the first gear and the engine output torque is 450 Nm, we calculate the driving force obtained by light truck A and light truck B at this time:

Light truck A driving force = (450×6.32X3.72X0.98)/0.3937=26384.55 (Newtons)

Light truck B driving force = (450×6.32X4.33X0.98)/0.3937=30653.36 (Newton)

When the engine is in 1st gear and the engine output torque is 450 Nm, the driving force obtained by light truck A is 26384.55 Newtons, which is generally speaking about 2692 kilograms (kg) of thrust (1 kg-force = 9.8 Newtons); the driving force obtained by light truck B is 30653.36 Newtons, which is generally speaking about 3128 kilograms (kg) of thrust (1 kg-force = 9.8 Newtons). Obviously, light truck B with a larger rear axle speed ratio obtains greater driving force, and naturally has stronger climbing power.

The above is a rather boring theoretical derivation. To put it in a more vivid way, if a truck is compared to a person, the rear axle speed ratio is a bit like the leg bones. If the rear axle speed ratio is small, the truck can run fast with a light load and the running frequency is high; if the rear axle speed ratio is large, the truck can run forward with a heavy load and the running frequency is low.

From the above analysis, it can be seen that the rear axle speed ratio is small, the climbing force is small, and the fuel consumption is low; the rear axle speed ratio is large, the climbing force is strong, the speed is slow, and the fuel consumption is high.

In the current domestic market, the combination of “high horsepower and small speed ratio rear axle” is the mainstream, and it is applicable to more scenarios. Unlike before, the engine horsepower was small, there were many overloads, and there were many mountain roads and dirt roads, so people tended to choose a large speed ratio rear axle.

Nowadays, transportation is mainly based on standard loads, efficient logistics, and highways. “The only way to defeat all martial arts in the world is to be fast.” When a high-horsepower engine car is driving at high speed, with a small speed ratio rear axle and the overdrive gear of the gearbox, the engine speed does not need to be very high to reach a speed of more than 90 miles per hour.

In addition, we also know that the rear axle speed ratio has the effect of reducing speed and increasing torque. If a high-horsepower engine has sufficient power reserve and itself has large torque and strong explosive power, the effect of relying on the large speed ratio of the rear axle to increase torque can be weakened. After all, the gearbox can also play the same role.

The high-horsepower, high-speed-ratio rear axle has very high fuel consumption and is suitable for use in special working conditions such as dump trucks, cement mixer trucks, and vehicles that frequently drive on mountain roads.

So when we buy a truck, is it better to buy a larger or smaller rear axle ratio? It still depends on your own usage.

For some transportation routes and loads that are relatively fixed, it is easier to choose a model with a suitable speed ratio. For some individual transporters who travel around the country, the routes and loads are not fixed, so it is relatively difficult to choose. You need to flexibly choose a medium speed ratio according to your own usage.

Post time: Aug-24-2024