How to choose the rear axle ratio?

When buying a car, some friends will be hesitant to choose the rear axle speed ratio of the vehicle, is it better to have a bigger speed reduction ratio or a smaller speed reduction ratio. Some friends may know that vehicles with small reduction ratios are faster, while vehicles with large reduction ratios are slower, but with greater climbing strength. Today we are on the rear axle speed reduction ratio size selection problem to discuss with you, I hope the choice of speed reduction ratio can be more in line with their own needs.

1 What exactly is a rear axle speed ratio

Rear axle speed ratios are formally known as main reduction ratios, which refer to the gear ratios in the vehicle's drive axle that slow down and increase torque by attenuating the RPMs on the driveshaft through the meshing of the gears. For example, if a truck has a rear axle speed ratio of 3.727, then if the driveshaft speed is 3.727 rpm, it will be reduced to 1 rpm. However, when comparing rear axle speed ratios, the comparison must be done on the same model year, otherwise, the final result will likely be inaccurate.

2 Rear axle speed ratios are small to run fast

Disregarding external factors such as load, wind resistance, uphill drag, etc., and considering only the transmission ratio, we can derive the speed of the car by a formula:

Vehicle speed = 0.377 x (engine output RPM x tire rolling radius)/(gear speed ratio put in the transmission x rear axle speed ratio), where 0.377 is a fixed factor.

For example, if the same vehicle A and B, configured with a rolling radius of about 0.3822 meters of tires, as well as a transmission with six forward gears and a reverse gear, the highest speed for the overdrive gears, the gear ratio of 0.78, the rear axle of the vehicle A ratio of 3.727, the rear axle of the vehicle B speed ratio of 4.33. Then the transmission in the highest gear, the engine speed of 2000rpm, by the above formula, we calculate the speed of vehicle A and vehicle B respectively:

Vehicle A speed = 0.377 x (2000 x 0.3822) / (0.78 x 3.727) = 99.13 (km/h)

Vehicle B speed = 0.377 x (2000 x 0.3822) / (0.78 x 4.33) = 85.33 (km/h)

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

3 Rear axle speed ratio is big and climbing strong

Strong climbing ability refers to the vehicle's strong driving force and being more powerful. The theoretical formula for truck driving force is:

Driving force = (engine output torque × gear speed ratio × main reduction gear ratio × mechanical transmission efficiency)/wheel radius

Or the above continues above-mentioned A, and B vehicles as an example, the wheel radius of about 0.3937 meters, if the A vehicle and B vehicle respectively in the first gear, the engine output torque of 450 Nm, we were calculated under the A vehicle and the B vehicle at this time to obtain the driving force:

A Vehicle drive force = (450x6.32X3.72X0.98)/0.3937 = 26384.55 (Newtons)

B Vehicle drive force = (450 x 6.32 X 4.33 X 0.98) / 0.3937 = 30,653.36(Newtons)

B vehicles with large rear axle speed ratios get more driving force and naturally climb more strongly. If the truck is compared to a person, the rear axle speed ratio is a bit like a person's leg bone, if the rear axle speed ratio is small, light load fast running, running frequency is high; if the rear axle speed ratio is large, the weight forward, running frequency is low.

4 Select the speed ratio according to the working road conditions

From the market situation in some countries, "large horsepower small speed ratio rear axle" with the mainstream, more suitable for flat roads, high-horsepower engine vehicles at high speeds, with a small speed ratio of the rear axle and gearbox overdrive block, the engine speed does not have to be very high can also run to more than 90 yards of speed. In addition, we also know that the rear axle speed ratio has the role of speed reduction and increases the role of torque, then the high-horsepower engine has enough power reserves, and the torque itself is a big burst of strength, if you rely on the rear axle speed ratio to increase the role of torque can be weakened a little bit, after all, the gearbox can also play the same role.

And the high-horsepower, big-ratio rear axle, with awesome gas mileage, is suitable for vehicles that run a lot of mountain roads.

The optimal gear ratio facilitates the engine to work at optimal working conditions and also keeps the gearing at minimum wear. For some transportation routes and loads are relatively fixed, it is easier to choose the model with a suitable speed ratio. For some of the national drift of individual transporters, the route is not fixed, and the load is not fixed, relatively speaking, the choice is difficult, according to their use of flexibility to choose a center of the speed ratio.

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