To understand the features of all-wheel drive crossovers, you need to start, oddly enough, with their mono-drive counterparts.
So, a modern front-wheel drive crossover is a regular car with a hatchback or station wagon with wheels slightly increased in diameter and a little (only 25 - 75 mm) increased ground clearance. In addition, it is most often created on the platform of a popular passenger car. On all crossovers created in this way, the engine is transverse, and the front wheels, of course, will be front.
However, not all firms agree with the presence of only such “non-powered” crossovers in the range of cars produced.
Four-wheel drive modern crossover
Practice shows that the recipe for creating an all-wheel drive crossover for all non-premium companies is approximately the same. The engine is mounted transversely on the right side of the engine compartment. The gearbox (no matter, mechanics, CVT or automatic), of course, also turns out to be installed transversely - only already in front of the driver. All shafts inside it are also located across the longitudinal axis of the car. And the drive shaft, which transmits torque to the rear axle, passes along this axis, through the entire machine. Therefore, it is necessary to use an angular gear in front and a cardan shaft going to the rear axle.
The driveshaft is connected to a clutch that controls the transmission of torque to the rear axle. Further, since rotation must be transmitted at an angle of 90 degrees, the use of a second angular gearbox combined with a differential is inevitable. From the differential, the drives transmit the rotation to the rear wheels. Such a scheme is used on cars of Nissan (Terrano, Qashqai, X-Trail) and Renault (Duster, Kaptur and Koleos).Transmission scheme: 1 - engine; 2 - clutch; 3 - gearbox; 4 - drive the right front wheel; 5 - angular gear; 6 - cross-axle differential of the front axle; 7 - cardan transmission; 8 - electromagnetic clutch; 9 - drive the right rear wheel; 10 - cross-axle differential of the rear axle; 11 - rear gear; 12 - drive the left rear wheel; 13 - drive of the left front wheel Transmission scheme: 1 - engine; 2 - clutch; 3 - gearbox; 4 - drive the right front wheel; 5 - angular gear; 6 - cross-axle differential of the front axle; 7 - cardan transmission; 8 - electromagnetic clutch; 9 - drive the right rear wheel; 10 - cross-axle differential of the rear axle; 11 - rear gear; 12 - drive the left rear wheel; 13 - drive the left front wheel Transmission: 1 - gear housing; 2 - clutch housing; 3 - a primary shaft; 4 - an epiploon of a leading shaft of a transfer case; 5 - mounting plane of the gearbox Transmission: 1 - gear housing; 2 - clutch housing; 3 - a primary shaft; 4 - an epiploon of a leading shaft of a transfer case; 5 - mating plane of the gearbox The mounting plane of the gearbox. The arrow shows the splines of the gear drive. The mounting plane of the gearbox. The arrow shows the splines of the gear drive.
After the primary and secondary shafts of the gearbox, the main gear is installed, which reduces the speed to the speed necessary for the wheels to rotate. Recall that communication with front-wheel drives is not direct, but through the differential.
The gearbox, whether it is automatic, a CVT or manual shift, is somewhat different from the usual one installed on front-wheel drive cars. To the left of the gearbox is the spline and cylindrical part of the housing of the left-hand drive inner hinge. It's all like any front-wheel drive car. But on the right side, it was necessary to connect the drive of the right wheel and carry out, as is customary to say with respect to trucks, “power take-off” for the drive of the rear axle.
For a more compact arrangement of nodes, a scheme is applied in which the drive to the rear axle comes from splines cut into the hole of the differential box. Docked to the gearbox, the transfer case is actually a conventional bevel gear with hypoid gear. The only not-so-traditional solution is the use of a hollow drive shaft. Such a design has been adopted in order to let the passage shaft through, on the splined end of which the body of the inner right-hand drive hinge is worn.
The cardan transmission serves to transmit torque from the transfer case to the rear gearbox at varying angles and is designed to compensate for the vibrations experienced by the power unit suspended on relatively soft supports.
On the subframe of the rear independent suspension, the rear axle gear is fixed. Torque is transmitted to the rear wheels through two identical drives. In the rear gearbox there is a main gear with differential and an electromagnetic clutch.Rear gearbox: 1 - rear gearbox cover; 2 - gear housing; 3 - electromagnetic clutch housing; 4 - flange of the electromagnetic coupling; 5 - block wiring harness control electromagnetic clutch; 6 - an epiploon of a drive of the left wheel. Rear gearbox: 1 - rear gearbox cover; 2 - gear housing; 3 - electromagnetic clutch housing; 4 - flange of the electromagnetic coupling; 5 - block wiring harness control electromagnetic clutch; 6 - an epiploon of a drive of the left wheel. Related Materials 6 main misconceptions about all-wheel drive
The main gear is hypoid. The drive gear of the main transmission is made integral with the shaft, the splined end of which is connected to the hub of the control clutch of the electromagnetic clutch. The pinion shaft is mounted on two roller tapered bearings. The driven gear of the main gear is attached to the flange of the differential box with special bolts. The differential transmits torque to the rear wheel drives and allows rotation of the drives with different angular speeds. This allows the wheels to pass different paths when slipping without slipping. The differential box rotates in two tapered roller bearings. Two differential gears and two satellites in constant gear are installed in the differential box. Satellites rotate around an axis fixed in the differential box.
The housing of the electromagnetic clutch for connecting an all-wheel drive is bolted to the main gear housing. The friction electromagnetic clutch is designed to connect the rear axle on command from the four-wheel drive control module. The leading part of the clutch is connected to the driveshaft, which, recall, always rotates when the gear of the main transmission of the gearbox rotates. The driven part of the clutch is connected by splines to the shaft of the drive gear of the main gear. To control the compression force of the clutch disks, a cam mechanism is applied that changes the clamping force. The voltage applied to the clutch solenoid closes the clutch disks and connects the rear axle. The amount of transmitted torque is controlled by the friction force of the friction discs in the clutch.Electromagnetic clutch: 1 - Input shaft, combined with the clutch housing; 2 - Main clutch; 3 - Control clutch; 4 - Electromagnet; 5 - Balls; 6 - The shaft of the rear axle drive. Electromagnetic clutch: 1 - Input shaft, combined with the clutch housing; 2 - Main clutch; 3 - Control clutch; 4 - Electromagnet; 5 - Balls; 6 - The shaft of the rear axle drive.
A kind of servo is applied here, that is, significant efforts are controlled with a small control action. An analog in electrical engineering is a relay. The control electromagnetic clutch causes the plate to move. Balls are located in the grooves of a specially selected wedge profile. The displacement of the balls causes them to wedge in the grooves and axial movement of the other plate, which in turn compresses the main sleeve. Torque is transmitted to the drive shaft of the rear axle gearbox.
Technical features of the coupling:
- The clutch transmits torque in both forward and reverse gear, as grooves wedging balls made symmetrical.
- To operate the clutch, you need at least a slight "backlog" of the rear wheels from the front.
- The electromagnet is controlled by the supply of voltage pulses; the force is regulated by the latitudinal modulation of these pulses.
- When applying partial power to the electromagnet, the clutch provides an incomplete short circuit and is capable of turning.
- When the voltage is fully applied, even a fully closed coupling can transmit a torque limited by the frictional forces in the coupling.
- There are no temperature sensors in the clutch, and its “overheating” shutdown occurs when the control unit “sees” through ABS sensors for a long time that when the clutch is fully powered, the rear wheels still do not rotate and the front wheels rotate at a considerable speed.
Features of the operation of all-wheel drive car Renault / NissanRelated Materials 22 responses about operation of Renault (with comments of experts of Renault Russia)
- In 2WD mode, no voltage is applied to the clutch under any driving conditions.
- Installation of rear wheels with a smaller outer diameter is not permissible. Otherwise, the rear axle will not be connected up to a significant slipping of the front wheels.
- Regardless of the included transmission control mode, all shafts, all gearboxes, all pairs of gears in the transmission part from the gearbox to the rear wheels of the car always rotate. So significant fuel economy in 2WD mode on good roads should not be expected. And, nevertheless, the most correct mode in the summer on the highway is 2WD.
- Operation of a car with a locked clutch on asphalt will cause its heating and wear, as well as excessive fuel consumption, tire and transmission wear. According to the instructions, Duster blocks the clutch to 80 km / h. Nissan X-Trail - up to 40 km / h. After exceeding the threshold, the system enters AUTO mode.
- When towing a trailer, increase the pressure in the rear wheels to get at least a little torque on them in AUTO mode.
What could break?Related Materials All-wheel drive rating: roller tests All elements of all-wheel drive are reliable enough if you don’t “jeep” every day. It is necessary to periodically replace the grease in the front bevel gear and in the rear unit, which combines the electromagnetic clutch main gear and differential. The frequency of replacement depends on the number of off-road feats, and in the limit may be required every 30, 000 km. The cardan shaft and both gearboxes may fail. In large industrial cities, there are firms that change crosses in cardan shafts and produce balancing. The price of such a service can reach 9, 000 - 12, 000 rubles. Moreover, it is often possible for the same money, having handed over your shaft, to get completely sorted out and balanced.
The front gearbox, in which the seals not only flowed, but the destruction of bearings and gears began, the easiest way to replace the contract. The price will be between 8, 000 and 15, 000 rubles.
The rear gearbox is much more complex in design and massive. Therefore, prices for a contractual gearbox can reach 20, 000 rubles. Here it is better to play it safe and go through the gear before putting on the car.