JPH0515575B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention provides four wheels that drive front wheels and rear wheels.
The present invention relates to a transfer structure in a wheel drive vehicle, particularly a four-wheel drive vehicle equipped with a center differential that allows differential movement between front and rear wheels.

(Prior art) In a four-wheel drive vehicle in which the power output from the engine via the transmission is divided to drive the front wheels and the rear wheels respectively, in order to eliminate the so-called braking phenomenon during cornering, may be equipped with a center differential. This center differential consists of an input element into which power is input from the engine or transmission, and output elements for front wheels and rear wheels that divide the power input into the element and transmit it to the front wheels and rear wheels, respectively. By performing differential operation between both output elements, the difference in rotational speed between the front and rear wheels during cornering is absorbed.

However, when a four-wheel drive vehicle is equipped with a center differential as described above, a problem arises in that when one of the front wheels or the rear wheels slips, power is not transmitted to the other wheel. Therefore, for example, as shown in Japanese Patent Application Laid-Open No. 57-118927, a so-called deflock mechanism is provided,
In the above case, the mechanism prevents the differential operation of the center differential so that power is transmitted equally to the front and rear wheels.

(Problems to be Solved by the Invention) However, in the conventional differential lock mechanism, the center differential can be manually operated in a completely free state (differential operation is possible) or in a completely locked state (differential operation is possible). Therefore, it is not possible to automatically control the differential operation of the center differential depending on the driving condition of the vehicle. Therefore, for example, if you neglect to unlock the center differential, a braking phenomenon will occur when cornering, and conversely, if you leave the center differential free, the difference in rotational speed between the front and rear wheels will be excessively tolerated, especially when cornering suddenly. If the condition becomes unstable or one of the front wheels or rear wheels slips when starting, a situation may occur where the vehicle cannot start or the required starting acceleration cannot be obtained.

The present invention addresses the above-mentioned problems in four-wheel drive vehicles equipped with a center differential, and in particular, a plurality of center differentials having different power split ratios to the front wheels and rear wheels are provided, and these center differentials are In 4-wheel drive vehicles that can be used selectively, instead of the conventional manual differential lock mechanism, the system is equipped with a differential limiting means that automatically controls the differential operation of the center differential depending on the driving condition. Combined with the selective use of the center differentials mentioned above, it not only improves the driving performance of this type of four-wheel drive vehicle, but also increases the occupied space especially when equipped with differential limiting means in addition to multiple center differentials as mentioned above. In order to cope with this problem, it is an object of the present invention to reduce the size of the center differential and the differential limiting means, and to ensure the operation of the differential limiting means.

(Means for Solving the Problems) That is, the present invention provides an engine, a transmission into which power from the engine is input, an input element driven by the output member of the transmission, and an input element driven by the output member of the transmission. In a four-wheel drive vehicle equipped with a center differential having a front wheel output element and a rear wheel output element that divides input power and transmits it to the front wheels and rear wheels, respectively, the center differential includes a front wheel output element and a rear wheel output element. A plurality of center differentials with different power division ratios to the output elements can be selectively used, and the output element and the input element between or between the front wheel output element and the rear wheel output element in these center differentials. A differential limiting means for limiting the differential operation of the center differential is interposed between the center differential and the differential limiting means, and a speed increasing means is provided in a power transmission path from the output member of the transmission to the plurality of center differentials and the differential limiting means. It is characterized by having the following.

The differential limiting means is composed of, for example, a viscous coupling, and when the difference in rotational speed between the two elements in the center differential exceeds a predetermined amount, it locks both elements in response to an increase in the speed difference, thereby increasing the differential of the center differential. It has the effect of restricting movement. The power to the center differential and the differential limiting means is input in a state where the speed is increased by the speed increasing means.

(Function) According to the above configuration, when cornering where the difference in rotational speed between the front and rear wheels is relatively small, the differential limiting means allows differential operation of the center differential, making smooth cornering possible. With,
When either the front or rear wheels slip when cornering sharply or starting suddenly,
When the difference in rotational speed between the front and rear wheels increases beyond a predetermined limit, the differential limiting means limits the differential operation of the center differential, improving driving stability and starting performance during sharp cornering. will be done.
In this way, together with the selective use of the center differential, the running performance of the four-wheel drive vehicle is improved. and,
In particular, power is input to the center differential and differential limiting means through the speed increasing means and the input torque is reduced, so that the center differential and differential limiting means can be small and small in capacity. Therefore, the difference in rotational speed between the center depth elements, that is, between the front and rear wheels, is amplified and input to the differential limiting means, thereby ensuring the differential limiting operation by the means. This will improve your sexuality.

(Example) Examples of the present invention will be described below.

First, the overall configuration of the four-wheel drive vehicle according to this embodiment will be explained with reference to FIG. It is equipped with a power plant. The engine 1 is arranged so that its output shaft 4 extends in the width direction of the vehicle body. Further, the transmission 2 is also arranged such that an input shaft 6 connected to the engine output shaft 4 via a clutch 5 and an output shaft 7 parallel thereto extend in the width direction of the vehicle body. Gear trains 8 1 to 8 ₅ , 8r for _1st to 5th speeds and reverse speed, which are selectively put into a power transmission state, are provided between the input and output shafts 6 and 7 of the transmission 2, and An output gear 9 is provided at one end of the shaft 7 for transmitting power from the transmission 2 to the transfer device 3.

On the other hand, the transfer device 3 divides and outputs the power input from the transmission 2, drives the left and right front wheels 12, 13 via the left and right front axles 10, 11, and also drives the left and right front wheels 12, 13 in the longitudinal direction of the vehicle body. Left and right rear wheels 18 and 19 are driven through an extending propeller shaft 14, a rear wheel differential device 15, and rear axles 16 and 17 extending left and right from the device 15.

Next, the internal structure of the transfer device 3 will be explained with reference to FIG. Shafts 21 and 22,
An intermediate shaft 23 arranged parallel to the rear of these;
A rear wheel drive shaft 24, which is arranged in the longitudinal direction of the vehicle body and projects rearward, is rotatably supported. The left and right front wheel drive shafts 21 and 22 are the first
The front axles 10, 11 shown in the figure, and the rear drive shaft 2
4 are connected to the propeller shaft 14, respectively.

Further, a bevel gear type front wheel differential device 30 is disposed around the opposing ends of the left and right front wheel drive shafts 21 and 22, and a first and a planetary gear type front wheel differential device 30 are disposed on the intermediate shaft 23. 2 center differential 4
0,50 are arranged.

In the front wheel differential device 30, the boss portions 31a, 31b extending on both the left and right sides are connected to the left and right front wheel drive shafts 21, 2.
A differential case 31 is rotatably fitted and supported on 2.
In the case 31, the front wheel drive shafts 21, 2
2, a pair of pinion gears 33 and 34 rotatably supported by the shaft 32, and a pair of left and right output gears 35 meshed with both pinion gears 33 and 34, 36, both output gears 35,
The end portions of the left and right front wheel drive shafts 21 and 22 are connected to the shaft 36, respectively.

A large-diameter idle gear 61 is rotatably fitted and supported on the outside of the differential case 31 in the front wheel differential 30 and is meshed with the output gear 9 of the transmission 2. is meshed with a small-diameter input gear 62 rotatably supported on the intermediate shaft 23. Furthermore, this input gear 62 is integrally provided with a first transmission gear 63, and the first transmission gear 63 and
A second transmission gear 64 rotatably supported on the right front wheel drive shaft 22 via the boss portion 31b of the differential case 31 is meshed with the second transmission gear 64, and a second transmission gear 64 is coaxially disposed adjacent to the second transmission gear 64. 1. Second center differential drive gears 65 and 66 are rotatably arranged, and a switching mechanism 7 is provided between the second transmission gear 64 and the first and second center differential drive gears 65 and 66.
0 is set.

This switching mechanism 70 is provided integrally with the first and second center differential drive gears 65 and 66 so as to be located close to the outer periphery of a spline member 71 fixed to the second transmission gear 64 and on both sides thereof. Splines having the same specifications are formed around the outer peripheries of the first and second spline portions 72 and 73, and a sleeve 74 is fitted to be slidable across these splines. And the sleeve 7
4 is slid to the left as shown, the spline member 71 and the first spline portion 7
The second transmission gear 64 and the first center differential drive gear 65 are connected via the spline member 71 and the second spline portion 73, and if the sleeve 74 is slid to the right from the illustrated position, the second transmission gear 64 and the first center differential drive gear 65 are connected via the spline member 71 and the second The transmission gear 64 and the second center differential drive gear 66 are connected to each other.

On the other hand, a first center differential 40 is mounted on the intermediate shaft 23.
is composed of a single pinion type planetary gear mechanism, and as shown in FIG. Pinion shaft 43...43 respectively
It has a carrier 44 supported through the pinion gears 42...42, and a ring gear 45 that is disposed outside of each pinion gear 42 and meshes with them.As shown in FIG. It is meshed with the 1 center differential drive gear 65. Further, the second center differential 50 is composed of a double pinion type planetary gear mechanism, and as shown in FIG.
...52, and a plurality of second pinion gears 53...53 that mesh with these pinion gears 52...52, respectively.
and each of the first and second pinion gears 52...52,5
3...53 respectively pinion shaft 54...54,5
5...55, and a carrier 56 supported via the second
It has a ring gear 57 that is disposed outside of the pinion gears 53 and meshes therewith, and an input gear 58 formed around the outer periphery of the ring gear 57 is meshed with the second center differential drive gear 66.

And the ring gear 45 of the first center differential 40
and the carrier 56 of the second center differential 50 are coupled to a bevel gear-shaped rear wheel first output gear 81 rotatably supported on the intermediate shaft 23, and the gear 81 is connected to the rear wheel A second output gear 82 for the rear wheels, also in the form of a bevel gear, is provided at the end of the wheel drive shaft 24 and is meshed therewith. In addition, the first and second
The sun gears 41, 51 of the center differentials 40, 50 are integrated and spline-fitted to the intermediate shaft 23. A first output gear 83 for front wheels is fixed to the intermediate shaft 23, and the output gear 83
and a front wheel second output gear 84 rotatably supported on the left front wheel drive shaft 21 via the boss portion 31a of the differential case 31. Also,
On the boss portion 31a of the differential case 31, there is a sun gear 91 and a plurality of pinion gears 9 that mesh with the sun gear 91.
2...92, a carrier 94 that supports these pinion gears 92...92 via pinion shafts 93...93, and a ring gear 95 that meshes with each pinion gear 92...92. There is. The sun gear 91 in the mechanism 90 and the second output gear 84 for front wheels are integrated, and the ring gear 95 is connected to the case 2 of the transfer device 3.
0, and the carrier 94 is fixed to the boss portion 31a of the differential case 31 in the front wheel differential device 30.
is combined with

In addition to the above configuration, this transfer device 3
A viscous coupling spring 100 is provided as differential limiting means for the first and second center differentials 40 and 50. This viscous cut spring 100 is
An outer case 102 is connected to the rear wheel first output gear 81 supported on the intermediate shaft 23 via a connecting member 101, an inner case 103 is spline connected to the intermediate shaft 23, and both cases 10
2, 103 and a number of plates 104...104 arranged between and alternately engaged with these plates,
In addition, the plates 104...104 are filled with viscous fluid. When the rotational speed difference between the inner case 102 and the outer case 103, that is, between the rear wheel first output gear 81 and the intermediate shaft 23 is small, the relative rotation between them is allowed, but when the rotational speed difference is When the amount exceeds a predetermined amount, the resistance of the viscous fluid acts to limit the relative rotation.

Next, to explain the operation of this embodiment, the power output from the engine 1 via the transmission 2 is transferred to the output gear 9 of the transmission 2.
to the idle gear 6 in the transfer device 3
1 to the input gear 62 on the intermediate shaft 23, and further to the first and second transmission gears 63, 64.
The signal is input to the switching mechanism 70 via.

The sleeve 74 in the switching mechanism 70
When is slid to the left as shown,
The power is transmitted to the carrier 44 of the first center differential 40 via the first center differential drive gear 65 and the input gear 46.
The power is input to the sun gear 41 and the ring gear 45, and the power divided to the sun gear 41 side is transmitted to the intermediate shaft 23 or the first output gear 8 for front wheels.
3, the power divided to the ring gear 45 side is transferred to the second
The signals are outputted to the rear wheel first output gear 81 via the carrier 56 of the center differential 50, respectively. Furthermore, when the sleeve 74 in the switching mechanism 70 is slid to the right, the power is transferred to the second center differential drive gear 6.
6 and the second center differential 50 via the input gear 58
The power is input to the ring gear 57 and is divided into the sun gear 51 and the carrier 56, and similarly to the first center differential 40, the power divided to the sun gear 51 side is sent to the intermediate shaft 23 or the first output gear 83 for the front wheels. The power divided to the carrier 56 side is output to the rear wheel first output gear 81, respectively. and,
In either case, the intermediate shaft 23 or the first
The power output to the output gear 83 is transmitted to the sun gear 91 of the reduction mechanism 90 via the second output gear 84 for front wheels.
After being decelerated by the mechanism 90, the signal is transmitted from the carrier 94 to the differential case 31 of the front wheel differential device 30, and drives the device 30. As a result, the left and right front wheel drive shafts 21 and 22 are driven via the differential device 30, and the front axle 1 is further driven.
Left and right front wheels 12 and 13 are driven via wheels 0 and 11, respectively. Further, the power output to the first output gear 81 for rear wheels is transmitted to the rear wheel drive shaft 24 via the second output gear 82, and is further transmitted to the propeller shaft 14, the rear wheel differential 15, the rear axle 16, The left and right rear wheels 18 and 19 are driven via the motor 17.

In that case, when the power is divided by the first center differential 40, the power input to the carrier 44 of the center differential 40 is distributed to the sun gear 41 and the ring gear 45 according to the ratio of the number of teeth of both gears 41, 45. For example, since the split is 3:7, the power transmitted from the ring gear 45 to the rear wheels 18, 19 is greater than the power transmitted from the sun gear 41 to the front wheels 12, 13, so that the so-called torque distribution is The distribution is suitable for sporty driving with a bias towards the side. On the other hand, when the power is divided by the second center differential 50, if the ratio of the number of teeth between the sun gear 51 and the ring gear 57 is set to, for example, 1:2, the power input to the ring gear 57 is divided between the sun gear 51 and the carrier 56. Therefore, the torque is distributed equally between the front and rear wheels, which is suitable for normal driving.

In any case, during relatively gentle cornering, the viscous cut spring 10
0 allows relative rotation between the intermediate shaft 23 and the first output gear 81 for the rear wheels. A differential operation is performed between the two output elements, the sun gear 51 and the carrier 56, so that the difference in rotational speed between the front and rear wheels due to cornering is absorbed and the braking phenomenon is eliminated. become.

On the other hand, when cornering sharply or when either the front or rear wheels slip, the front
When the rotational speed difference between the rear wheels exceeds a predetermined amount, the viscous cut spring 100
and the relative rotation of the first output gear 81 for rear wheels, that is, the first
Since the differential operation is limited between the two output elements in the center differential 40 or the second center differential 50, the center differential 40 or 50 approaches the locked state, eliminating or suppressing the rotational speed difference between the front and rear wheels as described above. will be done. Therefore, during sharp cornering, it is possible to prevent the driving condition from becoming unstable due to excessive rotational speed difference between the front and rear wheels, and when either the front or rear wheels slip, the other Power is transmitted to, for example, making it possible to escape from a slip condition or obtaining the required starting acceleration. In this manner, the differential operation of the center differentials 40, 50 is controlled according to the driving condition by the action of the viscous coupling spring 100.

However, in this transfer device 3,
Since power is input from the output gear 9 of the transmission 2 to the small-diameter input gear 62 via the large-diameter idle gear 61, the first center differential 40 or the second center differential is selectively linked to the input gear 62. 50, and a viscous coupling 100 interposed between these two output elements, the output rotation of the transmission 2 is once decelerated and then speeded up again and input thereto. Therefore,
Compared to the case where the output rotation of the transmission is inputted to the center differential or the viscous coupling spring in a decelerated state, the torque capacity of these can be made smaller.
0 and the viscous cut spring 100 can be downsized. In addition, as the input rotation to the center differentials 40 and 50 is increased, when a difference in rotational speed occurs between the front and rear wheels, that difference is amplified.
Therefore, the relative rotation between the outer case 102 and the inner case 103 in the viscous coupling spring 100 increases for a constant rotational speed difference between the front and rear wheels. As a result, the differential limiting action of the coupling 100 on the center differentials 40, 50 is ensured, and as a result, the reliability of controlling the differential operation as described above according to the driving condition is improved. become.

In addition, in response to the fact that the input rotation to the center differentials 40, 50 is increased in speed as described above, the output rotation of one of these is transferred from the intermediate shaft 23 to the front wheel differential device 30 or the front wheel drive shafts 21, 22. A deceleration mechanism 90 is interposed in the power transmission path, and the output rotation of the other side is decelerated by the rear wheel differential 15. Therefore, transmission 2
The speed ratio between the output rotation of the front wheel and the rotation of the front and rear wheels is maintained at a predetermined ratio.

Next, a second embodiment of the present invention shown in FIG. 5 will be described. The transfer device 3' according to this embodiment also has left and right front wheel drive shafts 21', 22' extending in the width direction of the vehicle on the same axis. , an intermediate shaft 23' arranged parallel to the rear of these, and a rear wheel drive shaft 24' extending in the longitudinal direction of the vehicle body. A sun gear 31', a plurality of pinion gears 32'...32' that mesh with the sun gear 31', and a carrier 33' that supports these pinion gears are disposed between the opposing ends of the left and right front wheel drive shafts 21' and 22'. , each pinion gear 3
A planetary gear type front wheel differential 30' consisting of a ring gear 34' meshing with 2'...32' is interposed,
In the device 30', a carrier 33' is connected to a left front wheel drive shaft 21', and a sun gear 31' is connected to a right front wheel drive shaft 22'.

Further, on the intermediate shaft 23', as in the above embodiment, a sun gear 41', pinion gears 42'...4
2', a first center differential 40' composed of a single pinion planetary gear mechanism consisting of a carrier 44' and a ring gear 45', a sun gear 51', a first pinion gear 52'...52', a second pinion gear 53'...53', Carrier 56' and ring gear 5
A second center differential 50' constituted by a double pinion type planetary gear mechanism consisting of 7' is disposed, and sun gears 41', 51', which are one output element of these center differentials 40', 50', are connected to the intermediate shaft 23'. is combined with A large-diameter idle gear 61' that meshes with the output gear 9' of the transmission is rotatably supported on the right front wheel drive shaft 22', and a small-diameter input gear 62' that meshes with this gear is rotatably supported on the right front wheel drive shaft 22'. It is rotatably supported on an intermediate shaft 23', and the input gear 62' is connected to a carrier 44' and a ring gear 57', which are input elements of the first and second center differentials 40' and 50.

Also, the ring gear 45' and the carrier 5, which are the other output elements in both center differentials 40' and 50',
Gears 46' and 58' are formed around the outer periphery of the right front wheel drive shaft 2, respectively.
They are meshed with first and second center differential output gears 63' and 64', respectively, which are rotatably supported on 2'. Between these gears 63' and 64', first and second spline parts 72' and 73' are provided integrally with both gears 63' and 64', respectively, and a second spline part 72' and a second spline part 73' are disposed between them. A switching mechanism 70' is provided, which is comprised of a spline member 71' and a sleeve 74' that is slidably spline-fitted over these members, and the position of the sleeve 74' allows the ring gear 45' of the first center differential 40' to Alternatively, either one of the carriers 56' of the second center differential 50' is connected to the spline member 71'. Further, this spline member 71' is a cylindrical second member rotatably fitted to the right front wheel drive shaft 22'.
A first output gear 81' for rear wheels is coupled to the intermediate shaft 25', and a first output gear 81' for rear wheels is fixed to the second intermediate shaft 25', and a first output gear 81' for rear wheels is connected to the second intermediate shaft 25'. It is meshed with the second output gear 82'.

Further, a first output gear for front wheels 83' is fixedly installed on the intermediate shaft 23', and a second output gear for front wheels with a large diameter is formed around the outer periphery of the ring gear 34' in the front wheel differential 30'. 84'.

In this embodiment, the carrier 44' and the ring gear 57' are the input elements of the first and second center differentials 40' and 50', and the output elements to the intermediate shaft 23', that is, the front wheels. A viscous coupling spring 100' is interposed between the sun gears 41' and 51'.

According to this embodiment, the power from the output gear 9' of the transmission is transferred to the large diameter idler gear 6.
1' and a small diameter input gear 62'.
Carrier 44' in center differential 40', 50'
and is input to ring gear 57'. If the sleeve 74' in the switching mechanism 70' is positioned on the left side as shown in the figure, the first center differential 40' divides the input from the carrier 44' and outputs the input to the sun gear 41' and the ring gear 45', for example. The output is output at a ratio of 3:7, and the output to the sun gear 41' is output to the intermediate shaft 23', and the first and second output gears 8 for front wheels.
3', 84', and the front wheel differential 30' to the left and right front wheel drive shafts 21', 22' or the left and right front wheels. In addition, the output to the ring gear 45' is transmitted to the gears 46', 63', the switching mechanism 70', and the second intermediate shaft 2.
5', and first and second output gears 81', 8 for rear wheels.
2' to the rear wheel drive shaft 24', and further transmitted to the left and right rear wheels via a propeller shaft, rear wheel differential, etc. (not shown).

On the other hand, if the sleeve 74' of the switching mechanism 70' is slid to the right position, the second center differential 5
0' divides the power input to the ring gear 57' and transmits it to the sun gear 51' and the carrier 56' at a ratio of, for example, 1:1, and these powers are respectively transmitted to the first center differential 40'. It is transmitted to the left and right front wheels and the left and right rear wheels through the same route as in the case of splitting.

Also in this embodiment, a viscous cut spring 100' interposed between the input element of the first and second center differentials 40', 50' and the output element to the front wheel side is connected to the center differentials 40', 50'. Since the differential operation of ' is controlled according to the driving condition, good driving performance such as improved stability during sharp cornering can be obtained, and the above-mentioned first and second center differentials 40', 50' and viscous The coupling ring 100' is connected to a large-diameter idle gear 6 that rotates from the output gear 9' of the transmission.
After being decelerated at first glance by 1', input gear 6 is
2', the rotation from the transmission is inputted while being decelerated, so the first and second center differentials 40', 5
0' and the viscous cut spring 100' can be made smaller, and the first,
The differential limiting effect on the second center differentials 40', 50' is ensured.

(Effects of the Invention) As described above, according to the present invention, in a four-wheel drive vehicle equipped with a plurality of center differentials having different power split ratios to the front wheels and the rear wheels, Because it is equipped with a differential limiting means that automatically limits the differential operation of the center differential when it exceeds a predetermined amount,
Since the differential operation of the center differential is controlled according to the driving condition of the vehicle, for example, when stability during sharp cornering is improved, driving performance is improved in combination with selective use of the center differential. It turns out. In particular, since the power to the plurality of center differentials and differential limiting means is inputted through the deceleration means, it is possible to downsize these center differentials and differential limiting means, thereby making it possible to reduce the size of these center differentials and differential limiting means. The layout of the transfer device in a vehicle can be simplified and made more compact. Moreover, the differential limiting action of the differential limiting means is ensured, and the reliability of this type of four-wheel drive vehicle is improved.

[Brief explanation of the drawing]

1 to 4 show a first embodiment of the present invention, FIG. 1 is a schematic diagram showing the overall configuration of a four-wheel drive vehicle according to the embodiment, and FIG. 2 is a longitudinal cross-sectional view of the main parts thereof. ,
3 and 4 are schematic diagrams showing the configurations of the first and second center differentials, respectively, in this embodiment. Also, the fifth
The figure is a schematic diagram of main parts showing a second embodiment of the present invention. 1...Engine, 2...Transmission,
9, 9'... Transmission output member (output gear), 40, 40'... First center differential, 50,
50'...2nd center differential, 61, 62, 61',
62'...speed increasing means (idle gear, input gear),
100, 100'...Differential limiting means (viscous cut spring).

Claims (1)

[Claims] 1. An engine, a transmission into which power is input from the engine, an input element driven by the output member of the transmission, and the power from the input element is divided and transmitted to the front wheels and rear wheels, respectively. A four-wheel drive vehicle equipped with a center differential having a front wheel output element and a rear wheel output element, wherein the center differential includes a plurality of center differentials having different power split ratios to the front wheel output element and the rear wheel output element. In addition, when the difference in rotational speed between the front wheel output element and the rear wheel output element in these center differentials, or between either output element and input element, is greater than a predetermined amount, the center differential A differential limiting means for limiting the differential operation of the transmission is provided, and a speed increasing means is provided in a power transmission path from the output member of the transmission to the plurality of center differentials and the differential limiting means. Transfer structure for 4-wheel drive vehicles.