1. Field of the Invention
The present invention relates to a traction control apparatus for vehicles that exerts braking force on each driving wheel so as not to excessively increase a slip amount of each driving wheel, when a vehicle is in the state to be driven.
2. Description of the Prior Arts
A traction control apparatus has conventionally been known that exerts braking force on each driving wheel so as to place a slip amount of each driving wheel within an allowable slip amount for keeping satisfactory running through performance and running stability, in case where a vehicle is in the state to be driven and the slip amount of each driving wheel exceeds the allowable slip amount. This allowable slip amount is desirably set to a suitable amount for effectively transmitting driving force to a vehicle body according to a state of a road surface on which a vehicle runs.
In case where a vehicle travels on a normal paved road (hereinafter referred to as “on-road”), a friction force between a tire of each driving wheel and a road surface becomes uniform to thereby make a slip amount of each driving wheel approximately same, since a road holding per each wheel is satisfactory and uniform. In other words, a difference in wheel speeds among each of the driving wheels is maintained low when each driving wheel is slipped. It has been found in this case that setting the allowable slip amount to a relatively great value so as to uniformly produce a relatively great slip on each driving wheel can effectively transmit driving force to a vehicle body.
On the other hand, when a vehicle travels on an unpaved road (hereinafter referred to as “off-road”), the road holding of the wheel is frequently poor so that a part of the driving wheels frequently separates from the road surface. Therefore, a part of the driving force from a driving source is used (consumed) for causing a slip of a driving wheel that is not grounded due to operation of a differential gear. As a result, the driving force is hard to be transmitted to the grounded driving wheel that can transmit the driving force to the vehicle body. In other words, the driving force is hard to be effectively transmitted to the vehicle body with a state where a difference in wheel speeds among each driving wheel becomes large when each driving wheel is slipped.
In this case, setting the allowable slip amount to a rather small value exerts braking force on the driving wheel a little earlier after the driving wheel that is not grounded starts to slip, whereby great driving force is transmitted due to the operation of the differential gear to the grounded driving wheel according to a degree that the slip amount (wheel speed) of the driving wheel that is not grounded is limited small. Consequently, the driving force is effectively transmitted to the vehicle body, and hence, satisfactory running through performance can be maintained. As understood from the above, it is generally preferable that the allowable slip amount is set rather small in the off-road travel situation compared to that in the on-road travel situation.
In case where a vehicle travels on the off-road, a running resistance becomes great compared to the case of travelling on the on-road, thereby requiring greater driving force. Therefore, there has been known a vehicle provided with a transmission including a sub-transmission having a high-speed mode for travelling on the on-road and a low-speed mode for travelling on the off-road in which a reduction gear ratio is greater than that in the high-speed mode, whereby driving force from the driving source is transmitted to each driving wheel via this transmission. A driver driving the vehicle provided with this sub-transmission generally sets the sub-transmission to the low-speed mode upon travelling on the off-road for transmitting greater driving force to the vehicle body to thereby enhance running through performance of the vehicle.
In view of the above-mentioned circumstance, a traction control apparatus disclosed in the following patent document 1 is applied to the vehicle provided with such sub-transmission, wherein an allowable slip amount is set small when the sub-transmission is in the low-speed mode compared to the case where the sub-transmission is in the high-speed mode. By this configuration, the driving force is effectively transmitted to the vehicle body by setting the sub-transmission to the low-speed mode upon travelling on the off-road, thereby being capable of enhancing running through performance of the vehicle.
[Patent Document 1]
Japanese Laid-open Publication No. 2000-344083
The road condition of the off-road greatly varies depending upon the types of the off-road. When the road surface of the off-road has relatively great inequalities and relatively great surface roughness, for example, the road holding of a wheel is insufficient, so that a difference in wheel speeds of each driving wheel becomes great when each driving wheel is slipped. In this case, the allowable slip amount is preferably set rather small as described above.
On the other hand, in case where the road surface of the off-road has relatively small inequalities and relatively small surface roughness such as a sand ground, for example, the road holding of the wheel is satisfactory and is liable to be uniform. Accordingly, there may be the case where the difference in wheel speeds of each driving wheel can be maintained rather small when each driving wheel is slipped, even upon travelling on the off-road. In this case, the allowable slip amount is preferably set rather great even upon travelling on the off-road like the case upon travelling on the on-road.
However, the traction control apparatus disclosed in the above-mentioned publication teaches that, when the sub-transmission is set to the low-speed mode, the allowable slip amount is uniformly set rather small. Therefore, when a driver sets the sub-transmission to the low-speed in the case of travelling on the above-mentioned off-road having satisfactory road holding of a wheel such as a sand ground, efficiency for transmitting driving force to the vehicle body is reduced, thereby entailing a problem that the running through performance of the vehicle may be deteriorated.