WO2008063103A1

WO2008063103A1 - An air-bag unit

Info

Publication number: WO2008063103A1
Application number: PCT/SE2006/001323
Authority: WO
Inventors: Klaus-Peter KÜHNE; Karl-Heinz Sommer; Thomas Reiter; Marcus Weber; Mark Schock; Jörg ALBERT
Original assignee: Autoliv Development Ab
Priority date: 2006-11-22
Filing date: 2006-11-22
Publication date: 2008-05-29

Abstract

An air-bag unit (1) is disclosed which comprises a housing (5), an air-bag (7) and an inflator (6) for inflating the air-bag. The air-bag is initially packed within the housing (5) so that, upon inflation of the air-bag, at least part of the upper region (24) of the air-bag inflates before substantial inflation of the lower region (26). A restraining tether (16) is provided within the air-bag and is secured, at a first anchor region (18), to a generally central region of a front panel of the air-bag. The tether is also secured, at a second anchor region (22), to another part of the air-bag unit (1) so that, at least during an initial stage of inflation of the air-bag, the restraining tether (16) is placed under tension, thereby restraining inflation of the central region of the front panel, in order to create a centrally-positioned recess in the inflating air-bag.

Description

Title: An Air-Bag Unit

Description of Invention

The present invention relates to an air-bag unit, and more particularly relates to an air-bag unit intended to form part of a safety arrangement in a motor vehicle in order to provide protection for an occupant of the motor vehicle.

It has been proposed previously to provide an air-bag located in front of a passenger in a motor vehicle, the air-bag being arranged to inflate in the event of an impact involving the motor vehicle, in order to provide protection for the passenger. Such "passenger air-bags" are generally located within the dashboard of the motor vehicle and, in normal use, are covered by a region of dashboard trim in a known manner.

Whilst it is known that such an air-bag may provide good protection for an occupant who, at the instant of impact, is sitting in an "ordinary" position, it has been found that the air-bag may actually injure a seat occupant who, at the moment of impact, is not in the "ordinary" position but, instead, might be leaning forwards in order to gain access to a glove box mounted in the dashboard of a motor vehicle, or may perhaps be leaning sideways in the occupant's seat.

It has therefore been proposed previously that an arrangement should be provided to detect when a vehicle occupant is out of the ordinary position and to moderate inflation of the air-bag, or even inhibit inflation of the air-bag, in such a situation in order to reduce the risk of the air-bag itself injuring the occupant. Various such proposals have been proposed previously which involve the use of detectors to detect when a vehicle occupant is "out of position", the detectors being in some way operatively connected to the air-bag in order that the inflation of the air-bag can be controlled in response to signals received from the detectors. Such prior arrangements are, therefore, "active" systems, in the sense that they actively monitor the position of a vehicle occupant.

In some instances, it is considered desirable to provide a safety arrangement in which an air-bag is configured to inflate in such a manner as to avoid significant injury to an "out of position" occupant, without the need for complicated and expensive detector and sensor arrangements.

The present invention seeks to provide an improved air-bag unit.

According to the present invention, there is provided an airbag unit comprising: a housing, an airbag, and an inflator for inflating the airbag, the airbag being packed within the housing such that, upon subsequent inflation of the airbag, at least part of an upper region of the airbag inflates prior to substantial inflation of a lower region of the airbag, the arrangement further comprising a restraining tether provided within the airbag and secured, at a first anchor region, to a substantially central region of a front panel of the airbag and secured, at a second anchor region, to another part of the airbag unit, whereby, at least during an initial stage of inflation of the airbag, the restraining tether is placed under tension to restrain inflation of the central region of the front panel, thereby creating a centrally positioned recess in the inflating airbag.

Preferably, the tether is secured, at the second anchor region, to a rear panel of the airbag. Advantageously, between said anchor regions, the tether passes through part of the housing for sliding movement through said part of the housing.

Conveniently, the tether is provided in the form of a loop, secured at both ends to the front panel.

Preferably, the loop is arranged so as to pass substantially around the inflator, at least during an initial stage of inflation of the airbag, so as to serve as a gas- diffuser during inflation.

Advantageously, the tether is secured, at the second anchor region, to the housing.

Conveniently, the tether is releasably secured to the front panel at said first anchor region so as to be released from the front panel when the lower region of the airbag is inflated to a predetermined degree.

Preferably, the tether is secured to the front panel at said first anchor region by a releasable connection comprising at least one tear-seam configured to rupture when the lower region of the airbag is inflated to said predetermined degree.

Advantageously, the or each tear-seam is configured to rupture under a peel- force applied to the seam by the inflating airbag, but to remain substantially intact under a shear-force applied to the seam by the inflating airbag.

Conveniently, the airbag is packed within said housing so as to form at least two fold-packets, a first of said fold-packets being arranged for substantially direct inflation by said inflator to define the upper region of the airbag, and the second of said fold-packets being arranged so as to be pulled from the housing by the first fold packet as the first fold packet is inflated such that the second fold-packet defines the lower region of the airbag.

Preferably, the housing comprises a divider, and wherein the inflator and said first fold-packet are arranged on one side of the divider and said second fold- packet is arranged on the other side of the divider.

Advantageously, the first anchor region extends a distance d transversely across the airbag, where d is at least equal to width of the head of a 95 percentile male dummy.

Conveniently, the tether is formed as a fabric sheet.

Preferably, the fabric sheet has a substantially uniform width equal to distance d.

So that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which :-

FIGURE 1 is a partially cut-away perspective view of an air-bag unit in accordance with the present invention, illustrating the air-bag of the unit during an initial stage of deployment; FIGURE 2 is a schematic cross-sectional illustration showing an air-bag unit similar to that of Figure 1 during an initial stage of deployment;

FIGURE 3 is a view corresponding generally to Figure 2, but illustrating the arrangement during a subsequent stage of deployment;

FIGURE 4 is a front view illustrating the profile of the air-bag presented to an occupant during initial stages of deployment of the air-bag arrangement illustrated in Figures 2 and 3; FIGURE 5 is a view corresponding generally to that of Figure 4, illustrating the air-bag during a subsequent stage of deployment;

FIGURE 6 is a view corresponding generally to that of Figures 5, but illustrating the air-bag substantially fully deployed; FIGURE 7 is a schematic cross-sectional illustration showing a modified form of the embodiment illustrated in Figures 1 to 6;

FIGURE 8 is a schematic cross-sectional illustration showing an air-bag unit in accordance with another embodiment of the present invention during an initial stage of deployment of the air-bag; FIGURE 9 is a view corresponding generally to that of Figure 8 illustrating a subsequent stage of deployment;

FIGURE 10 is a view corresponding generally to that of Figure 9, illustrating a further stage of deployment;

FIGURE 11 is a view corresponding generally to that of Figure 10, but illustrating the air-bag substantially fully deployed.

FIGURE 12 is a schematic cross-sectional illustration showing a further embodiment of the present invention, illustrating the air-bag during an initial stage of deployment;

FIGURE 13 is a view corresponding generally to that of Figure 12, illustrating the air-bag during a subsequent stage of deployment;

FIGURE 14 is a view corresponding generally to that of Figure 13 illustrating the air-bag during as still further stage of deployment; and

FIGURE 15 is a view corresponding generally to that of Figure 14, illustrating the air-bag substantially fully deployed.

Referring initially to Figure 1 , there is illustrated an air-bag unit 1 installed in the dashboard of a motor vehicle so as to be located behind the dashboard trim 2, generally below the windscreen 3 of a motor vehicle. The air-bag unit 1 is located generally below an upwardly-directed opening 4 formed in a top region of the dashboard trim 2. The opening 4 is, in a normal condition, closed by a removable or puncturable cover (not shown) so as to be openable upon actuation of the air-bag unit in the event of a crash situation.

The air-bag unit 1 comprises a substantially rigid housing 5 which can be made from metal or a convenient plastics material. The housing 5 is mounted to the dashboard structure behind the dashboard trim 2 and contains an inflator 6 such as a gas generator of a type known per se. Prior to actuation, the housing 5 also contains an inflatable air-bag (indicated generally at 7), such as a fabric air-bag which is initially packed within the housing. The inflator 6 is arranged so as to direct a supply of inflating gas into the interior volume of the air-bag 7, so as to inflate the air-bag upon receipt of an actuation signal in the event of a crash involving the motor vehicle.

As can be seen in Figure 1 , the inflator 6 is located in the lower region of the housing 5 and is positioned generally centrally in a fore-aft direction in the housing such that a forward compartment 8 is defined between the inflator 6 and a forward-facing (i.e. towards the front seat passenger of the motor vehicle) wall 9 of the housing, and so that a rear compartment 10 is defined between the inflator 6 and the rearmost wall 11 of the housing.

The air-bag 7, prior to inflation, is packed within the housing so as to form a first fold packet in the rear compartment 10 and in the space above the inflator 6, and to form a second fold packet located in the forward compartment 8. As will be explained in more detail with particular reference to Figures 2 to 6, as the air-bag 7 is inflated, gas is directed from the inflator 6 into the first fold packet so as to inflate the first fold packet in order to define an upper region (indicated generally at 12 in Figure 1) of the air-bag. The second fold packet located in the forward compartment 8 is arranged so as to be pulled from the housing 5 by the inflating upper region 12 such that the second fold-packet is then allowed to inflate in order to define a lower region of the air-bag (indicated generally at 13 in Figure 1 ). The inflator 6 is spaced slightly above the base 14 of the housing 5 so as to define a passage 15 between the inflator 6 and the base 14. A generally elongate sheet of fabric 16 passes through the passage 15 and is substantially free to slide through the passage 15. The fabric sheet 16 is provided inside the internal volume of the air-bag 7 and is secured, at one end 17, to the front panel of the air-bag 7 so as to define a first anchor region 18. The first anchor region 18 is provided at a generally central position across the width of the air- bag 7 (transversely across the motor vehicle) and has a width d. In a preferred arrangement, the width d of the anchor region 18 is at least equal to the width of the head of a 95 percentile male crash test dummy.

The connection between the forward end 17 of the fabric sheet 16 and the front panel of the air-bag 7 comprises one or more rupturable tear seams 19 formed as lines of stitching between the fabric sheet 16 and the front panel of the air-bag 7.

The preferred form of connection between the fabric sheet 16 and the front panel of the air-bag 7, at the first anchor region 18 is specifically configured so that the or each tear seam is adapted to rupture under a peel-force applied to the seam by the inflating air-bag (as will be described in more detail hereinafter), but to remain substantially intact under a sheer-force applied to the seam by the inflating air-bag.

As also illustrated in Figure 1 , the rear end 20 of the fabric sheet 16 is secured to the rear panel 21 of the air-bag at a second anchor region 22. Whereas the connection between the forward end 17 of the fabric sheet 16 and the front panel of the air-bag, at the first anchor region 19, is configured to be releasable, the connection between the rear end 20 of the fabric sheet 16 and the rear panel 21 of the air-bag, at the second anchor region 22, is significantly stronger and is intended to remain permanent and hence intact during inflation of the air-bag 7. The connection at the second anchor region preferably comprises one or more stitched seams 23.

It should be appreciated that Figure 1 illustrates only a relatively narrow transverse slice of the air-bag 7 during an early stage of inflation of the air-bag 7. Turning now to Figures 2 to 6, the mode of deployment of the air-bag 7 will be described in more detail.

Figure 2 illustrates the same stage of deployment shown in Figure 1. The inflator 6 has been actuated so as to direct inflating gas into the first fold- packet of the air-bag, thereby initially partially inflating the upper region 24 of the air-bag. It should be appreciated that this region of the air-bag was initially folded, as part of the first fold-packet, in the space above the inflator 6, and has been inflated such that the sheet of fabric 16 which effectively defines an internal tether, has been drawn taut. As the tether 16 is drawn taut in this manner, its connection to the front and rear panels of the air-bag by the first and second anchor regions 18, 22 respectively, serves to prevent subsequent further inflation of the upper region 24. Indeed, Figure 2 illustrates that the remaining part of the first fold-packet is still folded and contained in the rear compartment 10 of the housing 5.

However, because the tether 16 has a width d, and so does not extend across the entire transverse width of the inflating air-bag, only the central part of the upper region 24 is constrained by the tether against further inflation. The side regions of the air-bag are substantially free to inflate and so a side-bubble 25 of the air-bag is formed on each side of the central constrained part of the upper region 24.

As the central part of the upper region 24 inflates to its constrained extent illustrated in Figure 2, the first anchor region 18 is moved outwardly of the housing 5, dragging with it part of the folded air-bag which forms the second fold packet located within the forward compartment 8 of the housing 5. This allows inflating gas from the inflator to pass into the part of the air-bag forming the second fold-packet, thereby inflating that region of the air-bag to define a lower region 26 which is shown beginning to inflate downwardly, across the forwardmost surface of the dashboard.

With reference to Figure 3, inflation of the air-bag 7 continues in this manner with the second fold-packet initially located in the forward compartment 8 continuing to unfold during the further inflation of the lower region 26 of the air- bag, the lower region continuing to move downwardly across the front surface of the dashboard. During this stage of inflation, the internal tether 16 continues to be held under tension by the pulling force exerted on it by the central part of the upper region 24 of the air-bag. This therefore allows the lower region 26 of the air-bag to continue to inflate, whilst the central part of the upper region 24 is constrained against further inflation.

At this stage of inflation, the top part of the lower region 26, indicated generally at 27, extends inwardly of the inflating air-bag at the transverse centre of the airbag, so as to remain secured to the tether 16 at the first anchor region 18, whilst the side bubbles 25 continue to inflate to either side of the central part of the upper region 24. A recess 28 is thus formed, as seen most clearly in Figure 4 which illustrates the partially inflated air-bag from the point of view of the passenger 29. The recess 28 is thus formed in the central region of the air-bag (in the transverse sense), above the lower region 26, and between the two side bubbles 25. Because of the width d of the tether 16, the recess 28 preferably has a width substantially equal to or slightly greater than the width of the head of a 95 percentile male crash test dummy.

Figure 5 illustrates a subsequent stage during the deployment of the air-bag 7, with the second fold-packet initially contained within the forward compartment

8 having been completely unfolded so as to define the inflated lower region 26. At this stage during deployment, the top part 27 of the lower region 26 still extends inwardly of the inflating air-bag with the first anchor region 18 remaining intact. Therefore, the remaining part of the air-bag forming the first fold-packet remains substantially folded and housed within the rear compartment 10 of the housing 5. However, continued inflation of the air-bag, from the point illustrated in Figure 5, causes further inflation of the lower region 26 to a predetermined degree such that the top part 27 is caused to peel away from the forward end 17 of the tether 16. The peel force thus applied to the tear seams of the first anchor region 18 causes the tear seams to rupture, thereby releasing the tether 16 from the front panel of the air-bag 7. When this happens, the tension previously applied to the internal tether 16 is released and so the upper region of the air-bag becomes free for further inflation such that the remaining part of the first fold-packet is paid out from the rear compartment 10 whilst the tether 16 is pulled through the passage 15 formed between the inflator 6 and the base 14 of the housing.

Figure 6 illustrates the air-bag 7 in its fully deployed condition in which, following release of the initial connection between the forward end 17 of the tether 16 and the front panel of the air-bag, the upper region of the air-bag has become free for substantially complete inflation such that the recess 28 formed during the mid stages of inflation disappears. During the entire deployment of the air-bag, the second anchor region 22 remains intact such that the tether 16 remains secured to the rear panel of the air-bag 7.

It should therefore be appreciated that the above-described configuration and arrangement of the internal tether 16 serves to restrain inflation of a central region of the front panel of the air-bag, thereby creating a centrally positioned recess 28 during the mid-stages of deployment of the air-bag, but becomes released during later stages of deployment so that the front panel of the air- bag is no longer restrained, thereby allowing the upper region of the air-bag to become substantially fully inflated such that the recess 28 disappears. It should therefore be appreciated that the air-bag arrangement is configured so that, upon actuation of the unit, an initial start bubble is created above the dashboard of the motor vehicle in an upper region, whereafter the lower region of the air-bag 26 is caused to inflate, with the upper region of the air-bag only becoming free for substantially complete inflation once the lower region of the air-bag has become inflated to a predetermined degree sufficient to provide proper protection to the abdominal region of the occupant 29. Restraining substantially full inflation of the upper part of the air-bag in this manner prevents aggressive inflation of this part of the air-bag towards the head of the occupant 29, thereby reducing the risk of whiplash injuries, facial injuries from "bag-slap" or the like. With the creation of the centrally-located recess 28 during the initial stages of deployment of the air-bag, with a side bubble 25 building on either side of the recess, it is thought that the inflating air-bag will provide increased protection to an out of position occupant.

Figure 7 illustrates, in schematic form, a slight variation of the arrangement described above with reference to Figures 1 to 6. In the arrangement of Figure 7, the fabric sheet 16 defining the internal tether is formed as an initially closed loop of material, again passing through the passage 15 formed between the inflator 6 and the base 14 of the housing. In this arrangement, both ends 17, 20 of the sheet 16 are secured to one another and to the front panel of the air- bag 7 at the first anchor region 18, whilst the second anchor region 22, which forms a permanent connection between the sheet 16 and the rear panel of the air-bag 7 is created at a position spaced from the end 20. The first anchor region 18 is again formed to be releasable in substantially the same manner as in the arrangement of Figures 1 to 6.

During initial stages of inflation of the air-bag 7, the sheet 16 forming the internal tether will thus be held, whilst the tear seams of the first anchor region 18 remain intact, in the form of a loop passing substantially around the inflator

6, the loop having a width d. The resulting loop thus serves a gas-diffusing function which has been found to improve the creation of the side bubbles of the inflating air-bag without subjecting the restrained upper region to an excessively aggressive flow of gas from the inflator.

As the lower region 26 becomes sufficiently inflated, the tear seams of the first anchor region 18 again rupture in substantially the same manner as with the arrangement of Figures 1 to 6, but in this embodiment, rupture of the tear seams also breaks the connection between the two ends 17, 20 of the tether 16. The tether 16 thus becomes free for withdrawal through the passage 15 in substantially the same manner as with the embodiment of Figures 1 to 6.

Turning now to consider Figures 8 to 11 , a further embodiment of the present invention will be described with the same reference numerals being used for identical or equivalent parts of the air-bag unit.

With particular reference to Figure 8, in this embodiment, the inflator 6 is mounted within the housing 5 at the rear of the housing so as to abut both the base 14 and the rearmost wall 11. A substantially vertical divider 30 is provided within the housing 5 in order to divide the internal volume of the housing 5 into a forward compartment 8 and a rear compartment 10, the rear compartment 10 being located above the inflator 6. Although not illustrated in Figure 8 (which shows the unit during an initial stage of deployment of the air- bag 7), the first fold-packet is initially packed within the rear compartment 10, whilst the second fold-packet is shown initially packed into the forward compartment 8.

In this arrangement, an internal tether 16 is again provided within the internal volume of the air-bag 7, the tether 16 again preferably taking the form of a fabric sheet. The tether 16 again has a transverse width (across the air-bag) d (not shown in Figures 8 to 11 ). The tether 16 is releasably secured, at its forward end 17, to a central region (in a transverse sense) of the front panel of the air-bag at a first anchor region 18. This releasable connection between the tether 16 and the air-bag, at the first anchor region 18, is again provided by one or more rupturable tear seams. However, in this embodiment, the rear end 20 of the tether 16 is secured, in a non-releasable manner, to the upper region of the divider 30, although it is also possible to secure the tether to another part of the housing 5, to the inflator, or the air-bag.

Figure 8 illustrates the arrangement of this embodiment during an initial stage of deployment in which the first fold-packet has been deployed from the housing 5 under the inflating action of the inflator 6 such that an upper region 24 of the air-bag 7 is inflated in the manner of a start bubble, in the region above the dashboard and below the windscreen 3.

As the inflator 6 continues to direct inflating gas 24 into the air-bag, the air-bag becomes further inflated such that the second fold-packet initially packed within the forward compartment 8 is pulled from the housing 5, thereby allowing inflation of the lower region 26 of the air-bag. This stage is illustrated in Figure 9 and it can be seen that the connection between the front panel of the air-bag and the tether 16, at the first anchor region 18, remains intact during this early stage of inflation.

Figure 10 illustrates a subsequent stage of inflation in which all of the second fold-packet initially packed within the forward compartment 8 has been withdrawn from the housing and such that the lower region 26 has become further inflated. The connection between the front panel of the air-bag 7 and the tether 16 is still shown as remaining intact at the first anchor region 18 at this stage of deployment and so it can be seen that the top part 27 of the lower region 26 again extends inwardly of the inflating air-bag towards the first anchor region. Because the tether 16 is only secured to a central region of the front panel, across a width d which is substantially less than the overall width of the air-bag 7, a recess 28 is again formed, above the lower region 26 and between a pair of side bubbles 25, such that the air-bag again presents a form generally similar to that illustrated in Figure 4.

Further inflation of the air-bag, from the condition illustrated in Figure 10, results in a substantial peel force being applied to the tear seams of the first anchor region 18 under the pressure of the inflating gas bearing against the front panel of the air-bag. This peel force is sufficient to rupture the tear seams in the first anchor region, thereby releasing the forward end 17 of the tether 16 from the front panel of the air-bag. With the front panel of the air-bag thus released from the tether 16, the air-bag is substantially free to inflate to the fully deployed position illustrated in Figures 11 in which the centrally located recess 28 has disappeared.

Figures 12 to 15 illustrate a further embodiment of the present invention which represents a modification of the embodiment described above with reference to Figures 8 to 11. In this embodiment, the housing 5 takes the same general form, being provided with a central divider 30 and with the inflator 6 located at the rear, lower region of the housing. The tether 16, which again preferably takes the form of a fabric sheet having width d, is secured to the housing, and most preferably to the divider 30, at its rear end 20, in a substantially permanent manner.

The forward end 17 of the tether 16 is bifurcated so as to define a pair of branched regions 31 , each of which is permanently secured to a central part of the front panel of the air-bag 7, for example by way of one or more non- rupturable seams. The connection between the forward end 17 of the tether and the front panel of the air-bag defines the first anchor region 18, whilst the connection between the rear end 20 of the tether 16 and the housing defines the second anchor region 22. As can be seen from Figures 12 to 14, the initial deployment of the air-bag of this embodiment is substantially identical to that of the embodiment described above with reference to Figures 8 to 10, such that a central recess 28 is defined within the front region of the air-bag, between a pair of side bubbles 25 and above the inflating lower region 26. However, Figure 15 shows the air- bag 7 substantially fully inflated and, as can be seen from this figure, because the tether 16 is relatively short when compared to the overall dimension of the fully inflated air-bag 7, and because the connection between the tether 16 and the front panel of the air-bag at the first anchor region 18 is a permanent connection, the recess 28 formed between the side bubbles 25 remains, even when the air-bag is substantially fully inflated. However, with the lower region 26 of the air-bag fully inflated to the degree illustrated in Figure 15, it should be appreciated that the recess 28 remaining after substantially full inflation of the air-bag 7 is relatively small when compared to the overall volume of the air-bag and is located only at the very top of the front region of the air-bag, adjacent the windscreen 3.

In each of the embodiments described above with reference to Figures 1 to 11 , the releasable connection between the tether 16 and the front panel of the air- bag 7, at the first anchor region 18, can take any convenient form. One such form for this releasable connection involves the use of two different weights of stitching thread, for example, M30 thread and M20 thread. As will be appreciated by the person of skill in the art, M30 thread is significantly weaker than M20 thread. The thicker, and hence stronger, M20 thread is used to stitch together the fabric of the tether 16 and the fabric of the front panel of the air-bag 7 in the manner of a series of stitches, whilst the thinner, and hence weaker M30 thread is used to overlock the M20 threads. Upon application of a peel force to the anchor region, by the front panel of the air-bag, the M30 threads are ruptured such that the overlooking is removed, thus weakening the connection. Further application of force to the connection is then sufficient to rupture also the M20 threads.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

Claims:

1. An airbag unit (1) comprising a housing (5), an airbag (7), and an inflator (6) for inflating the airbag, the airbag (7) being packed within the housing (5) such that, upon subsequent inflation of the airbag, at least part of an upper region (24) of the airbag inflates prior to substantial inflation of a lower region (26) of the airbag, the arrangement further comprising a restraining tether (16) provided within the airbag (7) and secured, at a first anchor region (18), to a substantially central region of a front panel of the airbag and secured, at a second anchor region (22), to another part of the airbag unit (1), whereby, at least during an initial stage of inflation of the airbag (7), the restraining tether (16) is placed under tension to restrain inflation of the central region of the front panel, thereby creating a centrally positioned recess (28) in the inflating airbag.

2. An airbag unit according to claim 1 , wherein the tether (16) is secured, at the second anchor region (22), to a rear panel of the airbag.

3. An airbag unit according to claim 2, wherein, between said anchor regions (18,22), the tether (16) passes through part (15) of the housing (5) for sliding movement through said part (15) of the housing.

4. An airbag unit according to any preceding claim, wherein the tether (16) is provided in the form of a loop, secured at both ends (17,20) to the front panel.

5. An airbag unit according to claim 4, wherein the loop is arranged so as to pass substantially around the inflator (6), at least during an initial stage of inflation of the airbag, so as to serve as a gas-diffuser during inflation.

6. An airbag unit according to claim 1 , wherein the tether (16) is secured, at the second anchor region (22), to the housing (5).

7. An airbag unit according to any preceding claim, wherein the tether (16) is releasably secured to the front panel at said first anchor region (18) so as to be released from the front panel when the lower region (26) of the airbag is inflated to a predetermined degree.

8. An airbag unit according to claim 7, wherein the tether (16) is secured to the front panel at said first anchor region (18) by a releasable connection comprising at least one tear-seam configured to rupture when the lower region (26) of the airbag is inflated to said predetermined degree.

9. An airbag unit according to claim 8, wherein the or each tear-seam is configured to rupture under a peel-force applied to the seam by the inflating airbag, but to remain substantially intact under a shear- force applied to the seam by the inflating airbag (7).

10. An airbag unit according to any preceding claim, wherein the airbag

(7) is packed within said housing (5) so as to form of at least two fold-packets, a first of said fold-packets being arranged for substantially direct inflation by said inflator (6) to define the upper region (24) of the airbag, and the second of said fold-packets being arranged so as to be pulled from the housing (5) by the first fold packet as the first fold packet is inflated such that the second fold- packet defines the lower region (26) of the airbag.

11. An airbag unit according to claim 10, wherein the housing (5) comprises a divider (30), and wherein the inflator (6) and said first fold-packet are arranged on one side of the divider (30) and said second fold-packet is arranged on the other side of the divider.

12. An airbag unit according to any preceding claim, wherein the first anchor region (18) extends a distance d transversely across the airbag, where d is at least equal to width of the head of a 95 percentile male dummy.

13. An airbag unit according to any preceding claim, wherein the tether (16) is formed as a fabric sheet.

14. An airbag unit according to claim 13 as dependant upon claim 12, wherein the fabric sheet (16) has a substantially uniform width equal to distance d.