WO2005088152A1

WO2005088152A1 - Brake disc

Info

Publication number: WO2005088152A1
Application number: PCT/GB2005/000394
Authority: WO
Inventors: Jonathan James Simmonds; Ian Antony Nash
Original assignee: A P Racing Limited
Priority date: 2004-03-13
Filing date: 2005-02-04
Publication date: 2005-09-22
Also published as: GB0618073D0; GB2426304A; GB2426304B; GB0405697D0

Abstract

A brake disc (9) has an inner hub portion (10) and an outer disc portion (11). The outer disc portion is mounted to the hub portion by means of a plurality of circumferentially spaced strap means (12) such that the outer disc portion is able to move axially relative to the hub portion. The axial movement of the outer disc portion relative to the hub portion during normal use is limited to a maximum of 1mm or less either side of a central free running or neutral position. In certain embodiments, the axial movement of the outer disc portion relative to the hub portion during normal usage is limited to a maximum of 0.75 mm or 0.5mm either side of a central free running or neutral position. The axial movement of the outer disc portion relative to the hub portion during normal usage may be determined by the spring rate of the strap means and/or my means of physical stops which may be adjustable. Each strap means comprises a plurality of individual planar strap members one on top of each other to form a laminated strap.

Description

BRAKE DISC

The present invention relates to a brake disc. In particular, but not exclusively, the present invention relates to a brake disc for use with high performance vehicles including high performance motor cars and motorcycles.

It is known to provide a brake disc in which an outer disc portion is rigidly mounted to an inner hub portion. For use with standard production vehicles, the brake outer disc portion and hub are often produced as a one-piece component. For higher performance vehicles, it is known to produce a brake disc having an outer disc portion that is rigidly attached to the hub, either by means of bolts or rivets.

It is also known to provide a brake disc of the floating type, in which the outer disc portion is mounted to the hub portion so that it can move axially relative to the hub portion to a limited extent. Floating discs offer a number of advantages over brake discs in which the outer disc portion is rigidly connected to the hub portion. These include: • improved thermal stability as a result of removing the radial constraint present in any rigidly mounted outer disc portion, this leads to a reduction in coning;

• reduced tendency for build up of disc thickness variation (DTV) as axial movement of the outer disc portion relative to hub effectively eliminates run-out of the outer disc portion relative to the hub;

• reduced transmission of vibration created at the interface of the outer disc portion and pad;

• resistance to cracking which can be caused by stresses induced in a rigidly mounted outer disc portion by thermal variations.

These and other advantages of floating type brake discs are well known in the art. However, the arrangements used to drivingly connect the hub to the outer disc portion in known floating type brake discs tend to be complicated and hence are expensive to manufacture and not always reliable in use. Such known arrangements often include components that slide relative to one another with small clearances between them. In such arrangements there is a tendency for the clearances to become clogged with corrosion by-products so preventing the parts from sliding freely. Also, such mechanisms are prone to wear which can result in the need for the components, and in particular the inner hub, to require frequent replacement.

It is also known provide a friction disc in which an outer disc portion is connected to an inner hub by means of a plurality of straps. This arrangement is commonly used in clutch friction plates but GB 1 371 158 discloses a friction disc that is described as being adapted for use with either brake discs or clutches. In the arrangement disclosed, an outer disc portion is drivingly connected to a hub portion by means of four torsion straps made of a relatively thin metal to allow the outer disc portion move axially relative to the hub to compensate for wear of the friction material in fixed pad assemblies. Typically, brake pads can have up to 10mm of wear over their useful life and so the torsion straps described in GB 2 371 158 would be expected to enable the outer disc portion to move axially by a corresponding amount either side of a central neutral position in order to compensate for this wear. Such levels of axial movement would be impractical in most brake disc applications. It is an object of the present invention to provide an improved form of brake disc, which is able to accommodate stresses due to differential thermal expansion and contraction and to align itself axially with an associated brake caliper.

Thus according to the present invention, there is provided a brake disc having an inner hub portion and an outer disc portion, the outer disc portion being mounted to the hub portion by means of a plurality of circumferentially spaced strap means such that the outer disc portion is able to move axially relative to the hub portion, the arrangement being such that the axial movement of the outer disc portion relative to the hub portion during normal use is limited to a maximum of 1mm either side of a central free running or neutral position. As will be appreciated, a brake disc in accordance with the present invention is capable of accommodating differential thermal expansion between the inner hub portion and outer disc portion and allows the outer disc portion to float axially, to a limited extent, relative to the inner hub to align itself with an associated brake caliper. Thus a disk brake in accordance with the invention provides many of the advantages of a floating type brake disk but is simpler and more cost effective to manufacture and more reliable in use. Because the strap drive arrangement does not incorporate components that slide relative to one another, there is no risk of the mechanism becoming clogged and no wear takes place. Thus it is possible to retain and re-use the inner hub when the outer disc is replaced. Furthermore, as there are no loose parts or clearances in the mechanism, no noise is generated which makes the arrangement particularly suitable for use on road cars. The axial movement of the outer disc portion relative to the hub portion may be limited to a maximum of 0.75 mm or even 0.5mm either side of a central free ranning or neutral position during normal usage. The maximum axial movement of the outer disc portion relative to the inner hub portion during normal usage may be determined solely by the spring rate of the strap means. Alternatively, or in addition, stop means may be provided to physically restrict the axial movement of the outer disc portion relative to the inner hub portion. The stop means may be adjustable so that the maximum permitted axial movement of the outer disc portion relative to the inner hub portion can be selectively varied.

Each strap means may comprise a plurality of individual strap members one on top of each other. In which case, the individual strap members may each be substantially planar and extend substantially parallel to a brake pad contact surface of the outer disc portion and to each other when the outer disc portion is in a central, neutral position relative to the inner hub portion.

There may be eight to twelve circumferentially spaced strap means.

The strap means may extend substantially tangential to a circle drawn about an axis of rotation of the disc and are arranged so that, in use, the straps are stressed in tension when the disc is subjected to braking forces whilst rotating in a forward direction.

The inner hub portion may have a plurality of radially outwardly projecting lugs and the outer disc portion may have a plurality of radially inwardly projecting lugs, each strap means being attached between one of the lugs on the inner hub and an adjacent lug on the outer disc portion. The two portions of the disc may be located radially relative to each other by means of a controlled clearance between one of the sets of lugs and a peripheral portion of the other disc portion which extends between the lugs on the other disc portion. The controlled clearance may be provided between the radially inwardly projecting lugs on the outer disc portion and the valleys formed between adjacent outwardly extending lugs on the inner hub portion.

Each strap means may be attached to the outboard faces of adjacent lugs. Alternatively, each strap means may be attached to the inboard faces of adjacent lugs.

The lugs may be arranged so as to substantially fill the gap between the inner hub portion and the outer disc portion and lugs on either or both of the inner hub and outer disc may be recessed to provide clearance for the strap means to operate whilst maximising the strength and integrity of the lugs.

In an alternative arrangement, the outer disc portion may be provided with an array of radially inwardly projecting lugs, the outer diameter of the inner hub overlying the lugs and being provided on an inboard surface with a plurality of bosses separated by recesses, the arrangement being such that the lugs on the outer disc portion are aligned with the recesses on the inner hub portion and each strap means being assembled between one of the bosses and an adjacent lug. The present invention is primarily designed for use with metal brake discs but could also be used with discs in which one or both disc portions is formed from a carbon based material such as a carbon/carbon composite material or a carbon fibre reinforced ceramic material. Several embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Fig. 1 is a perspective view of a first embodiment of a brake disc in accordance with the present invention;

Fig. 2 is a front view in the direction of arrow A of the disc of Fig. 1 ;

Fig. 3 is an enlarged cross sectional view taken on line A-A of Fig. 2;

Fig. 4 is a view similar to that of Fig. 1 but showing a second embodiment of a brake disc in accordance with the invention;

Fig. 5 is a perspective view from the rear of the brake disc of Fig. 4;

Fig. 6 is a front view of a third embodiment of a brake disc in accordance with the invention;

Fig. 7 is an enlarged cross sectional view taken on line C-C of Fig. 6;

Fig. 8 is a perspective view from the rear of the brake disc of Figs. 6 and 7;

Fig. 9 is a front view of a fourth embodiment of a brake disc in accordance with the invention;

Fig. 10 is an enlarged cross sectional view taken on line A-A of Fig. 9; and Fig. 11 is a perspective view from the rear of the brake disc of Figs. 9 and 10.

Referring to the Figs. 1 to 3 of drawings, a first embodiment of a motor vehicle brake disc 9 in accordance with the invention comprises an inner hub 10 and an outer disc portion 11 that are connected together by a plurality of circumferentially spaced strap means 12. The hub 10 has a series of radially outwardly projecting lugs 13 whilst the outer disc portion 11 is provided with radially inwardly extending lugs 15 that are arranged to interleave with the lugs 13 on the inner hub. One end 12a of each strap means is secured by means a rivet 14 to one of the lugs 13 on the inner hub, whilst the other end 12b of each strap means is secured by means a rivet 16 to an adjacent lug 15 on the outer disc portion. In the present embodiment, the interleaving lugs are arranged so that they substantially fill the gap between the inner hub and the outer disc portion to ensure the maximum lug strength and integrity. In order to provide sufficient clearance for the strap means 12 to operate effectively, the outboard faces of the radially outwardly projecting lugs 13 on the inner hub are recessed 13a. Depending on the material used to make the hub, the lugs 13 could be reduced in size to provide a more open structure.

Although in the present embodiment the strap means 12 are attached to the lugs 13 on the hub 10 and the lugs 15 on the outer disc portion 11 by means of rivets 14, 16, it should be understood that any suitable fastening means could be used. For example, the strap means 12 could be attached to the lugs on either or both of the hub and the outer disc portion by means of bolts.

The number and arrangement of the strap means 12 is selected to ensure that they are capable of transmitting the maximum required brake torque for the particular vehicle application and to ensure that the axial movement of the outer disc portion 11 relative to the hub is limited during normal use to a maximum of 1mm or less either side of a central free running or neutral position. The central free running or neutral position being the axial position adopted by the outer disc portion 11 relative to the hub portion 10 as a vehicle to which the disc is fitted travels in a straight line without braking. In this condition, the disc 9 is rotating but is not subjected to either braking or cornering forces. In the embodiment shown, there are eleven strap means 12 circumferentially spaced about the hub 10. The precise number of strap means 12 provided can be varied depending on the particular application and the size of the brake disc. However, it is preferred that eight to twelve strap means 12 are used.

As shown in Figure 3, each strap means 12 consists of a number of separate strap members one on top of each other to form a laminated strap means. In the present embodiment there are three strap members 12c-12e but the actual number of strap members can be varied as required. By using more than one strap member 12c-12e, it is possible to ensure that the strap means 12 are capable of transmitting the required levels of torque whilst providing a spring rate that will enable the outer disc portion to float axially relative to the hub up to the permitted maximum travel in normal use. The strap members are planar and are arranged to extend parallel to the brake pad contact surfaces of the outer disc member and to each other, at least when the outer disc portion is in the central free running or neutral position. Preferably, each strap member is made of stainless steel; however, any suitable material could be used. The disc is 9 is designed so that the strap members 12 are stressed in tension when the disc is subjected to braking forces applied whilst the disc is rotating in a forward direction. The forward direction of the disc being the direction in which the disc rotates when fitted to a vehicle which is itself travelling in a forward direction. This arrangement could be reversed in certain vehicle applications.

By appropriate selection of the dimensions and material for each strap member and the number of strap members in each strap means 12, the spring rate of the strap means can be varied to suit any required vehicle application. The spring rate selected should be such that it allows the outer disc portion 10 to move axially relative to the hub portion 11 to compensate for run-out of the outer disc portion but resists movement beyond the permitted maximum travel, which in the present embodiment is 1mm in either direction from the central neutral position, in normal use. In certain applications, the spring rate may be selected such the maximum permitted axial float of the outer disc portion relative to the hub in normal use is 0.75mm or even 0.5mm either side of the central free running or neutral position. The term "normal use" should be understood to encompass situations in which the disc 9 is subjected to braking and cornering forces up to the maximum predicted for the particular vehicle application for which the disc is designed. It will be understood by those skilled in the art that it may be possible to move the outer disc portion 10 axially relative to the hub portion 11 by more that the specified limit if excessive forces beyond those that would be experienced in normal use of the vehicle are applied. Excessive forces might be applied where a vehicle to which the disc is fitted strikes a kerb or other surface irregularity at high speed, for example. When calculating the spring rate for the strap means 12, it is helpful to consider the overall spring rate for the disc assembly, i.e. the rate to deflect the outer disc portion 10 relative to the hub portion 11, rather than a spring rate for each strap member 12c-12e or strap means 12. This overall spring rate can be referred to as the assembly spring/deflection rate. Typical values for the assembly spring/deflection rate are expected to range from lOON/mm to 300N/mm, and in particular from 150N/mm to 250 N/mm, although rates outside of these ranges could be used as required. The assembly spring/deflection rate can be tuned to meet the requirements of a specific vehicle application dependant on the amount of suspension/bearing deflection experienced on the vehicle.

The torque capacity of a brake disc in accordance with the invention is dependent not only on the number of strap means 12 and the number of strap members 12c-12e in each strap means but also on a number of other variables such as the radius on which the strap means are mounted, which equates to the brake disc diameter. Furthermore, when calculating the torque capacity for any particular application, attention should be paid to the buckling limit for the strap means 12 if the direction of applied torque were reversed rather than the tensile capacity of the forward direction. In other words, the brake disc 9 should be designed so that it is capable a taking full brake torque even if fitted to the vehicle the wrong way round or if the vehicle is braked heavily whilst travelling in a reverse direction. In a typical application, the brake disc 9 will be designed so that it can handle brake torque of 2kNm to 4kNm in the reverse direction but torque capacities outside of this range are also within the scope of the present invention.

As shown in the drawings, the outer disc portion 11 is of a ventilated design with individual ventilation passages 17 that extend between the inner and outer peripheries of the disc portion in a conventional manner. It will be appreciated, however, that the outer disc portion could be of a non- ventilated design. In normal use, the inner hub and outer disc portion are self-centring.

However, during assembly of the disc 9, the concentricity of the inner hub and outer disc portion can be ensured by means of a controlled clearance between the inner periphery 15a of the radially inwardly extending lugs 15 and the valley portions 11a of the inner hub between the radially outwardly extending lugs 13. In the preferred embodiment, the controlled clearance is in the range of 0.6mm to 2.5 mm on diameter, although controlled clearances outside of this range may be used.

A second embodiment of a brake disc 109 in accordance with the invention will now be described with reference to Figs. 4 and 5. Similar reference numerals but increased by 100 are used to identify similar components.

The brake disc 109 is similar to the brake disc 9 described above except that the strap means 112 are connected to the inboard faces of the lugs 113, 115 on the inner hub 110 and the outer disc portion 111.

The lugs 113, 115 are arranged to substantially fill the gap between the inner hub and the outer disc portion to aid the channelling of cooing air through the disc vents 117 rather than it spilling out between the hub and outer disc portion. By mounting the strap means 112 inboard, they are positioned in the cooling airflow, which can be advantageous. Also, in some applications it may be more aesthetically pleasing to have the strap means 112 hidden behind the inner hub 110 and outer disc portion 111. In this embodiment, the rear or inboard faces of the lugs 113 on the inner hub are recessed at 120 to ensure that the strap means have sufficient clearance to operate effectively.

As shown in Figs. 4 and 5, the brake disk 109 has twelve strap means 112 that are connected to the lugs 113, 115 of the inner hub 110 and outer disc portion 111 by means of bolts 122. However, as already discussed, the number of strap means 112 can be varied to suit the particular application and the strap means 112 could be attached by any suitable means, such as rivets. Figs. 4 and 5 also show an array of grooves 130 formed in the brake pad contact surfaces of the outer disc portion 111, these grooves can be omitted altogether or a different groove pattern adopted as required.

A third embodiment of a brake disc 209 in accordance with the invention will now be described with reference to Figs. 6 to 8. Similar reference numerals but increased by 200 are used to identify similar components to those described above in relation to the first embodiment.

The disc 209 differs from the previously described embodiments in that the inner hub 210 does not have radically outward protecting lugs on its outer diameter. Rather, the outer diameter of the inner hub is increased so as to overlie the radially inwardly projecting lugs 215 on the outer disc portion 211. The inboard face 224 of the inner hub 210 is relieved to form a series of raised bosses 226 separated by recess 228. The strap means 212 are assembled between the bosses and the outboard faces of the lugs 215 on the outer disc portion, with die lugs 215 aligned opposite the recesses 228.

The arrangement in accordance with the third embodiment aids in channelling cooling air through the vents 217 of the outer disc portion 211, whilst allowing air to flow from inside the disc through to the outboard face of the outer disc portion. This can be beneficial in certain installations. As with the second embodiment, the third embodiment may be more aesthetically pleasing as the strap means 212 cannot be seen from the front of the disc when installed on a vehicle.

An array of grooves 230 may be provided on the brake pad contact surfaces of the outer disc portion 211 as shown in Fig. 6. However, these grooves 230 can be omitted, as shown in Fig. 8, or a different groove pattern adopted as required.

A fourth embodiment of a brake disc 309 in accordance with the invention will now be described with reference to Figs. 9 to 11. The fourth embodiment is similar to the third embodiment described above with reference to Figures 6 to 8 and similar reference numerals but increased by 100 are used to identify similar components to those described above in relation to the third embodiment.

As mentioned, the brake disc 309 is similar to the disc 209 described above with reference to Figures 6 to 8 and so its construction will not be described in detail. The disc 309 differs from the disc 209 principally in that it is provided with stop means for limiting the axial float of the outer disc portion 311 relative to the inner hub portion 310. The stop means in this embodiment comprises twelve restrictor plates 332 arranged to limit the inward axial movement of the outer disc portion 311 relative to the hub 310 and twelve adjustable screw members 333 arranged to limit the outward axial movement of the outer disc portion relative to the hub 310.

Each of the restrictor plates 332 is made of stainless steel, or any other suitable material, and is mounted to the inboard surface of the inner hub portion 310. Conveniently, each restrictor plate 332 can be mounted to the inner hub portion by means of one of the bolts or rivets 314 that fasten one end of a strap member 312 to a respective one of the raised bosses 326 formed on the inboard surface of the inner hub portion 310. Each restrictor plate is shaped such that a portion of the plate 332a overlies an inboard face of a respective one of the lugs 315 on the outer disc portion. The overlapping portion 332a of each restrictor plate is arranged to lie substantially parallel with the inboard face of its respective lug 315 and is set at a predetermined distance from the lug when the outer disc portion is in the central or neutral position.

Whilst the restrictor plates 332 are shown as being separate components in the present embodiment, those skilled in the art will appreciate that a restrictor plate could be formed integrally with one of the drive straps 312a. Each of the adjustable screw members 333 comprises a generally cylindrical body having a screw thread 334 over at least part of its length for engagement with a corresponding thread formed in a through bore in the inner hub portion 310. An inboard end of each screw member is arranged to engage an outboard face of a respective one of the lugs 315 to limit outward axial movement of the outer disc portion 311 relative to the inner hub portion 310. A hexagonal blind bore 335 is formed in the outboard end of the screw member to receive a hexagonal socket wrench or the like, to enable the screw member to be adjusted inwardly and outwardly to vary amount of axial float permitted. A locking nut 336 is provided to lock the screw member in position following adjustment.

The stop means 332, 333 acts to physically limit the axial float of the outer disc portion 311 relative to the inner hub portion 310. This is advantageous in ensuring that the outer disc portion 311 does not move axially relative to the inner hub portion 310 more than the maximum permitted axial float of 1mm or less either side of a central or neutral position even where the disc is subjected to forces in excess of those expected during normal use. This may be particularly advantageous where the disc is to be used on a racing vehicle where it is more likely to be subjected to very high cornering and braking forces and where the vehicle may clip a track kerb whilst cornering. It will be appreciated, however, that such stop means can have application in brake discs intend for none racing applications. Where stop means are provided, the stop means can be used to limit the axial float of the outer disc portion 311 even under normal operating conditions rather than relying on the spring rate of the strap means. This means that the strap means can have a lower spring rate tuned to provide optimum performance in compensating for run-out of the outer disc portion, whilst the stop means are used to control the degree of axial float during normal use as well as under abnormal conditions such as a kerb strike.

Although the present embodiment has stop means associated with each of the lug 315 on the outer disc portion this is not essential and the number of stop means can be varied as desired. Also the design of the stop means is not limited to those used in the present embodiment but any suitable form of stop means for physically limiting the axial movement of the outer disc portion 315 relative to the inner hub portion 310 can be used. For example, the stop means 332 acting to limit the inward axial movement of the outer disc portion may be arranged so as to be adjustable. This could be achieved by providing a range of restrictor plates 332 each providing a different clearance between the overlapping portion 332a and the inboard face of the lug 315 on the outer disc portion. In this arrangement, the maximum permitted inward axial float is adjusted by fitting the appropriate restrictor plates. Alternatively, a different form of stop member may be used to enable the inward axial float to be adjusted in-situ. Such an adjustable stop member may incorporate a screw adjuster, for example. Whilst the ability to adjust the stop members to vary the maximum permitted axial float of the outer disc member 311 relative to the inner hub member 310 can be advantageous in certain applications, for example when used on racing vehicles where the ability to tune the performance of the disc on the vehicle is desirable, this is not essential and the stop members may be fixed. For example, suitable stop means may be formed integrally with the inner hub member 310 and the outer disc portion 311 to provide a preset and permanent restriction of the axial float of the outer disc portion 311. Thus the stop means may be machined or cast into the inner hub portion 310 and the outer disc portion 311.

Stop means for restricting the axial float of the outer disc portion 11, 111, 211 may be incorporated into any of the embodiments described above in relation to Figures 1 to 8. Whilst in the particular examples illustrated both the inner hub portion

10, 110, 210 and outer disc portion 11, 111, 211 are made from metal such as aluminium, aluminium alloy, steel, or cast iron, the hub and outer disc portion can be made of any suitable material. For example, either or both of the outer disc portion and the hub portion can be manufactured from metal matrix composite or a carbon-based material such as carbon-carbon composite material or from a carbon ceramic composite material.

As indicated above, the present invention allows radial expansion of the outer disc portion free from constraint by the inner hub so that distortion of the disc ("coning") is kept to a minimum thus minimising "knock-back" of the pistons of the associated brake calliper and mamtaining consistency of pedal position/feel. The axial float of the disc portion relative to the inner hub counteracts

"knock-back" caused by deflection of bearing pack and suspension components under braking and cornering loads. Problems such as vibration and judder are much improved due to reduced stiffness between the brake faces and the disc mounting.

It is a further advantage of the preferred embodiments of the invention, in which there are a eight to twelve strap means 12, 112, 212, 312 each comprising more than one strap member, that the manufacturing tolerances of the individual strap members will tend to average out. As a result, the manufacturing tolerances of the individual strap members can be larger than would be acceptable using only a single or a double strap member at each location, whilst still ensuring adequate control over the positioning of the outer disc portion relative to the hub portion. This helps to reduce manufacturing costs.

Whereas the invention has been described in relation to what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention.

Claims

1. A brake disc (9; 109; 209; 309) having an inner hub portion (10; 110; 210; 310) and an outer disc portion (11; 111; 211; 311), the outer disc portion being mounted to the hub portion by means of a plurality of circumferentially spaced strap means (12; 112; 212; 312) such that the outer disc portion is able to move axially relative to the hub portion, characterised in that the axial movement of the outer disc portion relative to the hub portion during normal use is limited to a maximum of 1mm either side of a central free running or neutral position.

2. A brake disc (9; 109; 209; 309) according to claim 1, in which the axial movement of the outer disc portion (11; 111; 211; 311) relative to the hub portion (10; 110; 210; 310) during normal usage is limited to a maximum of 0.75 mm either side of a central free running or neutral position.

3. A brake disc (9; 109; 209; 309) according to claim 1, in which the axial movement of the outer disc portion (11; 111; 211; 311) relative to the hub portion (10; 110; 210; 310) during normal usage is limited to a maximum of 0.5 mm either side of a central free running or neutral position.

4. A brake disc (9; 109; 209)according to any one of claims 1 to 3, in which the maximum axial movement of the outer disc portion (11; 111; 211; 311) relative to the inner hub portion (10; 110; 210; 310) during normal usage is determined by the spring rate of the strap means (12; 112; 212; 312).

5. A brake disc (309) as claimed in any previous claim, in which stop means are provided to restrict the axial movement of the outer disc portion (11; 111; 211; 311) relative to the inner hub portion (10; 110; 210; 310).

6. A brake disc (309) as claimed in claim 5, in which the stop means are adjustable so that the maximum permitted axial movement of the outer disc portion (11; 111; 211; 311) relative to the inner hub portion (10; 110; 210; 310) can be selectively varied.

7. A brake disc (9; 109; 209; 309) according to any previous claim, in which each strap means (12; 112; 212; 312) comprises a plurality of individual strap members (12c, 12d, 12e) one on top of each other.

8. A brake disc (9; 109; 209; 309) according to claim 7, in which the individual strap members (12c, 12d, 12e) are each substantially planar and extend substantially parallel to a brake surface of the outer disc portion (11; 111; 211; 311) and to each other when the outer disc portion is in a central, neutral position relative to the inner hub portion (10; 110; 210; 310).

9. A brake disc (9; 109; 209; 309) according to any previous claim, in which there are eight to twelve circumferentially spaced strap means (12; 112; 212; 312).

10. A brake disc (9; 109; 209; 309) according to any previous claim, in which the strap means (12; 112; 212; 312) extend substantially tangential to a circle drawn about an axis of rotation of the disc and are arranged so that, in use, they are stressed in tension when the disc is subjected to braking forces whilst rotating in a forward direction.

11. A brake disc (9; 109) according to any previous claim, in which the inner hub portion (10; 110) has a plurality of radially outwardly projecting lugs (13; 113) and the outer disc portion (11; 111) has a plurality of radially inwardly projecting lugs (15; 115), each strap means (12; 112; 212; 312) being attached between one of the lugs on the inner hub and an adjacent lug on the outer disc portion.

12. A brake disc (9; 109) according to claim 11, in which the two portions (10, 11; 110,111) the disc portions are located radially relative to each other by means of a controlled clearance between one of the sets of lugs (15; 115) and a peripheral portion (11a; Ilia) of the other disc portion which extends between the lugs (13; 113) on the other disc portion.

13. A brake disc according to claim 12, in which there is a controlled clearance between the radially inwardly projecting lugs (15, 115) on the outer disc portion (11; 111) and the valleys formed between adjacent outwardly extending lugs on the inner hub portion (10; 110).

14. A brake disc (9) according to any one of claims 11 to 13, in which each strap means (12) is attached to the outboard faces of adjacent lugs (13, 15).

15. A brake disc (109) according to any one of claims 11 to 13, in which each strap (112) means is attached to the inboard faces of adjacent lugs (113, 115).

16. A brake disc (9; 109) as claimed in claim 14 or claim 15, in which the lugs (13, 15; 113, 115) are arranged so as to substantially fill the gap between the inner hub portion (10; 110) and the outer disc portion (11; 111).

17. A brake disc (9) as claimed in claim 14 or claim 16 when dependant on claim 14, in which the outboard face of each lug (13, 15) on either or both of the inner hub portion (10) and the outer disc portion (11) is partially recessed.

18. A brake disc (109) as claimed in claim 15 or claim 16 when dependant on claim 15, in which the inboard face of each lug (113, 115) either or both of the inner hub portion (110) and the outer disc portion (111) is partially recessed.

19. A brake disc (209; 309) as claimed in any one of claims 1 to 10, in which the outer disc portion (211; 311) is provided with an array of radially inwardly projecting lugs (215; 315), the outer diameter of the inner hub ( 210; 310) overlying the lugs and being provided on an inboard surface with a plurality of bosses (226; 326) separated by recesses (228; 328), the arrangement being such that the lugs on the outer disc portion are aligned with the recesses on the inner hub portion and each strap means (212; 312) being assembled between one of the bosses and an adjacent lug.

20. A brake disc (9; 109; 209; 309) according to any previous claim in which both portions of the disc are metal.

21. A brake disc constructed and arranged substantially as hereinbefore described with reference to and as shown in Figures 1 to 3, or 4 and 5, or 6 to 8, or 9 to llof the accompanying drawings.