FIELD OF THE INVENTION 
   The present invention relates to a braking band and to a disc for a ventilated disc brake, particularly but not exclusively for applications in the automotive field. 
   BACKGROUND OF THE INVENTION 
   As is known, a disc of the type specified above is constituted by two coaxial portions. A first portion, the support bell, is for connection to the wheel hub of a vehicle, and the remaining, peripheral portion, the so-called braking band, is for cooperating with the disc-brake calipers to exert the braking force on the vehicle. 
   It is also known that, during the operation of the brakes, the friction between the pads of the brake calipers and the surface of the braking band generates a large amount of heat which has to be dissipated. The heat generated in fact causes many undesired phenomena such as, for example, the formation of cracks on the braking surface, permanent deformations after cooling, or localized changes in state which in turn lead to vibrations. 
   In particular, in applications on high-performance motorcars with high braking efficiency, the energy to be dissipated is very high and there is an even greater need to dissipate the heat generated during braking. 
   This need for increased cooling efficiency must, however be reconciled with the further need to keep the space occupied by the discs unchanged to avoid modifications to the suspension of the vehicle for which they are intended. 
   For this purpose, ventilated discs have undergone continuous development, particularly with regard to the number and configuration of the so-called ventilation ducts which are defined between the two plates that conventionally make up ventilated discs, and between fins which connect the two plates transversely. 
   For example, fins constituted by curved walls repeated at a modular frequency, in which the module itself is constituted by two fins of which one is longer than the other are known from U.S. Pat. No. 5,427,212. 
   The above-mentioned example of a disc-brake disc cannot ensure effective cooling such as to overcome the above-mentioned disadvantages completely. 
   The problem underlying the present invention is therefore that of devising a disc-brake disc which has structural and functional characteristics such as to satisfy the above-mentioned requirements and at the same time to prevent the problems mentioned with reference to the prior art. 
   SUMMARY OF THE INVENTION 
   This problem is solved by a braking band according to claim  1  and by a disc-brake disc according to claim  24 . 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages of the disc-brake disc according to the invention will become clear from the following description of a preferred embodiment thereof, provided by way of non-limiting example with reference to the appended drawings, in which: 
       FIG. 1  is a perspective view of a disc-brake disc according to the invention, 
       FIG. 2  is a partially-sectioned front view of a detail of  FIG. 1  in the rough, unfinished state, 
       FIG. 3  is a section taken on the line III-III through the detail of  FIG. 2 , 
       FIG. 4  is a section taken on the line IV-VI through the detail of  FIG. 2 , 
       FIG. 5  is a front view of a finished detail of  FIG. 1 , 
       FIG. 6  is a section taken on the line VI-VI through the detail of  FIG. 5 , and 
       FIG. 7  is a perspective view of a disc-brake disc according to a possible variant. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to the above-mentioned drawings, a ventilated disc according to the present invention, for use in a disc brake (not shown) of a vehicle such as a motorcar, is generally indicated  10 . 
   The disc  10  has a substantially circular shape and extends about an axis indicated Z-Z in  FIG. 1 . 
   The disc  10  comprises a support bell  12  and a braking band  14  coaxial with the bell  12 . The support bell  12  and the braking band  14  may be connected to one another either rigidly or by means of connections which permit differential expansion. An example of a bell  12  and a braking band  14  in the assembled condition is shown in  FIG. 7 . Preferably, the braking band  14  is made of cast iron of the type usually used for brake discs such as, for example, cast iron with a high carbon content. 
   The support bell  12  comprises a central portion  16  for connection to the wheel hub of a vehicle, in conventional manner, and an annular, peripheral portion  18  which projects from the central portion  16  in a direction substantially parallel to the axis Z-Z. 
   The braking band  14 , which is intended to cooperate with the disc-brake calipers in order to exert the braking force on the vehicle, comprises a first plate  20  and a second plate  22  arranged face to face so as to form an air-outlet space  24 . The first plate  20  is disposed on the same side as the support bell  12  and the second plate  22  is disposed on the opposite side. 
   The second plate  22  is fixed to the first plate  20  by spacer elements and, in the embodiment in question, by fins  26 ,  28 ,  30  and  32  which are shaped so as to define ventilation ducts  34 . 
   The fins  26 ,  28 ,  30  and  32  form a module which is arranged at predetermined intervals around a circle in the braking band  14  so as to form a plurality of ventilation ducts  34  distributed uniformly around the space between the two plates. 
   The first fin  26  connects the first plate  20  and the second plate  22  continuously, as can be seen from  FIG. 4 . 
   An inner end  26   a  of the first fin  26  is “V”-shaped and has its concavity facing towards the centre of the braking band  14 , whereas the outer end  26   b  of the first fin  26  has a profile substantially perpendicular to the first plate  20  and to the second plate  22 . 
   In greater detail, the “V”-shaped end comprises two straight portions inclined substantially at 45° to the first plate  20  and to the second plate  22  and connected in the region of a centreline of the first fin. 
   The inner end  26   a  is positioned on a median circle, that is, a circle with a diameter corresponding to a middle value between the values of the inside diameter and of the outside diameter of the braking band  14 , in the rough, unfinished state ( FIG. 2 ). The outer end  26   b  is positioned in the vicinity of the outer circumference of the braking band  14 . 
   As shown in  FIG. 2 , that is, in a plane parallel to the plates  20  and  22 , the first fin  26  extends along an arcuate line, indicated  36 , relative to which a concave surface  38  and a convex surface  40  of the first fin can be defined. 
   An arrow representing the principle direction of rotation of the disc  10 , that is, the direction of rotation corresponding to the forward direction of movement of the vehicle, is indicated  42 . 
   Relative to this direction of rotation, the first fin  26  is arranged substantially in a manner such that the concave surface  38  and the convex surface  40  are upstream and downstream of the arcuate line  36 , respectively. 
   The arcuate line  36  extends continuously along two circles which are defined below with reference to the system of Cartesian axes X and Y shown in  FIG. 2 . 
   In fact, the inner portion of the first fin  26 , that is the portion closest to the axis Z-Z, extends along a portion of a circle of radius R 1  and of centre A 1 , whereas the outer portion of the first fin  26 , that is the portion farthest from the axis Z-Z, extends along a portion of a circle of radius R 2  and of centre A 2 . 
   The radius R 1  of the inner portion of the first fin  26  is greater than the radius R 2  of the outer portion of the first fin  26 . 
   The positions of the above-mentioned two centres A 1  and A 2  are also indicated in  FIG. 2 . In particular, the centre A 1  is disposed on a circle of radius R 3 , outside the braking band  14 , whereas the centre A 2  is disposed on a circle of radius R 4  having a diameter between the inside diameter and the outside diameter of the rough, unfinished braking band  14 . 
   The outer portion of the first fin  26  is thus more arcuate than its inner portion and the outer portion of the respective ventilation duct  34  consequently deviates further in the direction contrary to the direction of rotation of the disc  10 . 
   In  FIG. 2 , at the dimensional control stage, the centre A 1  is given by the intersection of the circle of radius R 3  and a straight line extending through the centre of the system of axes X and Y and inclined at an angle α of about 14° to the axis X. Similarly, the centre A 2  is given by the intersection of the circle of radius R 4  and a straight line extending through the centre of the system of axes X and Y and inclined at an angle β of about 40° to the axis X. 
   The first fin  26  also comprises two projections  44   a  and  44   b  which project from the concave surface  38  towards the interior of the ventilation duct  34 . The first projection  44   a  is positioned approximately centrally relative to the length of the first fin  26 , whereas the second projection  44   b  is positioned in the vicinity of the outer end  26   b.    
   The first fin  26  is repeated around the space  24  at predefined intervals which permit the production of a disc with about 20-50 first fins. 
   The further fins  28 ,  30  and  32  which, for simplicity of explanation, will be indicated as a second, a third, and a fourth fin, are positioned along further arcuate line, indicated  46 . 
   This further arcuate line  46  is offset relative to the arcuate line  36  by a distance equal to half of the intervals at which the arcuate lines  36  are repeated. The alternating arrangement of the first fin  26  along the arcuate line  36  and of the other fins  28 ,  30  and  32  along the further arcuate line  46  permits the production of a disc with approximately 40-100 ventilation ducts  34 . 
   The further arcuate line  46  also extends continuously along two circles which are defined below with reference to the system of Cartesian axes X and Y shown in  FIG. 2 . 
   In fact, the inner portion of the arcuate line  46 , that is the portion closest to the axis Z-Z, extends along a portion of a circle of radius R 1  and of centre B 1 , whereas the outer portion of the further arcuate line  46 , that is the portion farthest from the axis Z-Z, extends along a portion of a circle of radius R 2  and of centre B 2 . 
   The radius R 1  of the inner portion of the arcuate line  46  is greater than the radius R 2  of the outer portion of the arcuate line  46  and, in particular, these radii correspond to the respective radii defining the arcuate line  36  of the first fin  26 . 
   The outer portion of the further arcuate line  46  is therefore more arcuate than its inner portion and the outer portion of the respective ventilation duct  34  consequently deviates further in the direction contrary to the direction of rotation of the disc  10 . All of the ventilation ducts  34  therefore deviate in the same direction, contrary to the direction of rotation of the disc. 
   The positions of the two above-mentioned centres B 1  and B 2  are indicated in  FIG. 2 . According to the preferred embodiment shown in  FIG. 2 , the centre B 1  is disposed on a circle of radius R 3  outside the braking band  14 , whereas the centre B 2  is disposed on a circle of radius R 4  having a diameter between the inside diameter and the outside diameter of the braking band  14 . As can readily be inferred from the foregoing statements, the centre A 1  and the centre B 1  preferably lie on the same circle of radius R 3  and, similarly, the centre A 2  and the centre B 2  preferably lie on the same circle of radius R 4 . 
   In  FIG. 2 , at the dimensional control stage, the centre B 1  is given by the intersection of the circle of radius R 3  with a straight line extending through the centre of the system of axes X and Y and inclined at an angle γ of about 19° to the axis X. Similarly, the centre B 2  is given by the intersection of the circle of radius R 4  with a straight line extending through the centre of the system of axes X and Y and inclined at an angle δ of about 34° to the axis X. 
   As mentioned above, the second fin  28 , the third fin  30 , and the fourth fin  32  are disposed along the further arcuate line  46 . 
   The second fin  28  connects the first plate  20  and the second plate  22  continuously. Moreover, the second fin  28  has an inner end  28   a  disposed in the vicinity of the inner diameter of the braking band  14  in the rough, unfinished state and an outer end  28   b  corresponding to the output edge and disposed slightly beyond the above-mentioned median circle, that is, the circle of mean diameter between the inside and outside diameters of the braking band  14  in the rough, unfinished state. 
   In a substantially finished braking band  14 , the second fin  28  adopts the configuration shown in  FIG. 5 , in which the machining undergone by the first end  28   a  is clear. 
   The third fin  30  is disposed along the same arcuate line  46  in a manner such that an inner end  30   a  thereof is disposed slightly beyond the above-mentioned median circle and an outer end  30   b  thereof is disposed in the vicinity of a circle of mean diameter between the diameters of the above-mentioned median circle and the outer circumference of the braking band  14 . 
   Both the inner end  30   a  and the outer end  30   b  have portions  48  which are enlarged in comparison with the thickness of the fin itself. 
   As shown in  FIG. 2 , the third fin  30  is disposed substantially in an annular region between the two projections  44   a  and  44   b  of the first fin  26 . 
   Finally, the fourth fin  32  is disposed along the further arcuate line  46 . This last fin has an inner end  32   a  disposed on a circle substantially corresponding to the second projection  44   b  of the first fin  26  and an outer end  32   b  disposed in the vicinity of the outer circumference of the braking band  14 . 
   The fourth fin  32  is structurally identical to the third fin  30 , since both its inner end  32   a  and its outer end  32   b  have portions  48  which are enlarged in comparison with the thickness of the fin itself. 
   The disc-brake disc according to the invention satisfies the above-mentioned requirement and prevents the problems mentioned with reference to the prior art. In fact, as explained above, the structure of the disc according to the invention increases cooling efficiency, as is shown by the test results obtained and given in the following table, with a disc-brake disc produced in accordance with the teaching of U.S. Pat. No. 5,427,212 as a control: 
                                                                     U.S. Pat. No.   Present               5,427,212   invention                                        Weight (kg)   5,900   5,780           Disc temperature [° C.]   490   470           Pad temperature [° C.]   480   450           Caliper-fluid temperature [° C.]   130   120           Pad wear [mm]   6.9   6.7           “Disc stiffness” parameter   5.4   5.3                        
The second column relates to a disc having the characteristics described in U.S. Pat. No. 5,427,212, that is, a ventilated disc forming part of the prior art and selected as a control. The third column, on the other hand, relates to a disc which has the above-described characteristics and hence to a disc according to the present invention.
 
   The individual parameters taken into consideration have the following significance. In the first place, the term “weight” means the weight of the machined, finished braking band  14  ( FIG. 5 ). 
   The disc temperature corresponds to the temperature of the surface of the braking band  14  measured at the start of one of the braking operations after the disc has been brought to steady-state conditions during the test cycle. 
   The pad temperature corresponds to the mean temperature of the pad measured at the same moment at which the disc temperature was measured. 
   The term “caliper-fluid temperature” means the maximum temperature of the brake fluid in the caliper, measured at the end of the test cycle. 
   The term “pad wear” indicates the mean wear of the pads upon completion of the test cycle. 
   Finally, the expression “disc stiffness” defines an index of the travel of the brake pedal, evaluated as the pressure which the pump manages to develop for each millimetre of travel of the caliper piston. 
   The results given above were obtained with a test cycle on a dynamometric bench, the test parameters being kept constant in both of the cases considered. In particular, the parameters which were kept constant were the energy dissipated upon each braking operation, the time between the start of one braking operation and the start of the next braking operation, the torque developed during each braking operation and the number of braking operations. 
   As can be appreciated from the foregoing description, it is clear that the novel configuration of the fins, and consequently of the ventilation ducts, enables greater cooling efficiency to be achieved. 
   A further advantage of the disc-brake disc according to the invention lies in the fact that it is structurally and functionally simple so that it is inexpensive to produce and ensures reliable operation over time. 
   Naturally, in order to satisfy contingent and specific requirements, a person skilled in the art may apply to the disc-brake disc according to the invention many modifications and variations all of which, however, are included within the scope of protection of the invention as defined by the appended claims. 
   For example, the projections or the enlarged portions may have shapes, sizes and positions other than those described and illustrated. 
   Moreover, the braking surfaces may equally well be smooth, perforated, or with either linear or arcuate grooves, or may be both perforated and grooved.
A braking band ( 14) of a disc-brake disc ( 10), which has an unusual ability to improve the cooling efficiency of the disc brake, comprises a first plate ( 20) and a second plate ( 22) arranged face to face so as to form a space ( 24). A set of spacer elements ( 26 - 32) which connect the first plate ( 20) and the second plate. ( 22) constitutes a module which is repeated around a circle coaxial with the braking band. The set comprises a first fin ( 26) which connects the first plate ( 20) to the second plate ( 22) and which extends along an arcuate line ( 36). The set of spacer elements further comprises a second fin ( 28), a third fin ( 30), and a fourth fin ( 32) which connect the first plate ( 20) to the second plate ( 22) and which extend along a further arcuate line ( 46) interposed between two adjacent arcuate lines ( 36).