CA1156568A

CA1156568A - Thermally balanced rotor

Info

Publication number: CA1156568A
Application number: CA000355606A
Authority: CA
Inventors: Peter J. Soltis
Original assignee: Kelsey Hayes Co
Current assignee: Kelsey Hayes Co
Priority date: 1979-07-18
Filing date: 1980-07-07
Publication date: 1983-11-08
Also published as: JPS5618130A; FR2461847B1; GB2057609A; MX148253A; DE3026354A1; SE8005143L; FR2461847A1; BR8004438A; JPH0158779B2; IT1131711B; GB2057609B; DE3026354C2; IT8023519A0

Abstract

ABSTRACT
A disk brake rotor includes material or mass, preferably in the form of metal generally integrally cast with and disposed generally adjacent the outer periphery of the rotor which thermally balances the heat sink and inherent heat sinking function of the central mounting section portion of the disk rotor. In a vented disk brake rotor, this balancing mass may be disposed in the wall of the rotor most proximate the mounting section by proportioning the thickness of the wall at a given point in relation to the radial distance from the center of the rotor. In a solid disk brake rotor, thermal balancing mass may be disposed adjacent the periphery of the rotor beyond the wall areas of the rotor swept by the disk brake pads. The invention may also be practiced by providing an unswept area of a rotor sidewall proximate its outer periphery by reducing the radial width of a brake pad thereby providing a region which functions as a heat sink to compensate for the heat sinking function of the mounting section rather than a source of frictional heat.

Description

The invention relates generally to disk brakes and more specifically to disk brakes having a thermal balancing mass disposed on the rotor or disk. The thermal mass functions as a heat sink to balance the heat sink provided by the support and mounting structure which is utilized to secure the rotor or disk to the wheel hub.
The frictional heating and heat dissipation function of disk brakes in vehicle service is accurately described as cyclical. The brakes are intermittently and often relatively infrequently activated to stop the associated vehicle and this duty cycle provides substantial intervals during which the frictionally heated components may cool. The mass of a solid disk brake represents a substantial and accessible heat sink into which heat generated along the walls of the disk by frictional engagement with the brake pads may be transferred.
Nevertheless, the support structure which represents a substan-tial heat sink generally adjacent the inner edge of the disk having no corresponding or compensating heat sink on the outer edge of the disk will generate a radial temperature gradient acro~s the disk.
Prolonged application of disk brakes, of course, produces substantial quantities of heat which may be better dissipated by incorporating a plurality of radially disposed venting passageways within the disk which is then designated a rotor. (Although the term disk brake is commonly used generically to refer to both solid and vented rotor brake as-semblies, it should be noted that the term disk is properly used to refer only to a solid assembly and the term rotor is used in reference to a vented assembly. This distinction is generally maintained throughout the following specification except where repetition of disk and rotor would be ob-viously redundant or where the term disk brake is used generically to refer to a brake having a rotating circu-lar planar member frictionally clamped by a pair of opposed brake pads). The radial passageways provide additional heat transfer surface and air circulation within the rotor. A vented rotor will thus generally dissipate heat into the surrounding air more rapidly than a solid disk due to its greater heat transfer sur-face and air circulation~ Relative to the unified massof a solid disk, the inner and outer walls of a vented rotor represent thermal masses which exist in different thermal environments inasmuch as they are separated by a plurality of radial ribs or webs. Furthermore, the rotor hub and/or mounting structure is generally fabricated with or secured to at least one of the two rotor walls and represents a substantial thermal mass or heat sink.
Thus, a vented rotor is also subject to radial thermal temperature gradients in the wall adjacent or connected to the mounting and support structure.
An object of this invention is to provide a brake rotor or disk which exhibits a minimal radial tem-perature gradient.
According to one aspect of the invention there is provided a brake comprising a rotor assembly, the rotor assembly having a central mounting structure and a rotor section on the mounting structure, the rotor section comprising a rotor or a solid disk, and a caliper assembly having brake pads and means for advancing such ~,~

pads into contact with the rotor section, the central mounting structure being in heat transmitting relation-ship with the rotor section around the central mounting structure and the rotor section including mass means distributed on the rotor section for thermally balancing the thermal effect of the central mounting structure, whereby radial temperature gradients within the rotor section are reduced.
As will become apparent, the thermal balancing mass provided in accordance with the invention may be integrally cast with the rotor or disk or secured to the rotor or disk at any convenient step in the fabrica-tion process by any suitable fastening means. Further-more, the material of the balancing mass may be but need not be the same as the material of the disk or rotor itself. For example, if space were a consideration, a material of high heat capacity (specific heat) could be employed. In other applications, a lightweight material might be desirable. All such variations which provide heat sinking mass to balance the heat sinking mass of the rotor or disk support structure are deemed to be within the scope of the instant invention.
In a vented rotor, the balancing mass may be disposed on the wall of the rotor most proximate the support means, generally the outer wall, such that the outer wall thickness increases with increa~ing radial distance from the rotor center. Alternatively, heat sink mass may be disposed along a rotor wall in any fashion which achieves the desired heat sink balance with that of the rotor support structure and minimizes radial temperature gradients.
Alternatively, the vented rotor, or solid disk, may include mass or material about the periphery which is not acted upon by the brake pad. This may be accomplished by either reducing the area of frictional engagement between the brake pad and the rotor or disk wall by diminishing the pad's radial width thereby providing an unswept annulus about the outer periphery of the rotor or disk which functions as a heat sink or by adding additional material about the periphery of the disk in any convenient configuration. The distribution (i.e. cross section) of the peripheral mass may be adjusted to conform to particular space and thermal balancing require-ments.
A feature of a vented brake rotor in accordance with an embodiment of the invention is that it exhibits a minimum radial temperature gradient and can be manufactured by conventional and common manufacturing techniques.
Further features and advantages will become apparent by reference to the following description and appended drawings, in which:-Figure 1 is a perspective view of a disk brake and vented rotor employing the invention;
Figure 2 is a fragmentary, cross sectional view of a vented brake ro~or and caliper assembly taken along line 2-2 of Figure l;
Figure 3 is a fragmentary, perspective view of a vented brake rptor according to the invention illustrating the radial air passageways;

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Figure 4 is a fragmentary, perspective view of a vented disk brake rotor according to the invention illustrating an alternative embodiment mass aistribution and radial air passageways;
Figure 5 is a cross-sectional, diagrammatic view of a portion of a vented brake rotor illustrating the general heat transfer theory of the invention;
Figures 6 to 9 are fragmentary, cross-sectional views of solid brake disks in accordance with alternative embodiments of the invention.
Referring to Figure 1, a disk brake assembly accord-ing to the invention is generally designated by the reference numeral 10. The disk brake assembly 10 includes a caliper assembly 12 and a rotor assembly 14. The rotor assembly 14 includes a rotor section 16 having generally planar and parallel outer and inner walls 18 and 20, respectively. The rotor assembly 14 further includes a frusto-conical support structure or mounting section 22 in uninterrupted heat conductive rela-tion with the rotor section 16 around the mounting section 22 and which is generally cast integrally with the rotor section 16. The mounting section 22 defines a plurality of openings 24 which cooperate with a plurality of lug bolts 26 and other com-ponents of a wheel hub 28 to achieve securement of the rotor assembly 14 to the wheel hub 28 in a conventional fashion. The support structure for the rotor section 16 as well as the means for connecting it to the wheel hub 28 may take many forms and be produced by various manufacturing processes in addition to those described and illustrated. For example, the support structure may consist of a planar, inward extension of a rotor or disk axially slidably mounted upon a spindle or axle shaft by means of a fluted interconnection, a distinct insert 1 15~56~

which is secured to the rotor or disk by any appropriate securement means including integral casting, or a structure having a U-shaped cross section which attaches to the periphery of the disk or rotor. The invention may be practiced with all of the foregoing and many other mounting configurations.
Referring now to Figures 1 and 2, the caliper assembly 12 is generally C-shaped and includes a caliper bridge 30 interconnecting an outboard leg 32 and an inboard leg 34. The caliper bridge 30 extends across the rotor section 16 and is there positioned and retained by guides (not illustrated) disposed parallel to the axis of the rotor assembly 14 which are in turn secured to an anchor plate 35. The guides provide a floating mounting for the caliper assembly 12 according to conventional disk brake practice.
The inboard leg 34 of the caliper assembly 12 defines a cylinder 36 within which is disposed a mating piston 38. According to conventional practice, pressurized hydraulic fluid is supplied to the cylinder 36, advancing the piston 38 toward the rotor section 16. Adjacent the mouth of the cylinder 3~, an annular groove 40 provides seating for an annular seal means 42 disposed therein. The annular seal means 42 is preferably fabricated of an elastomeric material such as rubber and prevents loss of hydraulic fluid from the cylinder 36. The piston 38 includes an annular groove 44 which provides a retaining means for a complementarily configured portion of a dust boot 46. The dust boot 46 comprises an integral, one-piece seal molded of elastomeric material such as rubber. The folded, bellows-like dust boot 46 provides a protective seal about the periphery of the piston 38 to the caliper assembly 12 while permitting substantial relative axial motion therebetween.

An outboard disk brake pad assembly 48 and an inboard disk brake pad assembly 50 are slidably disposed on the anchor plate 35 and frictionally engage the outer wall 18 and inner wall 20 of the rotor section 16.
The invention does not reside in the caliper assembly 12 and associated components, and the foregoing description is intended only to be representative of the possible componentry with which it may be combined. The invention may be successfully practiced with widely varying caliper assembly and piston configurations as well as with other motive energy sources such as pneumatic, mechanical or electrical devices. Furthermore,since the structure and function of the caliper assembly 12 and associated components are well known, they will not be further described.
Referring now to Figures 2 and 3, the rotor section 16 includes an outer wall 18 and an inner wall 20, as has been previously explained. The rotor section 16 is vented and, as such, includes a plurality of radially disposed passageways 54 and a like plurality of ribs 56 which extend transversely between the outer wall or discoid 18 and inner wall or discoid 20 of the rotor section 16. In this embodiment of the invention, the inner wall 20, or more broadly, that wall most distant the mounting structure or `mounting section 22 and separated therefrom by the ribs 56, has a uniform wall thickness. The outer wall 18, or more broadly, that wall of the rotor section 16 most proximate and connected to the mounting structure or mounting section 22, however, increases in thick-ness with increasing distance from the center of the rotor assembly 14. The wedge~shaped mass of the outer wall 18 ther-mally balances the mass of the mounti~g seation 22 and thus _ ~, _ 115~5~8 minimizes radial temperature gradients across the outer wall 18 of the rotor section 16.
Referring again to Figure 2, and particularly Figure 3, it is apparent that increasing thickness o~ the outer wall 18 produces a corre8ponding reduction in the width of the air passageways 54. Assuming inlet and outlet lengths of the passageways 54 were equal, such a reduction in the width of the passageways 54 would result in a substantially smallar outlet and choke or impede air flow through the passageways 54. The choking of the air flow may, of course, be eliminated by maintaining the area of the air passageway outlets approximately equal to or greater than the area of the air passageway inlets.
By adjusting the cross sectional area of the ribs 56, a substantially constant or radially increasing cross sectional area of the air passageways may be maintained. Thus, thermally balancing the heat sink of the mounting structure and maintain-ing good air flow through the radial air passageways may both be achieved.
Referring now to Figure 4, an alternative embodi-ment of a rotor, designated by the reference numeral 60, is illustrat0d. It should be apparent that a vented rotor such as the rotor 16 need not incorporate substantially rectangular ribs nor substantially rectangular air passageways in order to practice the invention. In this regard, the alternative embodiment rotor 60 includes an inner wall or discoid 62 having a constant thickness at its thinnest portion, and an outer wall or discoid 64 having an outwardly increasing thickness. The rotor 60 includes air passageways 66 having a generally elliptical cross section. The major axes of the elliptical passageways 66 are aligned circumferentially about the periphery of the rotor 60. The major axes of the elliptical passageways 66 at the inner surface of the rotor 60 are parallel to the axis of the rotor 60 and thus at right angles to the major axes of the elliptical passageways 66 at the outer surface of the rotor 60. It should be noted that such a configuration may also exhibit constant or radially increasing cross sectional area air passageways in addition to the heat sinking mass and also achieves minimized radial temperature gradients in the wall of the rotor 60 adjacent the mounting structure while maintaining good air flow in the elliptical passage,ways 66.
It should further be noted that neither the evenly tapering wall of the preferred embodiment rotor adjacent the mountin,g section and its generally rectangular-~ir~passage-ways nor th,e scallopped, tapering wall of the alternative embodiment rotor and its generally elliptical air passageways are to be construed to be limitations of the scope of the invention. Rather, any configuration of the walls of a rotor ~or disk) which includes a heat sinking mass for balancing the mass and heat sinking effect of the rotor (or disk) mounting structure in order to minimize radial temperature gradients within the rotor (or disk) is deemed to fall within the ambit of the invention.
Figure 5 is further illustrative of the thermal balance achieved by disposing mass on a wall of a vented brake rotor to balance the inherent heat sink provided by the rotor mounting structure and thus minimize radial temperature gradients within the rotor. It should be appreciated that the following calculations are illustrative and exemplary in nature and are provided only to further clarify and explain the invention.

1 ~

1 15~t;8 The accuracy of the result is commensurate with the apparent complexity and sophistication of the mathematical analysis.
The result should be considered a first approximation which can be improved upon by more sophisticated mathematical and empirical analysis as well as experimental testing such as dynamometer testing.
Figure 5 illustrates an outer rotor wall and mounting section similar ~o that which might ~e~ utlized iB
a heavy duty vehicle brake. The ribs and inner wall of the rotor are illustrated in phantom lines for reference purposes only and are not included,in the following calculations inasmuch as their heat transfer and temperature gradient characteristics are deemed to have a negligible effect on the radial heat transfer and temperatur~ gradient characteristics of the wall of the rotor adjacent the mounting structure. As has been noted, the mounting section of the rotor designated elements 1, 2, and 3 of Figure 5, provides a heat sink for the frictional heat generated on the surface of and absorbed into the adjacent outer wall of the rotor which is designated element 4. Element 5 of Figure 5 represents a tapering mass disposed on the rotor wall which, with the mass of element 4, balances the heat sink provided by elements 1, 2 and 3 of the ~ounting s,ection.
The following calculations quantify this balancing relationship and determine an approximate width or thickness T
for the triangular element 5 for given dimensions of elements 1,

2, 3 and 4 of the mounting section and rotor section, respe~ti~e-ly. The radii and other dimensions of the elements of the mountinq-~eetion and rotor ~re those illustrat~d in Figure 5.
The Theorem of Pappus which states that the volume of a solid of revolution is the product of the generating area and the distance travelled by the centroid of the area is utilized to calculate the volumes of the various elements and thus implicitly the mass and heat capacity of these elements since it is assumed that the density and specific heat of the rotor are constants.
For element 1 of the moùnting aection, the~area (Al) equals .5 in x 2.4 in or Al = 1.2 in2, the centroid radius (Yl) equals 1.5 in + 1.2 in or Yl = 2.7 in and the distance travelled by the centroid ~Dl) is 5.41tin. According to the Theorem of Pappus, the product AlDl equals the volume (Vl) of element 1 and AlDl = 6.48~ in . Likewise, for element 2, the area (A2) equals .5 in x 2.0 in or A2 = 1.0 in2, the centroid radius (Y2) equals 3.9 in + .25 in or Y2 = 4.15 in and the distance travelled by the centroid (D2) is 8.3 ~in. The product A2D2 equals 8.3~t in3, which is the volume (V2)of element 2. Again for element 3, the area (A3) equals .5 in x 1.2 in or A3 = .6 in2, the centroid radius ~Y3) equals 3.9 in + .6 in or Y3 = 4.5 in and the distance travelled by the centroid (D3) is 9.0 ~ in. The product A3D3 equals 5.4~ in3 and the volume (V3) of element 3. Finally, the total volume (and proportionately mass) of elements 1, 2 and 3 (V123) equals AD or 6.48~f in3 + 8.31t in3 + 5.41t in3 = 20.18~ in3.
Performing these same calculations for elements 4 and 5 of the rotor section, the area of element 4 (A4) equals .5 in x 2.6 in or A4 = 1.3 in , the centroid radius (Y4) equals 5.1 in + 1.3 in or Y4 = 6.4 in and the distance travelled by the centroid (D4~ is 12.8~ in. The product A4D4 e~uals 16.6~ in . Likewise for element 5, the area (A5) equals .5 x 2.6 in x T or A5 = 1.3T in, the centroid radius ~Y5) equals 5.1 in + .667 x 2.6 in or Y5 = 6.8 in and the distance .,. 1~

1 158~f~8 travelled by the centroid ~D5) is 13,6 ~ in. The volume ~V5) of element 5 equals the product A5D5 or 17.7~ T in3.
For a static thermal balance to exist, the volume (and mass) of elements 1, 2 and 3 should approximate the volume (and mass) of elements 4 and 5 or Vl + V2 + V3 ~b V4 + V5 20.18~ in3 z~ 16.61~ in3 + 17.7~T in3

3.56~ in3 % 17.7~ T in3 .2 in ~ T
L0 Thus a maximum width of triangular element 5 of the rotor section of .2 in is a first approximation of mass thickness which achieves a thermal balance in a rotor assembly of the stated dimensions.
The foregoing description has been directed to vented rotors having generally radially oriented air passage-ways disposed between the walls or discoids of a brake rotor.
The concept of the invention, however which is the placement of mass on the brake disk or rotor to compensate for the inherent heat sinking characteristic of the mounting structure, ~0 may also be applied to solid brake disks. As such, the invention comprehends either a specifically formed peripheral extension to the solid disk or, alternatively, providing a brake pad on the side of the disk most proximate the mounting structure ox on both sides which does not frictionally engage the entire side wall of the disk but leaves a peripheral band unswept. So configured, this area of the disk, rather than being a source of heat from the frictional contact of the brake pad, becomes merely a situs of additional mass which functions as a heat sink to compensate for the inherent heat ~0 sinking characteristic of the mounting structure.

_ ~gr_ .
1 1~6568 Figures 6, 7, ~ and 9 disclose four solid disk profiles which may be utilized to practice the invention.
Specifically, Figure 6 discloses a solid disk 70 which is secured to an axially off set integrally cast ~ounting section 72 which is only partially shown. The frictionally engaged portion of the disk 70 corresponding to the effective radial width of the brake pads (not illustrated) is designated by the brackets denominated reference numeral 74. It is apparent that the disk 70 extends beyong this swept area.
Specifically, the outer edge of the disk 70 is obliquely formed and defines a pointed, triangular section 76 about its periphery. The triangular section 76,having a majority of its mass disposed adjacent the side of the disk 70 most proximate the mounti~g sectio~ provides slightly ~re~r heat sinking to this side of the rotor than its opposite side and thermally balances the disk 70.
Similarly, Figure 7 illustrates a disk 80 having a ~ounting section 82 and swept rotor surfaces 84. Again, mass designated by the numeral 86 is disposed on the disk 80. In a ~0 fashion similar to the preferred solid disk embodiment illustrated in Figure 6, the mass 86 is non-uniformly disposed about the outer surface of the disk 80 and provides somewhat greater heat sinking to that portion of the disk 80 most proximate the mounting section.
Since by its nature, a solid disk will tend to display a more uniform temperature than will a ven~ed rotor, applications may exist where a thermally balancing mass should be uniformly added to the periphery of a disk. Furthermore, a mounting structure such as the radially extending planar _ ~ _ :,""'.. 1 l~

115~568 structure described previously,. if secured substantially symmetrically about the axial midplane of the disk, will be generally optimally balanced by a correspondingly disposed heat sink mass. In Figure 8, a solid brake disk 90 is formed with an integral planar mounting 92 as described above.
: Again, the frictionally engaged area of the disk 90 is designa-ted by the brackets and numerals 94. An annulus 96 having a generally rectangular cross section is uniformly disposed about the periphery of the disk 90. The annulus 96 may be integrally cast or subsequently secured to the disk 90 and may be fabricated of the same or different material as the disk 90. It should be apparent that the mass of the annulus 96 will function relatively uniformly as a heat sink with regard to , the left and right faces of the solid disk 90 as will the j:~ planar mounting 92 due to the uniform distribution of material .
about the axial midplane of the disk 90.
Figure 9 illustrates another embodiment in which a solid disk 100 has a mass disposed about its periphery to thermally balance the inherent heat sink represented by the integrally cast mounting section 102. In.this embodiment, the additional mass is formed in a triangular section 104 and tends to uniformly absorb and dissipate heat from both the left and right surfaces of the disk 100 in a manner similar to the annulus 96 illustrated in Figure 8. It should thus be apparent that additional mass, incorporated into either a solid disk or a vented rotor, may be disposed about the periphery of the rotor in any fashion consistent with the goal of thermally balancing the inherent heat sink created by the mounting structure of the rotor. Once the required thermal balance heat transfer characteristics have been established 1 15~G8 the specific material, heat capacity, mass, cross section or thickness may be dictated by such considerations as casting and manufacturing techniques, cost or energy conservation.
As has been previously noted, the inventive concept may also be practiced by reducing the amount of surface on the side of the disk or rotor most proximate the mounting structure or on both sides of the disk or rotor that is frictionally engaged by the brake pad or pads. Referring briefly to Figure 9, the conventional brake pad swept surface is illustrated on the right and designated by numeral 106.
On the opposite side of the disk 100 a reduced area of frictional engagement designated by the numeral 108 is utilized - to practice the invention inasmuch as the non-engaged area designated by the numeral 110 is no longer a source of frictional heat but in fact represents an area and associated mass which functions as a heat sink. It should be noted that the radial width and thus frictionally engaging area of both left and right brake pads may be reduced thus providing heat sinking on both faces of the brake disk or rotor.
The foregoing disclosure is the best mode devised by the inventor for practicing the instant invention. It is apparent, however, that devices incorporating modifications and variations to the invention will be obvious to one skilled in the art of disk brakes. Inasmuch as the foregoing disclosure is intended to enable one skilled in the pertinent art to practice the invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the scope of the following claims.

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Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A brake, comprising a rotor assembly, said rotor assembly having a central mounting structure and a rotor section on said mounting structure, said rotor section comprising a rotor or a solid disk, and a caliper assembly having brake pads, and means for advancing such pads into contact with said rotor section, said central mounting structure being in heat transmitting relationship with said rotor section around said central mounting structure and said rotor section including mass means distributed on said rotor section for thermally balancing the thermal effect of said central mounting struc-ture, whereby radial temperature gradients within said rotor section are reduced.

2. A disk brake assembly for a motor vehicle com-prising a rotor assembly having an axis of rotation and a caliper assembly having a pair of brake pads and means opera-tively linked with said pads for advancing said pads into frictional contact with said rotor assembly, said rotor assembly including a rotor section and a centrally disposed mounting section, said rotor section having a first circular wall on said mounting section and in heat conductive relation with said mounting section around said mounting section, a second coaxially disposed circular wall spaced along said axis from said first wall and a plurality of rib means for inter-connecting said first and second walls, said first wall having an outer surface disposed normal to said axis and an inner surface obliquely disposed with respect to said axis, whereby said first wall increases in thickness with increasing dis-tance from said axis thereby providing a mass to thermally balance the effective mass of said mounting section.

3. The disk brake assembly of claim 2 wherein said first wall, said second wall and said rib means define radially disposed air passageways of substantially uniform cross-section.

4. The disk brake assembly of claim 2 wherein said rib means have a substantially uniform cross-section along their radial length.

5. A disk brake assembly comprising a rotor assembly having an axis of rotation and a caliper assembly having brake pads and means for advancing said brake pads into contact with said rotor assembly, said rotor assembly having a centrally disposed mounting section, a rotor section in heat conductive relation with said mounting section around said mounting section and heat sink means disposed radially outwardly from said mounting section for at least partly thermally balancing the thermal mass of said mounting section, thereby to counteract radial temperature gradients in said rotor section.

6. The disk brake assembly of claim 5 wherein said rotor section is solid and said heat sink means includes material disposed generally adjacent the outer periphery of said rotor section.

7. The disk brake assembly of claim 6 wherein said material has a substantially triangular cross-section.

8. The disk brake assembly of claim 6 wherein said material has a substantially rectangular cross-section.

9. The disk brake assembly of claim 5 wherein said rotor assembly includes first and second spaced apart walls and radially disposed air passageways therebetween and said heat sink means is disposed along at least one of said walls.

10. The disk brake assembly of claim 9 wherein said heat sink means has a thickness which increases with increasing distance from said axis.

11. A rotor assembly for a brake, comprising a rotor section, a mounting structure connected to said rotor section in uninterrupted heat conductive relation around said central section, and mass means distributed on said rotor section for thermally balancing the thermal effect of the mass of said mounting structure whereby radial temperature gradients within said rotor section during use of said brake are counteracted.

12. A rotor assembly for a disk brake, said assembly having a central axis and comprising a rotor section and a centrally disposed mounting section, said rotor section having a first circular wall in uninterrupted heat conductive relation with said mounting section around said mounting section, a second coaxially disposed circular wall spaced along said axis from said first wall, and a plurality of rib means for inter-connecting said first and second walls, said first wall having an outer surface disposed normal to said axis and an inner surface obliquely disposed with respect to said axis, whereby said first wall increases in thickness with increasing distance from said axis thereby providing a mass to thermally balance the effective mass of said mounting section.

13. A rotor assembly according to claim 12 wherein said first wall, said second wall and said rib means define radially disposed air passageways of substantially uniform cross-section.

14. A rotor assembly for a disk brake, said assembly having an axis of rotation and comprising a rotor section, a centrally disposed mounting section, said rotor section being in uninterrupted heat conductive relation with said mounting section around said mounting section, and heat sink means dis-posed radially outwardly from said mounting section for sub-stantially thermally balancing said mounting section thereby to reduce radial temperature gradients in said rotor section.

15. A rotor assembly according to claim 14 wherein said rotor section is solid and said heat sink means includes-material disposed generally adjacent the outer periphery of said rotor section.

16. A rotor assembly according to claim 15 wherein said material has a substantially triangular cross-section.

17. A rotor assembly according to claim 15 wherein said material has a substantially rectangular cross-section.

18. A brake rotor having a central mounting structure and frictional engagement means for engagement with brake friction pads, wherein said frictional engagement means includes a first friction pad engaging discoid uninterruptedly conjoined with and extending radially outward from said mounting structure, whereby a direct heat flow path exists between said first dis-coid and said mounting structure, a second friction pad en-gaging discoid axially spaced from said first discoid and means for conjoining said first and second discoids to permit passage of air therebetween for convective cooling of said first and second discoids wherein said first discoid provided with mass to thermally balance the thermal effect of the central mounting structure mass thereby to minimize radial temperature gradients of said first discoid during brake application.