GB2217408A

GB2217408A - Brake mechanisms

Info

Publication number: GB2217408A
Application number: GB8809285A
Authority: GB
Inventors: Brian Timothy Denvir
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-04-20
Filing date: 1988-04-20
Publication date: 1989-10-25
Also published as: GB8809285D0

Abstract

A brake mechanism, particularly for bicycles, comprises a hinged quadrilateral of members 11 upon which a sheathed cable 10 operates. The quadrilateral arrangement of members 11 in turn operate upon levers 13 upon which brake blocks 15 are mounted which grip the wheel rim 19 when the brake is applied. The brake may be released by a compression or tension spring acting between the members 11 or by torsion springs each associated with a respective lever 13. <IMAGE>

Description

Bicycle Brake Mechanism This invention relates to brake mechanisms for wheeled conveyances, particularly but not exclusively for bicycles.

Two principal forms of bicycle brake are currently in widespread use, both of them linking a sheathed cable to pivoted arms on which brake blocks are mounted. The two current forms of mechanism are known as "centre-pull" and "side-pull" brakes. In the case of the centre-pull mechanism, a short separate piece of cable pulls two separately pivoted arms with brake blocks mounted on their furthest ends, gripping the rim of the bicycle wheel. In the case of the side-pull mechanism, the sheathed brake cable squeezes in scissor fashion a pair of arms with a single pivot, so that brake blocks mounted at their furthest ends likewise grip the rim of the bicycle wheel.

The centre-pull brake has a number of disadvantages. Firstly, there is a relatively large amount of slack unsheathed cable to take up in operating the brake. Secondly, the force operating the pivoted arms is applied through the pivots and part of the bicycle frame, reducing efficiency and increasing strain on some parts of the bicycle. Thirdly, as the brake is applied, the mechanical advantage of the levers is reduced, owing to the increasing angle.with the cable.

The side-pull brake also has disadvantages. Its asymmetry results in its centring, i.e. the equal and simultaneous application of each brake block, being hard to establish and maintain. On the other hand, as the brake is applied, the mechanical advantage of the levers slightly increases.

According to one aspect of the invention, a conveyance has a frame, a braked wheel and a braking mechanism comprising a pair of levers located on opposite sides of the wheel and pivoted to the frame, the levers carrying brake pads for engagement with the wheel, a pair of links pivoted to each other and to the levers to form a toggle, and a transmission cable connected to the centre pivot of the toggle, whereby tension on the cable tends to straighten the toggle to apply the brake pads to the wheel. Preferably, the braking mechanism includes a second pair of links pivoted to each other and to the levers to form a second toggle, the links of the two toggles forming a quadrilateral, and in which the cable is partly contained in a sheath, the sheath being connected to the centre pivot of the second toggle.

According to another aspect of the invention, in a bicycle brake mechanism the braking force is transferred from a sheathed cable to levers bearing the brake blocks by means of members forming a hinged quadrilateral. Thus, the levers on which the brake blocks are mounted are separately pivoted, and operated by a further set of members hinged together in-the form of a quadrilateral. The ends of the levers on which the brake blocks are mounted are attached to the pivots which form the hinges at the ends of one of the diagonals of the hinged quadrilateral. The sheathed brake cable in turn acts on the other diagonal of this hinged quadrilateral, squeezing it together.

The present invention, in at least some of its embodiments, has the advantages which are specific to both the centre-pull and side-pull mechanisms: the arrangement is symmetrical, facilitating centring of the operation there is a relatively short length of exposed unsheathed brake cable, reducing the slack to be taken up: and the force applied to the levers holding the brake blocks does not act through the bicycle frame. Additionally the mechanical advantage of operating the levers increases substantially as the brakes are applied and the hinged quadrilateral alters shape: at first the mechanical advantage is low, enabling a rapid take-up of slack, then as the quadrilateral flattens it increases, allowing greater force to be exerted through the brake blocks.

In a preferred example the sides of the hinged quadrilateral are all equal. Although this is a natural approach to constructing the invention, it is not necessary: there could be advantages in unequal sides, for example with the lower sides shorter than the upper sides. Also, a number of means of causing the brake mechanism to return to the open position on release are possible. Three possible means are shown in the examples to be described.

The invention may be carried into practice in various-ways but one bicycle brake mechanism embodying the invention and a number of modifications thereto will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 is a front elevation partly in section of the mechanism showing it in the unapplied condition; Figure 2 is a view similar to Figure 1 but showing the brakes applied and Figures 3, 4 and 5 are views similar to parts of Figures 1 and 2 and showing various biasing means for returning the brake mechanism from the applied to the unapplied condition.

Turning now to Figures 1 and 2 there is shown in section part of a bicycle wheel comprising a rim 19, spokes 20 and a tyre 21. Above the tyre 21 is a mud guard 22 which is attached to the frame 23 of the bicycle only a small part of which is shown.

Rigidly attached to the frame 23 is a transverse brake assembly supporting bar 24 the outer ends of which carry pivot pins 14 on which pivot brake block carrying levers 13. At the lower ends of these levers there are brake blocks 15 which are positioned to engage the rim 19 of the wheel while their upper ends are pivoted at 12 to the outer ends of a first pair of toggle links lia having a central pivot lib and the outer ends of a second pair of toggle links lib having a central pivot lic.

The brake mechanism can be operated by means of a sheath or Bowden cable 10, the cable 10a of which is connected to the central pivot ila of the toggle links 11 and the sheath 10b of which is connected to the central pivot llc of the second pair of links llb. The opposite end of the sheath cable is connected to a brake lever and mounting on the handle bars of the bicycle in the usual way.

It will be appreciated that when the brake lever is pulled the sheathed cable causes the central pivot llc to move downwards and the central pivot lla to move upwards, thus moving the upper ends of the levers 13 outwards and moving the brake blocks 15 inwards into engagement with the wheel rim 19 as shown in Figure 2.

Various means may be provided to supply a returning force to remove the brake blocks from the wheel when the brake lever is relaxed. In the arrangement shown in Figure 3 a pair of mouse-trap or torsion springs 16 is provided, the coil parts of the springs being mounted on the pivot pins 14, one tail ofeach spring engaging the central screw 25 securing the rigid cross member 24 to the frame 23 and the other tail of each spring engaging a stop 26 formed on the respective brake shoe carrying lever 13. In the arrangement shown in Figure 14 the returning force is provided by a compression spring 17 extending between the pivots 12 at the outer ends of the toggle linkages. In the arrangement shown in Figure 5 the returning force is provided by a compression coil spring 18 through which the cable 10a passes and the opposite ends of which are connected to the centre pivots lia and llc of the toggle linkages.

As the brake blocks 15 wear, the central pivots lla and llc of the toggle linkages will come closer and closer together and means may be provided to prevent them approaching too close which might lead to locking. Accordingly abutment surfaces may be provided on two or more of the links to resist excessive movement. Thus Figure 4 shows abutments 27,28 on one link of each pair of toggle links. When the brake blocks are substantially worn these blocks will come togetherwhen the brakes are applied and prevent the toggle links moving into the in-line condition.

Claims

Claims:

1. A conveyance having a frame, a braked wheel and a braking mechanism comprising a pair of levers located on opposite sides of the wheel and pivoted to the frame, the levers carrying brake pads for engagement with the wheel, a pair of links pivoted to each other and to the levers to form a toggle, and a transmission cable connected to the centre pivot of the toggle, whereby tension on the cable tends to straighten the toggle to apply the brake pads to the wheel.

2. A conveyance as claimed in Claim 1 in which the braking mechanism includes a second pair of links pivoted to each other and to the levers to form a second toggle, the links of the two toggles forming quadrilateral, and in which the cable is partly contained in a sheath, the sheath being connected to the centre pivot of the second toggle.

3. A bicycle brake mechanism in which the braking force is transferred from a sheathed cable to levers bearing the brake blocks by means of members forming a hinged quadrilateral.

4. A bicycle brake mechanism as in Claim 3 in which the return on release is effected by means of a pair of torsion springs operating on the levers bearing the brake blocks.

5. A bicycle brake mechanism as in Claim 3 in which the return on release is effected by means of a tension spring operating on the hinged quadrilateral.

6. A bicycle brake mechanism as in Claim 3 in which the return on release is effected by means of a compression spring operating on the hinged quadrilateral.

7. A bicycle brake mechanism as in Claims 3, 4, 5 or 6 in which the brake is prevented from locking in the braked position by interengagement of abutment surfaces on two of the said members.

8. A bicycle brake mechanism substantially as described herein with reference to Figures 1-5 of the accompanying drawings.