Hollow crank arm for a bicycle and process for manufacturing the same

Abstract

A hollow crank arm for a bicycle comprising a body of elongated shape, at the ends of which are formed, respectively, an area for connection of the bottom bracket spindle and an area for connection of the pedal. The body includes a cavity of elongated shape and two solid end portions which are the aforesaid connection areas for the bottom bracket spindle and for the pedal. The crank arm is preferably formed through a forging process.

1 . A crank arm for a bicycle comprising an elongated body at the ends of which are formed, respectively, an area for connection to the bottom bracket spindle and an area for connection of the pedal, wherein the crank arm comprises between the end areas a portion with a hollow cross section and a portion with a solid cross section.  
   
   
       2 . A crank arm according to  claim 1 , wherein the portion with hollow cross section is adjacent to the area for connection to the bottom bracket spindle.  
   
   
       3 . A crank arm according to  claim 1 , wherein the portion with solid cross section comprises the area for connection to the pedal.  
   
   
       4 . A crank arm according to  claim 1 , wherein, in the portion with hollow cross section, the cavity extends for a predominant part of the section of the crank arm body.  
   
   
       5 . A crank arm according to  claim 1 , wherein, in the condition where it is installed on a bicycle, the portion with hollow cross section projects towards the frame of the bicycle more than does the portion with solid cross section.  
   
   
       6 . A crank arm according to  claim 1 , wherein the portion with hollow cross section is surrounded by a wall with a thickness smaller than the thickness of the portion with solid cross section.  
   
   
       7 . A crank arm according to  claim 1 , wherein the portion with hollow cross section and the portion with solid cross section each have longitudinal axes that are inclined with respect to one another.  
   
   
       8 . A crank arm according to  claim 1 , which has a predetermined useful length between the centres of the areas for connection of the bottom bracket spindle and for connection of the pedal and a predetermined length of said solid portion, wherein the ratio between said length of the solid portion and said useful length of the crank arm is equal to or greater than 0.2.  
   
   
       9 . A crank arm according to  claim 8 , which has a predetermined useful length between the centres of the areas for connection of the bottom bracket spindle and for connection of the pedal and a predetermined length of said solid portion, wherein the ratio between said length of the solid portion and said useful length of the crank arm is equal to or smaller than 0.5.  
   
   
       10 . A crank arm according to  claim 1 , which has a predetermined useful length between the centres of the areas for connection of the bottom bracket spindle and for connection of the pedal and a predetermined length of said solid portion, wherein the ratio between said length of the solid portion and said useful length of the crank arm is comprised between 0.3 and 0.4.  
   
   
       11 . A crank arm according to  claim 1 , which has a predetermined useful length between the centres of the areas for connection of the bottom bracket spindle and for connection of the pedal and a predetermined length of said hollow portion, wherein the ratio between said length of the hollow portion and said useful length of the crank arm is equal to or greater than 0.3.  
   
   
       12 . A crank arm according to  claim 1 , which has a predetermined useful length between the centres of the areas for connection of the bottom bracket spindle and for connection of the pedal and a predetermined length of said hollow portion, wherein the ratio between said length of the hollow portion and said useful length of the crank arm is equal to or smaller than 0.8.  
   
   
       13 . A crank arm according to  claim 1 , which has a predetermined useful length between the centres of the areas for connection of the bottom bracket spindle and for connection of the pedal and a predetermined length of said hollow portion, wherein the ratio between said length of the hollow portion and said useful length of the crank arm is comprised between 0.6 and 0.7.  
   
   
       14 . A crank arm according to  claim 1 , wherein said solid portion has a first predetermined thickness different than a second thickness of the hollow portion.  
   
   
       15 . A crank arm according to  claim 14 , wherein the ratio between said first thickness and said second thickness is equal to or greater than 1.2.  
   
   
       16 . A crank arm according to  claim 14 , wherein the ratio between said first thickness and said second thickness is equal to or smaller than 3.0.  
   
   
       17 . A crank arm according to  claim 14 , wherein the ratio between said first thickness and said second thickness is comprised between 1.5 and 1.7.  
   
   
       18 . A crank arm according to  claim 14 , wherein said portion with hollow cross section has walls having a predetermined thickness and in that the ratio between said wall thickness and the thickness of said portion with hollow cross section is equal to or greater than 0.1.  
   
   
       19 . A crank arm according to  claim 14 , wherein said portion with hollow cross section has walls having a predetermined thickness and in that the ratio between said wall thickness and the thickness of said portion with hollow cross section is equal to or smaller than 0.3.  
   
   
       20 . A crank arm according to  claim 14 , wherein said portion with hollow cross section has walls having a predetermined thickness and in that the ratio between said wall thickness and the thickness of said portion with hollow cross section is substantially equal to 0.2.  
   
   
       21 . A crank arm according to  claim 1 , wherein it is made of a material chosen in the group comprising: aluminium and its alloys, titanium, structural-fibre-based fabric embedded in a matrix of plastic material, in which said fibres are chosen between carbon fibres, glass fibres, aramidic fibres, boron fibres, ceramic fibres, and any combination thereof.  
   
   
       22 . A crank arm according to  claim 1 , wherein it comprises, between the end areas, a second portion with solid cross section.  
   
   
       23 . A crank arm according to  claim 1 , wherein the hollow section with an area outside the crank arm through an opening located on one side of the crank arm in an area comprised between the areas of connection to the bottom bracket spindle and to the pedal.  
   
   
       24 . A crank arm according to  claim 1 , wherein the hollow section has a blind bottom.  
   
   
       25 . A crank arm according to  claim 1 , wherein the crank arm comprises a pair of holes for connection of the bottom bracket spindle and of the pedal, in which the holes do not communicate with the hollow section.  
   
   
       26 . A crank arm according to  claim 1 , wherein the area for connection of the pedal is formed in the solid portion having a longitudinal axis substantially aligned to the longitudinal axis of the hollow section.  
   
   
       27 . A crank arm according to  claim 1 , wherein the area for connection of the pedal is formed in a solid portion having a longitudinal axis inclined with respect to the longitudinal axis of the hollow section.  
   
   
       28 . A crank arm for a bicycle comprising an elongated body at the ends of which are formed, respectively, an area for connection to the bottom bracket spindle and an area for connection of the pedal, wherein the crank arm comprises between the end areas a portion with a longitudinally-extending hollow cross section and a portion with a longitudinally-extending solid cross section.  
   
   
       29 . The crank arm of  claim 28 , wherein the longitudinally extending cross-sections overlap in an intermediate portion within the crank arm.