On 28 Feb 2005 19:11:24 -0800, Ed <[email hidden]>
Quoted message said:In article <[email hidden]>, [email hidden]
says...Quoted message said:
Robin Hubert said:[email hidden] wrote:
> On Mon, 28 Feb 2005 01:19:31 GMT, Robin Hubert[snip]
Quoted message said:> Dear Robin,
>
> Can you measure the arms in a few dimensions?
>
> That is, what are the distances from the middle of the brake
> pad to the pivot and from the pivot to where the cable
> action is applied?
>
> With those, we could work out the actual mechanical
> advantage--the difference alone at one point isn't enough.
>
> Long reach pad to pivot: xx mm
> Long reach pivot to cable: xx mm
>
> Short reach pad to pivot: xx mm
> Short reach pivot to cable: xx mm
>
> The leverage may not be the culprit, but you seem to be
> saying that everything is about the same except that one
> rear brake is a short reach and the other is a long reach.
>
> Carl FogelHi Carl,
These are measurments from the pads.
74/53mm for the long reach, and 80/49mm for the short reach. I reckon
the pivot to cable (which I take to mean the top of the arm at the bbl
adjuster) can be exrapolated from the above to 21mm and 31mm
respectively (I hesitate to run downstairs again).Is this the information you're looking for?
Thanks,
Robin HubertDear Robin,
That's getting closer, but not quite what's needed.
Sheldon's reply is a pretty good explanation, and here's an
etch-a-sketch diagram:cable attaches
C
.
. brake arm pivot
P .
x .
x .
x |
x |
x |
B | <--brake pad on slotted arm
|
|
On a Shimano 105 I measured C to P as 52mm, P to B as 52mm. Also Cable to P as
44mm per Sheldon. All measurements +/- 1mm.
Dear Ed,
If I'm following you, then the cable side of the see-saw (or
the handle side of the pliers) is 44 mm directly from cable
to pivot, and the brake-pad side of the see-saw (or the jaws
of the pliers) is 52 mm.
Cable x---------- -----------x Brake Pad
44 ^ 52
Pivot
It doesn't matter if the line is straight or angles at the
pivot--just the straight-line distances from the pivot to
each end-point. A brake arm's leverage will work just as
well at an angle, as shown below, or even with one arm
running right past the other (which is how the endless set
of sun-ray gear-teeth on the front chain-ring work,
including the tooth right next to the longer lever arm of
the pedal, with the long levers of the spokes on the rear
wheel doing the same thing next to the much shorter levers
of the cogs).
/
/
\ /
\ /
\ /
\ /
\ /
\ /
\/
pivot
This would produce a mechanical disadvantage. A 10 pound
force on the cable would produce (11/13) * 10 pounds at the
brake pad, reducing it to about 8.5 pounds (over a wider
arc).
If I'm misunderstanding you and both distances are and
identical 52 mm, then the see-saw leverage is dead even and
there's no change beyond the slight frictional loss at the
pivot.
Carl Fogel