Re: Analysis of Dual Pivot Brake

Joe Riel wrote:
Yes, it is getting boring, however I repeat my question because
despite having been asked it over half a dozen times, you have yet
to give any sort of answer that is consistent with the fundamentals
of mechanics. A freely pivoting 1:1 lever has forces on its ends
in a 1:1 ratio, but you seem to believe that it can support a 2:1
ratio instead.

Your question is reasonable, and I don't understand jim's response
(the lever is not "locked by the rim"). However, your question seems
flawed. Near as I can tell, there is no "freely pivoting 1:1 lever"
in the assembly.

The question's not flawed, rather you hit right on the whole
point of the question. jb's assertion has been that the Y arm receives
no force from the centering linkage, yet somehow behaves as a fixed
stop: my question has been for him to make a consistent picture out
of his contradictory assertions.

(8) F/T = ((1-k)*L1 + k*L3)/((1-k)*L2 + k*L4)

Note that if the arms' point of contact lies on the line
between pivots, then the numerator and denominator of this equation
correspond to the distance from this point of contact to the cable, and
to the line through the pads respectively. This point of contact can be
treated as the pivot of a single-pivot caliper with the same cable and
pad location. This makes sense since when the contact lies on the line
between pivots, there is no cosine scrubbing between arms, thus the
incremental relative motion of one arm to the other looks as though it
were just a pivot there.
If the point of contact is away from the line between pivots,
then the equivalent point would be the intersection between this line,
and a line normal to the surface at the contact.

F/T = 1/2*(L1/L2 + L3/L4)

So, with k set so the pads move equally, the gain of
the brake is the average of the gain of the two
lever arms without the connection (the connection is required
for operation.

Thank you for confirming this. I've said the same thing at least
three times already in this thread, but the philistine is unconvinced.

-Luns
.

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