Re: Analysis of Dual Pivot Brake

jim beam <spamvortex@xxxxxxxxxxxxxxx> writes:

Joe Riel wrote:
jim beam <spamvortex@xxxxxxxxxxxxxxx> writes:

Joe Riel wrote:
As a followup, we can look at the relative pad movement. Let x1 be
the movement of the left pad, x2 the movement of the right pad. For
small movements, the vertical movement of the contact point of the
two arms is

dy = dx1*L5/L2 = -dx2*L6/L4.

The (negative) ratio is

-dx1/dx2 = L4*L5/L2/L6.

Expressed in terms of k we get

-dx1/dx2 = L4/L2*k/(1-k)

Ideally we'd like this ratio to be 1, so that both pads open equally.
that is ideal, but presumptive. the y-arm is actually free to compress
independently.


Solving for k we get

k = L2/(L2+L4)

Plugging that into the gain ratio and simplifying we get

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.
the connection is required to help swing the two arms together when
free, but under load, that connection is small beer compared to the
force exertion from compressing the rim. the connection never sees more
than caliper spring loading.

The force is considerable and directly computed from my previous formulae.
If the "optimal" ratio I suggest is used, then it is

f/T = (L3/L4 - L1/L2)*1/2*(L2+L4)/L7

Assuming L3/L4 = 2 and L1/L2 = 1, this gives

f/T = 1/2*(L2+L4)/L7.

Note that the force is proportional to the applied cable tension.

absolutely!

I'm confused. You previously said it was "small beer", "no more than
spring tension". It is not. It looks to be about half the force
on the pads.



The
spring has nothing to do with it, its only function it to open the
arms.

indeed.

If what you said were true, then with the spring disconnected
and the brake applied hard there would be zero force between the little
screw and the pad that it hits. Tightening it would be easy.
Try it. I've done so...

i obviously did not succeed in being clear. if it was acting as
braking force leverage, /loosening/ it would be hard because it would
continue to be the lever point as pressure is applied - but it loosens
no problem.

That doesn't follow. The diameter of the bolt is 0.145 in. Assume
coefficient of friction in threads is 0.2. With 50 lb force,
the torque required to unscrew the bolt should be about 0.7 in-lb.

--
Joe Riel
.

Relevant Pages

  • Re: Analysis of Dual Pivot Brake
    ... the movement of the left pad, x2 the movement of the right pad. ... Ideally we'd like this ratio to be 1, so that both pads open equally. ... lever arms without the connection (the connection is required ...
    (rec.bicycles.tech)
  • Re: Analysis of Dual Pivot Brake
    ... the movement of the left pad, x2 the movement of the right pad. ... Ideally we'd like this ratio to be 1, so that both pads open equally. ... lever arms without the connection (the connection is required ...
    (rec.bicycles.tech)
  • Re: Thermal Relief in Protel
    ... First you must be sure if it is a plane or a poured polygon, ... connection rule for either Power Plane Connection Style or ... You could write the rule for your PAD using the Footprint/Pad ...
    (sci.electronics.cad)
  • Re: Analysis of Dual Pivot Brake
    ... the movement of the left pad, x2 the movement of the right pad. ... Ideally we'd like this ratio to be 1, so that both pads open equally. ... Plugging that into the gain ratio and simplifying we get ... lever arms without the connection (the connection is required ...
    (rec.bicycles.tech)
  • Re: Analysis of Dual Pivot Brake
    ... we can look at the relative pad movement. ... Ideally we'd like this ratio to be 1, so that both pads open equally. ... Plugging that into the gain ratio and simplifying we get ... lever arms without the connection (the connection is required ...
    (rec.bicycles.tech)