Trevor Boicey wrote:
>
> Guys, can we think a little before we spread misinformation? This
> isn't an opinion question, there is only one right answer.
>
> Please remember that the back cylinders are connected. The
> above logic depends on a false truth, that if the master moves
> 10cc to the rear wheels that each cylinder will get 5 so the small
> will move farther and brake first. This is simply false.
Well, I won't belabor the point too long, but the issue is about the
relative sizes of the rear wheel cylinders, as I understand it. One
larger, one smaller. To reduce the problem to its simplest variables is
a help to describe the theory. The comment above suggests that this is
an issue of balance between the rear cylinders, and for argument's sake,
let's say that both of the rear wheel brake assemblies are identical and
identically adjusted. In that case, the actuation will be the same for
both.
Now, extrapolate that fixed circumstance to the two differently-sized
wheel cylinders. The smaller bore cylinder _will_ extend more for a
given amount of fluid delivered than the larger cylinder, and its
applied force will be less, and because its displacement is more, it
will extend further for a given volume of fluid. That's hydraulics.
> Each cylinder will get as much fluid as will be admitted
> AT A GIVEN PRESSURE. If one brake pad is much fatter, that
> cylinder will get less fluid. If one slave cylinder is
> a different diameter, it will get less fluid.
Again, this assumes a variable which isn't at issue. The issue was
larger vs. smaller cylinders. You are quite right in saying that
relative clearance between shoe and drum (or pad and rotor) has an
effect on travel. But this does _not_ have an effect on the fluid
delivered to one half or all of the system. That is wholly dependent
upon the amount of fluid delivered by the master cylinder, and that is
dependent upon the distance of the delivered stroke of the master
cylinder.
> When all the dead space is consumed and ALL pads contact
> the braking surfaces, then the pedal will start to
> feel harder and real pressure is being exerted. Then it's
> the leverage problem. This is a TR3 with one hydraulic
> system for the whole car, so it's very simple.
Nevertheless, it confuses the issue a bit. Smaller bore cylinders will
extend further and sooner than larger bore cylinders, and therefore will
contact the drum sooner, and will cause the hydraulic system to reach
fluid displacement stasis earlier, at the point of balanced pressure.
Yes, I agree that at the point that all friction material is in firm
contact with braking surfaces is the point at which pressure builds.
However (and this is a big however), further extension of the master
cylinder _past_ the point of firm contact of all surfaces displaces more
fluid. That amount may be trifling, but, regarding the relative sizes of
the wheel cylinders, is still measurable in its effect. Smaller
cylinders extend further than larger cylinders, provided that the limits
of force have not yet been exceeded. If they are exceeded, the shoe
doesn't move, because insufficient force is available to _make_ it move.
If there is still sufficient force to make it move, it will move at a
greater rate of travel than of a larger cylinder, which was the original
issue.
Further, if one looks at the problem dynamically, rather than
statically, weight transfer is also an issue. The force required to lock
up the rear brakes decreases as weight shifts forward under braking.
Assuming both cylinders are capable of applying sufficient force, under
the reduced loads created by weight transfer, to lock up the rear
brakes, the cylinder extending further under pedal travel (dependent on
fluid displacement) will lock the brakes sooner, because that's what
locks up the brakes--not absolute pressure, but rather, sufficient
actuation, i.e., real movement (distance traveled) of the shoe against
the drum. If the shoe moves in closer contact with the drum, friction
goes up. If there is insufficient force to create that movement, it
doesn't occur. My assumption is that both cylinders can create enough
force to create that movement, and with that assumption, the smaller
cylinder will lock up the brakes more quickly than the larger.
Cheers.
--
My other Triumph doesn't run, either....
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