On 19 Apr 2006 09:54:33 -0700, "
[email protected]"
<
[email protected]> wrote:
>Werehatrack wrote:
>
>"Big cars use big
>converters because big converters are cheaper to make than small ones
>that can do the same job; if you don't have to keep the tolerances as
>tight, and you don't need to worry about weight as much, the bigger
>converter is the better choice."
>
>not true at all. If the cost was main factor then economy cars would
>use big torque converters,
Actually, some small cars *do* use a bigger TC than they really need.
Often this is because the carmaker decides that it's not worth the
expense to tool up to make a smaller unit when there is an existing
design in production that will serve adequately. On occasion, as in
the celebrated suit against GM decades ago, this sort of reasoning
also results in a carmaker using a TC that's really not as big as it
ought to be.
>and car makers would certainly find a way to
>stuff the cheaper part in.
There are multiple costs to pay for having a big TC; today, one
significant reason to avoid a larger TC than needed is that the
overall drivetrain width and weight needs to be kept down. But for
that matter, look at what was in the Vega; the TCs in those were much
larger than needed precisely because it was cheaper to use what was on
the shelf than to design new bits for the application. Where the
makers can get away with using cheap-but-heavy, they do it. The
Gremlin and the Pacer epitomized that philosophy end-to-end.
>Cost is not the the factor. People that buy
>luxury cars dont want to feel the the engine shift, so the torque
>converters are big. People that buy performance cars don't care if they
>feel the engine shift, and in most cases prefer it, so the converters
>are smaller, but no less expensive.
Actually, the performance-car converters are smaller and *more*
expensive. Making it smaller, for the same power-handling, generally
means it can't be made as cheaply.
>People that buy cheap economy cars
>don't expect performance or smoothness, so the converters are cheap and
>light.
Huh? You ever sell cars for a living? Lots of cheap economy cars
have smooth shifting.
>This, I realize, is a gross oversimplification but without
>getting into the technical details of torque converter design will have
>to do.
Having had to deal with the technical details of TC design a couple of
times, the reason that small economy cars have small converters is
that they don't need a bigger one; a smaller unit does the job just
fine for a low-output motor. The TC's slippage isn't the big reason
that the shifting is damped well in luxury cars or any other cars for
that matter; that's also a product of the sheer mass of the vehicle,
the tuning of the valve body, the choice of friction materials for the
clutches, the use of good isolation mounts, and the choice of a low
stall speed for the converter. GM sold a lot of mid-price and even
inexpensive cars for a long time on the strength of the fact that
their THM unit's typical valving was so well designed that it damped
the shift effects beautifully, "just like a Cadillac". Even in a
ratbox bottom-of-the-line Chevy. On the other hand, to make a small
converter do the work of a big one requires much closer engineering of
the components and mandates the use of better materials; this
increases the cost *without* improving the end result relative to the
big converter in any area except weight. Cost is important to
manufacturers; you don't spend money where you don't need to. So, for
a big car with a big mass, a big engine, and no weight issues, you use
a big but cheaply manufacturable converter; for a small car with a big
engine and no performance issues, you still use a (relatively) big
converter; if the performance issues are important, you spend the
money to make a smaller converter do the job. Small non-performance
cars can use smaller versions of the cheap-to-make big-car converters,
and they generally are so equipped. Over the last 40 years, TC design
has improved considerably overall in response to the need for improved
fuel economy, but the biggest gain was with the lockup converter,
where the TC is taken out of the picture as anything more than a
flywheel. (And that brings us to the other reason for using a big TC
in a luxury car; a big flywheel makes for a smoother-feeling
drivetrain in any event.)
>"None of this has a lot of relevance to the
>current discussion beyond the fact that the water-filled tire will
>mean that the wheel's inertia will increase, with the accompanying
>predictable effects. The fluid will, at these levels of acceleration,
>behave enough like a solid that the hydralic effects can be ignored."
>
>Again, not true at all. Try this experiment: Fill two identicle round
>bottles with water. Freeze one of them.
Poor simulation. Try it with a narrow torus of significant diameter.
The magnitude of the resulting difference will change quite a bit. At
bike wheel sizes, speeds and accelerations, I suspect you won't see a
difference at all.
>Place them next to each other
>on an incline plane and see which one accelerates faster. If the plane
>is long enough to evoke the entire mass of liquid water to spin, then
>you could see which one goes futher after the end of the incline. If
>the liquid gives the same results as the solid, then the laws of
>physics are different in your part of the universe than everywhere
>else. Besides that, how would one make the determination that the fluid
>dynamic effects are insignificant through empiracle testing? You would
>need solid tires of approximately the same density of water to make
>that comparison. Otherwise, the fluid dynamic effects will be there,
>and must be accounted for. These accelerations are slow enough to
>actually calcualte the effect of fluids with different viscosites.
The jar of water has the fluid mass at the center of rotation; there
is no impetus to put it into angular motion except the surface effects
of the rotation of the jar, which are minimized in a cylinder of
significant diameter. This isn't all that similar to the effect with
a small-volume toroid, where the fluid is much more likely to move
with the containing vessel. At any given point in the torus, the
fluid sees much more of the local effect of the wheel's motion.
Torque converters are a different matter; they work in a range where
hydraulic effects are the goal. The bike wheel at typical speeds and
accelerations will not go there.
>"If the test is with road tires, I think the effects will not be
>dramatic. If the test is with fat cruiser tires, I expect that it
>would be more noticeable"
>
>This is true.
--
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