Gears & Gearing March/April 2004 |
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Wednesday, April 25 2007 12:26 |
What do people mean when they talk about putting higher gears in their
rear end? Why do they do it (and if it is so good, why didn't Ford do
it in the first place?)
Everyone seems to want to as fast as they can.
Higher gears can help them do that. If 3.55 gears are good, wouldn't 3.00 or 2.50 be even better?
In
engineering, as in life, most things are a compromise. The higher the
gear ratio, the faster the vehicle can go, at least in theory. The
engine has to have enough power to achieve and maintain this higher
speed. Even if the engine can get the vehicle to the higher speed,
with the higher gears it will take longer to do so. If it has lower
gears, it will be able to accelerate to a given speed quicker.
Consider
a teeter-totter. If it is pivoted in the center and two people of the
same weight sit at the same distance from the pivot, they will balance
each other. The rotational force that they produce will be equal. When
they go up and down, each moves the same distance and at the same speed.
If
one person weighs twice as much s the other, the heavier person would
have to sit one-half as far from the pivot for the teeter-totter to
balance. But, when this combination moves up and down, the person
twice as heavy, sitting half as far from the pivot, would move only
half as far up and down as the lighter person. The heavier person
would also move only half as fast as the lighter person.
This
is basic mechanical advantage. You can see that the lighter person can
produce the same rotational force as the heavier person only if the
lighter person travel farther. Because this happens in the same amount
of time, you can see also that the lighter person is moving faster.
The lighter person requires the additional mechanical advantage of
moving farther and faster to balance the heavier person.
How
does this apply to our vehicles? Think of the engine as being the
lighter person. The torque available to move a 2500 - 3500 lb vehicle
is in the 200 - 300 foot-pound area. The engine needs help, in terms of
mechanical advantage, to get that weight moving. It requires less help
to keep the weight moving at a steady speed. That is why you need lower
gears to start from a stop than you do for maintaining cruising speed.
That is also why trucks, which are heavier -- especially when loaded --
usually have lower gear ratios than passenger cars.
Gear ratios state the mechanical advantage that the engine
employs to move the vehicle. A rear end ratio of 4:1 means that when
the transmission is in top gear, the engine turns four times for each
one time that the rear wheels turn. If the transmission has a ratio of
3:1 in first gear, then the overall ratio is 12:1, good for moving from
a stop. The total or overall ratio is determined by multiplying the
rear end ratio by the transmission ratio.
Lower gears give
better acceleration and are easier on clutches, but are harder on
tires. Higher gears may give higher cruising speeds with less noise
and perhaps better fuel economy and less wear on the engine.
Ford
probably used lower gears than would be suitable today because of
poorer roads with lower speeds and the need to provide more pulling
power for the everyday needs of folks "back then".
Using gears that are too high can cause poorer fuel economy,
engine overheating, and damage.
The renowned Lincoln Zephyr gears
What
is the reason for Lincoln Zephyr transmission gears? They are not
stronger than Ford gears, but first and second are higher ratio. That
means that they are spaced closer to each other, and to third gear. If
you have increased the power of your engine and your vehicle is
relatively light, you may not need as much overall reduction in first
gear to get moving. Having the ratios closer can keep the RPM more
closely in its efficient or effective RPM range.
That is why they were, and are, popular in the Dry Lakes, drag race cars, and hot street cars.
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