How Transmissions Work

This is especially for Jason Helmick, who uses the word “power” incorrectly – a lot – with his new Dodge Challenger. You’ll need a piece of paper, and a pen or pencil, to follow along.

On that piece of paper, draw a short horizontal line – about two inches long. In the middle, along the bottom of the line, draw a small triangle, so it looks as if the line is balanced on the tip of that triangle.

You have drawn a lever. In this case, it isn’t an especially useful lever, because the fulcrum is right at the middle. That means applying 10 pounds of downward force to one end will result in exactly 10 pounds of upward force on the other end. If you press down one inch, the other end will rise one inch. This is a neutral lever. 

Draw the line again, and move the fulcrum to just near the right-hand end. This is a more traditional lever. Applying 10 pounds of downward force to the left end will result in, oh, say 40 pounds of upward force on the other end. However, you pay for that mechanical advantage – in order to get the right end to rise an inch, you’d have to press the left end down four inches. This is a 4:1 lever, meaning every unit of input force will be multiplied by 4, and your input distance will be divided by 4 on the output side.

Now draw a “plus” sign. Draw an “X” over it, so you have something approximating an asterisk. In a crude sense, you have drawn a wheel, or a gear. Mechanically, wheels and gears are just an infinite number of levers, radiating out from a common center. You calculate mechanical advantage exactly the same. For example, a 4:1 gear, where you apply force to the outside edge, will apply 4 times that force to the interior hub, and you will have to move that outside edge 4 times the distance of the inside edge.

Traditional transmissions, both automatic and standard (but not continuously variable) have several sets of these gears – usually at least 4, sometimes as many as 6 in passenger vehicles (reverse gear doesn’t count in this list; it usually mimics 1st gear but reverses the drivetrain direction).

1st gear is usually a tall gear, meaning it has a high ratio. My old Jeep Wrangler had a roughly 4:1 1st gear. The idea is that the engine’s output, applied to the gear, results in a lot of output force, or torque, but not a lot of output speed. That is, every rotation of the engine’s output shaft translates into few rotations of the gear’s output, which is connected to the drivetrain, which is connected to the wheels. 1st gear is torquey, which means it’s designed to get a lot of weight moving very slowly. 2nd gear is a bit less torquey – it can move a bit less weight, but it can move it a bit faster. 2nd gear relies on the inertia created by 1st gear – the car has already overcome its dead-strop inertia. That pattern continues up, usually through to the next-to-the-last gear. Your last gear is often overdrive, meaning it is a negative ratio, often something like .9:1. This is designed solely for speed, relying heavily on the inertia of previous gears. It’s more fuel-efficient, and it’s designed to keep the car going – usually on a flat or downhill surface.

If you start applying opposing forces – like gravity, or a load of cargo in the trunk – you end up staying in lower gears. Each gear is essentially optimized for a weight:speed combination. 4th gear might get you up to 75MPH, but only if you don’t need to overcome more than X pounds of weight and drag (including gravity acting against you on an uphill climb). If your weight/drag goes over X, then you’ll end up downshifting to apply more torque, paying for that in speed – just as with our original lever, where you paid in distance to achieve more force. Remember, for wheels and gears, the “distance” the lever moves translates to the rotational speed of the wheel or gear.

Jason’s car is heavy, but its gears are designed to move that weight – and only that weight. Contrast that to my Tundra, whose gears are designed to move the truck as well as a few tons of cargo or tow load. In 1st gear, my truck’s tires never move very quickly. The engine can rev all the way to 5k RPM (close to redline), and the truck won’t exceed 5MPH. But it can do that 5MPH up a hill, hauling another truck or two. Jason’s car, on the other hand, will spin the tires rapidly in 1st gear, because the gears themselves are designed to favor speed over torque as much as possible. The only compromise toward torque is the bare minimum needed to get the damn thing moving.

Thus, Jason has a great fear of pulling out of my 30-degree-uphill, graveled driveway at my cabin. He fears spinning out and flinging gravel everywhere, because his little car wants to go fast as soon as possible, and is geared thusly. Also, Jason comes off the clutch really hard. And steps on the gas really hard. Bad combo. However, if he feathers the clutch a bit, the car – and he’s demonstrated this – rolls evenly up the hill pretty effortlessly, without flinging gravel everywhere. While his 1st gear is geared more for speed than for torque, it’s not totally useless on a hill. It has enough torque to overcome the extra drag of gravity, and it’s not so totally focused on speed that it spins out the tires immediately.

Power – the word Jason kept overusing in our argument about whether his car could make it out or not – is a word the automotive industry uses to refer to the engine’s raw output. It’s usually expressed in horsepower, because Henry Ford. But this is the raw, pre-transmission output. It doesn’t matter for any practical purposes. What matters is how that power is applied, through he transmission’s gears, to create either torque or speed.

3 thoughts on “How Transmissions Work

  1. jamesbirley

    Nice blog Don. Very similar to the way people (especially ISP’s) use the word “speed” instead of bandwidth. For example: “Checking my email is so much faster, now that I’ve upgraded to FIOS Gigabit service. My old 100 Megabit service did not provide me with enough speed.” (WRONG)!!

  2. wanderson75

    Why doesn’t Jason just put it in second (or third) to start. Then he should be able to make it up the driveway with his lead foot without digging a ditch in your driveway?

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