The Magic of Shifting Gears

There's something supremely satisfying about manual transmissions. Whether it's the joy of blipping the throttle to rev-match perfectly as you enter that hairpin or the feeling of dumping the clutch and leaving a layer of your Goodyears on the asphalt, it's a feeling of power and control no true gearhead will deny. In this post I am going to explain the basics of driving a manual transmission: what exactly you're doing when you push in the clutch and pop it into gear.

A modern internal combustion engine can only operate efficiently at a limited range of speeds, for example, 500RPM up to 6000RPM. Engines typically make the most power and have the greatest efficiency at a specific RPM, which is a result of the design of the engine. The automobile operates at a wide range of speeds, from stop-and-go traffic to travelling hundreds of miles an hour. A transmission serves to change the speed of the wheels in relation to the speed of the engine. 

So, for example, in first gear, say the maximum speed the car can travel is 20MPH. That means that at 20MPH the engine is spinning its maximum speed. To potentially go faster, the ratio of the engine speed to the wheel speed must change. So another gear is added to the transmission that makes the engine spin half as fast at the same speedometer reading. You can now go 40MPH when the engine spins 6000RPM. By changing the ratio of these gears, you also change the amount of torque the engine can deliver to the ground. You will generate twice as much torque at the wheels in first gear as you will in second in this example.

Think about a bicycle. If you want to climb a hill, you shift to a lower gear. You have to spin your legs faster to go the same speed, but you can produce more torque with the same amount of leg force. Just like a human, a car engine makes the most amount of torque when it's running at a specific speed. Too fast, and it's inefficient. Too slow, and you cant make enough force to speed up. This range of engine speeds where the engine operates most efficiently is commonly called the power band. The power band usually starts somewhere in the 3000RPM range for most cars, and stops around 6,000RPM (or more with higher performance engines.)  While you can operate the engine below this speed just fine, the torque output and resulting acceleration is very limited.


Another design challenge of the IC engine is that it must be started by some external energy - when you turn the key, a motor engages and turns the engine to a speed where it can pump enough air in to sustain itself, and then the electric motor disengages. There must be a way to disconnect the engine from the wheels to allow the engine to keep idling when the car is not moving, and to transition it from neutral (engine is not connected to wheels by any gears) into gear. This is called a clutch, so called because small friction-generating pads "clutch" at a rotating plate connected to the engine, known as a flywheel. Using a combination of springs and centrifugal force, the pads grip the wheel and in turn spin the wheels. by use of a foot-pedal, the clutch pads are lifted from the flywheel, and the engine spins free of the tires. 

Think of the clutch pedal like a brake pedal - you wouldn't just want to stop the wheels instantly, you want to ease into the brakes gently. The clutch operates much the same, just in reverse.

So, in the typical driving cycle of a car, you sit down, depress the clutch to disconnect the engine from the transmission and wheels, and start the engine. If you were to leave the engine connected to the wheels, the electric motor would try to spin the motor and try to move the entire car, either pushing your car into the garage wall or eventually burning out the starter motor by working it too hard. Almost all cars have a lockout that will not let you start the car without the clutch depressed to prevent this very thing from happening.


You then put the engine in gear. This connects the clutch to the tires. You then let the clutch out. As the clutch contacts the flywheel and grabs onto it, connecting the engine to the wheels, the engine slows down and eventually stalls, coming to a stop. Congratulations, you've killed your car. It won't be the last time. To transition from a standstill to moving, the clutch must be slowly let out while power is added by depressing the accelerator. The clutch will slowly rub against the flywheel, giving a little energy to the car until it starts moving. You want to let the clutch rub slowly until the car speeds up to match the engine speed - rev the engine too high and you'll wear the pads out, but not enough and it will stall the motor. 

This is something that you just have to feel and practice. For a beginner, a good strategy is practice putting the engine at about 2000 RPM, and let the clutch out slowly until the car just starts to move. At that point, just keep both feet steady, and let the friction slowly speed the car up. As the car speeds up, slowly let the clutch the rest of the way out. The engine is now fully hooked to the wheels, and you can mash on the gas just like an auto. 


Depending on how far the gas pedal is pushed down, you will shift in one of two places - the first is if you are driving like a normal person. The engine will accelerate at whatever pace you want, and then as the car speeds up the engine speed will level off. You'll feel it as a decreasing acceleration. When you feel like you're not accelerating any more, release the gas pedal, push in the clutch, and shift to the next gear. Then let the clutch out slowly, adding gas as you do so. After several jerks because you once again let the clutch out too fast, you'll be in second gear. This can be continued until you either get pulled over, crash into the car in front of you, or reach your car's top speed.


So what happens when you want to slow down? Well, the same thing as in an auto car. Let off the gas. If that's not enough, use the brakes. Remember though, as you slow down the engine will spin slower and slower. You can either clutch in and use the brakes only until you are slower, then shift into a proper gear for your speed, or downshift one gear and let the engine slow you down. The faster the engine is spinning when you let off the gas, the faster your car will naturally slow down. This is called engine braking.

A note about engine braking: Because an engine is a self-powered air pump, when you have your foot off the gas the fuel is cut off and the car's momentum actually turns the engine instead of burning fuel. The faster the engine is going when you let off the gas, the faster it pumps air through it, and the faster the car slows down.

Because the car is turning the engine when you are engine braking, it is possible to shift into too low a gear and make the engine spin too fast. Imagine if you were on an exercise bike and your legs were tied to the pedals. If someone sped it up enough, very bad things would happen. The same thing happens if you downshift to a gear that is too low - always be cautious when doing so. One gear is usually enough to get sufficient engine braking. The best time to use engine braking is coming up to a red light, when you have plenty of warning and can slow down gently by sequentially downshifting and letting the engine slow you down, the reverse process of accelerating. As always, let the clutch out slowly to let the engine change speeds without jerking the car

Rev-matching and Double Clutching:  When you are downshifting, as you shift to a lower gear the engine will speed up, because the transmission is making it spin faster in relation to the wheels. To keep the car from suddenly speeding the engine up and stealing the car's momentum, you can push the clutch in, give the gas pedal a slight tap to raise the engine RPM, shift to the lower gear, and let the clutch out smoothly and quickly. With practice, you can match the engine speed to the car speed in the lower gear and you will have a smooth, fast, undisturbed shift. A more advanced technique is called "double clutching." Depressing the clutch once, you pull the gearshift into neutral, and release the clutch. You then blip the throttle to rev-match, and clutch again and shift to the lower gear. This is smoother and induces less wear on the clutch by allowing the clutch to speed up to engine speed without spinning other shafts and bearings inside the transmission. When racing, it is important to prevent suddenly shocking the tires with engine braking, so the double clutch technique is to prevent losing traction of the drive tires. Contrary to the Fast and Furious movie, it does not make you faster in a drag race, where all your shifts are up.

Panic stopping is inevitable, and the most common way to do so is "put both feet in and pray." While I don't advocate leaving your braking and steering up to Fate, the adage is correct in that you should depress the clutch with your left foot and the brake with your right foot, simultaneously disconnecting the engine and slowing with the brakes. You should always actively steer and monitor your braking input so you don't skid into something you could have simply steered around. 


There are other more advanced techniques that you can learn, but it comes down to one basic thing: practice. The most important things you can learn are how to control your pedals. Learning just how much gas you need to give to get moving and how far you have to release your clutch pedal before the clutch starts to grip the flywheel is something you just have to learn, and it differs greatly from car to car. It is for this reason that, to some degree, you must re-learn every car you want to drive. An experienced driver does this experimentally - just add a little bit of gas, and be delicate with the clutch. Hill starts come with practice - by knowing exactly where your clutch pedal starts to let the clutch grip the flywheel, you can add gas and quickly release to that just-gripping point, and from there slowly release it as usual. 


As it is, expect to give it way too much gas, to stall it out multiple times, and to shift into the wrong gear many times, even after you get the hang of it. It is a matter of feeling and listening to what your car is telling you, both on the dash and in your bones.