On manual cars, Why can’t a car start in a higher gear?

Question

As the title says, I know that different shifts mean different gear sizes bein used, but I don’t understand why it makes you unable to start moving the car. I have been able to start a couple of cars on the 2nd shift as an experiment and I understand that I could damage the car and I do it just once for testing purposes but I don’t understand why I cannot do so on other shifts. To clarify, I mean start as in start moving the car and not just turning the car on. Thanks

In: Engineering

Answer 1

In short, mechanical advantage. Think of the gears of a transmission in a vehicle as kind of like a bicycle with multiple gears. When you’re in a very low gear on a bicycle, with the rear on the biggest sprocket and the front on the smallest sprocket, it’s very easy to push the pedals no matter whether you’re going up a hill or starting from a stop. You do have the issue of only being able to pedal your legs so fast, and it’s really easy to get your legs going as fast as you can comfortably pedal but the bike isn’t moving very fast because it takes a lot of turns off the pedal crank to equal one turn of the rear wheel. At the other end, if you’re in a very high gear, like when the rear is on the smallest sprocket and the front is on the largest, you can pedal fewer revolutions to make one revolution of the rear wheel, but it’s much more difficult to press those pedals and if you’re going uphill it may be so difficult you can’t continue in that gear. A car transmission works in the same principles, just with an engine turning a crankshaft instead of legs pushing a pedal what and all the gears are in one place (except for the final drive, but that doesn’t change in on-road passenger cars so we’re pretending it doesn’t exist for this example) instead of spread out. A lower gear allows the engine to more easily turn the wheels due to mechanical advantage but the tradeoff is that the engine will run out of usable rpm range before the vehicle is moving very fast, so then you can shift gears to provide a little less mechanical advantage but you get a higher top speed, and so on until you run out of either gears or the power to push the car any faster in the gear you’re in.