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
To understand this it is probably best to understand what gears do and why. Imagine you have a circle with a radius of 1 unit. It will have a circumference two times the radius multiplied by pi, or ~6.28 units. Now imagine you have a second circle of radius 3 units, which means a circumference of ~18.85 units.
Suppose that our two circles are actually gears that mesh together, so when one turns they cannot slip and the other turns as well. The first, smaller gear has a circumference about 1/3 of that of the larger gear which means it needs to turn about 3 times before the larger gear turns once. This doesn’t just mean we can change the *speed* at which things turn in relation to each other, it also changes other aspects about how they turn. Even though that large gear will be turning more slowly it will be doing so more forcefully.
In the context of a car the car engine only works properly within a certain range of speeds. Usually they will idle between 600 and 1000 rpm, while on the highway they go up to 1500 to 2000 rpm. Car engines tend to have some problems around 5000 rpm. Note the ratio between those speeds is not that high; if you idle at 1000 rpm and go up to 3000 rpm while driving normally, if your car moves at 5 mph while idling then why don’t you top out at 15 mph at 3000 rpm? That is where gears come in.
At low gear your car engine spins a lot compared to how quickly the wheels spin, translating to more torque but lower speed. At high gear the wheels are spinning faster compared to the car engine which translates to more speed but lower torque. This means that when you are trying to start a car in a high gear you are trying to make the car spin the wheels *really fast* while the engine is barely moving itself. It just doesn’t work, the engine doesn’t have the power to do that.
For the same reason it is hard to start pedaling a bike in the highest gear. You may even have to put your whole weight on it.
The lowest gear has the most leverage to start the car from a stop.
You can if you have enough torque. Many big trucks optimized for towing can skip first great entirely unless there’s a heavy load.
They can. I accidentally started an old late 90s Nissan Altima in 4th gear once. Thing had a very forgiving transmission.
You can do it, you just have to let the clutch out extremely slowly (or rather, hold the clutch at the bite point for quite some time until you have reached a high enough speed) and press the accelerator enough.
Source: I started going in 5th once just for the fun of it.
Disclaimer: This will wear out your clutch.
It comes down to some math, and power curves.
Gas cars have a narrow range where they can make power (say 800 RPM to 6000 RPM). If you put your in a high gear your engine can turn slowly and the wheels can turn fast (say highway speeds your doing 2500 RPM). No wthink about being stopped, and having it in a high gear where at 2500 rpms the wheels need to turn 65mph…and you are going 0mph. There is no way they can spin that fast and accelerate the weight of the car. Those forces are so high, they will stall the engine. So you choose a lower gear so that the engine can make power at very very low speeds and not stall.
In a roundabout way, a transmission is like a system of levers. A long lever lets you move a heavy object. A low gear is like a long lever. A higher gear is like a shorter lever. And while it is able to spin faster, it does not give you as much “leverage,” (which is, in this case, Torque.) Which means that, depending on the engine, it might not have enough mechanical advantage to move the weight of the vehicle.
And now for something completely different.
Let’s say you have a 4 cylinder car. Each time your engine turns over, each cylinder delivers a certain amount of energy to get your car going, or to keep it going.
In low gear, you are delivering a whole lot of cylinders of energy in a short time. So if your engine delivers, say, 100 cylinders of energy per second in low gear, it’ll get you moving a lot faster than if it delivers only 10 cylinders of energy per second, which it would in a higher gear.
When you get moving, you find you are using a lot more cylinders per second than you need, so you shift to a higher gear. You go from 100 cylinders per second to maybe 50 cylinders per second. Shortly you shift again and are going from 50 cylinders per second to 25, and so forth.
If, as in the original post, you start to move in a higher gear, you are using only 50 cylinders per second. You CAN get going this way but as someone pointed out, it wears out the clutch and other parts quickly.
Because cars with manual transmissions have a “dry clutch”.
Dry means there’s no fluid that would be able to cool it down when it heats up from friction.
And a clutch is the thing that decoupled the gearbox from the engine, allowing changing gears or stoping. The gearbox has its speed dictated by the wheels.
Now, when starting your wheels are at 0 speed, so same for transmission. So when starting you basically close the speed gap between the wheels and transmission.
If you have a lot of difference, it means a lot of friction in the clutch, so a lot of wear and heat. Plus, in a taller gear (higher gear number), it will take more torque from the engine to get the car moving, and MUCH more feathering of the clutch. You could, in theory, start in top gear, but it’s hard as hell, and most likely you’ll need to change the friction plates.
I hope that helped.
[Because gears are just a bunch of spinning levers.](https://youtu.be/JOLtS4VUcvQ?si=1JMQdYxThh_rch5R)
This video is a great ELI5 if you’re a visual learner like me.
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