In a manual car, why does going switching down gears make it easier/possible to to uphills?
In: Engineering
Someone will nail this but an automatic car does the same thing just slower….and regulated/programmed
Switching down gears is similar to riding a ‘mountain’ bike up a hill. More chain on a smaller cog allows the wheel to rotate with less resistance.
Because going uphill requires more power in order to maintain the same speed or accelerate. Switching gears gives your car’s engine more mechanical advantage by allowing it to turn faster.
Imagine you are spinning with a five foot bamboo in one hand.
Now if I add weights to the far end of the bamboo, you will start to slow down. I cut the bamboo in half. You are able to spin again.
That’s what happens when you move to a lower gear. The car is able to push itself up a steeper incline, albeit at the cost of speed.
Engine speed is measured in RPM.
Engine power is measured in torque, how much turning force the engine produces.
Horse power is some math between Torque and RPM.
So on level ground, your cruising at 65 in 5th gear in a Toyota Camry at 2,000 rpm. Your fine because your current horsepower can maintain your speed.
You go up a light hill, your engines Torque cannot maintain their difficulty in pushing up the hill and you begin to slow down. So, you down shift to 4th; RPMs jump to 3000, your engine is producing more horsepower to counter act the drag of the hill. And, the downshift will give you a mechanical advantage requiring less torque from the engine.
But of course it’s a camery and 4th wasn’t good enough so you drop to 3rd. Now your at 4000 RPM and have plenty horsepower on tap so you put the peddle down till the valves cry at 6500 – and now your passing traffic again … that barely acknowledged the hill you just conquered.
Automatics do the same thing, but depends on the generation.
It’s exactly the same process that happens with a pushbike.
If you move into a lower gear, for the same speed your legs move, you now move more slowly.
You can put a given amount of force in to turn the crank once.
If you need more power to get up hills, you put the bike in a lower gear, so that means now to move forward one meter, for example, you have to turn the crank 3 times.
If you’re in top gear on a pushbike, you might only turn the crank 1/8 of a turn to move forward a meter.
So considering you put out a certain amount of force to turn the crank once, in low gear, you’ve now got three times that force to move 1 meter. The gears allow you to exchange speed for torque, or pulling power.
In top gear, once the bike is moving fast, you don’t need anything like as much power to maintain that speed, so you can move into the top gear, spreading your body’s force out along more distance.
The car’s engine works the same way. If you move down the gears, a given number of revolutions of the engine now are responsible for moving the car less distance, so the car has that much more power to move.
Forget the angles and the car and all of that.
Imagine trying to lift a piano. Naturally it would be very difficult. Now think about your grade school science class and specifically levers.
If you put a piano onto a teeter totter (lever) but have the part holding the teeter totter up (axis) closer to the piano you get what is called a mechanical advantage.
It becomes easier to move the heavy piano, but at a cost, you can’t move the piano as far because it is on the short side of the lever.
This is what the lower gears of the car are designed to do. They are made to make it easier to get the car moving from a stop, if you’ve ever tried to start moving in second gear you’ll understand it takes a significant amount more rpm to prevent stalling.
When you shift up, you are essentially moving the part holding the teeter totter up closer to the center, losing the mechanical advantage you had, and normally it is okay because your car is already moving.
When you start going uphill it is like shifting the part holding the axis of the teeter totter towards you, you are now at a mechanical *disadvantage* and start slowing down, and to compensate you must downshift, to get the mechanical advantage back, but like the teeter totter, the downside is you move slower.
Gear ratios. Low speed = high torque and high speed = low torque. To get up a hill you need more torque. Lower gears dont turn as fast so they are lower speed
Every time the engine rotates, it delivers a certain amount of power and force, the transmission applies that over a certain distance.
A car in high gear traveling at 75 mph @5000 rpm moves about 16 inches every engine rotation. The same car in low gear going 25@5000 only travels 5.3 inches. The same amount of engine output is being applied over a shorter distance, allowing the car to overcome the greater resistance.
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