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Understanding GEAR RATIOS

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    Ok, gear ratios are a bit tricky but I found a great bit of information on "How Stuff Works."...

    Quote:
    Understanding the Concept of Gear Ratio
    Understanding the concept of the gear ratio is easy if you understand the concept of the circumference of a circle. Keep in mind that the circumference of a circle is equal to the diameter of the circle multiplied by Pi (Pi is equal to 3.14159...). Therefore, if you have a circle or a gear with a diameter of 1 inch, the circumference of that circle is 3.14159 inches.
    The following figure shows how the circumference of a circle with a diameter of 1.27 inches is equal to a linear distance of 4 inches:

    View picture on the actual link here

    Let's say that you have another circle whose diameter is 0.635 inches (1.27 inches / 2), and you roll it in the same way as in this figure. You'll find that, because its diameter is half of the circle's in the figure, it has to complete two full rotations to cover the same 4-inch line. This explains why two gears, one half as big as the other, have a gear ratio of 2:1. The smaller gear has to spin twice to cover the same distance covered when the larger gear spins once.

    Most gears that you see in real life have teeth. The teeth have three advantages:

    They prevent slippage between the gears. Therefore, axles connected by gears are always synchronized exactly with one another.

    They make it possible to determine exact gear ratios. You just count the number of teeth in the two gears and divide. So if one gear has 60 teeth and another has 20, the gear ratio when these two gears are connected together is 3:1.

    They make it so that slight imperfections in the actual diameter and circumference of two gears don't matter. The gear ratio is controlled by the number of teeth even if the diameters are a bit off.



    So, to get the actual gear ratio of a specific gear box, from 1-2 you count the number of teeth on the 1st gear and divide it by the number of teeth on the 2nd gear....and so on (2-3, 3-4, 4-5...).

    The higher the initial gear ratio, the more pull off the line the car can have.

    Quote:
    The final drive gear ratio is the number of times the motor needs to turn to power the wheels for one full rotation.



    Here's our gear ratio setup:
    Final-drive ratio: 4.24:1
    Gear, Ratio, Mph/1000 rpm, Max test speed
    I, 3.54, 4.7, 29 mph (6200 rpm)
    II, 2.05, 8.2, 51 mph (6200 rpm)
    III, 1.33, 12.6, 78 mph (6200 rpm)
    IV, 0.97, 17.2, 107 mph (6200 rpm)
    V, 0.78, 21.6, 127 mph (5900 rpm)

    For our final drive to be 4.24:1, that means our motor has 4.24 full revelutions to make just to get the tire to make 1 full revelution.

    Compared to a Porsche 911 Turbo:
    Final Drive 3.44:1
    1st Gear Ratio 3.82:1
    2nd Gear Ratio 2.05:1
    3rd Gear Ratio 1.41:1
    4th Gear Ratio 1.12:1
    5th Gear Ratio 0.92:1
    6th Gear Ratio 0.75:1


    Amazingly, our gear ratio isn't far off from the 911 setup. It has a higher 1st gear ratio, so it will explode more from the start, has more in the middle gears as well and a similar final gear setup. Only thing is, it's got a final drive of 3.44 so it takes about 1 less engine rotation to get the tire spinning than it does ours.


    As you can see, it's all like a bike gear setup. You start off in the biggest gear b/c that's where you get more of a 1:1 (not exact but closer) engine to power delivery conversion. If you started in the highest gear of a bike, you'll notice that it takes less rotations to spin the rear tire but a LOT more force to get it spinning. Same concept in cars. Start on the big gear b/c from the start, it takes less power to get spinning and can get you moving faster with less power. As you move up in the gears, the ratio actually drops b/c the gear size from the previous gear to the next is getting smaller. With the engine (or the person on the bike) in full rotation now, the smaller the gears means that the engine can utilize the power it's creating by allowing the smaller gears to spin the tires at a higher rate with less actual rotation.

    As you get closer to the end, the gears are much smaller than when you started, which allows the engine to spin less but to have more power delivered to the ground with less effort. In the FINAL drive ratio, you are in the smallest gear and that's where (as stated above) you are spinning the engine as little as possible to get the most forward motion out of the vehicle.

    Please ask questions if necessary.
    Wow Web, you've done it again. Very good stuff.

    So let me relate this to other concepts. In the write up, there was something that said that if the initial gear ratio is higher, it will start off the line faster. Or something to that effect. So does that mean its better to use the cars "peak torque" when in the gears with a gear ratio higher than one?

    If my years of physics classes serve me, because the engine is using a larger gear to turn a smaller one, it requires more torque.
    Oh god. I got the idea of gear ratios backwards. Nevermind what I had just said.

    No wonder it wasn't making sense to me.
    hahaha..............well, you want a higher gear initially b/c torque is what gets you off the line. As you advance the gears, your engine hp (continuous and building power supply) is what keeps the gears turning and that's why you can go so smaller gears. The hp keeps the engine turning and increasing after it gets started. The torque is what gets it moving from the start in the higher gear.
    Alright, I think I found out what was confusing me. Straight from Wikipedia:

    Quote:
    The first number in the ratio is usually the gear that power is applied to. In an automobile the first number is the gear receiving power from the engine.


    which seems to contradict:

    Quote:
    Gear Ratio
    1st gear 2.97:1

    In 1st gear, the engine makes 2.97 revolutions for every revolution of the transmission’s output.


    See what its saying? The statements contradict each other!
    thanks for making this, I can honestly say I know more about gear ratios after reading this
    Mayo wrote:
    Alright, I think I found out what was confusing me. Straight from Wikipedia:

    Quote:
    The first number in the ratio is usually the gear that power is applied to. In an automobile the first number is the gear receiving power from the engine.


    which seems to contradict:

    Quote:
    Gear Ratio
    1st gear 2.97:1

    In 1st gear, the engine makes 2.97 revolutions for every revolution of the transmission’s output.


    See what its saying? The statements contradict each other!


    The problem is, most examples detail an almost 1:1 power delivery system. The engine spins 1 time and the wheels spin 1 time. But, we also have to take into acount the gears for the transmission will add to that and actually cause more engine rotation per tire rotation. if you take the transmission out or have a single gear transmission that has a gear that matches the overall gear output of the engine, then you will be almost 1:1. But, since it's almost impossible to have that, although some cars come REALLY close, we have to deal with what we have now.
    so in a ratio of 2.05 : 1 that 2.05 is the number of the engine revolution? RPM?
    Orin wrote:
    so in a ratio of 2.05 : 1 that 2.05 is the number of the engine revolution? RPM?


    Quote:
    So if one gear has 60 teeth and another has 20, the gear ratio when these two gears are connected together is 3:1.


    That is simply the ratio of the two gears that it is describing.

    Perfect is 1:1 and that's when both gears you are looking at have the exact same amount of teeth/diameter. That means for each revolution of one, the other does 1 revolution also.

    2.05:1 means: One gear is of a specific diameter/tooth count, and the other divided by the lower gear (previous gear) becomes 2.05:1. So, it takes 2.05 engine revolutions to make 1 revolution in the gear box.


    YES, to answer your question
    Alright another question thats been bothering me. Its hard to make it tC specific, some Im going to use simply a general car example.

    Say it takes a car about 1.5 seconds to get from 10mph to 20mph (2nd gear, 2k rpm to 3k rpm)

    But that same car takes 3 seconds to get from 50mph to 60mph (5th gear, 2k rpm to 3k rpm)

    ^ That may not be a tC, but its decently realistic for a car. Same amount of throttle for both examples.

    My question is why it takes longer to go from 50-60mph than it does from 10-20mph. If you think about it, the gear ratio in 2nd gear is much larger than the gear ratio in 5th gear. 5th gear makes fewer engine strokes per wheel revolution. So shouldn't 50-60 mph be faster than 10-20 mph? (I know its not, haha, but what the hell am I missing?)
    Mayo wrote:
    Alright another question thats been bothering me. Its hard to make it tC specific, some Im going to use simply a general car example.

    Say it takes a car about 1.5 seconds to get from 10mph to 20mph (2nd gear, 2k rpm to 3k rpm)

    But that same car takes 3 seconds to get from 50mph to 60mph (5th gear, 2k rpm to 3k rpm)

    ^ That may not be a tC, but its decently realistic for a car. Same amount of throttle for both examples.

    My question is why it takes longer to go from 50-60mph than it does from 10-20mph. If you think about it, the gear ratio in 2nd gear is much larger than the gear ratio in 5th gear. 5th gear makes fewer engine strokes per wheel revolution. So shouldn't 50-60 mph be faster than 10-20 mph? (I know its not, haha, but what the hell am I missing?)


    Ok.........as you move up in gears, it's easier for the gears to spin b/c they are smaller BUT.....it takes more engine revolutions to make them complete 1 full spin. So....2nd gear only takes that little time b/c the larger gear spins closer to that 1:1 ratio with the engine and transmission compared to 5th gear. The further you get from 1:1 power utilization, the more power it's going to take (and time) to utilize the power output.

    The higher gears have smaller diameter/tooth count gears so it takes more engine revolutions to get those smaller transmission gears to be able to transfer that power from the engine to the wheels.
    In the higher gears, that is where Horsepower comes into play. It takes more Horsepower to go from 50-60 than it does to go from 10-20. That's another reason why going from 100-120 takes longer than 50-60........gear ratio (final drive gear) is the main reason but Horsepower is considered in that as well.
    Web wrote:
    Mayo wrote:
    Alright another question thats been bothering me. Its hard to make it tC specific, some Im going to use simply a general car example.

    Say it takes a car about 1.5 seconds to get from 10mph to 20mph (2nd gear, 2k rpm to 3k rpm)

    But that same car takes 3 seconds to get from 50mph to 60mph (5th gear, 2k rpm to 3k rpm)

    ^ That may not be a tC, but its decently realistic for a car. Same amount of throttle for both examples.

    My question is why it takes longer to go from 50-60mph than it does from 10-20mph. If you think about it, the gear ratio in 2nd gear is much larger than the gear ratio in 5th gear. 5th gear makes fewer engine strokes per wheel revolution. So shouldn't 50-60 mph be faster than 10-20 mph? (I know its not, haha, but what the hell am I missing?)


    Ok.........as you move up in gears, it's easier for the gears to spin b/c they are smaller BUT.....it takes more engine revolutions to make them complete 1 full spin. So....2nd gear only takes that little time b/c the larger gear spins closer to that 1:1 ratio with the engine and transmission compared to 5th gear. The further you get from 1:1 power utilization, the more power it's going to take (and time) to utilize the power output.

    The higher gears have smaller diameter/tooth count gears so it takes more engine revolutions to get those smaller transmission gears to be able to transfer that power from the engine to the wheels.


    kinda like when your riding your bike in the highest gear: you peddle 50 times and only go two feet?
    Thanks for taking the time to explain. I hope other people are wondering the same thing cause I feel dumb asking.

    Just making sure you realize how thoroughly confused I am, heres my train of thought:

    2000-3000 rpms in 2nd gear (2:1) yields 'X' wheel revolutions

    2000-3000 rpms in 5th gear (0.5:1) yields '>X' wheel revolutions becauce the gear receiving the power from the engine is smaller, making the wheels spin faster.

    lemire04 wrote:
    Web wrote:
    Mayo wrote:
    Alright another question thats been bothering me. Its hard to make it tC specific, some Im going to use simply a general car example.

    Say it takes a car about 1.5 seconds to get from 10mph to 20mph (2nd gear, 2k rpm to 3k rpm)

    But that same car takes 3 seconds to get from 50mph to 60mph (5th gear, 2k rpm to 3k rpm)

    ^ That may not be a tC, but its decently realistic for a car. Same amount of throttle for both examples.

    My question is why it takes longer to go from 50-60mph than it does from 10-20mph. If you think about it, the gear ratio in 2nd gear is much larger than the gear ratio in 5th gear. 5th gear makes fewer engine strokes per wheel revolution. So shouldn't 50-60 mph be faster than 10-20 mph? (I know its not, haha, but what the hell am I missing?)


    Ok.........as you move up in gears, it's easier for the gears to spin b/c they are smaller BUT.....it takes more engine revolutions to make them complete 1 full spin. So....2nd gear only takes that little time b/c the larger gear spins closer to that 1:1 ratio with the engine and transmission compared to 5th gear. The further you get from 1:1 power utilization, the more power it's going to take (and time) to utilize the power output.

    The higher gears have smaller diameter/tooth count gears so it takes more engine revolutions to get those smaller transmission gears to be able to transfer that power from the engine to the wheels.


    kinda like when your riding your bike in the highest gear: you peddle 50 times and only go two feet?


    Yeah, when you're in 1st chain and 1st gear, the gears are almost the same diameter so that's why you can spin the hell out of the tire but you don't really go anywhere. To counter that, you need to be able to multiply the amount of force YOU are producing to the rotational force the bike/gears are producing. This is where the smaller gears come into play.
    I suppose another simpler way to look at it is


    lower gears like first require the engine to spin more to turn it

    smaller gears like 5th allow your engine to relax because not as many revolutions are required to spin it


    Mayo wrote:
    Thanks for taking the time to explain. I hope other people are wondering the same thing cause I feel dumb asking.

    Just making sure you realize how thoroughly confused I am, heres my train of thought:

    2000-3000 rpms in 2nd gear (2:1) yields 'X' wheel revolutions

    2000-3000 rpms in 5th gear (0.5:1) yields '>X' wheel revolutions becauce the gear receiving the power from the engine is smaller, making the wheels spin faster.



    That's where you're confusing yourself. The ENGINE is what's spinning faster and not the transmission gear. The transmission gears spin faster in LOWER gears, not higher gears. Just like a mountain bike. The higher you go in gears requires MORE force from you to go even faster. If you could spin the highest gear of a bike at the rate that you spin the lowest gear (1:1 respectively), you would be able to cruise at a HIGH speed and have HUGE quads. Just like cars, it takes more power to go faster when you're in a higher gear b/c of the ratio to engine/pedal gear compared to transmission/tire gear.
    Orin wrote:
    I suppose another simpler way to look at it is


    lower gears like first require the engine to spin more to turn it

    to get it moving off the line, yes......the engine revolves much faster in lower gears along with the transmission gears

    smaller gears like 5th allow your engine to relax because not as many revolutions are required to spin it

    yes, unless you're accelerating, which requires more engine revolutions at a higher speed to be able to "catch up" to the overall output rotation of the tires




    Key thing...........the faster you go in each increasing gear takes more horsepower than the previous/lower gear to continue forward.
    Oh man does that clear things up. Thanks.
    ...No problem.
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