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I have a very simple mental model of a car which is applying torque around an axle -- perhaps through a gear train -- onto the wheels. Then KE gained = torque * number of wheel rotations. The car gains a unit of velocity squared for each unit of distance it travels.

I can see that this makes sense for pretty much any car I can think of -- electric motor, gasoline, etc -- which all map some force-producing element onto wheel torque.

Here's what I'm stuck on: what's the point of the geartrain, in this simple model? Why wouldn't I just have my car on the highest gear ratio all the time so it can get fast extremely quickly, since the torque is so high?

I'd ideally like to add pieces to this intuitive model until I can explain everything about cars (or any other machine that moves).

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  • $\begingroup$ Stuff I know I'm not considering: internal friction, air resistance, reflected load, torque/speed curves Analogies: electrical transformers, hydraulic piston $\endgroup$
    – Matt
    Aug 19 at 12:22
  • $\begingroup$ There's also something about 'fast-geared motors being easy to stop with your hand' which I don't completely understand $\endgroup$
    – Matt
    Aug 19 at 12:29
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    $\begingroup$ You don't need gears. I know some guys who run a restored 100-year-old steam powered truck (with the suspension modified a bit for modern road surfaces). Well, to be pedantic there are "gears" but you can't change gear when moving. You select a speed range (0-40mph or 0-80mph) and direction (forward/reverse) when stationary, and then just drive. (And yes, it will do 80 mph in reverse, if you are brave enough to try.) The acceleration from a standing start will beat any "boy racer" away from traffic lights. The point being that a steam engine has maximum torque at zero RPM. $\endgroup$
    – alephzero
    Aug 19 at 13:00
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    $\begingroup$ ... and what is even more confusing for the boy racers is that compared with a gasoline engine, it is completely silent. There is no warning it is going to set off like a jack-rabbit from a standing start! (Note, as well as no gears, there is no clutch or torque converter either). $\endgroup$
    – alephzero
    Aug 19 at 13:05
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    $\begingroup$ " why does a steam engine have maximum torque at zero RPM " This question has an interesting answer. But it deserves to be asked as a separate question, rather than trying to answer it through comments. ALthough fair warning, it might be migrated to the engineering stack exchange as it is more engineering than physics. To be fair, even this question is more engineering than physics, i am glad it has not been migrated to engineering stack exchange $\endgroup$ Aug 20 at 7:04
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The need for gears depends upon the nature of the propulsion unit and the vehicle being propelled. Some sources of propulsion, such as gasoline engines, have performance characteristics that depend upon the speed at which they run. If you did not have gears then you would not be able to select the optimum engine speed to match a given road speed. For example, driving up a very steep hill requires more power than driving on a level road- to cope with that task you need to increase the power generated by the engine (ie raise its revs), while decreasing the power necessary to maintain the motion of the car (ie reduce its speed). You would not be able to increase engine rpm while reducing speed if the motor were directly coupled to the wheels. There are countless other similar considerations that make gearing essential.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Buzz
    Aug 20 at 15:30
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Why wouldn't I just have my car on the highest gear ratio all the time so it can get fast extremely quickly, since the torque is so high?

Gears serve to multiply the torque provided by engine at the cost of reducing the rpm provided by the engine.

Input torque * input rpm = Output torque * output rpm

where, Input torque/rpm is the torque/rpm provided by engine
output torque/rpm is the torque/rpm provided to the drive axle

In case of lower gears, the output torque is a higher multiple of input torque, than in case of higher gears and correspondingly the output rpm is a lower multiple of input rpm than in higher gears.

So, lower gears work perfectly when the car is just starting, since at those low speeds, you need low output rpm and high output torque.
Once, the car has gotten to a higher speed, you need high output rpm. So , you shift to higher gears to get the higher output rpm, with the tradeoff that you get a lower output torque.

If your car only had the highest gear, then when you start your car i.e. at lower velocities, the output torque would be too small to get your car moving conveniently. So, it would be a constant pain to drive the car at lower velocities.

You can try it out in any manual transmission car. Try to start the car with the gear set not at 1st gear, but at a higher gear, and see how difficult it is to get the car moving.

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  • $\begingroup$ > you need low output rpm and high output torque But in the simplest model (I put it in the question) we haven't even added a squared resistive force, so setting output rpm doesn't make sense yet. $\endgroup$
    – Matt
    Aug 20 at 10:32
  • $\begingroup$ Yeah, i saw that part, i must admit i do not completely understand what you meant by that part. So, i tried to answer just the question as best i could $\endgroup$ Aug 20 at 12:19
  • $\begingroup$ @Matt Your tires have a fixed radius, so for a given speed your output rpm (i.e. the rotational speed of the tires) is also fixed. This is something you need to consider in your model well before resistive forces. $\endgroup$
    – Chris
    Aug 20 at 19:55

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