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I have basic understanding of gearboxes and why we use them, but what is baffling me is that after about 4000 RPM (at least in my car) it doesn't accelerate further (or it does slowly)... This seems to apply to any kind of gearbox. For example in bicycles, when we select a lower gear after a while we feel with no resistance under our feet and the bicycle will not accelerate more. If we add more revs, why don't we get more speed?

  • What is the make/model/year of your vehicle? Is this a manual transmission? How many gears does it have? Thanks for asking your question. Cheers! – DucatiKiller Mar 20 '16 at 8:02
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    Any correctly functioning manual gearbox will have a direct relationship between RPMs, gear and resultant speed. Automatics have some slip built into them, but the majority of that slip is at lower RPMs or during deceleration. Bicycles are limited by the RPM at which one can pedal, but again have the same direct relationship as a manual gearbox vehicle. Are you referring to the power band? Please clarify what you're actually asking about. – Lathejockey81 Mar 20 '16 at 18:27
  • I'm understanding the question well: you get a whatever gear box, manual transmission etc, (you are not talking strictly about cars but you just mean the same kind of mechanism) you rev up the power source and you are asking why it feels like there is no acceleration when above a certain number of RPM? I have the impression the answer about the bicycle is that you cannot move your legs faster than a certain speed and once the bicycle moves at a speed where the tire rotation matches the maximum speed you can move your legs ( through the gearbox) simply cant do more than that – Erik vanDoren Mar 21 '16 at 2:20
  • Needs clarification. We are departing the meaning. The summary itself is confusing to me. I have to assume the meaning. – DucatiKiller Mar 21 '16 at 7:45
  • I think this might be better suited to engineering.stackexchange.com as it relates to the general system (despite the title) rather than any sort of repair – Erik vanDoren Mar 21 '16 at 13:36
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The amount of acceleration you feel depends on the amount of power the engine can deliver, and the speed you're driving at.

Power

Internal combustion engines are more efficient at some speeds than others. To illustrate, here's a random power and torque curve:

Torque curve

Torque (which is the property you feel when you accelerate) peaks around 3000 rpm, and then starts to drop: the engine gets less efficient at higher rpm.

Speed

Drag increases with the cube of speed, so it rises quickly when you accelerate. The faster you go, the more difficult it becomes to accelerate (the more torque you need). At some point, all of the engine's torque is spent counteracting drag (and other losses), and none is available anymore to accelerate.

Bikes

On a bike, when you select a low gear and start pedaling as fast as you can, you run into the rev limit of your legs: the point where you can't pedal any faster because your nerves and muscles can't expand/contract any faster.

In a car something similar happens when you accelerate in low gear. Usually, the engine has a rev limiter to prevent damage. 4000 rpm is a very low limit for a petrol engine, but diesels can have limits in that region.

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First the bicycle example: resistance is acceleration essentially. If you feel it, you accelerate. You can spin your pedal only so fast, so after some time, you can no longer increase the speed of your pedals, and thus the speed of the bike, that's the time when your efficiency dropped, and you need to shift.

Now the cars: Similar concept applies to car engines. Each engine type has it's own efficiency range. If you noticed, car manufacturers would specify peak torque and HP readings at a certain RPM. For some engines it's at 4k RPM, for some at 6.5k RPM. It depends on the engine design, displacement, type, etc.

Also, efficiency range can be adjust with different profile camshafts for different engines types, it's usually diled specifically for the vehicle needs - if it's a city vehicle, it would be making power rather early and feel snappy of the lights, but will run out of steam in the higher RPM. On the other side, sport vehicles would have peak power available rather late in the RPM.

Once you are out of the efficiency range, the forces acting on the engine components like friction and inertia increase, efficiency decreases and you feel loss of power.

It is very expensive to make an engine that is efficient in most of the RPM range, but many companies actually have variable timing like V-TEK for Honda, VVTI for Toyota, MIVEC for mitsubishi. This feature adjusts the cam-shaft profiles based on pre-programmed tune and helps to increase the range in which engine makes the power most efficiently. Alternatively, Constant Variable Transmission are also there to always keep you in the most efficient range.

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