RPM vs speed vs torque


Ris91
07-30-2009, 02:31 PM
Hi friends
I'm new to the community........:)

1. I wanna ask how can a wheel at higher speed can be at lower rpm........i mean when we accelerate high on a lower gear the rpm is very high but after shifting up at same speed rpm is less.........how is it possible...........:banghead:
rpm means how many rotations we take per minute ..........so if we are at a higher speed that means wheel is rotating fast so we are definitely at higher rpm (which is seen within same gear) but then why after shifting however we are at high or same speed, we are at lower rpm
And secondly.....
2. How peak torque can be achieved at lower rpm.............
when rpm is higher i.e we get more no of rotations which is only possible if engine gives the wheel more coupling force i.e torque so greater the rpm greater should be torque............but this is not so i guess.....................
thanx in advance for a good answer............
and plz tell me in general language not ny equations and all...

MagicRat
07-30-2009, 05:26 PM
The transmission contains several different gear ratios.
These gears provide mechanical advantage.

In low gears, like first gear, the engine completes relatively MORE revolutions for each revolution of a car tire. This means the car feels like it has lots of power and acceleration, but cannot go at high speed.

As one shifts into higher gears, the gear ratio changes. The engine produces relatively FEWER revolutions for each revolution of the tire. Therefore, the car feels like it has less power, but the car can travel at higher speed.

The easiest way to see this at work is with a bicycle that has multiple gears.

http://en.wikipedia.org/wiki/Transmission_(mechanics)
http://en.wikipedia.org/wiki/Gear_ratio

shorod
07-30-2009, 10:14 PM
Ris91, the tachometer displays the engine speed (aka RPM), not the wheel speed. The horsepower and torque ratings for the vehicle are for the engine at a particular engine speed, not ratings at the wheels. For those numbers, you'd need to have your car tested on a dynamometer (aka dyno). The vehicle speedometer measures the wheel speed in miles per hour or kilometers per hour. This is not a direct correlation to the tachometer. The vehicle speed is usually measured at the output of the transmission or via ABS wheel speed sensors. Because it is not measured by the outside circumference of the wheel, the speedometer reading can vary if non-stock tire sizes are used as well as by the air pressure in the tires.

-Rod

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