Authored by: hockeystix
Submitted by: Johnf514
gearing is a very simple concept. you take 2 connected gears. spin one, other will spin. depending on how big the gears are, the other gear can spin faster, slower or same speed as the gear you turned. this creates different gear ratios. output gear divided by the input gear is the gear ratio
heres 3 scenarios that i drew up real fast. input is on the left, output is on the right. circles represent the actual gears, size of circles is somewhat proportional to what they would look like in real life. numbers are the number of teeth on that gear. more teeth=bigger (physically bigger) gear.
heres a possibility of a 1st gear.
input gear is connected to the engine, and has 40 teeth on it. output gear has 180 teeth in is. do the math. 180/40=4.5 that is the gear ratio for that gearset. it will take 4.5 turns of the input gear to make the output gear turn a complete circle.
now lets apply that to a real situation. your engine is spinning 3000 rpms and producing 100ft/lb of torque at 3000 rpms.
gearing works on the principle of conservation of energy. you need more torque in 1st gear, so you sacrifice rpms. thus, 3000/4.5 = 667. your transmission output shaft will be spinning at 667 rpms. now, 100ft/lb X 4.5 = 450 ft/lb torque.
now we know that if you are in 1st gear and your motor is turning 3000 rpms and making 100 ft/lb torque at the crank, your transmission output will turn 666 rpms and have 450ft/lb torque. you traded rpms for torque. this is why your 1st gear has great acceleration but runs out of rpms very fast. 1st gear is a very short gear.
now for 2nd gear. not as drastic jump.
input gear has 60 teeth, output has 100. now, it takes 1.66 turns of input gear to turn the output a comlete circle. lets plug some real numbers in.
3000 rpms /1.66 = 1807 rpms at the output shaft
100 ft/lb X 1.66 = 166ft/lb at the output shaft
now you dont have as much torque at the output shaft, but you have more rpms. this is why your 2nd gear lasts much longer than your 1st gear. 2nd gear is a taller gear.
now lets jump up to 5th gear thats most likely is a overdrive gear
now your input gear has 60 teeth and output has 50. it takes .83 turns of the input to turn the output a complete circle. now for the numbers
3000 rpms / .83 = 3614 rpms at the output shaft
100ft/lb X .83 = 83 ft/lb at the output shaft
now you have almost no torque. but with having no torque, your output shaft also spins faster than your engine. now your wheels can spin faster while the engine turns slower. in the environment where you dont have fast acceleration and dont need a lot of torque (highway) overdrive gears will keep engine speeds lower and wheel rpms faster.
then, after all that, is the final drive gear, but for the simplicity, i wont get into it. its just another gear that once again cuts down rpms and increases torque some more. whatever your output shaft numbers are, feed them thru the final drive and you will have your wheel numbers. this would be true for a perfect mechanism, but since thats impossible, you will always have drivetrail losses.
but thats the idea behind gearing. and understanding how this all works in a manual transmission is a huge part of becoming a high end stanardshifter.
hope this helps and isnt too confusing.
and for changing gear ratios, it involves taking the transmission apart and replacing the gearsets inside the transmission.
or your could put in a different final drive and alter all of your gears at the same time. but for stuff like making your 2nd gear a little shorter or making 5th a little taller, thats replacing the actual gears.