torque vs horsepower?

[ivymike1031]

let's quickly review how a lever works:

A force is applied at one point on the lever, which is at a given distance from the fulcrum (x1). A force is applied at another point on the lever, which is at another distance from the fulcrum (x2). The ratio of the forces is inversely proportional to the ratio of the distances (f1/f2 = x2/x1, or f1x1 = f2x2). It doesn't matter what x1 and x2 are individually, it is only the ratio that matters. Thus if x1 = 6 and x2 = 3, the ratio is 2. If you wanted a ratio of 3, you could shrink both sides and still get there: x1 = 3 and x2 = 1 gives ratio = 3.

If you draw a gear pair as a couple of circles touching each other, with radii r1 and r2, and the tangential contact force F, then the torque on each gear is the force times the radius (t1 = r1f and t2 = r2f). Since the force acting on the gears is the same, we know that the ratio of torques is proportional to the ratio of the radii (t1/t2 = r1/r2). Thus if a large radius gear acts upon a small radius gear, torque output of the smaller gear is less than torque input to the larger gear. If the smaller gear is the input, then the output will be multiplied by the ratio. You don't need to double the size of the larger gear to double the ratio - you could halve the size of the smaller gear and accomplish the same thing. In a differential, the absolute sizes of the gears would depend on the loads that they needed to carry, but the number of teeth on each would be picked to give you the right gear ratio. Since the tooth pitch has to match, the diameters of the gears will be in the same ratio.

The torque you're measuring on the dynamometer will depend on the specifics of how you're measuring it and what you're plotting against. If you plot output torque vs engine rpm, it will remain constant. If you plot wheel torque versus wheel speed, the high-ratio graph will shrink horizontally and grow vertically compared to the low-ratio graph. On a standard acceleration dyno, if you change the gears in the diff, you'll need to change the drive ratio that you input into the software to get the right engine torque curve out. Do you have a specific output plot that you're thinking about?

[ivymike1031]

Out of curiousity, did you have an alternative explanation for how changing gearing affects vehicle acceleration? I know you said earlier that it "makes it easier for the engine to turn the wheels." Did you have any thoughts on how that might happen physically, or what that means in regard to torque?

[FYRHWK1]

the explanation i had was, comparing a 2:! ratio to a 4:1, the 2:1's rear wheel only gets 2 engine RPM, or every 1/2 turn of the rear wheel recieves 1 engine RPM, meaning if the engine puts out 200 ft lbs, the wheel recieves 200 ft lbs every 1/2 RPM, whereas in the 4:1 ratio, the wheel recieves 200 ft lbs every 1/4 RPM. and i dont quite follow your post about leverage, the only way i'm aware of, to increase leverage, is to increase distance from the center, or to move the fulcrum point closer to the center.
as to the chart, no, i've not got a dyno chart in mind, just from looking at ones i've seen, i've never seen one put out the kind of torque ratings that they should be, several were from very tall geared drag cars, as to the dyno settings, i dont quite follow how the gearing being inputted would be the reason, unless they didnt input the actual gearing the car has.

[ivymike1031]

okay, well see if you can answer this:

if you apply 200 lbf to one end of a lever, and get 500 lbf from the other end of the lever, how long is the lever?

[ivymike1031]

It sounds like you could use a little more info about how levers work.

First, reading something like this might help: http://cosmos.colorado.edu/~urquhart/play/lever.html Remember to read the "teacher's guide."
or maybe this: http://www.atlans.org/elements/machine/pg02.html
brief, but helpful: http://www.ceeo.tufts.edu/curriculum...oom/lever.html

gears in brief: http://www.ceeo.tufts.edu/curriculum...oom/gears.html

gears as they apply to this question:

http://www.btc-bci.com/~billben/lincoln.htm

Because the maximum torque developed by the engine is puny compared to the weight of the car, mechanical advantage must be obtained through leverage to move the car and this is accomplished with gears. By multiplying the torque available at the flywheel through speed reducing gears, 300 pounds feet of torque can become as much as 3000 (or more) pounds feet at the driving wheels.


http://www.miata.net/sport/Physics/03-Basic-Calcs.html

A transmission is nothing but a set of circular, rotating levers, and the gear ratio is the leverage, multiplying the torque of the engine. So, at the output of the transmission, we have of torque. The differential is a further lever-multiplier, in the case of the Corvette by a factor of 3.07, yielding 3100 foot pounds at the center of the rear wheels (this is a lot of torque!).

[FYRHWK1]

Originally posted by ivymike1031
okay, well see if you can answer this:

if you apply 200 lbf to one end of a lever, and get 500 lbf from the other end of the lever, how long is the lever?

well, i'd guess 2.5' long, but i'm not positive if the length of the lever after the fulcrum point is taken into acocunt, or how exactly to overcome that, i have a guess but thats for another discussion.

but, i've cleared up the gearing issue, had a discussion with a friend last night, was forgetting the fact torque is work over time and you need to add each revolution of the gear into the one of the wheel to get your torque numbers, i was believing the gear was sending that much torque to the wheel each revolution of its own, however many thousands of lb-ft. that may be. and thanks for the links, i'll have to read them when i get home.

[ivymike1031]

It's actually not possible to answer the question as I posed it. The only thing you know about the lever from what I told you is that one side is 2.5 times as long as the other side. (each side is measured from the fulcrum to where the force is applied)

Possible solutions:

side1 is 2.5 inches long, side2 is 1 inch long (total = 3.5 inches)
side1 is 25 feet long, side2 is 10 feet long

etc...

[454Casull]

torque is work over time

No.

[texan]

Horsepower is basically power = energy x time, torque is energy = force x distance.

[FYRHWK1]

doh, not time, distance traveled, thanks texan.

[Jimbo_Jones]

this is pretty much why 4 cylinder cars need to be at about 3000 rpm to take off comfortably, and in a 350 big block you can take off at about 1200... just about idle