Find HP from Force and Displacement?

Mike,

It is the force you need to know (but more specifically the torque at the wheels), the mass of the vehicle, and the wind resistance.

The mass of the vehicle and the torque will let you determine what your acceleration is. That is, how quickly you can get up-to-speed.

The wind resistance (as a function of speed) will be the determining factor in how much torque is needed to keep the vehicle moving at a certain speed (assuming other sources of friction are small).

This is not an easy thing to calculate. The best thing for you to do is to see what others are using for motors and batteries and start there. You can't be the first one to do this, I wouldn't think.

FYI -- the power (or horsepower) can be used, too, but it's a more indirect method. The torque is proportional to the power divided by the rotational speed (rpm). To me, it's easier to look at a torque curve to compare vehicle-to-vehicle performance, but the power curve will work, too. You just need to do one more calculation.

Bob

This depends on many factors such as angle your driving on, wind resistance, friction, etc...

Horsepower is all about acceleration. Think of cars. All cars can go 60mphs but not all cars can accelerate 0-60 in 3s.

But factors such as wind resistance depend non-linear on the velocity... this means that at low velocity they are weak but at high velocity they are strong. This effect prevents you from reaching any arbitrary velocity.

Even with a .000000001HP motor you could reach any velocity(

Assuming the force is constant over that displacement...

Power is also related to force by P = f*v. (differentiate the above equation you have w.r.t time assuming f is constant)

If you know the mechanical power(total) then P/v = f

now f is the sum of resistive forces + gravity(if on incline), etc...

I guess the 50lb's then is due to "friction" like forces?

what force?

Yes.

Yes... or P/v = F or F = P*v

Yes, for a good first approximation... except your starting values might be completely wrong. (This is why wind-tunnels are so important because its very difficult to determine the drag forces involved.)

It's also best to overrate it because you can always back off the throttle but you can't push the motor past its maximum.

I'd suggest you look at other go-carts and see what they use and there top speed.

Remember those that power is really about acceleration and that a 10hp motor won't give double the acceleration as a 5hp motor. But of course the top speed will increase somewhat because you can overcome wind resistance(but its not proportional because usually the force depends on the square of the velocity once you get up past a certain point).

The main point I'm trying to get across is that there are many factors involved. You can look at many cars that have widely different HP but very similar max speed and/or accelerations and vice versa. (depends on weight, aerodynamics, power transmission efficiency, torque response, etc...)

Dear amdx:

This would be proportional to drag force (wind resistance + rolling resistance when fully loaded) * velocity. The drive system would have to supply this, and you'd have to factor in some losses between the wheel and the motor itself.

No. See above.

I'd mock it up. Drag it behind a car, with a force gauge, at the speed desired.

David A. Smith

The force I was asking about is what is "how much force does it take to push

Electric motors have there maximum torque at 0 rpm this helps the acceleration. See this;

Others use 1 hp to 34 hp, 1-12v battery to 10-12v batteries. Just looking for the minimum motor to create some excitement. 30 mph four inches from the ground would be exciting until you do 60mph.

How do you want to push it? Start really fast then drop down smoothly to 0 towards the end or start smoothly really slow then ramp up really quick or what?

"amdx"

Going up-hill will require a lot more than travelling on the level.

The actual force is not calculable since it depends on the wind drag of your vehicle plus driver.

..... Phil

This is because when gravity is involved on inclines the power needed is greatly increased. IIRC a 1500lb car going 30mph up an incline of 15 degrees requires almost 2 times the HP to continue at the same speed. (I guess basically the thing wants to slide back down so you have to overcome that too... which contributes mg*sin theta to the force

I might not have my numbers right but the point is that going up inclines takes a lot more HP to continue at the same speed. This is why 30HP-50HP is the average HP used by a car going 60mphs(basically the required force needed to overcome drag + friction) but if thats all you got you won't be able to make it up a hill without reducing speed.

Dear amdx:

The mass of the vehicle is 450. Rolling friction is n1 * 450, with n1 < 1. The wind resistance is n2 * speed.

The force required is: ( n1 * 450 ) + ( n2 * ft / sec )

The shaft horsepower required to move that vehicle that fast is: ( ( n1 * 450 ) + ( n2 * ft / sec ) ) * ft / sec / 550

David A. Smith

| | | This is because when gravity is involved on inclines the power needed is | greatly increased. IIRC a 1500lb car going 30mph up an incline of 15 degrees | requires almost 2 times the HP to continue at the same speed. (I guess | basically the thing wants to slide back down so you have to overcome that | too... which contributes mg*sin theta to the force | | I might not have my numbers right but the point is that going up inclines | takes a lot more HP to continue at the same speed. This is why 30HP-50HP is | the average HP used by a car going 60mphs(basically the required force | needed to overcome drag + friction) but if thats all you got you won't be | able to make it up a hill without reducing speed. |

An incline of 15 degrees is a one in 3.7 slope (1/tan(15) ), VERY steep. Most people would demand steps.

The steam engine was originally developed for vertical mines, pumping out water and lifting tin ore in Cornwall or coal in South Wales to the surface. The horsepower was defined (550 ft lb per second) to give steam engine customers an intuitive sense of the power of steam engines in terms of what they were familiar with, i.e. the horse. A man can develop 1 hp by running up stairs, lifting his own weight against gravity, but he can't keep that up for very long (at least I can't). A horse can develop about 10 hp, but will soon tire. On level ground no horsepower is needed because nothing is lifted, but clearly a car has to overcome the enormous air pressure it experiences on its windscreen and the rest of its cross section when travelling at

Now you want to complicate that by combining lift with resistance against air at 30 mph, presumably at standard air pressure and temperature, so the cross-sectional area and streamlining of the car plus its weight (1500 lb) becomes an important factor. What this comes down to is a rule of thumb "IIRC a 1500lb car going 30mph up an incline of 15 degrees requires almost 2 times the HP" which you have not calculated but guessed, and that is just what I advocated the OP did.:-)

Unfortunately the question is so under constrained (and incorrectly stated if you pay attention to dimensions) that it is meaningless.

If you have a 450 pound-mass (i.e. about 14 slugs) of material zooming through the vacuum at 44 ft/sec, then the amount of force you need to push it to keep it going that speed is exactly zero.

If you have a 450 pound-mass carbon-fiber/kevlar sea anchor that's designed to give the maximum possible drag at 44ft/sec without tearing up, and you want to drag through the water at 44 ft/sec, then you may need a bit more force.

If you want to push 450 pound-mass straight upward in the earth's gravity field then you need at least 450 lbs (of force, since that's what a lb is).

Vehicle design has a _lot_ of variables, so it's hard to reduce it down to a few simple rules. You either spend a lot of time measuring and tinkering, or you just start by copying what works for others.

(balance snipped)

Well, just from the back of the envelope and personal experience, being

You should be able to get a 1-2HP motor at some motor repair shop of some kind. Maybe the lawnmower shop, and ask around - they might know somebody.

Of course, this would only serving the purpose of getting some HP/ speed curves - when you find your optimum, then that's the size of the electric motor you want to buy.

Then sell the engine back to the guy, or make a generator to charge the kart batteries with. ;-)

Good Luck! Rich