Is acceleration a function of horsepower and weight?
In this case, here is what I don't understand:
- hp can be expressed in lb.ft/s
- weight is in lb
- if acceleration is hp/weight, it should be measured in lb.ft/s/lb = ft/s, and that's a unit of speed, NOT acceleration.

What am I missing?

 

What is the /s in lb.ft/s? lb/ft is torque.

 

and I thought horsepower is weight * distance by time (lb*ft/s). Do I have everything mixed up?

 

but i have seen a calculator online that will compute horsepower based on weight and speed at the end of the quarter mile (trap speed i believe it's called). based on your term cancellations, this calculation makes sense to me.

 

consider in any discussion of acceleration? Or, do you mean some sort labratory measurement of acceleration?

 

and the units don't work out as far as I can tell.

acceleration = ft/s^2
hp = (ft-lb)/s
Torque = lb-ft
weight = lb

hp/weight = ft/s which, like you said is not acceleration.

While the two may be related in some manner, I'm not sure acceleration can be computed directly from only those two variables.

garreTTk

 

How can acceleration be directly calculated as HP/weight w/o factoring in gearing?

 

"right" way it should be written! lol

This question is best answered by SmallTT's as he is an automotive engineer.

 

that's why I asked what I'm missing

 

It is best to view the situation as though you have frozen the vehicle in time and are looking on the forces acting upon it at a particular instant. Yes, the acceleration is related to both the horsepower and weight of the vehicle. It is easier to grasp the concept if analyzing by instantaneous torque. Newton's second law is Force = mass * acceleration. Therefore, acceleration = Force / mass. Acceleration is in units of ft/(s*s), force in units of pounds-force (lbf) and mass in units of pounds-mass (lbm). The force imparted on the vehicle can be calculated by multiplying the torque produced by the engine at the specified point in time and rpm, multiplied by the gear ratio, final drive ratio, and divide by the tire radius. Torque multiplied by the gear ratio will still be in units of torque (lbf-ft) until you divide by the tire radius (lbf-ft / ft = lbf). If the mass of the vehicle is known, acceleration at the given point can be calculated. This model neglects wind resistance and drive-train losses, which can be accounted for if all forces are added together and used as a net force in the equation above.

I hope this has answered your question and not caused any confusion. In brief: Is acceleration a function of mass and horsepower? sort of. Acceleration is a function of net force and mass. Net force is a function of engine torque. Engine torque is what is used to calculate horsepower.

--Tim--
Oh, yeah, I'm new. Picked up the car on Friday, will post my first question later today. Here is an example of hp/mass.
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