Max speed.
11-14-2008, 07:54 PM
#41
5th Gear Member
Join Date: Mar 2007
Location: Orlando, FL
Posts: 2,031
By education, I'm a civil engineer (structural).
By the last 30 years occupation, I'm somewhere between civil/structural and mechanical, working as a pipe stress analyst. Technical writing (aka preparing calculations and other reports) is a part of that job, as is reviewing calculations and reports prepared by other engineers.
Norm
By the last 30 years occupation, I'm somewhere between civil/structural and mechanical, working as a pipe stress analyst. Technical writing (aka preparing calculations and other reports) is a part of that job, as is reviewing calculations and reports prepared by other engineers.
Norm
11-14-2008, 08:59 PM
#42
3rd Gear Member
Join Date: Dec 2006
Location: Bullard, Tx.
Posts: 533
11-15-2008, 10:54 AM
#43
4th Gear Member
Join Date: Mar 2006
Location:
Posts: 1,470
Let's go through an analysis of the units.
1 HP is 550 ft-lb/sec, which can be rearranged to lb-ft/sec. That's a force times a velocity. When the total drag (force, lbs) times whatever speed (velocity in ft/sec) equals 550 times the peak HP of your engine, you are at the maximum possible speed for that car and engine. The details of whatever gearing, tire size, etc., it takes to put that car at that speed with that engine and its peak HP rpm are not involved. Powertrain efficiency, however, is involved.
HP does not care about engine rpm for determining theoretical top speed. Gear ratios are dimensionless and drop out. At top speed, rotational acceleration is also zero, so all of the rotational inertia things disappear as well. IOW, HP is directly tied to top speed, but indirectly related to acceleration (since you then need to know all the other stuff).
Torque is ft-lbs (or lb-ft, doesn't really matter). Since it is a rotational item and vehicle straightline acceleration is linear, you need to divide by a distance (ft) in order to get a force. The appropriate distance in this case is drive wheel radius, which leaves you with a force at the drive tires' contact patches. Opposing that is the total drag force. The difference represents the force available to accelerate the car mass.
Here, the gearing, efficiencies, torques lost to rotational acceleration consistent with the car's forward acceleration do matter, as they affect the force applied against the pavement.
You can work from torque to determine top speed, but this is the less direct method for doing so. Essentially you have to close in on the speed at which acceleration equals zero.
You could work from HP to determine acceleration, though you'd need to know the HP involved to rotate all of the rotational inertia things consistent with the forward acceleration. It can be done, but it's cumbersome.
Long post short, HP is more directly related to top speed, torque is more directly related to acceleration. So it's not a matter of either torque or HP being 'wrong' with respect to what you're looking to find out (speed or accel), just which one is better suited to doing the required math.
I hope this makes sense. I can't look at it from a newbie perspective any more. Been at it too long.
Norm
1 HP is 550 ft-lb/sec, which can be rearranged to lb-ft/sec. That's a force times a velocity. When the total drag (force, lbs) times whatever speed (velocity in ft/sec) equals 550 times the peak HP of your engine, you are at the maximum possible speed for that car and engine. The details of whatever gearing, tire size, etc., it takes to put that car at that speed with that engine and its peak HP rpm are not involved. Powertrain efficiency, however, is involved.
HP does not care about engine rpm for determining theoretical top speed. Gear ratios are dimensionless and drop out. At top speed, rotational acceleration is also zero, so all of the rotational inertia things disappear as well. IOW, HP is directly tied to top speed, but indirectly related to acceleration (since you then need to know all the other stuff).
Torque is ft-lbs (or lb-ft, doesn't really matter). Since it is a rotational item and vehicle straightline acceleration is linear, you need to divide by a distance (ft) in order to get a force. The appropriate distance in this case is drive wheel radius, which leaves you with a force at the drive tires' contact patches. Opposing that is the total drag force. The difference represents the force available to accelerate the car mass.
Here, the gearing, efficiencies, torques lost to rotational acceleration consistent with the car's forward acceleration do matter, as they affect the force applied against the pavement.
You can work from torque to determine top speed, but this is the less direct method for doing so. Essentially you have to close in on the speed at which acceleration equals zero.
You could work from HP to determine acceleration, though you'd need to know the HP involved to rotate all of the rotational inertia things consistent with the forward acceleration. It can be done, but it's cumbersome.
Long post short, HP is more directly related to top speed, torque is more directly related to acceleration. So it's not a matter of either torque or HP being 'wrong' with respect to what you're looking to find out (speed or accel), just which one is better suited to doing the required math.
I hope this makes sense. I can't look at it from a newbie perspective any more. Been at it too long.
Norm
11-22-2008, 06:25 PM
#44
4th Gear Member
Join Date: Mar 2008
Location: Alberta Canada
Posts: 1,432
norm your the man... Guess i should have read all the post before i asked a question that answers were already posted.
I would like to formally thank all the people with great amounts of knowledge who are happy enough to share with the others... To all of you THANKS
I would like to formally thank all the people with great amounts of knowledge who are happy enough to share with the others... To all of you THANKS
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