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HP required for speed?

HP required for speed?

HP required for speed?

I realize theres a few varibles. Cd being one, and rolling resistance of the road surface, even gear set friction in drive train etc. Is there a place that has a listing of various drag coefficients for various vehicle types as well as aircraft?
After watching some bonneville speed record videos, got curious about a few things.  

RE: HP required for speed?

The amount of HP needed to reach specific speeds depends, on weight of the vehicle, aerodyanamics of the vehicle, density of the surrounding air, and a multitude of other variables. Once the veriables have been pinpointed for each scenario you can plug them into an equation and figure the needed amount of power to achieve the the desired speed.

RE: HP required for speed?

So it is calculable? That's a start, I guess.

RE: HP required for speed?

In my experience, the rule of thumb type online calculators seem to work relatively well despite lacking some seemingly important data Most require some rough input to guess at frontal area and CD.

Weight is more to do with acceleration, although it does impact on rolling resistance, but the faster you go the less important that becomes as part of the overall equation as aero goes up exponentially and rolling resistance probably goes down due to lift unless you have big wings.

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RE: HP required for speed?

Since weight (mass) controls your acceleration rate, it indirectly controls the maximum speed attainable on a fixed length track or road.

- Steve

RE: HP required for speed?

If you're looking for "actual" top speed in a known vehicle with actual fixed parameters rather than the theoretical top speed it might attain if everything were optimized to that end, it might be easier to work from the torque curve than just peak HP.

If you're writing your own acceleration simulation and defining top speed as that where a = 0, be prepared to continually rewrite the whole thing as you decide that something else is important enough to consider.

Steve's right, and a halfway decent acceleration sim will show how long it takes to add the last couple of mph.


RE: HP required for speed?

"If you're writing your own acceleration simulation and defining top speed as that where a = 0, be prepared to continually rewrite the whole thing as you decide that something else is important enough to consider."

I have done this recently, started out as a perfectly simple gear ratio calculator and somehow ended up as a comparison tool to compare different setups taking into account weight, tyre profiles gear ratios and a full two step power curve 'I am a Honda fan and wanted something that could represent a VTEC power curve' to give distance and time measurements for half, quarter and full mile drags, 0-60, 0-100, 40-80 and top speed runs.

Unfortunately it is of limited accuracy as it ended up with Excel telling me I could not add any more variables as there was no more space in the cells!

Monster task though, the spreadsheet weight in 6MB!

RE: HP required for speed?

Let each cell take a smaller bite and use more rows/columns of them.

It'll always be an approximation. At 6MB, it sounds like some things are being polished a bit more than necessary.


RE: HP required for speed?

patprimmer, your comments about the effect of weight vs lift on rolling resistance are interesting and make a lot of sense. Even so, you always see breakdowns of the drag forces into a parabolic aero component and a linear "rolling resistance". Makes me wonder.
I also wonder about driveline drag which is significant, about 10-15% of horsepower, and being proportional to HP, if represented as a drag factor, should be proportional to the CUBE of velocity. I think it is accounted for as merely an excess power needed to reach a particular speed.

Anyway, related to aero lift is a tire slip factor that often foils speed projections at Bonneville.

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