Jim Hill schrieb:
..........
> The formula is, I suspect, merely a specific application of the fact that
> wind resistance varies as the cube of speed. There are, of course, other
> factors besides wind resistance that affect top speed - like drag
> coefficient, frontal area, friction from the tires, gearbox, universal
> joints, rear end, hubs and the like.
>
> But if one assumes that all other factors are constant (not unreasonable
> within a narrow speed range) then the horsepower required to produce a given
> top speed varies as the cube of the speed. The actual horsepower required
> can then be calculated as:
> Horsepower = [speed]^3 divided by [some constant]^3
> or, as in the formula given:
> HP = (top speed in kph)^3 / (37)^3
>
> Since the horsepower at the rear wheels is generally assumed to be some
> percentage of the brake or clutch horsepower, that percentage can simply be
> a part of the [constant] for purposes of this calculation. In a similar
> fashion Gerhard's formula can be converted from KPH to MPH simply by
> adjusting the constant (from 37 to 22.94).
>
> If you accept the basic concept, you can calculate the constant from
> published figures for engine HP and top speed of a similarly equipped (and
> geared) automobile.
>
> Note, however, that the horsepower figure you get will be the horsepower
> rating at the RPM reached at top speed. If you're going as fast as you can
> and you're not at the red line, there's probably more power left.
>
> Of course with a Triumph, you have to factor in the weight reduction as
> parts begin to fall off the car at high speeds . . .
>
> Jim Hill
> Amici Triumphi
> Madison WI
>
> PS: Wait a minute . . . YOU're the "P.E., Ph.D.", why aren't you explaining
> this to ME?
>
Jim found it out and wins a T-shirt. There is not much left to explain. The
required HPs will go up with Exponent 3 in relation to the speed and all the
other mentioned and unmentioned factors can be regarded as constant. I just had
to find a car test with reliable figures. I took AUTO MOTOR UND SPORT of 1969
because they had the DIN HPs on the clutch of a TR 6 PI and the top speed of
the
car. I know they use a good equipment and that they consider wind. All I had
to
do now was to find the total drag factor for the known speed/HP function untill
it hit the figures as measured by the motor magazin. And this can be done
similar with any car with top speed higher than 50 mph. Only the factor will
be
very special for each car ( 37 in the case of the TR 6)
And it was not even necessary to know the cw of the TR 6 which I really would
like to know. I contacted Karmann to find out if they had the TR 6 in the wind
tunnel. They were former a good source for questions about the TR 6 body. But I
noticed that since Wilhelm Karmann died some months ago, this source doesnīt
work any more for me. At last I found out that cw is in the range of limousines
and not in the range of sports cars, even compared to other 1969 cars.
However the formula works and I donīt think that there is an easyer, exacter
and more practicable way to get as close to the HPs of a TR 6 engine. Even
detailed calculations would not give a better result because you would have to
do a lot of assumptions of unknown figures(cw, drivetrain drag etc.) and use a
lot of mean values.
Regards
Gerhard Wiederholl
CC 31998 LO
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