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#1: Dyno charts, measured at the wheels? and in what gear?

Posted on 2006-07-18 18:16:11 by Mark

Question about the dyno charts seen in magazines showing torque and
horsepower vs RPM.

I assume these are measured at the drive wheel, correct?

Is the RPM value the RPM of the wheel or of the engine?

In what gear is the transmission for the measurement?

thanks

Mark

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#2: Re: Dyno charts, measured at the wheels? and in what gear?

Posted on 2006-07-18 20:36:56 by Binder Dundat

Mark wrote:
> Question about the dyno charts seen in magazines showing torque and
> horsepower vs RPM.
>
> I assume these are measured at the drive wheel, correct?

Yes, if a magazine does the test, it's going to be a rear wheel
horsepower reading.

A Japan Inc factory would have special output shafts to measure the
horsepower output of engines under development, but production
motorcycles are dyno tested to massage the egos of yuppy riders who
have more money than they know what to do with.

There are two types of dynos, the cheaper one uses a rotating drum
filled with water and gives the operator an erroneously high "flash
reading" since it's using the inertia of the rotating drum to get a
reading.

The other type is actually an electric generator and it converts the
rotational energy into electric power which is absorbed into eddy
currents in the generator itself. The operator can load the generator
by changing the amount of current to the generator's field windings and
get a steady reading for as long as it takes the dyno to get too hot to
continue.
>
> Is the RPM value the RPM of the wheel or of the engine?

The RPM on the chart is engine RPM, but the rear wheel may be turning
about 2400 RPM on the dyno if the motorcycle is capable of going 180
mph and has a 25-inch diameter rear tire.

> In what gear is the transmission for the measurement?

If the transmission has six gears, the operator will shift the
transmission into sixth gear. Don't forget that a 1000 cc engine might
be putting out around 80 foot pounds of torque and the sprocket ratio
is typically around 3 to 1, so the dyno has to be capable of absorbing
more than 240 foot pounds of torque to be able to test a 1000 cc
engine.

If the transmission was in 3rd, 4th, or 5th gears, the dyno would need
an even higher torque capacity. Think about the torque multiplication
of the higher ratio gears.

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