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Chassis dyno

  1. Aug 22, 2007 #1
    Can anyone explain me what kind of chassis dynos are used to measure shaft torque/power and how they do it.

    Things I allready known;

    I know that inertia brake dynos use a solid disc flywheel of a constant mass and radius and by meassuring time and flwheel speed (rpm), we can calculate flywheel kinetic energy and power and with car's wheel size, and drivetrain ratio we can calculate torque.

    I also known that we can calculate drivetrain and tyre losses by coasting down method(disengaged clutch), but all power/torque readings measured like that won't be accurate as we can't know car's rotational parts inertia (wheels and gears).

    So does chassis dynos use some extra el motor to measure frictional loses and thus calculate car's rotational inertia?
  2. jcsd
  3. Aug 23, 2007 #2


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    Yes. They'll typically either allow the operator to enter a (usually exaggerated) estimation of the car's frictional losses, or will 'motor' the drivetrain over to approximately measure the losses without fuelling the engine.
  4. Aug 24, 2007 #3


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    Some chassis dynos are similar to engine dynos in that they use a variable load instead of inertia to measure power. Some of these may have the drum(s) connected to a generator driving a variable load to measure power. If the generator can also act as a motor, then drivetrain drag could be measured.
  5. Aug 24, 2007 #4
    well as I know most chassis dynos are brake inertia - thus BHP.
    Measuring and calculating drivetrain drag is no problem, as drag is in LINEAR coleration with the speed, so you have two choises.

    el. motor - you apply some power (current) to the el motor (if it is AC motor you must know its "cos fi" - power data) and when it reach steady speed you read the flywheel rpm. Then you can calculate drivetrain drag for any given rpm.

    second method is so called "coast down" method with disengaged clutch. here you allow the drivetrain drag to eventualy stop the flywheel inertia, and because you know the flywheel inertia for any given rpm you can also calculate drag losses for any given rpm.

    That is all OK. But I was asking about car's rotational inertia caused by wheels and gears. You can't know its mass, shape constant and radius, so it is inposible to calulate it, but they do add some error to the final HP/torque readings!!!!

  6. Aug 24, 2007 #5


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    Engine dynos apply a load to "brake" the engine and hold it at a fixed rpm, thus BHP. Most chassis dynos are inertia only (no braking during a run). The rate of acceleration of a heavy drum is measured, and given angular acceleration versus the angular speed and angular inertia of the drum, power is calculated. By connecting a sensor to one of the spark plugs, engine rpms can be known, and net torque can be calculated. Including drum diameter, transmission and rear end gearing information, shaft torque and effective tire diameter can be calculated.

    Some chassis dynos wedge the driven tires between two long and light cylinders with a relatively small diameter (about 1 foot). Similar to engine dynos, the cylinders are applying a load so power can be measured at constant speeds. Since the tires are wedged in between the two cylinders, this affects tire losses, but I don't know by how much.

    Most rare is a single drum chassis dyno that also applies a load so speeds are constant during power measurement.

    Non-inertial, constant speed chassis dynos are advantagous for engine tuning.
    Last edited: Aug 24, 2007
  7. Aug 25, 2007 #6
    Yes I agree that inertial dyno is not most suitable for tuning purposes, but still they are most common, at least tunners in my surroundings use them (they aren't real tunners more "chip" tunners). And my appologise as I mixed inertial and brake dyno as same thing. It is because I found the division on some web site, that brake dynos can be - inertial brake, frictional brake and load brake (AC motor)

    back to the original question, as what is the method to measure power with inertial chassis dyno.

    none has give me the proper answer to that. There must be a way to do that, and as I know that those power readings can be quite accurate - how they do it.
    whit how they do it I mean this again poping thing - CAR'S ROTATIONAL INERTIAL FORCE.

    so is really so hard to answer the question.
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