Portable dynamometer (wheel dyno, road dyno) vs chassis dynamometer
As there are various solutions and devices available at the market for performance measurement of motorized vehicles (cars, trucks, bikes, agricultural machines etc), people interested in buying dynamometer, or dynamometer alternative may be confused. Is that a good idea to spend more on regular chassis dynamometer? Is so called „road dyno” or „wheel dynamometer” (device connected to one of car wheels) a good equivalent for proffesional power and torque measurement tool?
To be a devil’s advocate, I will list advantages of portable dynamometers, („wheel dynos”):
- cheap (maybe not all, because some well marketed gadgets may be quite expensive)
- really portable. You need a roll of double sided adhesive tape and you can stick such „dyno” to car’s wheel and voila! - you can start driving
There are no more real advantages.
Main disadvantages of „road dyno” aka „wheel dyno” are:
- wheel dynamometer does not measure true power and torque, but it’s equivalent – because to measure power and torque by driving you MUST know precise weight of the car, front surface size, and Cx (air drag coefficient, specific to model of car).
- you must drive customers car, accelerating at high gear (min. 3rd gear). If your company is in the town, police calls that „speeding” or „exceeding the speed limit”.
- road dyno is cheap until first accident with customers’ car, especially with 100km/h+ speed in the city. Then – it is expensive.
- repeatability of results increases when weight of vehicle increases. Results for small, light vehicles are more inacurate, because influence of other parameters increases with decreasing car’s weight.
- wind falsifies results, also air drag depends on Cx of vehicle and it’s front surface. Unsolvable – where to find proper data for any vehicle? How to switch off the wind?
- inexact flatness of surface falsifies results. Theoretically be corrected by driving second time back same road, but then wind drag will increase incorectness. So unsolvable.
- wheel friction depends on surface and is not constant.
It seems that to measure precisely, you need to know exact vehicle weight with driver, not to use too much fuel between moment of weighting and moment of measuring (as weight changes), use extremely flat roads, no wind, and wheels that do not change resistance so much. Also if your vehicle is heavy – results will be more repeatable.
Conclusion: wheel dyno is perfect. For trains.
By the way – immagine that a patient is comming to cardiologist, reporting problems with the cardiac output. And doctor says: „I have only stethoscope instead of real ECG (electrocardiograph). Because stethoscope is portable. And much cheaper.”
For sure patient will seek for another, professional cardiologist with pro tools and devices.