Originally Posted by
BobW
Perfect explanation hprt. People don't realize that most of the full scale car chassis dynos are Inertia (Flywheel) dynos. The drum the tires spin up is the inertia wheel.
Regarding your quote above I'm having 50mm and 60mm dia flywheels machined for the MD2. The inertia will be 2.5x and 5.5x greater than the 40mm flywheel that is provided. I have found that the spin up times are so fast you get very few data points at lower RPM with the 50hz sample rate. The larger flywheels will help this issue somewhat. I would be interested in your thoughts.
Hi Bob,
Using a flywheel with a larger moment of inertia will increase the number of readings per rotation as you already know. According to Newton, doubling the inertia will reduce by the same factor the angular acceleration, assuming the torque applied by the motor under test is the same for both cases. I quickly calculated the number of samples recorded during the same angular displacement.
For a flywheel with 2.5x the inertia of the OE unit, you would get about 148% more samples across the run. For the 5.5x inertia flywheel, you would end up with 234% more samples. The sampling frequency hasn't changed but the flywheel is taking longer to spool up and, as a result, we have more samples collected over the time period. If the total run time is 2 seconds with the OE flywheel, expect the new spool time to be about 1.48x (for the 2.5x flywheel) and 2.34x (for the 5.5x flywheel) longer, or 2.96 and 4.68 seconds respectively.
This increased number of samples per run will give more accurate results when plotting the motor performance, and based on the above, I would focus efforts on the 5.5x inertia flywheel. The 2.5x will not give significantly better resolution during the spool time.
This will put more load on the drive circuit. I don't think you will risk popping a 35 amp ATO fuse with a 21.5, but you might with a 17.5 or 13.5. I would run two batteries using the 5.5x inertia flywheel. Back to back dyno runs might actually see some effect from motor heat...
Let me know how you make out on this. I might have to get one made out here.