Several RCTech users are building, or have built, dynamometers ("dyno" for short), but there is no generic thread where we can all come together and share our experiences and show off our projects. So here it is!

To start the ball rolling, I'm re-posting this from another thread:

Yes, I do know of some expensive ones. They would be the ones made by the guy I know who owns an NC lathe!

Yes, larger diameter does give more MOI for the same mass. But it also gives more leverage on the motor shaft for secondary imbalances, more aerodynamic drag, and reduces the maximum operating speed. Spokes may look nice, but for drag and structural integrity, I would prefer to stick with a solid disk.

Since I'm going to be using a gear drive, I can easily select the MOI that the motor "sees" by varying the ratio, which makes it easier to get good results with a large variation in flywheel sizes. That's important when being forced to use what's available, rather than what is optimum.

Yeah, a disc is better, I just meant changing the shape so that the maximum amount of mass is at the outer diameter, to make the most of what mass you have. Not really concerned with "looking good"!

And it should be possible to compensate for the effects of gearing by looking at the deceleration curve when spinning down, right?

I'm now thinking it might not matter anyway, if we don't care about converting to real units? As in, if one setting goes from 5,000 to 5,075 RPM between two samples (at fixed time intervals), and another setting goes from 5,000 to 5,100 in the same interval, the second is better, and it doesn't matter if it could have gone to 5,150 without the gearing, right?

Maybe one of those "rolling roads" dyno from McPappy would be fine (with the right data capture and analysis), after all?

That's a big question at the moment. Determination of things like aerodynamic drag and bearing friction should be quite accurate while coasting down, since they are dependent on speed. Determination of gear losses might not be as accurate, if they are dependent on transmitted torque. The torque from the motor is very high during acceleration, and very low during deceleration, so it's apples and oranges.

I do like the idea to measure data on the track:
http://www.rctech.net/forum/nitro-ro...mes-alive.html

It is possible to compare data to see if there are improvements.

Back in the day (1987-1993) I built my own dyno's. Basically I used a Computer power supply for a 5v constant, slave motor, with 3 resistors in the circuit and controlled by 3 switches. Whole thing was about 1" x 3” x 4". Had meter lead receptacles to attach a multi-meter for voltage or amps. I bought the stuff from Radio Shack or another electronics store. Cost was about $15.
RPM was equal to output voltage from the slave motor. Swap the lead and read the amp draw which equaled torque.

I would run it in free mode then flip the switches one at a time and apply the resistive load to see how the motor responded to loads from a start and from a running motor. Worked very well to determine RPM motors and torque motors.
Plus it was great when you could change brushes, springs, timing, etc. Lot more tuning options than with today’s motors.

Thought of building one again, but with so limited tuning options, the G-Force motor analyzer reads timing and RPM plus amps, though does not check loads is sufficient for now.

The only thing with a slave motor and resistive load is that it can't go very low, and you have only very granular resistance, which will slow it down to a certain level (so you only get the torque at the RPM that it'll go down to).

Or does it?

Would a slave motor with a set resistance could be used in place of a flywheel to do a spin up, to get most of the torque curve? It wouldn't go all the way to zero (it'd stop short of the free running RPM), but would it generally work for the rest of the RPM range?

I'd be a bit worried that the torque reading wouldn't be linear through the RPM range, but if you optimize for each given RPM (use the timing that gives you the best torque at each RPM), it could make sense, right?

For blinky, there's not much to tune, but for boosted, there's a lot of information that you need to optimize the motor, and that's information a dyno can give you (more or less) easily.

The G-Force motor analyzer sounds good enough for blinky, IMHO. Getting the true timing measurement from each sensor is good for getting (more) consistent results from one motor to another, and the amp reading can let you see how much timing is too much.

It was only a $15 (poor man's) dyno It was a good indicator of motor performance (top RPM and AMP draw). To design what you are asking will cost $$$$$.

Ha! Yeah, that's not wrong!

For brushed motors, you could get a lot of mileage out of that kind of dyno setup, when tweaking brushes and such...

The RC Benchmark dyno that recently came out is just shy of US$450! It looks like the current version is aimed at other applications than RC cars (like quadcopters, robots, etc), but they seem interested in developing something that would be good for car racing...

That's awesome work-- definitely what we have come to expect from you!

It's not difficult nor terribly expensive to use an electronic current sink (controlled by a microprocessor, of course!) on the slave motor to vary the load on the motor under test. That's how the fancy dynos for brushed motors worked. But torque output needs to be measured with a separate sensor like a strain gauge to get good accuracy. Just measuring the current loading the slave motor isn't good enough.

The slave motor could simply replace the propeller as the load on the RCBenchmark dyno, since it already has a strain gauge to measure torque.

The flywheel dyno still gives more complete data in a shorter time, but if one needs to run a motor under a continuous load for an extended period of time (perhaps for stress testing), then the absorption dyno the proper device.

What I meant was to try to have the slave motor give a (high enough) constant load, and rather than run the tested motor continuously, as you normally would with a slave motor setup, instead do a full-throttle acceleration, like you would with a flywheel? You'd then analyze the data the same way as you would with a flywheel.

Like I said, calculating "real" torque won't be easy (or maybe even possible), and it won't spin up all the way to the free-spinning RPM, but for the range covered, you should be able to get data to compare different timing settings at the same RPM, and come up with some sort of reasonable timing advance curve.

Maybe the data at the top of the RPM range could be gathered with a lower load setting on the slave motor, and yet more runs?

Back in the day I had a Fantom Dyno. Thing was great! Could do alot with it based upon the data you were getting. There was a slight learning curve when you made changes. The total opposite happened that what you would think.. I figured alot of it out over time.

I see HobbyKing has a meter for props now.

http://www.hobbyking.com/hobbyking/s...arehouse_.html

I was thinking about buying a Novak Sentry Data Logger, but they're a bit old, maybe newer hardware would be better (high sampling rates, for example)? I wonder if some Arduino would able to do that? I'm not that good with hardware, I'm more of a software guy!

I see you're working with nitro cars, but could this be modified to gather some other information, putting probes on the motor power and sensor leads, and measure how much timing advance the ESC is applying?

A bit off-topic, but that whole telemetry/data recording business also seems very interesting, albeit in a palliative way. I could do drag runs, with various timing settings, and at least get my boost settings in the ballpark...