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Friday, September 25, 2009

Coastdown Testing, Etc.

I had been planning on doing various types of tests for later comparison as I change things on the bike, but somehow I never ended up getting around to it. One of those tests was a coastdown test, to determine comparison data for later rolling resistance and aerodynamics tests.

Today I tried to do them, but it turned out to be almost a waste of time, because due to technical difficulties on the way home after the last test, I lost the paper I'd written the results down on when I stopped to fix the problems at some point. However, I'll post what results I do have, for later perusal, for what use they may be.

For the current tests, I used this donated analog ammeter, apparently from an old charger:

It's wired on the battery side this time, rather than the motor side, since the current draws should be lower based on the PWM multiplication effect, and maybe would all stay within the range of the meter (12A).

I tried the red HF DMM that I'd fixed, but there's something seriously wrong inside, as it got hot and began smoking at the shunt (visible easily thru the hole I'd cut to both keep the shunt ventilated and to resolder it thru if it failed again later) even with only 3A current thru it, so I decided to not use it for these tests. If I spent the time to fix it now, I wouldn't get the tests done.

Unfortunately, ultimately all the startup currents were still out of range of the ammeter, though operational cruising currents were roughly the same as they had been with previous tests of motor side currents using the HF DMM, several posts back.

Now, for the coastdown tests, I accelerated to 20MPH on a relatively flat stretch of smooth paved road, then as I passed a marker on the side of the road I cut all power, started the stopwatch function on a wristwatch, and coasted down to 10MPH (the point at which I begin having a little trouble keeping it stable--at 9MPH it's difficult and below that it's really problematic) and stopped the timer. I repeated this in the opposite direction, and did each run twice, to help eliminate wind, even though it was essentially still air at the time, noting each time down on paper, because I have a mind like a steel trap (everything comes out crushed and mangled).

The first test was the 26" already on there, so when I rode home after that test, I changed the whole wheel out for the previous 24", using the MTB/road hybrid tire. It also has the identical rear cassette as the 26", for some current tests under different tire diameters. After that I rode home and changed out just the tire to the gray powerchair tire identical to the front one. Then it was too dark to continue, plus on the last trip home after that test, I had my technical difficulties mentioned above.

I cannot remember any of the actual times, but there was basically no difference between these three rear tires:
(Gray 24" powerchair knobby on front in all cases)
26" rear road
24" MTB/road hybrid
24" gray p/c knobby
Maybe I'll redo the tests with the last tire on it before I do the tests with the new slicks.

There was also no significant difference between the battery side current draw for any of the 24" rears. The 26" rear, however, at the same gears as the 24" (using the identical rear cluster), was I think about 2 amps more than the 24" for accelerations; cruising at speed generally seemed around the same but fluctuated a lot more on 26" than 24".

With the 26", I also did a braking test from 20MPH, squeezing both front and rear brakes really hard to skid it and see how far I would skid, but the number was lost with the paper.

I couldn't do the test with the 24s since I did not have a rear brake with them. That I still have to add a crossbar to mount them on, and would test them then.

I did do a test with the 24" front gray knobbly only, while I had the 26" rear on there, but again, it was lost with the paper. :( I had planned to compare that to the 24" slick once I changed them out.

The problems I had later will be in the next post, to keep this one short and to the point.

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