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Allan Wakefield
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Discussion Starter · #1 ·
Nuro and I were awaiting the arrival of Edo and the UK guys last Friday so we decided to test the New Fly Viper against the first one ever, on the Kelvin light test bench.
Then we added the results from the same test on the metal chassis viper I just built.

Please note the test gives no indication or allowance for handling but shows the top speed that IS available for each car.

Interesting to see...

Voltge 12.5

A - First Fly Viper (mag) - 5,200 RPM; 300Ma draw; 5.7 Metres per second
B - New Fly Viper (mag) - 8,040 RPM; 492Ma draw; 9.2 Metres per second
C - New Fly Viper (no mag) - 9,400 RPM; 304Ma draw; 10.3 Metres per second
D - Metal chassis Fly Viper (no mag)- 12,000 RPM; 920Ma draw; 12.7 Metres per second
(This had a Little Ripper motor, Fly starter set with 2.6 gear ratio. 10 lights also added to the current draw which were in parallel with the motor so would not affect performance if the power supply was good enough)

Also worth noting is the fact that with all three cars the current draw on initial acceleration exceeded .7Amp and with B and D this was over 1Amp, this shows homeset transformers are not up to the job and DO affect car performance.



 

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Beppe Giannini
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Quite interesting - the point, of course, is that the KL test bench provides some friction (gears, tires, is the roller idle?), but still less than what the car would face on a long straight

To put this into perspective, consider that at Verbano cars equipped with an NC3 clone were timed at 23 km/h down the straight on the Wincrono speed trap (divide by 3.6 to get m/s) - and these had 10/26 gearing, those with 9/27 topped out at 20 km/h

Theoretically, current draw is max. when the rotor is stalled (then it only depends on arm resistance) and it decreases linearly with speed down to the no load value

So, what you could do would be measuring stalled rotor current (I suggest you do it at say 3 V and then multiply it by 4 !!). Then measure no load speed and amps on the motor alone, and plot a line on a speed/amps graph
Your test bed reading should fall somewhere on that line, rather close to the no load limit

If it does, then you can put an amp meter in series with your controller, and you'll know your speed every moment !! I would expect speed/amps to be in the middle third of the range

Would do it myself, but I'm supposed to be busy building my track


Ciao
Beppe
 

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Allan Wakefield
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Discussion Starter · #3 ·
Cheers Beppe, excellent theory to adle my mind as always


The rolling road is idle yes so the only other point of friction missing is the sliding of the brushes along the track? everything else including mag drag would be the same on the track no?

I also can't go that far into it this week as I am supposed to be driving to the UK tonight and have not even began to pack yet (taking the kids so packing is THE big job)

Hehe I haven't even worked out how to fit the new roof rack yet!

see you all later....................
 

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QUOTE The rolling road is idle yes so the only other point of friction missing is the sliding of the brushes along the track?

Friction of air over the body work?

McLaren
 

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A couple of points from the posts above strike me as being interesting enough to highlight.
Swiss's Data
QUOTE B - New Fly Viper (mag) - 8,040 RPM; 492Ma draw; 9.2 Metres per second
C - New Fly Viper (no mag) - 9,400 RPM; 304Ma draw; 10.3 Metres per second
Note how simply removing the magnet permits a significant increase in maximum speed AND an even more significant reduction in the current required to achieve it.

Later, Swiss says
QUOTE everything else including mag drag would be the same on the track no? (This is by comparison with sitting on the Test Bench).
My OPINION (as opposed to fact) is that the magnetic effect is probably quite lot less on the Test Bench than on the track, as cars sit a little high above the rails on the Test Bench. If that is so, then the potential performance increase of No Mag over Mag would be even greater on the track than on the bench.

On the bench, the height of magnet above the rails is affected by exactly where on the roller, the drive wheels are placed. Most people would aim at a contact point at the highest point of the roller, but it's not difficult to lower it a little by carefully positioning the car's drive wheels just a little forward or backward of the roller's TDC ('Top Dead Centre'). Of course this also depends very much on the steel used in the track rails and their relative height also! One would have to measure the magnetic attraction of both bench and track, at exactly the correct rail heights, to be sure.
Life sure gets complicated, don't it!
But that's what makes it so interesting.
 

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does the magnet affect the test bench? surely this is variation in fly motors rather than because of the magnet?
 

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QUOTE (astro @ 2 Oct 2004, 12:15)does the magnet affect the test bench? surely this is variation in fly motors rather than because of the magnet?
<{POST_SNAPBACK}>

Regardless of variation in motors (Swiss might expand on that),
Yes, the magnet significantly affects the test bench, but I think less so than on track, as explained.
 

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Beppe Giannini
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For a given brushed DC motor at a given voltage, an increase in speed will always entail a decrease in current : it's counterintuitive, but that's the way it is

Now, Swiss' data (thanks Tropi for pointing out similarities between case B & C) say that the stalled rotor current is 1.6 A , and the arm resistance 7.8 ohm
The motor should be drawing around the track between 0.5 and 1.1 A

On second thoughts, however, there's something very odd : the results seem NC-1ish [and instead we know the motor's rated at 25-26,000 RPM no load], with the standard 3:1 gear ratio motor revs come out too high - the car could never go that fast !! Probably, the two measured points are too close for extrapolation
 

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QUOTE (Tropi @ 2 Oct 2004, 12:35)QUOTE (astro @ 2 Oct 2004, 12:15)does the magnet affect the test bench? surely this is variation in fly motors rather than because of the magnet?
<{POST_SNAPBACK}>

Regardless of variation in motors (Swiss might expand on that),
Yes, the magnet significantly affects the test bench, but I think less so than on track, as explained.
<{POST_SNAPBACK}>

I do not understand how the magnet can affect the test result on the test bench ?!
The magnet ads some weight and braking effect on the track but on the bench ??
Did you test the same car (not another car of same make) with/without magnet ?
Any idea what the weight is of the roller in the testbench ? The results on the testbench should be very close to theoretically values of a permanent magnet dc motor with no load. If the magnet is positioned very close to the motor it may have some effect on its behavior..
 

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It's all to do with the downforce sucking the car down.

For example, if you put an elephant inside a Nissan Micra, the top speed would be slower. Same problem with a magnetic test bench.

McLaren
 
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