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Old 02-02-2003, 01:45 AM   #16
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Thanks X-Ray for the post and kind words i just hope this stuff helps people and any corrections are made to further peoples enjoyment of RC Racing. Now we just need more people to do 12th scale. LOL.
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Old 02-02-2003, 02:02 AM   #17
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Default warning maths alert!

hi all,

the dyno is actually the king of determining maximum power. it does it very accurately, what it is cack at is determining how efficient the motor is across the rev range. So thats why those numbers on the labels mean next to nothing.

Now the maths bit, it is possible to determine how a motor will perform given the amp draw and the maximum power.

say for arguments sake the motor is 200watts and the amp draw is 40A.

so Power = Voltage x Current

a bit of algebra

so Voltage = Power / Current

to work out the torque, use tessla's law or weber's I can't remember but here is the formula

F = B I L sin (theta)

F = Force
B = Magnetic flux density (the strength of the magnets)
I = Current
sin (theta) = 1, this is because sin (90degrees)=1

so then you end up with


I realise that the B, and L is probably unknown but its best to make a rough estimate at say 5m (easy numbers). It doesn't matter provided youk eep the same numbers for each of the motors.

Torque = F x (Armature diamter / 2)

From that it is possible to work out the torque at each level of RPM so you get a rev map over the whole RPM spectrum. I am well aware tha h 90% of this board will have got lost somewhere but hopefully someone out there in webland knows what i mean!

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Old 02-02-2003, 03:45 AM   #18
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Hi guys. Greetings from Malta.

DARNOLD. Thanks for the simple explanation.

I had been looking at the revs all the time, though in fact a high revving motor does produce more power according to the dyno.

I suppose that with today's 3300 cells the amp draw is becoming a bit of an academic figure since it is virtually impossible to dump, even with mod motors, let alone with stock in 5 minutes.

Trying my car with 21 turn stock motor on the track i have reached an optimum gearing for the quickest laps which still gives me 2 minutes of run time after the race has ended.

Schumacher, I wish I remember my algebra from 45 years ago when I was at school, like you know it. I think you'll have to try and repeat that in layman's language if you don't want to loose us all.

Thanks again for all your help.


Joe from sunny Malta.
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Old 02-02-2003, 09:40 PM   #19
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I'm going to have to pull out my old math books.
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Old 02-02-2003, 09:41 PM   #20
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Hi Axiom5b and DArnold

I'm the only other CS dyno user at the track you race at.


I might have some business for you.
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Old 02-02-2003, 10:52 PM   #21
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Hey guys, how are things going?

Schuey- Think i may have followed you but who knows that was a pretty good discertation you gave. LOL! If i followed what you said you basically repeated what you had said earlier about the equation solving for torque. LOL!

Nanook- Cool send me a pm. BTW we have a new place for you to see that we were at today and you would love it.
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