Speed Calculator

[sleebus.jones]

Well, the problem is that the drag is not constant, it's exponential.

However, I did find a neat article here that might help ya out:

http://en.allexperts.com/q/Physics-1...ce-running.htm

[BrianG]

OK, I see that that it's not a true constant... sort of.

So, how about something like:

speed_loss = calc_speed^2 * 0.0015

For 45mph calculated speed, the actual estimated speed is ~42mph
For 60mph calculated speed, the actual estimated speed is ~53mph
For 100mph calculated speed, the actual estimated speed is ~85mph
For 140mph calculated speed, the actual estimated speed is ~111mph

Again, I want this to be just a rough estimation. Basically, I'm trying to develop a formula based on some calculated vs actual speeds from different existing setups. I know, kinda working backwards.

[snellemin]

I would say the calculator is pretty accurate up to around 50-60MPH with the right batteries. For trucks I subtract 10% (drag/resistance/traction loss) at 70mph, 80mph I subtract 30% and 85mph and up the resistance goes higher and higher.

[BrianG]

Added a rough estimate of speed reduction due to wind resistance, drag, etc to the top speed estimator. There is a disclaimer in the help link that says this value is very rough and many variables will affect the accuracy, but it's better than nothing IMO.

Also added power calculations (in watts and HP) to the top speed estimator.

[BrianG]

Updates:

Added a new page to calculate the needed resistor to eliminate the spark when hooking up the battery. It has a very creative name: "No Spark" Calculator.

Also grouped menu items because the list is starting to get a bit long now...

I think it's about time we send Brian some pizzas. What do you guys think?

[BrianG]

Thanks, but no thanks. It's hard enough to lose weight over the winter months without that! 75lbs lost so far (since last June), but still have 35 to go, and not going to get there eating pizzas!

[lutach]

You are the man.

[snellemin]

Cool man. I'll try the no sparky on my next setup.

[BrianG]

What is the "rule of thumb" regarding the minimum rpm a sensorless motor has to turn for the ESC to get the proper back-EMF signals? Something like kv * 0.1?

I want to try to include this calculation in my top speed estimator to notify the user that they are geared too high for proper ESC operation...

[BrianG]

Getting back on topic; anyone know this?

Originally Posted by BrianG

What is the "rule of thumb" regarding the minimum rpm a sensorless motor has to turn for the ESC to get the proper back-EMF signals? Something like kv * 0.1?

[GriffinRU]

Originally Posted by BrianG

Getting back on topic; anyone know this?
What is the "rule of thumb" regarding the minimum rpm a sensorless motor has to turn for the ESC to get the proper back-EMF signals? Something like kv * 0.1?

I want to try to include this calculation in my top speed estimator to notify the user that they are geared too high for proper ESC operation...

A simple rule is zero-crossing.
Zero crossing would be sync with every pole, so for given motor number of magnet poles and armature poles per rotation would tell you what min. RPM would be sufficient for ESC to get it going. But to get direction correct you might want to get one extra, but it can be much simpler as well. If hardware can do that, than software is your limiting factor...

[macr0w]

I figure the balloon effect is at least 1 inch?

Differential Ratio: 3.3333333333333335
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 12
Total Voltage: 14.8
Motor KV: 3300
Tire Diameter (inches): 4.25
Tire Ballooning (inches): 1
Motor Current Draw: 125
Motor coil Ω: 0
Spur/Pinion Ratio: 3.83 : 1
Total Ratio: 12.77778 : 1
Tire Circumference (inches): 19.63 inches (498.73 mm)
Total Motor Speed: 48840 RPM
Vehicle Speed: 71.07 mph (114.16 km/h)
Effective Speed Rate: 4.8 mph/V (7.71 kmh/V)
Estimated Adjusted Speed: 64 mph (103 km/h) - 10% loss
Effective KV Value: 3300
Motor Torque: 0.27 ft-lbs
Final Torque: 3.41 ft-lbs
Final Power: 1850 watts (2.5 HP)

[Serum]

75lbs lost so far

Concrats Brian. That's serious mass you've lost!

[BrianG]

Thanks! Yeah. I definitely feel better! Started at ~310lbs, and down to 240 right now. Gained about 7lbs over the winter, but am staying pretty constant. When springtime comes around, I want to get down to 199. Still "obese" by the BMI charts, but oh well. I REALLY want to see a number that starts with "1" - I haven't seen that since I was about 13-15 years old!