Speed Calculations

[Mozzy]

I posted this in the Speed Calc section too, but, didn't want to clutter it up if it got out of hand.

Does this seem right??
I have taken this data from when my CRT had the Neu 1515 2.5D F in it running 5S.

Input data:

Enter Front/Rear Differential Gear Ratio (?):3.30769230 Jammin CRT 43/13
Enter Transmission Gear Ratio (?):1 Direct drive or centre diff 1/1
Enter Additional Gear Ratio (?):1 Select preset other ratio
Enter Spur Gear tooth count (?):62 Truggy CRT (Mod 1)
Enter Pinion Gear tooth count (?):16
Enter total battery voltage (?):18.5v
Enter kv rating of brushless motor (?):1700
Enter tire diameter in inches (?):5.65 Crimefighter MTR 1/8 scale truck
Enter the added radius of tire
"ballooning" in inches (?):2 My standard radius is nearly 3. I didn't think this was correct.
Enter Current draw in amps (?):0
Enter motor resistance in Ω (?):0

Results:

Spur/Pinion Reduction Ratio (?): 3.88 : 1
Total Reduction Ratio (?): 12.81731 : 1
Tire Circumference (?): 30.32 inches (770.04 mm)
Rollout (?): 2.37:1
Total Motor Speed (?): 31450 RPM
Theoretical Maximum Vehicle Speed (?): 70.44 mph (113.16 km/h)
Calculated Loaded KV Value (?): 1700
Calculated KT Constant (?): 0.8 oz-in/A
Calculated Motor Torque (?): Amperage not specified...
Calculated Axle Torque (?): Amperage not specified...
Calculated Motor Power (?): Amperage not specified...

With 3 inch as my radius the Theoretical Maximum Vehicle Speed is: 85.04 mph (136.61 km/h)

[Mozzy]

Maybe it's time you went back to school then, huh? ;-)

Look for yourself & learn. :-)

http://www.scriptasylum.com/rc_speed/_top_speed.html

[BrianG]

Well, the numbers do add up, but that 2" of ballooning is really artificially increasing your speed. If you use the base value of 5.65" for tire size and not enter any ballooning, the speed is ~40mph. Don't forget, the ballooning value is that added radius you get. So, if your tires are 6" in diameter normally, and they become 8" in diameter when ballooning, that is a 1" change in radius. A 2" ballooning value on a 5.65" tire means that the tire is 9.65" in diameter, which is very doubtful. The whole ballooning thing is described in the little help questionmark.

Also, to make it easier to post that information here, there is an "Export Values" button. You can then copy/paste from that. Also, if you save this info somewhere on your computer, you can later repaste that info back in using the "Import Values" button. Anyway, here is a more reasonable output with more normal values:

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 62
Pinion Tooth Count: 16
Total Voltage: 18.5
Motor KV: 1650
Tire Diameter (inches): 5.65
Tire Ballooning (inches): .5
Motor Current Draw: 0
Motor Coil Resistance: 0.01
Spur/Pinion Ratio: 3.88 : 1
Total Ratio: 12.81731 : 1
Tire Circumference (inches): 20.89 inches (530.65 mm)
Rollout: 1.63:1
Total Motor Speed: 30525 RPM
Vehicle Speed: 47.12 mph (75.68 km/h)
Effective KV Value: 1650
KT constant: 0.82 oz-in/A
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...

[nitrostarter]

Your ballooning is OFF!! No way you should be calculating and additional 3 inches of radius to the tires.

Most people add .1-.3 inches for ballooning. I add .1

[Mozzy]

Ok, I see now.

I was adding the radius measurement at rest. Then I guessed the 3inches.

Now it's a whole lot more realistic.

Thanks.