thorwil

I would like to calculate the motor kv required to reach a top speed with a given gearing (knowingly ignoring losses due to drag and other inefficiencies).

After a fair amount of googling and going through it several times, my result is 2584 kv to reach 50 km/h with 4S, Serpent 811-E, stock gearing. Since the usual choice is about 1800 kv, this can't be right.

Where's the mistake in the following, that will surely make me feel silly, once you point it out to me?

Outer_tire_diameter_mm * pi = distance_per_tire_revolution_mm
required_tire_revolutions_per_hour = speed_in_km/h * 1000000 / distance_per_tire_revolution_mm
pinion_revolutions_per_tire_revolution = (axle_ring_gear_T / driveline_T) * (spur_T / pinion_T)
required_pinion_revolutions_per_hour = required_tire_revolutions_per_hour * pinion_revolutions_per_tire_revolution
required_pinion_revolutions_per_minute = required_pinion_revolutions_per_hour / 60
required_kv_at_4S = required_pinion_revolutions_per_minute / 14.8

With numbers:

Typical 1/8 buggy outer tire diameter: 108 mm
-> 339.292996587698 mm distance per revolution.

Tire revolutions per hour for 50 km/h: 50 * 1000000 / 339.292996587698 = 147365.688048051.

Tire to pinion revolution factor: (44 / 10) * (46 / 13) = 15.5692307692308. Since the front/rear ring gears have 44T, the driveline gears 10T, spur 46T, pinion 13T.

Pinion revolutions per hour: 147365.688048051 * 15.5692307692308 = 2294370.40468658.
- per minute: 38239.5067447764.
- divided by 14.8 for 4S -> 2583.75045572813 kv (revolutions per minute at 1 V).

mkiiina

iTrader: (71)

1 minor issue. 14.8 is the nominal pack voltage, fully charged would be 16.8v. This would get you down to a ~2200kv motor which is reasonable.

I think there's another issue in there but I'm too tired to work it all back.

ThePanda

iTrader: (68)

bold words are wrong. the font and rear diffs use a 44/13, not 44/10

I changed it to 44/13 and 46/15 and it gave me roughly a 1500 kv motor on a fresh 4s pack.

hope this helps.

mtpocketsracing

iTrader: (2)

this software will work for you,

http://www.calcrc.com/

thorwil

mkiina: I thought about that and figured that the packs don't stay at maximum voltage for long. Ideal would be average V, which might be distinct from nominal ... but that's the best I got, then. Thanks.

ThePanda: Many thanks for finding that error. So I checked the manuals and part numbers again.

(ring / driveline) * (spur / pinion)
Serpent 811 BE: (44 / 13) * (46 / 16)
"Sport" version, kit or RTR: (44 / 10) * (46 / 16)

For "Sport", 50 km/h @ 4S now leads to 2099 kv. 44 km/h would be the maximum with the 1850 kv motor included in the RTR.

ThePanda

iTrader: (68)

I had no idea it was different between the two versions. Huh. that seems very very slow. I can get a brushed bandit to go nearly that fast.

Stangas

iTrader: (11)

you might get a bandit to go that fast, but the 1/8th has so much more torque and get there much faster

ThePanda

iTrader: (68)

but 30mph, max? for a ⅛ electric buggy?