running lower rpm?

[tashpop]

i've been following the general idea of around 35k rpm on my motors, but i'm kindof in a bit of a bind here. what i'm looking at is ideas for my newly acquired lsp. i'm going to be running a mm esc, but i'm not fully decided on which motor i'm going to go with. i have a really nice mega 22/45 here that i would love to put on the truck. problem is the best i can spin it with the mm is around 24k rpm. will there be any adverse effects running it at a lower rpm like that and using gearing to get me up to speed? i know too much gearing will spike the amps, but i believe these are very effecient motors, and i'm going to try to keep the truggy somewhat light.

if i can't do it i guess i'll just do a little looking for something that spins faster, but i'd like to use the money saved else where.

[lutach]

Originally Posted by tashpop

i've been following the general idea of around 35k rpm on my motors, but i'm kindof in a bit of a bind here. what i'm looking at is ideas for my newly acquired lsp. i'm going to be running a mm esc, but i'm not fully decided on which motor i'm going to go with. i have a really nice mega 22/45 here that i would love to put on the truck. problem is the best i can spin it with the mm is around 24k rpm. will there be any adverse effects running it at a lower rpm like that and using gearing to get me up to speed? i know too much gearing will spike the amps, but i believe these are very effecient motors, and i'm going to try to keep the truggy somewhat light.

if i can't do it i guess i'll just do a little looking for something that spins faster, but i'd like to use the money saved else where.

Get a ACn22/45/1. It has a 2500Kv and with 4S (14.8V) it should be spinning at around 37000RPMs. Plus you will have the torque from a 6 pole motor.

[snellemin]

Bigger gearing is ok, if the load(vehicle weight+tire choice) is light enough.

Seeing that mega motor in action, I'm thinking about putting one in my Rustler.

[BrianG]

It seems there are three things to keep in mind when selecting a motor for a vehicle:

1) Motor size. Pick a motor that will have the torque to pull the weight around. For 8th scale MT/truggies, this means a motor with a longer can.

2) Max motor rpm. We all know the Feigaos like to run up to 30-35k max before they start losing efficiency. So, that's a no-brainer.

3) Min motor rpm. This is something we don't usually think about. If you use a motor with a kv that is too low, you have to gear higher for speed. Of course, taller gearing increases current, but it also affects ESC circuit effectiveness. If each motor revolution moves the truck 6", the ESC might have trouble locating rotor position and cog at lower speeds.

So, IMO, you want to pick a motor/voltage combo that will spin up to it's max rpm at the max speed. Spinning the motor at a slower max rpm means you'll have to gear down.

[kulangflow]

Brian, does this mean it would be a bad idea to run a 9XL on 4s (27K)?

Or does it just mean that you'd need to gear it down to stop cogging?

[BrianG]

Not necessarily a bad idea as long as you don't gear too high to compensate for the lack of overall speed...

[tashpop]

that's why i thought of this idea is because i've seen a few people list their feigao 9xl's on 4cell. i figured if a feigao can do it, my mega can.
i'm not stuck on running a mega, its just what i have here with me, so buying the 22/45/1 doesn't really matter, but i did look for one at the hobby shop, he didn't have any.

i might just throw it in there and see how it does.

[BrianG]

Yeah, the 9XL is a good choice for 4s to help limit the current through the MM IF the user doesn't gear up to compensate for lack of speed. If geared up to compensate, current goes up, and you might as well have gotten a 7 or 8xl...

[Arct1k]

How much is too much? 22/66 on emaxx trans revo?

Differential Ratio: 2.8461538461538462
Transmission Ratio: 1.7222222222222223
Other Ratio: 1
Spur Tooth Count: 66
Pinion Tooth Count: 22
Total Voltage: 14.8
Motor KV: 1853
Tire Diameter (inches): 5.8
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0.0092
Spur/Pinion Ratio: 3 : 1
Total Ratio: 14.70513 : 1
Tire Circumference (inches): 18.22 inches (462.82 mm)
Total Motor Speed: 27424.4 RPM
Vehicle Speed: 32.18 mph (51.69 km/h)
Effective Speed Rate: 2.17 mph/V (3.49 kmh/V)
Estimated Adjusted Speed: 31 mph (50 km/h) - 4% loss
Effective KV Value: 1853

and I just got this for my HB LSP - Hybrid outrunner 41mm / 60mm long:

Differential Ratio: 4.3
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 18
Total Voltage: 16.5
Motor KV: 1600
Tire Diameter (inches): 6
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0
Spur/Pinion Ratio: 2.56 : 1
Total Ratio: 10.98889 : 1
Tire Circumference (inches): 18.85 inches (478.78 mm)
Total Motor Speed: 26400 RPM
Vehicle Speed: 42.88 mph (68.89 km/h)
Effective Speed Rate: 2.6 mph/V (4.17 kmh/V)
Estimated Adjusted Speed: 40 mph (65 km/h) - 6% loss
Effective KV Value: 1600

Reason for smaller can is i flipped the diff on the LSP so only have 75mm clearance so too tight for an XL can and can't run to a neu atm...

[lincpimp]

18t pinion for the lsp will be too much with that motor at that speed.

[sleebus.jones]

FYI, i'm running a 1220kv with 5S A123 on my emaxx. Geared 19/66. So, that's 20k rpm. Runs fine, one *doink* when it starts and it's gone.

[Arct1k]

I think I have a 16T in my CRT .5 I could swap them over.... eeek to fast on CRT .5 on 3s - 2s would be ok...

Differential Ratio: 4.3
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 16
Total Voltage: 16.5
Motor KV: 1600
Tire Diameter (inches): 6
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0
Spur/Pinion Ratio: 2.88 : 1
Total Ratio: 12.3625 : 1
Tire Circumference (inches): 18.85 inches (478.78 mm)
Total Motor Speed: 26400 RPM
Vehicle Speed: 38.12 mph (61.23 km/h)
Effective Speed Rate: 2.31 mph/V (3.71 kmh/V)
Estimated Adjusted Speed: 36 mph (58 km/h) - 5% loss
Effective KV Value: 1600


Differential Ratio: 3.25
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 18
Total Voltage: 11.1
Motor KV: 3511
Tire Diameter (inches): 3.8
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0.0059
Spur/Pinion Ratio: 2.56 : 1
Total Ratio: 8.30556 : 1
Tire Circumference (inches): 11.94 inches (303.23 mm)
Total Motor Speed: 38972.1 RPM
Vehicle Speed: 53.05 mph (85.21 km/h)
Effective Speed Rate: 4.78 mph/V (7.68 kmh/V)
Estimated Adjusted Speed: 49 mph (79 km/h) - 7% loss
Effective KV Value: 3511

[MetalMan]

RPMs are the only thing being mentioned in this discussion - but what about the number of magnetic poles of a motor? Like outrunners, they won't necessarily have any issue being started even though they're turning at only 8000 RPM (for example). Would this carry over to the higher pole inrunners, like the 6pole Megas?

[tashpop]

heres what i just found out. i didn't adjust any settings on my controller to dial anything, but compairing my feigao against the mega. feigao was running a little higher rpm than most recomend, but on a 12t pinion. mega running low rpm, but on a 24t pinion. same voltage. both motors got a little hot, the mega heated up the speed controller more, and the high reving feigao had more punch and rpm.

this was compairing the 10L feigao 24XX kv against a mega 22/45/2 1380kv on same voltage 19.8 before load, under load i have no idea. the over geared mega being slower, it seems that perhaps the lower speeds and punch could have been caused by over drawing the batteries and dropping voltage. thats what that motor felt like, cheapish battery supply. it scoots like crazy in my other car on 8cells.

[BrianG]

@MM: Not sure what role pole count plays in all this, but the torque formula does not contain any factors that include pole count (torque=1352/kv*amps/192). All I know is that I generally run motors as high rpm as possible up to their efficiency roll-off point and gear appropriately. For feigaos, that's ~35k rpm; for Neus, that's ~55k.