A my calculations correct?

[bugone]

Just been looking on eBay at brushless motors. I saw this one...

I'm new to brushless motors, and have read that Kv x Volts = rpm. Is that correct.

So if I get a couple of lipos and put 22.2v through this motor, it will spin at 120,000 rpm?

Is there an ESC that could handle it?

[bromvw]

Just been looking on eBay at brushless motors. I saw this one...

I'm new to brushless motors, and have read that Kv x Volts = rpm. Is that correct.

So if I get a couple of lipos and put 22.2v through this motor, it will spin at 120,000 rpm?

Is there an ESC that could handle it?

more to the point is there a gearbox that would'nt self destruct ?

[SteveAdair]

Hi,

That advert says the motor is 540Kv. My understanding of brushless motors is that represents 540 RPM for each volt put in to the motor. So, if you put 22.5 volts through it, it should spin at 12,150, which does seem rather low. Anyone care to correct me ?

[bromvw]

i used to be into rc boats guys . having looked at the photo its a boat motor and totally unsuitable for a rc car . 6mm shaft is a dead give away

[speedy_w_beans]

I don't know if you noticed this, but this is not a standard 540-size motor that would typically be used in Tamiya models. A standard 540-size motor is 36 mm in diameter; this one is 55.8 mm in diameter. The length of this motor is nearly double the length of a standard 540 motor. This motor will not fit normal 1/10 scale RC models.

Other differences:

- Voltage from 5S to 20S (18.5 V to 74 V) versus 2S to 3S typically (7.4 V to 11.1 V)

- Max power of 5500 W versus a few hundred watts typically

- kV of 540 rpm/V versus higher kV of 2200+ rpm/V

- 6 mm shaft diameter vs. 3 mm shaft diameter

- Dimensions 55.8 x 92 mm versus 36 x ~43 mm

- Huge weight -- 880 grams -- versus 180-200 grams

I checked a few 1/8 scale systems and this motor is bigger than even those...

At 20S (74V) this motor might achieve 39,960 rpm maximum. At 5S this motor might achieve 9,990 rpm maximum. The size of this motor and power rating suggests it can deliver 10x the torque of a normal 1/10 brushless motor system, but at the expense of speed.

Note that brushless motors advertise a certain kV rating, but in practice the motor speed drops under load due to efficiency. I typically use ~70% of kV to estimate what the real speed will be in a model.

-Paul

[bugone]

I see. Ok, how about this one?

5800kv, and an handle 12v (10 cells), so that would make it Nearly 70,000rpm.

[bromvw]

I see. Ok, how about this one?

5800kv, and an handle 12v (10 cells), so that would make it Nearly 70,000rpm.

if its outright speed your after google Mamba Max pro 6900KV .

http://www.hobbies4life.com/ProductDetails...=CSE010-0066-08

http://www.hobbies4life.com/ProductDetails...=CSE010-0066-09

or if you want to go insanely fast they do a 7700kv

[wolfdogstinkus]

I would just spend a little extra and get an Ezrun combo, the 9T system is around £50.

Live links to ebay auctions are against the rules.

[bugone]

Do higher Kv have less torque?

[bromvw]

this is a bit complex to explain . have a read of this thread that explains things far better than i can

http://www.rcuniverse.com/forum/m_9622661/printable.htm

[chevyman3]

I would just spend a little extra and get an Ezrun combo, the 9T system is around £50.

Live links to ebay auctions are against the rules.

Those lesser priced HW combos are 380 motors in a 540 size can.

Do higher Kv have less torque?

yes the higher the KV the less the torque when you get in the high 4700s and up you can't use 3s either unless you want to fry things.

Iv'e seen 2900 KV motors geared up and they are fast don't be fooled by the KV

[bugone]

Thanks for all the info.

I am looking for outright speed, but I need a fairly torquey motor. I'm running a 16t pinion in my monster beetle instead of the 10t. It's going pretty well with the sport tuned, however obviously now lacks the kind of grunt needed to lift the front wheels (although still has enough to spin the wheels on tarmac).

If I were to go brushless, what kind of kv would give me the best top end, whilst still giving me the boost in torque needed due to the pinion change?

[chevyman3]

Thanks for all the info.

I am looking for outright speed, but I need a fairly torquey motor. I'm running a 16t pinion in my monster beetle instead of the 10t. It's going pretty well with the sport tuned, however obviously now lacks the kind of grunt needed to lift the front wheels (although still has enough to spin the wheels on tarmac).

If I were to go brushless, what kind of kv would give me the best top end, whilst still giving me the boost in torque needed due to the pinion change?

put a 3500KV to a 4200 kv in it and trust me even on 2s it will be more than enough of both torque and top end.

[bugone]

Thanks. Ordered an EZRUN 5.5 60a combo, and some deans connectors. .

[MadInventor]

Thanks. Ordered an EZRUN 5.5 60a combo, and some deans connectors. .

Dude, thats a 6000KV motor, and the speedo that comes with it can't be run on 3S with a 6000Kv motor. The recommended usage for the motor is 'on road' with that speedo, so you might not be able to run it off road without overheating the motor or blowing the speedo.

I've been going through motor selection problems with my thundershot. I'm running a 8.5T motor on a 10.8V NimH and it's V quick.

In theory the 5.5T motor 6000Kv * 7.2 = 43200rpm, the 8.5turn is 4000Kv, and that gives exactly 43200 on a 10.8V battery. So the 8.5T gives more torque from the higher wind, but exactly the same theoretical rpm from a 10.8 as the 6000KV running on 7.2V, and because of the higher voltage you get less current draw for the same speed, which means less cogging / heating issues.

I think you're going to run into cogging issues if you try to run that in the beetle, or possibly even fried electrics, especially on the 16T pinion.

[bugone]

Yer, you're probably right. Especially with the 16t pinion. Fortune may have struck, in that the seller is out of stock.

I might look to make my own combo. Thinking of the hobbywing 120a ESC and a slightly lower KV motor.

[matman]

LST mini 1/18th with C4 9200 & mamba esc with 11.1 li-po .

Went for a long time , could not control it etc .

Then one day , fire , can't see why that happened LOL .

Lucky it was not mine , It was a M8's one . I stuck with the 2s li-po

& that was nut's in this mini LST 1/18th with C4 9200 & mamba esc .

[chevyman3]

that's why castles gearing charts have "OMGWTFBBQ" on the higher rated kv motors.

[MadInventor]

Yer, you're probably right. Especially with the 16t pinion. Fortune may have struck, in that the seller is out of stock.

I might look to make my own combo. Thinking of the hobbywing 120a ESC and a slightly lower KV motor.

Good call, ezrun do a 120A combo with a 2700Kv motor for 4wd monster trucks, it's a low KV but will have bags of torque. The motor is bigger than a 540, 40mm diameter versus the standard 36mm, is a 4 pole motor, and it has a 5mm shaft. Can be picked up off ebay for £100. Be careful though, as some seller are listing a similar combo with the motor listed with 540 size dimensions, and these may have inferior performance if they are only 2 pole motors.