Motor info please.??

[BashingBrian]

Hi everybody,

Bit of a motor question..?? I'm tinkering around on a cheap project, lets call it my mini speed runner...?

Only 3s max..!!

So my question is.. which of these would work best with a 27T pinion and 57Spur..??

GoolRC 3680 1500KV.... ??

GoolRC 3674 2650KV.... ??

GoolRC 3670 2850KV.... ??

I understand its not going to be lightning fast but I'm trying to figure out which would be best..

ESC I'm using is BLX185, just incase that makes a difference..?

Any help or advice would be great..??

 

[bicketybam]

Hi everybody,

Bit of a motor question..?? I'm tinkering around on a cheap project, lets call it my mini speed runner...?

Only 3s max..!!

So my question is.. which of these would work best with a 27T pinion and 57Spur..??

GoolRC 3680 1500KV.... ??

GoolRC 3674 2650KV.... ??

GoolRC 3670 2850KV.... ??

I understand its not going to be lightning fast but I'm trying to figure out which would be best..

ESC I'm using is BLX185, just incase that makes a difference..?

Any help or advice would be great..??

Tire size/type and truck type would help.

Assuming the same gearing for all of those motors, the 2850kv motor is will be nearly twice the speed of of the 1500Kv motor. You would need to gear up that 1500kv motor. On 3s it will be way too slow

Given the limited info, I would pick the 3674 motor.

 

[BashingBrian]

I think the wheel size is about 100mm the are Typhon wheels..

 

[bicketybam]

I think the wheel size is about 100mm the are Typhon wheels..

Awesome. That's actually what I used when I was trying to calculate speeds. So you are looking at about 60 mph with the 2850kv motor, half that with the 1500 KV motor.

 

[BashingBrian]

My mistake the wheels are 115mm..!!

 

[sadlerbw]

Do you know the number of teeth on the differential ring gears and input pinions? Or just the gear ratio of the diffs? I’ve got a little spreadsheet somewhere I can drop all that info in to come up with a theoretical max speed. It won’t tell you if the motor will die in a ball of molten metal though!

BLX185 is probably fine from a power/thermal perspective. Not sure if being sensorless matters or not.

 

[BigBassBrent]

For 3s I'd run the highest kv motor out of all those then gear it appropriately.

You can multiply the kv rating times the voltage to get the max rpm of the combo with no load.

2850kv x 11.1v = 31635 rpm

So a lower kv motor will produce less rpm with the same voltage. Take the same formula and plug in the 1500kv motor on 6s power for comparison.

1500 x 22.2 = 33300 rpm

Then if you run the 1500kv on only 3s

1500 x 11.1 = 16650 This low of max rpm would require a huge pinion to produce any speed at all. Stock rpm on a 6s Arrma with the 2050kv motor

2050 x 22.2 = 45510 rpm

It wouldn't hurt to go higher than 2850kv and shoot for the 40000 plus rpm mark.

 

[LibertyMKiii]

Usually knowing 3s max I'd look at a higher kv motor for speed runs. (2000kv to 3500 range)

I hit 83mph on 3s with the castle 1515 (2200kv) in a SCT. Some cheap chinese copy motors might do the job well or some other options: SSS motor, leopard, blx 2050kv

 

[sadlerbw]

So, just as an example, if you were using the diffs out of a 3S ARRMA truck, the diffs would be 37T ring gear and 13T pinion for the diffs, so a ratio of about 2.85:1. In that case, with your 57T spur gear, 27T pinion, 100mm diameter tires and 3S battery, the theoretical maximum speed for the three motors would be 36.9mph, 65.1mph and 70.1mph (or 59.4kph, 104.8kph, and 112.8kph if you don't do freedom units!) Now, you wont ever hit those speeds in real life because the battery voltage will dip during load, the motor wont spin at the full no-load speed, and the friction losses will eat up some power...but at least the tires balooning a bit will help offset some of that! Anyway, as a very rough estimate, I'd say you should be able to get 75% of that theoretical maximum speed if everything is working well.

If you are using a different ratio of diff gears than what I assumed, here is how you you can convert from the speeds I came up with to whatever your diff ratio would do: Multiply my top speeds by 2.85, and then multiply that by the number of teeth on the diff input gear, then divide by the number of teeth on the diff ring gear. There is your new theoretical max speed.

 

[BashingBrian]

Would I be right in saying the longer can motors have more torque and less rpm, and therefore can turn a larger pinion..??

 

[bicketybam]

What exactly are you trying to build or is it top secret? ?

 

[BigBassBrent]

Would I be right in saying the longer can motors have more torque and less rpm, and therefore can turn a larger pinion..??

I can try to explain in the way I understand it. I'm not an expert. Longer can motors will have more torque because of the magnets larger surface area. The number or wraps and size of the copper wire inside the motor determine the kv. Kv summarised is how many times the motor turns when given a a set amount of power. Can't think of the number but it's something like 1 watt. A higher kv motor spins faster compared to a lower kv motor. But a lower kv motor is more powerful, it has more torque. So the trick is finding the balance between the two and using pinion size to fine tune things based the 3s battery you plan to run. Too high a kv motor with too large a pinion will cause overheating of the motor. Hope this makes sense.

 

[BashingBrian]

What exactly are you trying to build or is it top secret? ?

Top secret..????

 

[sadlerbw]

Would I be right in saying the longer can motors have more torque and less rpm, and therefore can turn a larger pinion..??

Longer can does mean more torque. It doesn't have to mean less RPM. KV ratings don't care what size the motor is, or how many turns it has or any of that. It's a straightforward measure of how fast the motor actually spins per volt of potential across it with no load. So, if you have a short motor with 2000kv and a really long one with 2000kv, and you put 10v across each one with no load, they both ought to spin at 20,000 rpm. The differences between the two are mostly in how much RPM they will drop under load, how much power they draw, heat flux, etc.

The goofy part, which is pretty much impossible to tell ahead of time, is what RPM the motor is actually going to reach under the real load of an RC car moving at 80mph. The motor certainly isn't going to spin at the maximum no-load speed the KV rating would give you. In fact, the more torque it has, the closer it will get to its theoretical maximum under load. Say you have some monster 1500kv long-can 40mm motor and a 2500kv 540-sized motor. If you stick both of them in a little 1/10, 2WD buggy with the same battery, and the 2500kv motor is going to end up spinning much faster and give you a better top end because the load is relatively small. Now, stick both motors in an X-Maxx and it is very possible that the 1500kv motor would end up pushing the truck to a higher top speed because the smaller motor is going to drop way more RPM trying to generate the torque needed to push that huge pile of truck.

Kinda sucks, but RC does really seem to be a guess-and-check type deal when it comes to motor sizing...at least from the consumer end. You pick a can that you think is big enough not to overheat in the size vehicle you have with a KV rating that supports the battery you want to use, and then keep throwing taller gear ratios at it until you start to overheat, or run out of space for bigger gears! If you overheat first, then you can try getting a bigger, lower-KV motor and make up for the difference with gearing, and keep doing that till you hit the 'can't fit a bigger gear' issue. C'est la vie, and all that.

 

[BashingBrian]

So I ended up ordering the 3670 2850KV as I'm limited on length space at the moment, I'll whack on the 27T and wait to see if everything cooks itself to death..????

2nd plan is get longer chassis then can length won't be a problem..??

What is roughly max safe temp for motor of that size 170 ish..??