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Brushless Motor Rating Frustration

New school stuff -- brushless, lipo, spektrum, etc...

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Basher67
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Brushless Motor Rating Frustration

Post by Basher67 » Mon Oct 09, 2017 4:11 pm

Change is always frustrating. I have been slowly and painfully converting over to brushless and lipo for the past couple years. I know this is old news and it's been discussed to death, but some of this is just a pain. I have been learning about the brushless motor ratings and I thought my understanding of the "kv" rating system was good enough to buy a motor that was suitable for my application along with an esc that was rated at a decent amperage to go with it. I started looking at some old familiar brands and now I'm more confused then ever. Castle uses "kv" ratings, trinity is using the old "number of turns" rating, Reedy uses the "number of turns" except for a couple spec motors which are rated with "kv", tekin is a mix of both. What the hell? :x Why the lack of consistency with motor ratings? Is there something I'm missing?

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Re: Brushless Motor Rating Frustration

Post by jwscab » Mon Oct 09, 2017 4:36 pm

it's because the kV rating and turns are not quite equivalent.

let's keep it to 1/10 scale and 2S. If you assume that, you can sort of rule of thumb it from there.

kV is 1000rpm/volt, so you can calculate how fast the motor will spin based on the operating voltage. you can match up equivalent 'turns' brushed motors from there. but bear in mind the brushless ones with have quite a bit more torque.

because they can make more torque and have no physical brushes, they do so while using less power(watts). this is why you can't really compare brushed to brushless directly. the brushed motor will lose more rpm when you gear it vs the brushless motor. you can gear up the brushless motor quite a bit.

http://www.rccaraction.com/brushless-mo ... kv-rating/

the link above is a kind of decent approximation of turns to kV

this list is a decent conversion of brushed to brushless equivalence:
27T brushed -> 17.5 brushless
19T brushed -> 13.5 brushless
13T brushed -> 10.5 brushless
10T brushed -> 6.5 brushless
7T brushed -> 3.5 brushless

so again, it's ballpark, but with those two pieces of info, you can approximate pretty closely.

I use the rc10 manual to adjust gearing based on brushed vs brushless since they have a nice chart to start from.(be sure to account for the trans ratio if using a different car).

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Re: Brushless Motor Rating Frustration

Post by jwscab » Mon Oct 09, 2017 4:43 pm

one other thing I should point out, the motor itself is described in the number of turns, physically, ie 10.5t etc. The motor construction and rotor however can affect how much power (kV) the motor actually produces, so some vendors prefer to rate the motor in kV vs turns. This is the long and short of why you can't directly compare.

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Re: Brushless Motor Rating Frustration

Post by Timmahhh » Mon Oct 09, 2017 5:08 pm

Most of the time, racing/sensored motors are rated in turns and basher/entry level/unsensored motors are rated in KV. I think of a KV rating kind of like the "it goes xx-mph" on the outside of an RC car box. It sort of means something, but there's also no accountability for the rating being accurate, so it really doesn't mean much to me. To me, sensored motors are better in every aspect except for being waterproof, and at this point, there isn't much of a difference in price.

So unless waterproofing is a big deal to you, I would highly recommend just skipping the "KV" motors entirely, and get a sensored motor rated in turns and sensored ESC. It will be smoother, have more tuning options, and honestly just fit in with any nice builds or race cars better than an unsensored motor.

With that out of the way, turns on a brushless motor mean about the same thing as turns on a brushed motor. I would venture to say that with the advances in brushless, its more of a 1:1 conversion on the speed of brushes turns vs brushless turns than the chart from car action (ie: 17.5 turn brushless is more like 17 turn brushed now). Some even say that 17.5 race classes are approaching the speeds of modified brushed motors from back in the day, but that might be a stretch.

So at this point, its just a matter of picking how fast you want to go, similar to how you might have done it back in the day.

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Re: Brushless Motor Rating Frustration

Post by Basher67 » Mon Oct 09, 2017 6:15 pm

You nailed it. I didn't see that before. Tekin lists their sensored motors by # of turns and sensorless by kv's. The light becomes a little clearer. :mrgreen: Great article too. At least that gets me in the ballpark. The velineon motor is deceiving with its 3500 kv, 10T specs. I already have an SC700-BL esc in the T4.2 I just picked up so I'm going to get a sensorless motor to go with it to get me started with this truck. I will eventually be getting a sensored setup for smoother throttle control. At $50-$150 a motor, I'm trying to eliminate the trial and error of buying the wrong motors. I recently picked up an HPI drift car at a yard sale with a 17.5t brushless motor. When I ran it, I was pretty disappointed at the power output. I have 19t brushed mod motors that would wax ass on that thing. :lol:

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Re: Brushless Motor Rating Frustration

Post by Timmahhh » Mon Oct 09, 2017 8:09 pm

One thing to keep in mind if that the gearing is much different between brushed and brushless too. I've seen up to 38 tooth pinions used with a 17.5 in a 10th buggy for racing, although its usually more like high 20s to low 30s. Unless you're racing, there isn't much reason to go with a fancy/expensive motor. a nice 17.5 might be 10% faster than a cheap 17.5, but a cheap 13.5 is faster than both of them.

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Re: Brushless Motor Rating Frustration

Post by Basher67 » Mon Oct 09, 2017 8:31 pm

I do plan to use the T4.2 to start racing again a little. (its been a while) Figured i would throw a cheap sensorless motor in it to start practicing and getting in some laps. I figure on getting a quality sensored setup for it down the road.

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Re: Brushless Motor Rating Frustration

Post by matt1ptkn » Tue Oct 10, 2017 11:20 am

Timmahhh wrote:
Mon Oct 09, 2017 8:09 pm
One thing to keep in mind if that the gearing is much different between brushed and brushless too. I've seen up to 38 tooth pinions used with a 17.5 in a 10th buggy for racing, although its usually more like high 20s to low 30s. Unless you're racing, there isn't much reason to go with a fancy/expensive motor. a nice 17.5 might be 10% faster than a cheap 17.5, but a cheap 13.5 is faster than both of them.
I was quite surprised the first time I swapped a 17.5 brushless into a car that I previously ran a 27T brushed in. I had to gear up so much, I thought I was doing something wrong. It was in a gearbox class oval car, and my pinion and spur were fast approaching the same size! :shock:
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Re: Brushless Motor Rating Frustration

Post by RCveteran » Tue Oct 10, 2017 4:22 pm

Timmahhh wrote:
Mon Oct 09, 2017 8:09 pm
although its usually more like high 20s to low 30s.
This was the hardest thing for me to get used to. I had to buy lots of pinions as I never had anything that big. I mean a 30T pinion on an old RC10, who'd a thunk, but yes, that is what I run with my 17.5T.

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Re: Brushless Motor Rating Frustration

Post by Timmahhh » Tue Oct 10, 2017 5:05 pm

A couple friends and I were out of racing from about 2001 until 2011, and when we got back into it, one of them bought the top of the line tekin setup at the time. So of course we geared it like we vaguely remembered gearing a stock motor. I think it was a 21t pinion on there or so. He took it out for the first run, and we all watched with excitement to see what $250 of motor and esc could do.... he comes onto the straight aaaaaannnndddd :? :? :? :? :? what? thats it? seriously? After adding about 10t of pinion to it, it was a lot faster though!

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