Brushless torque?

[Juggular]

Why haven't I seen specs on torque?  It would be nice to have some basis for comparison like below.  

Stock motor:  14,500 rpm, 255g/cm torque

Sport Tuned:  18,300 rpm, 350g/cm torque  

GT-tuned:  19,000 rpm, 500g/cm. (On paper, GT should be the fastest here.  But in reality, TT-02B on the stock motor goes up to 20km/h.  22km/h with GT, and 24km/h with Sport Tuned.  All at 8:1 gear ratio... so... specs are not that useful).   

But I'm still curious. 

Super Stock is supposed to have 27,500 rpm, 500g/cm.  Tamiya 10.5t brushless is rated for 3370kv, which should give it 24,264 rpm at 7.2v.  Based on RPM alone, 10.5t should run 12% slower than Super Stock.  But in reality, 10.5t is 10% faster.  

Which lead me to my rule of thumb, "when comparing, reduce the RPM of the brushed by 20%."  I can only guess that this is because a brushless motor would have a lot more torque --- 1) same RPM seems to go faster, which means the load isn't reducing the RPM as much;  2) people report being able to run larger pinions without ill effect.  

P.S. It occurred to me that I could look for a motor dyno.  Then I stumbled upon a video of a dyno.  2S (7.7v) rpm was 19,500.  That is 2500kv, which could be a 15.5t motor.  The torque was 286.43 N.mm, which translates to 2,920g/cm.  That's 6 times the upgraded motors, 12 times the torque of the stock motor!  Could this be right?  That seems unbelievable.  Has anybody seen torque specs on any brushless?  

 

[max69vk]

Not my answer but I found this reply somewhere on the web when looking into the same thing about drill motors. Whilst I can’t guarantee it’s correct, it’s does make a few good points...

 

A drill manufacturer claiming higher torque and efficiency from their brushless motor is probably telling the truth - however that doesn't mean being brushless is the only reason for the improvement. They could put a really bad brushless motor in there that's no better than a high quality brushed motor, but why would they?

All else being equal, a brushless motor should have more torque because none is wasted in friction between the brushes and commutator, and higher efficiency because no power is lost in brush heating and contact arcing. Without having to worry about burning up brushes, a brushless motor can handle more current - and since current creates torque that means a brushless motor can produce higher torque without having its lifespan reduced.

But that's not all. The weight and space saved by not having brushes and a commutator leaves more for copper windings, stator iron and rotor magnets - the parts that actually create torque. Thus a brushless motor can be more powerful than a brushed motor of the same size. 

A brushless motor can also spin faster because it doesn't suffer from brush bounce, has no commutator to fly apart and no armature windings to get flung off. The rotor can be made from a single cylinder of Neodym magnet, or several magnets bonded to a core and wrapped in Kevlar. Whereas brushed motors typically max out at around 20,000rpm, a well made brushless motor might manage 50,000rpm. This higher speed can then be geared down to produce more torque.

Further improvements are mostly due to the use of better materials - eg. Neodym magnets instead of Ferrite, thinner laminations made from higher quality silicon steel. The same improvements could be applied to a brushed motor, but it would still be limited by the brushes. Adding higher quality brushes would probably make it cost more than an equivalent brushless, so why bother.

[Saito2]

Great topic and I wish you luck in your quest for knowledge ('cause I've got the same type of questions). I've never found an accurate comparison between brushed and brushless. I still have a "feel" for brushed. In other words, 15 turn double vs 21 turn single, etc. Brushless...no idea. Gearing perplexes me even further. I familiar with gearing down for high turned brushed motors. In recent reads about increasing power brushless motors, folks are gearing up? So if I stick a brushed mod motor in my Super Shot, I'd drop down to a 13T pinion but if I drop in a brushless motor I go up above the stock 15T pinion? I'm too old and confused . There needs to be an encyclopedia for all this knowledge. 

[Juggular]

Yes, it's hard to wrap my old brushed head around this.  

Max's explanation makes a lot of sense.  I suppose if 550 has a lot more torque than 540, clearing the room for the brushes would have that much impact too.  And those little "silver magnets" are a lot more powerful than old "black magnets" that are in brushed motors.  Who knew that a tiny 1/16" thick magnet could almost hold up a screwdriver.  

What I didn't realize was how much stronger brushless motors became. 

One brushless = 12 silver cans!!? 

That's not just some progress, that's a "revolution." (pun intended) 

It a humble 2500kv brushless can produce 286.43N/mm (2,920g/cm) of torque?   That just blows 500g/cm Super Stock out of the water.  No wonder using a bit larger pinion doesn't slow it down.  I'm still reeling from a shock.  

 

[Fabia130vRS]

15 hours ago, Juggular said:

Then I stumbled upon a video of a dyno.  2S (7.7v) rpm was 19,500.  That is 2500kv, which could be a 15.5t motor.  The torque was 286.43 N.mm, which translates to 2,920g/cm.  That's 6 times the upgraded motors, 12 times the torque of the stock motor!  Could this be right?  That seems unbelievable.  Has anybody seen torque specs on any brushless?  

 

why not?

if a brushless FDR can go low as 1.85 on F1 or under 4 FDR on touring cars... when have you seen a brushed motor pull this kind of FDR's?

my driving pal had a 13,5T brushless on his F104 with 2,03FDR and top speed was above 80kmh.... then yesterday we tried his f104 with a brushed 21T can, on 40t pinion and 93T spur with a FDR of 2,32 and it went only 40kmph...

I have a plan to build my own dyno/motor tester.

http://www.rccartips.com/rc-car-dyno-cheap.htm

sorry for oftopic, but has anybody tried it? would give measures to compare..

[ThunderDragonCy]

Interesting subject. I don't have any firm data, but my anecdotal evidence from my own cars says brushless must have miles more torque than brushed. My Thunder Dragon on 13.5 brushless with 2S pulls the 17t pinion (6.7 fdr) no problem at all, and when the cheap sensored 4370kv system blew up in my DN01, I put another 13.5 in that, geared appropriately (6.8 fdr) and it seems no slower than the higher power motor on a lower gearing. It just lugs the big gear without any trouble. The other part of the equation here is that torque x Revs = power, so if the brushless motors are producing loads more torque at similar rpm to brushless it will be producing considerably more Watts of power. So that will keep the top end speed much better too. The torque will lug from low revs then the power will take over at the top end. 

[MadInventor]

Well, I've got a brushless motor that nearly the size of  a coffee cup in the War Rig, and that pulls around 30kg of Rig uphill at 18mph on 6S without getting hot, and has the torque lift the slack out of the front suspension when you open the throttle. Got a similar motor in my forestry machine, and I'd there is bags more torque in brushless motors. I can't remember exactly but seem to remember doing a rough torque calculation for the skidder, which came out 300Nm at the axles with a reduction ratio of about 150:1.

 

Edit: looked back at the War Rig build thread, and the motor in that is 2330Watts @22.2V, versus a silver can 540 which is about 60-65Watts @9.6V, so about 15 times the power of a 540 at equivalent voltages.