brushless motor advice

[rob m]

how do you know what torque or how fast a motor is if you dont know the max rpm?, example a losi 6.5 is 380w and the rpm per volt/kv is 4880,a novak velociti 6.5 is 390w and the rpm per volt/kv is 6400, does the rpm per volt/kv determind how much torque a motor has? and the watts is the speed? am i right in thinking that?

[Si Coe]

Well you could do all sort of fancy calculations and stuff, but its pretty pointless really because that data you have has no relation to a motors actual power band or how it operates under load (kV is a no load figure). Furthermore torque at the wheel relates not just to the torque effect of the motor, but also the gearing applied to it.
In theory a lower kV motor will have more torque, but since we would commonly give it a higher gearing to produce a similar top speed to a higher kV motor that benefit is lost.
The watts is the motors power, but its really just the electricity converted per second. Converted into what (no pun) is the question - you'd hope forward movement, but some is turned straight into heat. Power figures are no use with efficiency as well, telling you how much is converted into the form you want.

[Marvin]

Ignore the power rating - you cannot really gain much information from this.

What you need to know is the length and diameter of the rotor, in comparison with other motors of the same KV. That is the best way to determine the torque, along with the number of motor poles.

In general, a 4 pole motor will have more torque than a 2 pole.

A longer rotor will have more torque than a shorter rotor.

A larger-diameter rotor will have more torque than a smaller rotor.

[DCM]

Also, the air gap betermins torque too, the smaller the gap, the lower the KV but higher torque. Plus timing too.

[rob m]

ok im lost now,so all the motor specs like i posted above mean nothing,so how do you know when buying a motor witch one to get when they dont tell you the things that seem to be important? the reason why im asking this is in my b4 i had a losi 6.5,put a novak velocity 6.5in and it was a lot more agressive then the losi on throttle(same gearing),im thinking of upgrading the motor in my b44 but im lost on which motor is good for what! i have a losi 4.5 in it at the moment but after trying the novak in my b4 the losi motors seems weak in throttle,so when buying a motor how do you know what is good for what, lrp x11/x12 , novak, tekin etc,???????? how do you know which one gives what you want for a motor?

[mattb]

get one of those new lrp x12 6.5t octiwind rob

[Marvin]

Torque (or lack of) can be cured with changing the gearing to suit. Generally, a high revving motor will have less torque than a low revving motor (given the same size). You can gear the less torquey motor down so it 'produces' more torque, and gear the torquey motor up for more speed.

[rob m]

so idealy you would want a motor with a lot of torque so you can gear it high (bigger pinion) still have accelaration and faster top speed,but if you have a low torque motor you couldnt do it because it would struggle with the high gearing and have slow accelaration/over heat so you would have to gear it down which would slow the top speed but give you accelaration back.is that right? so the answer to my original question is that the specs the manufacturers put on paper are pointless becouse they give you no indication/comparison to how much torque/speed etc a motor of the same wind has from one brand to another?

[Si Coe]

Within reason.

Specs are vaguely useful within a range to let you identify the current wind for you, and specifically what the jump is between winds. For example for many brands 7.5t-5.5t covers between about 4500kV and 6000kV (not a huge change) and then 4.5t is suddenly 7500kV, which shows why we don't use them.

But there is some validity to using them to compare motors from different ranges too, if you assume they are based on equal technology. In theory if two manufactures sell 5.5t motors, one at 5500kV, the other 6500kV if would be fair to assume the lower kV one offered more torque. However it is possible the 6500kV features better magnets etc and actually offers the same torque (its just an all round better motor) .