By popular request we are going to look at the idea of changing kv to go faster. Building a faster RC vehicle is a lot of fun but building a setup only to find out it burns up in the first few runs can leave you scared. It’s important to be informed to make the best most reliable decisions to help increase your chances of success.
Most electrical topics in RC can be confusing and difficult to understand. Today’s question is no exception. Let’s go through the answer to our question, does a higher kv brushless motor make your RC Car faster. Even though we are going to be talking about an RC car in this article, the same principles apply for other RC vehicles.
How kv Impacts Output RPM
A motors kv constant tells us how many revolution the motor will make in one minute per volt applied. As we increase the voltage to the motor we get more RPM. However, if we increase the kv of the motor, we get more RPM out even if voltage is held constant.
Here is a simple example showing the difference between a 2200kv motor on 4s LiPo (14.8v nom) vs moving up to a 2650kv motor operating on 4s. A 2200kv motor will spin ( 2200 x 14.8 ) 32,560 RPM where a 2650 kv motor will spin ( 2650 x 14.8 ) 39,220 RPM. Both of these values are the unloaded RPM output of the motor at full speed. As you can see, the higher kv motor spins a higher amount of total RPM.
What does higher Output RPM Contribute to?
If we want our RC car to go faster, we simply need the tires to rotate faster. A faster spinning motor surely makes sense if we are trying to get higher axle speed out of the RC car. However, the higher output RPM that we get out of the motor does not come for free. Higher rotational motor speeds suggest that the motor can make more power, but where does this power come from? Power in watts is equal to voltage multiplied by current. We know that voltage in our example above is held constant, we are not changing it. The only variable that can increase in order to make sense in our power formula is current. Current by rise.
It makes sense for current to increase. An easy way to understand when current is increasing is to consider the load that the motor would be under. Higher load translates to higher current being drawn from the motor. Asking our motor to spin faster increases the amount of work that motor can do. Since we did not change the gearing on our RC car to maintain the same speed (want to go faster) the motor must be under additional load increasing the current.
Answering the question in the header above, higher RPM output of the motor contributes to a higher potential power output. If the speed of the RC car is increasing we know it is under additional load, making more power.
Does this Mean higher kv makes more Power?
Looking at the result makes it apparent that higher kv motor does increase the axle speed of our RC car. Higher axle speeds do increase the overall RC car speed. The simplistic answer is yes, higher kv does increase the speed of your RC car. But there is a catch.
Here is where the problem lies
What if I told you that in the above example, the 2200kv motor was almost at its maximum thermal limit? Meaning it runs about as hot as it can get away with. Does this change anything for you? Well, what if I mentioned that the 2650kv motor in the above example is actually a smaller motor. Length of the 2650kv motor is 68mm long vs 75mm long for the 2200kv motor. Now does this change anything for you?
We should have a change of heart on what motor we want to use in our example RC car. If we moved to the 2650kv motor, we would certainly let the magic smoke out, rendering the motor useless to us.
How to Select the Best Option for Increasing Speed
Knowing that in our example the motor is at its maximum thermal limit means one thing for sure. We must find a larger motor to cope with the increase in power that we wish to get out of it. Going faster means more power resulting in more waste heat the motor has to get rid of. Moving to a larger motor will help dissipate this waste heat. Once we select a motor model that is physically larger, we now have options.
Increasing the kv of the Motor to go Faster!
If you can spin the motor faster without going over a maximum rotational motor limit, increasing the kv is OK. Just make sure you aren’t going to increase kv by a ridiculous number. Stay within a total maximum unloaded RPM range that makes sense for your RC. This is by far a frequently made mistake by RC hobbyists. Find out what RPM range your RC requires and stay within that range!
Change Gear Ratio to go Faster!
If you moved up to the next available size motor, you could simply select the exact same kv option as your smaller motor. To increase the speed of the axle, just adjust the gear ratio. A larger pinion gear and / or a smaller spur gear, will allow you to change the final drive ratio resulting in faster speeds! Make sure that you set your gear mesh correctly to reduce wear and maximize performance.
Increase Voltage to go Faster!
Another avenue you may be able to take is increasing the amount of cells you plan to use. One thing to keep in mind if you do plan to increase voltage is the limits. Your motor/ESC must be capable of handling the increase in voltage. In addition if you aren’t changing the gear ratio of your RC car, current will also increase. Make certain that your setup can handle the large increase in power.
Conclusion
It is a lot more accurate to think in terms of increasing your power output of the motor in order to go faster. Simply trying to increase the kv of a motor does not paint the whole picture. It is a very easy and often misleading simplification that can get you in to trouble. First, find out where your setup is at thermally. (A motor that is running cool can simply benefit in a speed increase by just swapping gears ) Then decide which avenue you would like to try. There are options, you don’t have to go straight to a kv change.