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I have an engineering background however I am new to motors. I am looking to understand what the options exist for maximizing the top speed a motor can produce (say for an RC vehicle) without simply using the "brute force" option of "buy a bigger, heavier, more expensive motor with a higher number of maximum RPMs". Because after a point there is a step function in terms of size and cost that make selection impractical depending on application.

Seems like there is plenty of motorization in the world, so this has to be a solved problem--I just need some pointers to pursue learning more. Specifically: what options exist to maximize motor output for speed? What are the advantages and disadvantages of each?

All useful help appreciated--thank you!! Happy to clarify if anything is not clear.

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  • $\begingroup$ Yes, appropriate gearing is the right answer to your question. With that said I'm afraid that questions which are as broad as this are off-topic because answers would need to be too long. As it says in How to Ask If you can imagine an entire book that answers your question, you’re asking too much. We prefer practical, answerable questions based on actual problems that you face, so please take a look at How to Ask, tour and the Robotics question checklist for advice on writing a good question. $\endgroup$
    – Tully ♦
    Nov 22, 2022 at 17:53
  • $\begingroup$ Hi @Tully, yes I partially agree with you and I am not stranger to the Stack Exchange communities. I shared some of those same concerns from the outset (which is partly why I provided some edits and updates to the OP), however I will say that sometimes all you have is straws to grasp at when learning and there needs to be room for those sorts of questions, too. If you see something flagrant in the checklist, let me know (maybe you had something specific in mind). It also turned out my question and answer are both somewhat theoretical in nature and address concepts with very practical impacts. $\endgroup$
    – tniles
    Nov 23, 2022 at 18:57
  • $\begingroup$ I'm glad that you found your answer for your usecase. For reference though there are a lot of cases when picking the right transmission will be much better than just throwing a bigger motor at it. This is a large part of mechanical design, and there are whole courses on it. Which is why trying to summarize it here in one question is effectively impossible. $\endgroup$
    – Tully ♦
    Nov 24, 2022 at 9:15
  • $\begingroup$ @tully could you share some of those resources? Or at least a useful starting point? Seems like, if you were to write an answer, it would have to do with transmissions. $\endgroup$
    – tniles
    Nov 28, 2022 at 19:34
  • $\begingroup$ Terms to look for that might help you get into this area would be motor design and sizing, transmission and linkage types and optimizations. There's some good comments from Chuck below too. $\endgroup$
    – Tully ♦
    Nov 28, 2022 at 21:50

1 Answer 1

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Through my own research since originally posting I have learned what seems to be a rather practical answer. While one could provide some sort of a mechanical advantage in conjunction with a motor, the reality is that it would be less efficient than simply using a larger motor.

The tradeoff is essentially that losses of a larger motor are merely electrical in nature, whereas the losses of nearly any conceivable mechanical advantage (for a specific example, let's suppose a step-up gearbox) will likely be in the form of heat loss. Another potential "loss" could be in quality or reliability issues due to introducing more moving parts into the system. So at the end of the day, especially from a systems-level perspective, trading up in motor size, construction, or performance will almost always provide the better option. I say "almost always", because there very well could be an overly specific application out there where the only option is to pursue a mechanical advantage (such as if you had no control over choice of motor, or other odd physical space constraints).

In conclusion, the old adage still applies: sometimes all you need is simply a larger "hammer".

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  • $\begingroup$ There are many use cases that using a direct drive are not the most "efficient" depending on your use case and what are the most important design factors. Some examples such as robot arms and legs are almost never direct drive because using mechanical advantage or gearboxes is more efficient. $\endgroup$
    – Tully ♦
    Nov 24, 2022 at 9:18
  • $\begingroup$ I think it's a hard disagree here for me. Assuming the mechanical power of your motor is limited, you can maximize the top speed of a vehicle by increasing the wheel radius. This is because path length $s = r\theta$, and so vehicle speed $\dot{s} = r\dot{\theta}$. You get more distance per revolution if you get a larger radius. However, because torque is $\tau = r\times F$, if you have a motor power $P = \tau\dot{\theta}$, the propelling force you can apply for a given speed drops as the wheel radius increases. This means there's a tradeoff between top linear speed and acceleration. $\endgroup$
    – Chuck ♦
    Nov 28, 2022 at 20:38
  • $\begingroup$ Also hard disagree with your statements on motor losses being "merely electrical in nature." Blindly upgrading the motor can definitely be an option, but motors output higher power at higher rpm. Putting a larger motor in to boost the low-rpm power output is buying more motor than you need - and comes with the need to provide a high-power controller, larger batteries (higher C rating/power output), larger gauge wires, etc. There is a way to engineer it - create a representative drive cycle, set speed and acceleration specifications, and design the system from there. $\endgroup$
    – Chuck ♦
    Nov 28, 2022 at 21:33
  • $\begingroup$ The other adage: "When all you have is a hammer, everything looks like a nail." $\endgroup$
    – Chuck ♦
    Nov 28, 2022 at 21:35
  • $\begingroup$ @Chuck lol yes familiar with that adage, too. Which, oddly enough, was one element of what precipitated my question! $\endgroup$
    – tniles
    Nov 29, 2022 at 21:46

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