Adding more motors for more torque and speed?

[jam8280]

You've touched on a very complex topic with no easy answers. But there are some valuable rules of thumb. At the risk of getting too technical and way too long-winded, I'll share what I've learned from real-world engineering and from extensive experience and experimentation with working LEGO gizmos of all kinds, including vehicles operating on land and in the water. I'll focus on vehicles here.

May have missed it, but I didn't see anyone mention the critical role of mechanical power in a vehicle's top speed. In short, a vehicle reaches its top speed when the mechanical power collectively delivered by all the motors equals the power consumed by all losses between the motors and vehicle's interfaces with the real world (e.g., the ground and the air). That's because one way or another, total loss invariably grows with both speed and total vehicle mass. Once this power balance is reached at full throttle, no further acceleration is possible, and speed levels off.

So, there are 2 main strategies for increasing top speed: (A) Know your batteries, RC receivers, and motors and increase the total mechanical power coming out of the propulsion motors where you can. (B) Know your losses and reduce them where you can -- starting with the biggest losses -- while being mindful of the trade-offs involved at every step. Understanding all the losses in play and which ones are worthy of attack gets technical in a hurry, but Wikipedia is a great resource.

(A) On the motor end, the goal is to arrange -- through gearing and other choices -- for each and every propulsion motor to be operating near the peak of its power-speed curve as the vehicle approaches top speed. For reasons I won't go into, you'll know that you're near peak power in a LEGO motor when its shaft is turning at ~50% of its no-load speed (NLS) with the battery/receiver combo you'll be using. The good news: The power vs. speed peak is broad enough that 40-60% of NLS will be good enough.

If you do nothing else to maximize top speed, do this while trimming weight where you can!

For example, the NLS of an XL at 7.4V (PF LiPo battery voltage) is ~180 RPM. So you'll want that XL to be running at 72-108 RPM when the vehicle hits top speed. If it takes more gears to bring this about, so be it. The triple-screw powerboat below reached its highest top speed (~1.1 m/s) when each XL was turning at 90-100 RPM when the speed flattened out. That required a 3-stage 1:8.33 overdrive transmission between each XL and its prop, for a total of 3 gear pairs per prop. When I used fewer gears, the boat when slower. 

NB: Finding the motor/gearing/prop or motor/gearing/wheel combo that maximizes top speed in a particular vehicle is an iterative process based on exhaustive guess-and-check testing, changing only one thing (say, final drive ratio, or prop or wheel diameter) per trial. You'll need a cheap laser tachometer to get at motor shaft speeds and a way to measure or at least rank top speed results as well.

(B) When taking stock of your losses and what you can realistically do about them, it's helpful to divide them into internal and external losses.

The internal power losses in LEGO vehicles have several important sources: (i) Gear-to-gear and shaft-to-bearing friction -- the latter especially at the drive wheel or prop bearings. (ii) Continuous distortion of axles that bend or twist while turning under load. (iii) Unwanted motions of motor and bearing mounts during operation. So, use the fewest gears possible (preferably double-bevels) to attain the optimal motor shaft speeds at vehicle top speed (see above). Keep axles as short as possible and reinforce the ones that have to be long with bushes or axle joiners. Make all motor and bearing mounts as rigid as possible while keeping an eye on total mass. These measures are often worth the added mass that most entail. Count on having to run top speed trials to find the sweet spot in the rigidity vs. structural mass trade-off.

The external losses come mainly from total rolling resistance (not the same thing as friction) , water resistance, and even air resistance. Rolling resistance grows with weight and surface softness but generally varies little with speed. It increases sharply with tire or tread sinkage into the ground and to a lesser extent, with sidewall flex. Water and air resistances, on the other hand, grow very rapidly with speed. Water resistance also grows quickly with displacement (total mass), but displacement vs. resistance trade-offs in boat top speed are very complex.

Finally, whatever you decide to do about top speed, test, test test! The trade-offs involved are seldom straightforward and often counter-intuitive.

Edited August 17, 2017 by jam8280