Full Speed Ahead!

[amorti]

Flicking through the motors page at @Philo's site, this caught my eye:

Because rpm/torque curve is linear a motor provides maximum power when load slows it down to half of no-load speed

https://www.philohome.com/motors/motorcomp.htm

Now I'm a novice in this stuff, so hoping maybe Philo or someone else knows more and could explain.

If you have a battery and motor which output (for the sake of easy Maths) 1,000rpm unloaded which translates through the selected gears and tires to 50km/h, does the above mean you're more likely to actually hit 50 by gearing for 100?

[GerritvdG]

2 hours ago, amorti said:

does the above mean you're more likely to actually hit 50 by gearing for 100?

I'm no expert in this field, however, the gearing itself will make the drive train less efficient, so the power will be lost in the resistance of the gearing.

A solution would be to use big wheels 

[amorti]

Yes but there are always compromises. With big wheels and no gears comes a high CoG, and a large gyroscope which is fine until you hit a bump and it starts to oscillate.

[m2fel]

I am not sure but with the different gearing you double the rpm at the wheel but cut the torque in half. Even if the motor has its peak power at half the none loaded rpm the 50% loss in torque won't be compensated.

Another problem is you can't just go from 50km/h to 100km/h. You have to hit the sweet spot where the motor rpm is actually 50% of the none loaded rpm.

[allanp]

Interesting question. What if you geared the car to 100, and that works fine with the wheels spinning in the air, but you have so little torque the car doesn't move at all when you put it down? So I guess we assume Philo's take is correct (which I think it is) but we also assume that the achieved top speed is directly proportional to the power extracted from the motor by loading it via gearing (this I'm not entirely sure is accurate but let's assume it is). My gut feeling tells me that would only hold true when you have a motor that produces exactly the right amount of horse power to reach 50kp/h, no more or less. If you have less power required to reach that speed then of course you won't ever reach that speed, but some other slower speed (let's say the motor can make it go 40kp/h max), in which case you'd have to gear to twice that new lower speed (80kp/h), in order load the motor to 50%, to get 40kp/h, which in this case is as close as possible for this motor to get to the original 50kp/h target. Any higher or lower gearing in this case would result in the motor not spinning at 50% no load speed. So you're more likely to achieve your goal by loading the motor to half no load speed via gearing, but whatever speed that might be isn't necessarily going to be the speed target you set for yourself.

Edited April 29, 2022 by allanp

[Philo]

I don't have a definitive answer as it depends on so many parameters... I have the feeling that the 50% motor load will give the best compromise between acceleration to reach speed and top speed, but not necessarily the best top speed!

[Jurss]

I remember, there was topic of fastest (legal) lego car. It was really hard to reach some 40km/h , as significant role started to play aerodynamics.

It would be interesting to try now BMW tires. They could provide some advance to reach speed record.

One of topics

[MOC] 34,8 km/h worlds fastest LEGO-car (2 motor version) - LEGO Technic, Mindstorms, Model Team and Scale Modeling - Eurobricks Forums

[m2fel]

This question bugs me a lot and a looked a little bit into it. Again I am no expert. Your looking at this in the wrong way. Changing the gearing with the same motor doesn't help as you will loose torque. You would need another motor with the same characteristics but a none loaded rpm of 2000 instead of BUT the problem is that at 50km/h you don't have the stalled current it is the point where the motor has not enough power to further accelerated but keep the speed thus the whole thing does not work... 

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