bsodmike
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Hi all,
This topic has been discussed ad nauseam but do bear with me as I'm interested in both the theoretical and practical aspects.
There are many guides that talk about gearing, wheel circumference, and a calculated top-speed.
To start off, I'm going to take my Kyosho as a base reference running Bad Lands, with an OD of 117 mm. I'll be using the x10^(-3) notation to refer to 'mm', so 117x10^(-3) m.
Circumference is calculated as 2πr (r=wheel radius) or πd (d=diameter). So our circumference is π117x10^(-3) m. It's a direct pinion/centre diff (spur) setup, so the FDR is simply 12T pinion:46T spur which works out as 46/12 = 3.83 rounded to 2 d.p. This means every 3.83 revolutions of the pinion, the Spur makes a single revolution. The link I posted at the beginning has got this fact wrong in their article; the pinion is smaller, it has to turns many more times to get the spur to complete one rev. So 1 motor rev = 1/3.83 spur revolutions.
Q1. Does one complete revolution of the center CVD-axels translate to a full-revolution at the wheel knuckles? I'll check into this tomorrow.
Roll out is defined as wheel circumference/gear ratio. Therefore this can be calculated one of two ways:
1. π117x10^(-3) m/3.83 = 95.97 mm/motor rev roll out
2. π117x10^(-3) m*(12/46) for the same result. This is just simple algebra, i.e. x/(1/2) = 2x
I also thought of the 'ideal' case without any gearing at all, i.e. the pinion is the same size as the spur, a 1:1 gearing. This works out to 367.57mm/motor rev roll out. 367.57/3.83 = the result we got previously, so that's a decent check.
I won't get into the maths on the theoretical top-speed calculation but it works out 132.23 mph running 2,200 kV * 16.8V(4S on full-charge) = 36,960 motor RPM. However, I will note that it means the spur gear/centre axels will rotate at 36,960/3.83 = 9,650 RPM.
From the various articles I've searched to forum posts I've seen, there has been no reference to wheel mass, inertia, and the energy required (torque) to rotate the wheels from rest. The RC mass would also play into such considerations.
I've seen comments for increasing the tooth-count on the pinion gear have people say 'you're trying to run it at too high an RPM, hence it's getting hot'. Say going from 12T to 19T pinion - less torque, more-top speed.
Q2. Why does gearing the pinion lower cause the motor to get hotter, doesn't the RPMs we demand via the throttle determine the motor RPM? That said, one way to look at this is in terms of work-done and roll-out. A longer roll-out (say we increase the size of our tire's OD) means the RC will travel a longer distance per motor rev. Work = Force*distance, therefore the motor has to do more 'work' per rev. Thoughts?
It also makes me thinking about motor physics; starting a motor on a high-load needs a high in-rush current. Once a BL motor reaches continuous velocity (say you're cruising at constant speed) then the current-draw is minimal; these surge current spikes will happen when we blip the throttle to max! The ESC is a limiting factor in terms of max current available to the motor and this would be the limiting point as to the max weight of the buggy/wheels a BL motor could power.
I guess I'm interested in more (theoretical) details on the 'limits' and factors resulting in hotter motors. Practical details from our veteran RC'ers is also much valued.
Thanks again for your time.
Cheers M.
This topic has been discussed ad nauseam but do bear with me as I'm interested in both the theoretical and practical aspects.
There are many guides that talk about gearing, wheel circumference, and a calculated top-speed.
To start off, I'm going to take my Kyosho as a base reference running Bad Lands, with an OD of 117 mm. I'll be using the x10^(-3) notation to refer to 'mm', so 117x10^(-3) m.
Circumference is calculated as 2πr (r=wheel radius) or πd (d=diameter). So our circumference is π117x10^(-3) m. It's a direct pinion/centre diff (spur) setup, so the FDR is simply 12T pinion:46T spur which works out as 46/12 = 3.83 rounded to 2 d.p. This means every 3.83 revolutions of the pinion, the Spur makes a single revolution. The link I posted at the beginning has got this fact wrong in their article; the pinion is smaller, it has to turns many more times to get the spur to complete one rev. So 1 motor rev = 1/3.83 spur revolutions.
Q1. Does one complete revolution of the center CVD-axels translate to a full-revolution at the wheel knuckles? I'll check into this tomorrow.
Roll out is defined as wheel circumference/gear ratio. Therefore this can be calculated one of two ways:
1. π117x10^(-3) m/3.83 = 95.97 mm/motor rev roll out
2. π117x10^(-3) m*(12/46) for the same result. This is just simple algebra, i.e. x/(1/2) = 2x
I also thought of the 'ideal' case without any gearing at all, i.e. the pinion is the same size as the spur, a 1:1 gearing. This works out to 367.57mm/motor rev roll out. 367.57/3.83 = the result we got previously, so that's a decent check.
I won't get into the maths on the theoretical top-speed calculation but it works out 132.23 mph running 2,200 kV * 16.8V(4S on full-charge) = 36,960 motor RPM. However, I will note that it means the spur gear/centre axels will rotate at 36,960/3.83 = 9,650 RPM.
From the various articles I've searched to forum posts I've seen, there has been no reference to wheel mass, inertia, and the energy required (torque) to rotate the wheels from rest. The RC mass would also play into such considerations.
I've seen comments for increasing the tooth-count on the pinion gear have people say 'you're trying to run it at too high an RPM, hence it's getting hot'. Say going from 12T to 19T pinion - less torque, more-top speed.
Q2. Why does gearing the pinion lower cause the motor to get hotter, doesn't the RPMs we demand via the throttle determine the motor RPM? That said, one way to look at this is in terms of work-done and roll-out. A longer roll-out (say we increase the size of our tire's OD) means the RC will travel a longer distance per motor rev. Work = Force*distance, therefore the motor has to do more 'work' per rev. Thoughts?
It also makes me thinking about motor physics; starting a motor on a high-load needs a high in-rush current. Once a BL motor reaches continuous velocity (say you're cruising at constant speed) then the current-draw is minimal; these surge current spikes will happen when we blip the throttle to max! The ESC is a limiting factor in terms of max current available to the motor and this would be the limiting point as to the max weight of the buggy/wheels a BL motor could power.
I guess I'm interested in more (theoretical) details on the 'limits' and factors resulting in hotter motors. Practical details from our veteran RC'ers is also much valued.
Thanks again for your time.
Cheers M.
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