When using designing and using RC motors bike drives, it is important to understand what the throttle does, and how power is controlled.
First the throttle. I always thought the throttle controlled motor voltage. Experience has taught me otherwise. An example:
Where I live there is a 3 mile walk/bike path, along the ocean. Often, I want to ride slow, and enjoy the sights. Just instinctively. I go to a low gear and constant (fixed) partial throttle. If the throttle was controlling voltage, let's say at half throttle, the motor would go to half full rpm and stop accelerating. But I noticed that the motor goes to full rpm.
Leaving out all the technical details, what I'm actually doing is controlling the drive output power. The bike accelerates at constant rate until the motor back emf nearly equals the battery voltage, then the output power drops, and the speed stabilizes. If a grade is encountered (fixed throttle), the input power increases, with little difference in speed (output power). Its like cruise control. That is, if the power required is within the dialed in amount, the input power will adjust itself. If the grade requires more power than is dialed in, the bike will slow until the drive power required for climbing the grade equals the power dialed in. The input power will rise to maintain the output power, plus inefficiencies. In other words the throttle controls output, or drive power.
Bike riders are usually unaware of all this, and are perfectly happy. But, there is a more subtle form of this phenomenon that does matter, and can result in unhappy riders, with broken drive systems (burnt motors).
Some bench tests I ran illustrates this phenomenon. I have a motor/gearbox test stand that mounts the RC drive, which then turns a 26 inch rear wheel thru an 8 speed dérailleur. The wheel rolls on a Power Tap Pro Trainer, that can apply a controlled constant load (watts). I ran one test ( all tests were run at full throttle) that used a low derailleur gear (motor turning near max) and dialed in a 400 watt load. The speed was about 17 mph. The input power during this test was 500 watts, for an overall efficiency of 80%. So far so good. For the next test, I shifted up 2 or 3 gears, left the load power at 400 watts. For this test the motor was spinning much slower, but the bike speed was about the same. The input power was surprising. The input power went up to 730 watts at the same output power, for an overall efficiency of 55%. 30 seconds into the test, I smelled hot dust and oil, and shut it down. The motor was smoking hot. But why not, the motor drive power dissipation went from 100 watts to 330 watts.
The latter condition is similar to a hub motor running at low speed up a steep hill. The hub motor is also running inefficiently, but it has a lot of thermal mass and surface area, so it is going to get hot slower, and stabilizes at a lower temperature than the RC motor. A RC drive with a fixed drive ratio that is run under these conditions, that is run at low motor speed, with high load, will provide the power, but will heat up very quickly, and get very hot.
When designing a fixed drive ratio RC drive system, the design constraints have to be the steepest grade anticipated, weight of bike and rider, and the power dissipation limits of the motor (with attached structure), not the desired top speed on the flat. The speed up the grade (and on the flat) is set by these constraints, along with the parameters of the electronic components (battery, controller, motor, etc.).
If you have a thru the gears RC drive you should select a gear that keeps the motor spinning at high rpm for maximum efficiency.
These examples doe not take into account pedal power, which in all cases, lessens the load on the motor.
I hope this helps.
First the throttle. I always thought the throttle controlled motor voltage. Experience has taught me otherwise. An example:
Where I live there is a 3 mile walk/bike path, along the ocean. Often, I want to ride slow, and enjoy the sights. Just instinctively. I go to a low gear and constant (fixed) partial throttle. If the throttle was controlling voltage, let's say at half throttle, the motor would go to half full rpm and stop accelerating. But I noticed that the motor goes to full rpm.
Leaving out all the technical details, what I'm actually doing is controlling the drive output power. The bike accelerates at constant rate until the motor back emf nearly equals the battery voltage, then the output power drops, and the speed stabilizes. If a grade is encountered (fixed throttle), the input power increases, with little difference in speed (output power). Its like cruise control. That is, if the power required is within the dialed in amount, the input power will adjust itself. If the grade requires more power than is dialed in, the bike will slow until the drive power required for climbing the grade equals the power dialed in. The input power will rise to maintain the output power, plus inefficiencies. In other words the throttle controls output, or drive power.
Bike riders are usually unaware of all this, and are perfectly happy. But, there is a more subtle form of this phenomenon that does matter, and can result in unhappy riders, with broken drive systems (burnt motors).
Some bench tests I ran illustrates this phenomenon. I have a motor/gearbox test stand that mounts the RC drive, which then turns a 26 inch rear wheel thru an 8 speed dérailleur. The wheel rolls on a Power Tap Pro Trainer, that can apply a controlled constant load (watts). I ran one test ( all tests were run at full throttle) that used a low derailleur gear (motor turning near max) and dialed in a 400 watt load. The speed was about 17 mph. The input power during this test was 500 watts, for an overall efficiency of 80%. So far so good. For the next test, I shifted up 2 or 3 gears, left the load power at 400 watts. For this test the motor was spinning much slower, but the bike speed was about the same. The input power was surprising. The input power went up to 730 watts at the same output power, for an overall efficiency of 55%. 30 seconds into the test, I smelled hot dust and oil, and shut it down. The motor was smoking hot. But why not, the motor drive power dissipation went from 100 watts to 330 watts.
The latter condition is similar to a hub motor running at low speed up a steep hill. The hub motor is also running inefficiently, but it has a lot of thermal mass and surface area, so it is going to get hot slower, and stabilizes at a lower temperature than the RC motor. A RC drive with a fixed drive ratio that is run under these conditions, that is run at low motor speed, with high load, will provide the power, but will heat up very quickly, and get very hot.
When designing a fixed drive ratio RC drive system, the design constraints have to be the steepest grade anticipated, weight of bike and rider, and the power dissipation limits of the motor (with attached structure), not the desired top speed on the flat. The speed up the grade (and on the flat) is set by these constraints, along with the parameters of the electronic components (battery, controller, motor, etc.).
If you have a thru the gears RC drive you should select a gear that keeps the motor spinning at high rpm for maximum efficiency.
These examples doe not take into account pedal power, which in all cases, lessens the load on the motor.
I hope this helps.
