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The Gearing Advantage
Let's start with a few assumptions...
First let's say that you are racing in something like NEDRA where the voltage is limited for your class and you have to run a DC motor of some kind. You've done everything possible to create or buy a motor that delivers good performance. You've decided that you want range as well so you've made certain that the efficiency peak is equal to the power peak in your motor. You did this because this represents the best that a motor can ever achieve. While there are any number of alternative configurations (like allowing a very high current limit) for this study we are just trying to establish a "baseline" upon which we can compare results. Every change that can be done to the motor simply adjusts our baseline... the final result should still be the same.
For this theoretical experiment I used a little tiny 250 watt motor and pumped up the voltage to a ridiculous 96 volts. The current limit is set at 30 amps. Are these the only numbers I could have used? No, but again we have to agree on some baseline and so for this case this is it. (this motor would most likely burn up at this voltage)
Looking at the chart below we see the difference in power output at the range of speeds that the motor can cover. It's top speed is something like 56 mph for this demonstration and so the fixed gear bike simply has the bike stuck in 8th gear all the time. The multiple speed bike has 8 speeds and can use the appropriate gear for any given speed.
The results are interesting. For the exact same motor there is an overall 20% power gain using gears as opposed to using a fixed gear. This does not even begin to get into the issue of ability to climb a hill which is obviously going to favor the multiple speed solution. I'm also not dealing with efficiency which also should point towards an advantage for the multiple speed bike.
These power figures are "power available" at those speeds... it doesn't mean you need to use it all, but it means you have it there for you if you want it.
A reminder... this motor was configured for maximum efficiency... so the overall efficiency could not likely be improved upon significantly. (so all the solutions that involve increasing the current limit just end up creating more heat and driving down the range)
This is as they say: "As Good As It Gets".
Let's start with a few assumptions...
First let's say that you are racing in something like NEDRA where the voltage is limited for your class and you have to run a DC motor of some kind. You've done everything possible to create or buy a motor that delivers good performance. You've decided that you want range as well so you've made certain that the efficiency peak is equal to the power peak in your motor. You did this because this represents the best that a motor can ever achieve. While there are any number of alternative configurations (like allowing a very high current limit) for this study we are just trying to establish a "baseline" upon which we can compare results. Every change that can be done to the motor simply adjusts our baseline... the final result should still be the same.
For this theoretical experiment I used a little tiny 250 watt motor and pumped up the voltage to a ridiculous 96 volts. The current limit is set at 30 amps. Are these the only numbers I could have used? No, but again we have to agree on some baseline and so for this case this is it. (this motor would most likely burn up at this voltage)
Looking at the chart below we see the difference in power output at the range of speeds that the motor can cover. It's top speed is something like 56 mph for this demonstration and so the fixed gear bike simply has the bike stuck in 8th gear all the time. The multiple speed bike has 8 speeds and can use the appropriate gear for any given speed.
The results are interesting. For the exact same motor there is an overall 20% power gain using gears as opposed to using a fixed gear. This does not even begin to get into the issue of ability to climb a hill which is obviously going to favor the multiple speed solution. I'm also not dealing with efficiency which also should point towards an advantage for the multiple speed bike.
These power figures are "power available" at those speeds... it doesn't mean you need to use it all, but it means you have it there for you if you want it.
A reminder... this motor was configured for maximum efficiency... so the overall efficiency could not likely be improved upon significantly. (so all the solutions that involve increasing the current limit just end up creating more heat and driving down the range)
This is as they say: "As Good As It Gets".