Jethro56
100 W
Has anyone measured the amount of electrical power needed to overcome the drag of a direct drive hub? In other words How much battery power does it take to make a direct hub feel equal to a gear hub when it's freewheeling?
Battery power to overcome direct drive drag
- Thread starter Jethro56
- Start date
monkeybutt
10 mW
- Joined
- Nov 27, 2009
- Messages
- 20
Just a guess ,but what if you severely limited the Amps while keeping the voltage relatively high,would this not allow you to freewheel @ med. to high speeds?
dogman dan
1 PW
Not really, unless you are Lance Armstrong.
It may take much less than 50w to eliminate drag, but about 100 w eliminates some of the feel of the heavy motor and battery. FWIW, ever tried to hold your throttle on less than 150 w? Pretty much impossible with a 20 amp controller, unless you lowered voltage.
To freewheel at a high speed, you must give the motor enough watts to run at that speed, no load. As soon as you pedal faster than the no load speed, you start pushing the motor, and feel regen like resistance. So all you can do with dd, is set a speed, then pedal briskly to add 1-2 mph more. With a wattmeter, you see your pedal effort dramaticly lessen the watts used.
So you can ride fast, and pretty efficient with dd, but it still takes a minimum watts. Pedal hard, and it's possible to do 25 mph with 400 -500 watts, when you'd need 200 more to go that fast no pedaling. Bottom line, you'r effort is going to max out at about 200 watts. And now you are on a bike heavy enough to need a lot more watts to go somewhere.
It may take much less than 50w to eliminate drag, but about 100 w eliminates some of the feel of the heavy motor and battery. FWIW, ever tried to hold your throttle on less than 150 w? Pretty much impossible with a 20 amp controller, unless you lowered voltage.
To freewheel at a high speed, you must give the motor enough watts to run at that speed, no load. As soon as you pedal faster than the no load speed, you start pushing the motor, and feel regen like resistance. So all you can do with dd, is set a speed, then pedal briskly to add 1-2 mph more. With a wattmeter, you see your pedal effort dramaticly lessen the watts used.
So you can ride fast, and pretty efficient with dd, but it still takes a minimum watts. Pedal hard, and it's possible to do 25 mph with 400 -500 watts, when you'd need 200 more to go that fast no pedaling. Bottom line, you'r effort is going to max out at about 200 watts. And now you are on a bike heavy enough to need a lot more watts to go somewhere.
Did any one ever try to use to capacitors to overcome drag from a direct drive hub to store enough energy from breaking to allow the bike to freewheel for a while. Do I have something here? They charge faster than they discharge. It may work but im not electrical engineer so im just guessing here....anyone any thoughts?
No, capacitors charge at the same speed they discharge, typically, unless you aren't using hte energy from them and just letting it sit there. Plus like anything else, you have to input more energy than you can get back out of them.
wesnewell
100 GW
It's like this. As long as you are feeding power to the controller, there is no drag until you reach the point where the motor has actually turned back into a generator. If you can maintain that pace, you will recharge the battery, not use power from it. This is something you can't do with a geared motor since it freewheels once you do over the motors speed.
liveforphysics
100 TW
Lift the wheel up. Roll the throttle open very slowly so you can watch steady-state power vs speed.
Thats your no-load curve, and it's the power you need to overcome the wheel.
Thats your no-load curve, and it's the power you need to overcome the wheel.
dogman dan
1 PW
Revisiting this old thread, it's worth noting that holding a very low wattage on a bike that is medium wattage (600-1000w) is much easier if you have a low speed setting, like a three speed switch.
Put it in low speed, and it becomes a lot easier to hold a wattage on the throttle that is under 100w. The newer kits are starting to come out with little dashboards that include up to 5 assist levels. Very good for when you really need to get home on a near dead battery, or do a long ride on the minimum assist.
It's something well worth learning how to do, since the last 50wh of a battery can still creep you home with a lot less effort than pushing the motor, and more exhausting, the battery and motor weight if it's even a tiny bit uphill. Same for if it's upwind. a 50w assist sounds like nothing, but it's better than nothing.
Put it in low speed, and it becomes a lot easier to hold a wattage on the throttle that is under 100w. The newer kits are starting to come out with little dashboards that include up to 5 assist levels. Very good for when you really need to get home on a near dead battery, or do a long ride on the minimum assist.
It's something well worth learning how to do, since the last 50wh of a battery can still creep you home with a lot less effort than pushing the motor, and more exhausting, the battery and motor weight if it's even a tiny bit uphill. Same for if it's upwind. a 50w assist sounds like nothing, but it's better than nothing.
I thought I have seen on youtube some type of a capacitor that had a slow discharge. But I could be wrong. Could be charged in seconds but it would take 10-13min to discharge. It looked like a blue soda can. Again it was a while ago when I seen this video.... anyway it was just a thought
You can make any cap charge fast and discharge slowly, or vice-versa--but you do it by adding circuitry to it to slow the discharge down. That adds losses, space, expense, heat, etc.
Lots of caps look like blue soda cans, unfortunately.
If you want to ponder further about doing anything with caps, you're gonna want to read about how they work, and the different types, and how they store and discharge energy, etc. I'd start with Wikipedia for the basics, and then reading around ES in the many threads about using different types of caps for various projects.
Lots of caps look like blue soda cans, unfortunately.
If you want to ponder further about doing anything with caps, you're gonna want to read about how they work, and the different types, and how they store and discharge energy, etc. I'd start with Wikipedia for the basics, and then reading around ES in the many threads about using different types of caps for various projects.
Kingfish
100 MW
Creating the most efficient motor is every designers dream. 8)
I would agree with Luke on the in situ freewheel test, and add that if one could graph the result – the curve would fit the mathematical relationship of a typical Energy Balance equation.
To begin, the value to overcome direct drive drag is unique to each of us and relative to the equipment, environment, and style of riding. However we are able to develop a formula for calculating the losses of a system and thus estimate to a fine degree of accuracy the effects of drag.
The First Law of Thermodynamics deals with the conservation of energy, that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. Therefore we can state that the total Energy (E) of the system is equal to the amount of Heat (Q) and Work (W) added or removed.
In the case of the direct drive motor, we can define E as being the total Battery Power, W as horsepower/torque produced during freewheel (in the ideal) or watts/mile (in the practical application), and Q as friction, drag, waste heat.
For the direct Drive, Q can be resolved as Inertia, tire-to-pavement drag, EV aerodynamics which are notable and common to all, although more importantly are the losses relating to conversion of transmission of DC to the controller, DC to AC within the controller, transmission of AC to the Motor, AC to EMF, BEMF, eddy currents, and mechanical drag due to the physical assembly and contamination (effects of moisture on the bearings etc).
The Sum of these losses between the Battery and the shaft horsepower measured determines the efficiency of the System, and from that value we can determine the amount of battery power required over velocity range at STP. Aggregation of repetitive data provides a practical model against the ideal, and from that we are able to box in the amount of error; tolerance by another name.
Given that we are mainly DYI enthusiasts, the amount of variance between our rides is going to vary widely. Thus…
Without a formula, without tangible data, it is difficult to estimate the amount of “battery power to overcome drag”. The best measurement is the practical observation made by the observer at the helm over time upon the same or similar routes using the same equipment. By the same token, one could also determine the approximate range of their ride for said battery capacity. In my mind, the two goals go hand in hand. :wink:
In flux, KF
I would agree with Luke on the in situ freewheel test, and add that if one could graph the result – the curve would fit the mathematical relationship of a typical Energy Balance equation.
To begin, the value to overcome direct drive drag is unique to each of us and relative to the equipment, environment, and style of riding. However we are able to develop a formula for calculating the losses of a system and thus estimate to a fine degree of accuracy the effects of drag.
The First Law of Thermodynamics deals with the conservation of energy, that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. Therefore we can state that the total Energy (E) of the system is equal to the amount of Heat (Q) and Work (W) added or removed.
In the case of the direct drive motor, we can define E as being the total Battery Power, W as horsepower/torque produced during freewheel (in the ideal) or watts/mile (in the practical application), and Q as friction, drag, waste heat.
For the direct Drive, Q can be resolved as Inertia, tire-to-pavement drag, EV aerodynamics which are notable and common to all, although more importantly are the losses relating to conversion of transmission of DC to the controller, DC to AC within the controller, transmission of AC to the Motor, AC to EMF, BEMF, eddy currents, and mechanical drag due to the physical assembly and contamination (effects of moisture on the bearings etc).
The Sum of these losses between the Battery and the shaft horsepower measured determines the efficiency of the System, and from that value we can determine the amount of battery power required over velocity range at STP. Aggregation of repetitive data provides a practical model against the ideal, and from that we are able to box in the amount of error; tolerance by another name.
Given that we are mainly DYI enthusiasts, the amount of variance between our rides is going to vary widely. Thus…
Without a formula, without tangible data, it is difficult to estimate the amount of “battery power to overcome drag”. The best measurement is the practical observation made by the observer at the helm over time upon the same or similar routes using the same equipment. By the same token, one could also determine the approximate range of their ride for said battery capacity. In my mind, the two goals go hand in hand. :wink:
In flux, KF
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