How much current do you need to inject into a direct drive motor to simulate freewheeling?

[Bonzo Banana]

Just curious on the views on this. I think I read somewhere around 6-10W power depending on hub motor but I can't seem to see that information now. I wonder if there is some formula for working this out.

I think its Grin technology who state that a combination of injecting such current plus combining it with regenerative braking actually results in a overall improvement in range and riding experience on their direct drive motors but then they are not exactly unbiased. I think they may have been the source of the 6-10W figure.

To me that is a surprisingly low figure, would the basic direct drive hub motors, ebay sourced etc have higher current demands for this.

If you wanted to create a switch for freewheeling based on a resistor in the cable that bypassed the normal throttle what would such a circuit look like and would you need any more protective components in that circuit diodes or capacitors? I assume the throttle is just a giant potentiometer.

I could measure the range of that potentiometer and the wattage used to give me a rough idea of resistor value for freewheeling?

Has anyone else done this before as a experiment. I realise higher end motors allow freewheeling as a parameter adjustment in the firmware/software of the controller but not for more basic
controllers without such options.

 

[vfr400]

That sounds much too low to me. The no-load current is about 1 amp, depending on the winding speed, and probably half of that at 15 mph. Taking into account efficiencies, you'd have to put in the equivalent of about 0.375 amps. At 48v, that would be 18W.

Looking at it another way, a normal rider on a normal pedal bike outputs about 100w. It's very hard to pedal a DD bike without power at a normal cycling speed. that's partly because of resistance, but also because of weight. Comparing it with the results from my turbo trainer, I'd say equivalent to around 150w, so that means about 50w extra.If you're an 80kg rider, you'd have to pedal around 10% harder on any incline, just because of the weight.

 

[Bonzo Banana]

That sounds much too low to me. The no-load current is about 1 amp, depending on the winding speed, and probably half of that at 15 mph. Taking into account efficiencies, you'd have to put in the equivalent of about 0.375 amps. At 48v, that would be 18W.

Looking at it another way, a normal rider on a normal pedal bike outputs about 100w. It's very hard to pedal a DD bike without power at a normal cycling speed. that's partly because of resistance, but also because of weight. Comparing it with the results from my turbo trainer, I'd say equivalent to around 150w, so that means about 50w extra.If you're an 80kg rider, you'd have to pedal around 10% harder on any incline, just because of the weight.

That first paragraph sounds right. The second paragraph sort of sounds like you are taking the bike weight into account as well so almost like you are trying to calculate out the greater weight of the ebike as well as the freewheeling resistance if so that sounds right too at an extra 50W assuming that is what you meant. So I guess you could inject 50W or more to simulate having a zero weight ebike on the flats at least.

Thinking about it you could fit a secondary potentiometer on the bike not sprung return with a bypass switch on the normal throttle and have a dial with various markings on it like freewheel, zero weight bike on the flat, zero weight bike going up various gradients and then perhaps zero weight rider and bike over various gradients. I guess the final options might end up beyond the maximum power of the bike if its street legal.

I assume the resistance is relative to the size of the direct drive motor hub so those very small motor hubs that are direct drive with only 15Nm etc would give far less resistance than those larger direct drive motor hubs capable of 45Nm. So perhaps very small direct drive motor hubs would need a 6W injection compared to 18W for the larger hub motor? I see Grin technologies sell some very small direct drive hub motors so that might account for the 6W figure.

 

[vfr400]

That's right. it's the weight that makes the biggest difference. I wasn't sure whether your question was just about the motor resistance or the overall difference.

Riding my 22kg electric bike on level 1 (60 - 80w) takes about the same effort and goes about the same speed as my 8kg road bike. The road bike obviously has a wight advantage, which is very noticeable on hills, but it also has lower rolling resistance and lower air resistance.

 

[Bonzo Banana]

That's right. it's the weight that makes the biggest difference. I wasn't sure whether your question was just about the motor resistance or the overall difference.

Riding my 22kg electric bike on level 1 (60 - 80w) takes about the same effort and goes about the same speed as my 8kg road bike. The road bike obviously has a wight advantage, which is very noticeable on hills, but it also has lower rolling resistance and lower air resistance.

That's a useful comparison. Where would you put lets say a 14-15kg un-assisted mountain bike in there. I'm a heavy rider myself and don't feel soo much difference between bike weights as they become less significant because of my greater weight. When you are 120kg, 3kg difference on the bike becomes minimal especially as I can lose that weight in a week by a high level of exercise and reduced food consumption. Don't get me wrong the initial push off of the bike I can feel the lower weight but once I'm rolling the difference in weight doesn't seem as significant at all.

 

[vfr400]

Weight helps you going down hills. On the flat, it slows down your accelleration. It's only on up-hills, no matter how slight, that the weight has an immediate effect, where the effort to maintain any speed is more or less directly proportional to mass./weight.

An unassisted MTB at around 14kg would be about half way between the above two on weight, so if all were ridden uphill without power, it would be half way between with pedal effort but if the electric one engaged level one assist, it would be a very sad third; however, it could gain an advantage over the road bike on rough terrain or grass.