What's the most efficient speed?

[egroover]

I was out riding today on my yosepower conversion, cruising at about 18mph and pulling about 200w at the rear wheel. Hit the throttle and it upped it to 20mph but now pulling 400w.
It got me thinking, as in a car, the most efficient miles per gallon is at a steady 56mph, what's the most efficient speed or power draw to get the most miles on a charge assuming little or no effort from the rider?

 

[flecc]

I was out riding today on my yosepower conversion, cruising at about 18mph and pulling about 200w at the rear wheel. Hit the throttle and it upped it to 20mph but now pulling 400w.
It got me thinking, as in a car, the most efficient miles per gallon is at a steady 56mph, what's the most efficient speed or power draw to get the most miles on a charge assuming little or no effort from the rider?

It varies with the type of e-bike. It's easiest to know with hub motors where the point of maximum motor efficiency is commonly around 80 to 85% of maximum revs. That speed on a legal pedelec is around 12.5 to 13.5 mph where air resistance is very low, so a happy coincidence of factors.

That said, the Panasonic crank motored Giant Lafree was also very efficient, running at typically 12 to 13 mph average the consumption was very low at about 7.8 Wh per mile for most users. When we've surveyed pedelec consumption previously, it's most commonly around 12 Wh per mile, but the greediest e-bikes hitting 24 Wh per mile.
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[Nealh]

The most efficient speed is the one you can maintain above the cut off speed, then you will be only inputting your own watts.
Today I covered 162km/101 miles with my Yose powered Boardman and according to the watt meter I managed a respectable 9wh/m, my cut off is 33km/h / 20.5mph which is when I see the watts go to <10w. With the 25a controller I set the amps to 22.5 and pull about 95-105w in PAS1, Ideal I find for flat windless riding.

 

[vfr400]

I was out riding today on my yosepower conversion, cruising at about 18mph and pulling about 200w at the rear wheel. Hit the throttle and it upped it to 20mph but now pulling 400w.
It got me thinking, as in a car, the most efficient miles per gallon is at a steady 56mph, what's the most efficient speed or power draw to get the most miles on a charge assuming little or no effort from the rider?

With a hub motor, and the controller set to any one level, the faster you go, the less battery you use, so select level 1 and pedal as hard as you can if you want your bike to go a long way before the battery goes flat. I'm assuming that you have the controller set to current control, which gives a constant current regardless of speed up to the point that the back emf ramps down the current, and that's all using the pedal sensor, not the throttle.

With the throttle only (no pedalling), the slower you go, the less power you use, but not on hills or into strong winds. each set of external conditions will have a different optimum speed, where you're balancing efficiency, power and speed.

 

[egroover]

Thanks guys, really informative. So it sounds like the best balance for speed vs efficiency/range is maintaining a speed of about 12-13mph...obviously any rider peddling effort will offset battery usage. I know that the biggest hit on effort is overcoming wind resistance, so it was interesting to see the effect of that today on my ride when increasing speed by 2mph with an extra 200w needed at the hub to maintain that speed.
I'm looking to do a European tour at some point, I want to cover 90-100 miles per day with a couple of 36v/13ah batteries, Belgium, Holland, Germany etc, so mainly flat, I think it's doable, so will practice maintaining that speed to get the range I need
Cheers

 

[Andy-Mat]

I was out riding today on my yosepower conversion, cruising at about 18mph and pulling about 200w at the rear wheel. Hit the throttle and it upped it to 20mph but now pulling 400w.
It got me thinking, as in a car, the most efficient miles per gallon is at a steady 56mph, what's the most efficient speed or power draw to get the most miles on a charge assuming little or no effort from the rider?

You are very wrong about cars, each model of engine and body has its own most efficient point on the speed scale. It is not a fixed speed for all cars as you seem to imagine.....
With electric bikes, as they don't have inefficient IC engines, they are almost the same efficiency for any speed, only wind resistance changes the amount of power needed..... In a vacuum, with very minor changes of power needed, any speed would be of a similar efficiency.
The only difference might be how hard you accelerated....and for how long, to get to the wished for speed.
Lying down on your bike should demonstrate this needs less power than sitting up....
regards
Andy

 

[danielrlee]

Something that rarely gets a mention is that battery range and electrical efficiency is not the same thing.

To maximise battery range, you should run your motor at the lowest possible power level, while inputting as many human watts as possible.

Electrical efficiency (heat per unit of input energy) is dependent on specific motor/system parameters, but is usually highest approaching no-load speed.

 

[Woosh]

the most efficient speed depends on your target: most range, most speed or a combination of the two.
If you want best range, follow NealH's advice. If you want best speed (derestricted), you need a crank driven bike with rigid fork and thin tyres, if you want a combination of the two, you need a bike that you can set the pedal assist to zero and use throttle control for assist like on the Woosh Karoo or convert a road bike with the 48V TSDZ2T crank drive torque sensor motor fitted with a throttle like this one:
http://wooshbikes.co.uk/2018/tsdz2/giant.jpg

 

[Nev]

I was out riding today on my yosepower conversion, cruising at about 18mph and pulling about 200w at the rear wheel. Hit the throttle and it upped it to 20mph but now pulling 400w.

Something does not seem quite right with these figures. To increase the speed by a little over 10% (18 to 20 mph), requires a 100% increase in power (200 W to 400 W).

I know overcoming wind resistance becomes increasingly important as speeds increase, but I would be surprised if a 10% increase in speed needs a 100% increase in power when we are talking of speeds of 18 and 20 mph.

My guess would be to go from 18 mph to 20 mph would need an increase in power of about 30% but if someone has one of those on line calculators to hand and can work it out please post the results.

 

[Woosh]

Something does not seem quite right with these figures. To increase the speed by a little over 10% (18 to 20 mph), requires a 100% increase in power (200 W to 400 W).

I know overcoming wind resistance becomes increasingly important as speeds increase, but I would be surprised if a 10% increase in speed needs a 100% increase in power when we are talking of speeds of 18 and 20 mph.

My guess would be to go from 18 mph to 20 mph would need an increase in power of about 30% but if someone has one of those on line calculators to hand and can work it out please post the results.

most of the energy is used to overcome air resistance, about 5%-10% on friction with the road surface, joints and ballbearings.
for the non-technical person, all the calculations are done for you here:
https://www.ebikes.ca/tools/simulator.html
You input your weight, bike weight, road gradient and the tool calculates the load versus speed for you. The site explains also the assumptions on Cd (air resistance) and fomulae they use for their code.
For an average European rider (80kg + 20kg bike) wearing summer clothing, the load is 180W at 15.5mph. If you input half of that (eco mode setting on your LCD), the motor only needs to produce 90W.
at 18mph, load is 255W
at 20mph, load is 333W
Power required is proportional to speed power 3 (cubed).

 

[Nealh]

The 100% increase to achieve more speed( even 2mph more) is easy to do, it just means the bikes drive is doing all the work while you are simply applying little in the way of watts.
The ultimate speed will depend on the motor rpm with a throttle, a fit rider though can produce a faster speed after the motors maximum has been reached by simply applying more watts by sheer pedalling.

 

[Nev]

I just tried an on line simulator (this was for conventional bikes not pedal assist) and kept all the parameters the same except increasing the required speed from 18 to 20 mph. The power required went from 182.7 W to 237.7 W, so an increase of just over 30%. This was the type of increase I would expect, I am not sure how the OP is seeing an increase of 100%, anyone got any ideas?

 

[Nev]

The 100% increase to achieve more speed( even 2mph more) is easy to do, it just means the bikes drive is doing all the work while you are simply applying little in the way of watts.

Ah I think I get it now. So when the OP was doing 18mph he was doing a fair amount of the work in addition to the bike motor, but when he increased the speed to 20 mph, the motor was left to do all the effort.

 

[Nealh]

Ah I think I get it now. So when the OP was doing 18mph he was doing a fair amount of the work in addition to the bike motor, but when he increased the speed to 20 mph, the motor was left to do all the effort.

Yes, at 18mph he was using probably PAS 2 (OUT OF 5) so will have been apllying maybe upto 50w himself, the use of throttle by passes the PAS and gives max amps the controller can give less any efficiency from the final drive.

Ghost pedalling alone in PAS 2 with cadence systems (for instance ) will achieve up to about 15mph or so the motor does all he work nearly with a little effort the speed can be raised quite easily. You can pedal the b******s out of a cadence system but for little gain, except for a lot of human wasted energy for the gain of 3 or 4 mph. The only real time your effort pays off is if you can maintain a speed over the cut off level.
The cut off level is the legal 15.5mph set on a lot of systems, the cut off set by a rider (if applicable) or the max rpm the motor can give.

 

[Nealh]

With my Yose hub and 25a controller I have set the cut off at 33km/h or 20.5mph, I can in the right conditions reach this in PAS 1 or PAS 2 with some input in addition to the motor power, in benign conditions on the flat I can pedal past the cut off to about 24.5mph so I'm then inputting greater effort and watts.

PAS 3 - 5 I reserve for steeper inclines depending on their severity and let the battery supply the power and take a breather by ghost peadlling up. Depending on the amps I set for the controller I can see about 700w with inefficacy taken into account.

 

[Nealh]

I'm looking to do a European tour at some point, I want to cover 90-100 miles per day with a couple of 36v/13ah batteries, Belgium, Holland, Germany etc, so mainly flat, I think it's doable, so will practice maintaining that speed to get the range I need
Cheers

For yesterdays 162km/101mile ride I had the equivalent of a 14p10s PF cell battery so 40.6ah if you use the 2900mah rating, on E-cigs forum Mooch tests many cells and reckons the true rating for the cell is 2680/2700mah (37.8ah) . My three batteries I used were all hooked up in parallel to reduce any sag to minimal, all batteries fully charged to 4.17 - 4.19v prior to connecting and by rides end all were at an even 35.9v/29%.

My watt meter gave a used figure of 25.077 ah or 902.77 wh used, a revised wh/m usage relates to about 8.991 wh/m on my part. The reamining capacity suggests another 50 - 62miles of range (dependant on the true cell capacity) for complete packs depletion, so despite the use of a higher amp controller giving higher watt output for each PAS level 8.991wh/m shows I put in some good worthwhile effort.


My 21 -23 mile approx ride home due North from Brighton West Peir I didn't measure the wh/ah used but voltage wise the multi PF cells in P held up very well. I left the seafront with 36.9v registering across the whole pack range and arrived home with 35.9v measured at rest on each cell pack.

 

[vfr400]

Thanks guys, really informative. So it sounds like the best balance for speed vs efficiency/range is maintaining a speed of about 12-13mph...obviously any rider peddling effort will offset battery usage. I know that the biggest hit on effort is overcoming wind resistance, so it was interesting to see the effect of that today on my ride when increasing speed by 2mph with an extra 200w needed at the hub to maintain that speed.
I'm looking to do a European tour at some point, I want to cover 90-100 miles per day with a couple of 36v/13ah batteries, Belgium, Holland, Germany etc, so mainly flat, I think it's doable, so will practice maintaining that speed to get the range I need
Cheers

That depends a lot on whether you have your controller set to use power (current) control or speed control on the PAS (P3 parameter 1 or 0). With P3 set to 1 and the LCD set to level 1, your controller gives about 60W to the motor regardless of any external conditions. In that state, the faster you go, the greater the range of the battery.

When you have P3 set to 0 and level 1 set on your LCD, the controller will send more power to the motor up to the maximum allowed (written on the label) the slower you go, or the other way round, the faster you go, the less power you use. At 12 to 13mph, that will be zero power.

The same applies to the other levels set on the LCD except that with speed control, the cut-off speed for the power goes higher as the levels increase.

People still don't seem to be grasping this fundamental difference between speed control and current control and how it affects range and efficiency.

 

[Deleted member 25121]

For maximum range turn off the battery.
For maximum speed set the controller to the maximum assistance and pedal as hard as you can.

 

[Nealh]

For max range buy a pedal bike no use riding a heavy leccy bike turned off, it is counter productive to add several kg of motor/battery etc then riding a lighter bike.
As already mentioned the best use of a leccy bike for range is current control set to PAS1 , the stronger rider doesn't need as high watts draw so if lcd allows controller/amps reduction then that is the way to go.

On flat ground in benign conditions I can achieve max speed in PAS1 or 2, using PAS 5 just gives bags more torque/acceleration to arrive at the top speed quicker.

 

[Woosh]

People still don't seem to be grasping this fundamental difference between speed control and current control and how it affects range and efficiency.

why does it matter that much? aren't headwinds, gradient and a torque sensor more important considerations? after all, you need the Watts to help you go faster and the relationships between the Watts, motor and battery are well understood and independent from whether the LCD is programmed to maintain constant speed or constant current?