Assistance getting less the faster I go ?

[MikeHam]

Hi All,
So I have a Cyclotricity 250/500w 36v Mid-drive conversion on my road Bike.
I currectly have it at 500w with the speed limit set at 25mph but I'm noticing the more I go over 20mph the less assistance I'm getting ?
At lower speeds i'm getting 4-5 bars (out of 8 I think) but over 20mph I'm getting 2, sometimes 1?
I believe it's either running out of umpf, or maybe it because I only have a 42T front ring ?
So do I buy a 52T front ring, or a 48v battery ?
Many Thanks,
Mike.

 

[sjpt]

Motors have a natural peak power at some motor speed, and output falls off very quickly past that.
see https://www.ebikes.ca/tools/simulator.html to see the power graph.
You can set it for different motors, controllers etc, but the general shape remains similar.
(The initial graphs shown have an unusually hard edge at the peak, most are more gradual.)

Change to 52T and you will get more road speed at same motor speed; but suffer on hill climbing.

48v battery will very likely need new controller.

 

[MikeHam]

Thanks for the reply, my controller seems to be able to switch between 36v and 48v so that could be an option ? My commute is quiet flat so that not an issue

 

[Deleted member 25121]

sjpt is spot on, motors have a self induced rpm limit caused by something called back EMF.

Another thought - as an experiment, are you able to drop the power to 250W and see what happens over 20mph?

 

[MikeHam]

I can do that. I've also changed the wheel size in the controller but that makes no difference (speed measured by Garmin)

 

[Nealh]

Current ramps down as you hit the hubs terminal rpm limit (motor winding), as mentioned back EMF comes in to play. When you drop the speed the back EMF is no longer an issue so the hub rpm can increase again until it all begins over again.

What is the no load speed of the hub on full throttle ?

 

[MikeHam]

I'll find out the no load speed tomorrow and get back to you. I've used the simulator with a BBS02 as I think thats near the spec of my motor and power drops right off at 20mph with a 42t.

 

[Deleted member 25121]

Is this a new installation or has it recently started to show this behaviour?

 

[MikeHam]

It's new, I'm a new convert after buying a Focus Jam2 before Christams.

 

[Deleted member 25121]

OK, it's less likely to be a faulty component then. Let us know how you get on tomorrow with those experiments.

 

[MikeHam]

Deffo, and thanks for all your help so far. Would anyone know if the chain ring would be the same as the Bafang ones ?

 

[MikeHam]

I’m limited on what I can do today as I’ll be commuting by car today (starting at 13:00) So I’ve suspended the bike at max speed unloaded using the throttle is 25mph at 500w and 16mph at 250w, these are speeds shown on the controller and not my Garmin. I hope to do a road test tomorrow afternoon.
Cheers,
Mike.

 

[Nealh]

The 250w speed of 16mph suggests it is speed limited as the 500w setting is greater.
The mid drives are geared for certain rpm/cadence speeds so at 20mph when you feel the current reducing is because the cadence has reached the max for the drives rpm ratio.
A larger chain ring will reduce the cadence so you may be able to up the speed ramp down to a slightly higher speed. though you may risk frying the controller Mosfets as the drives like to be spun rather then laboured. You could try a 46 or 48t chain ring.

 

[MikeHam]

Thanks for your reply, so you think a larger front ring would help ?

 

[Nealh]

It should help as your cadence will be lower so rpm is lower you might get another 2 -3 mph before you feel the speed/current ramping down as you reach the BEMF limit again.

 

[Deleted member 25121]

I’m limited on what I can do today as I’ll be commuting by car today (starting at 13:00) So I’ve suspended the bike at max speed unloaded using the throttle is 25mph at 500w and 16mph at 250w, these are speeds shown on the controller and not my Garmin. I hope to do a road test tomorrow afternoon.
Cheers,
Mike.

The 16mph at 250W could be because the controller is limiting the speed to the legal limit of 15.5mph, I suggest you try riding the bike in the 250W mode and see whether the motor applies the same speed limit on different terrains.

 

[MikeHam]

A little update on this. I was struggling to get the correct sized front ring for my Cyclotricity kit so I ended up buying a Bafang one then a mate at work machinned them both and welded the original center into the Bafang 52 tooth one. My average speed has not gone from 22mph, to 26 mph. Very happy with this now. Long term I plan on going to a 48v battery as my controller can handle it but for now I'm more than happy. Thanks for all your input