Help! ebike won't reach full speed!

[DougalDogHead]

Hey all, I'm new here so apologies in advance if I break any rules.

I have recently bought a 2nd hand ebike and as the title says, it won't go full speed (15.5mph) with the assistance of the motor, topping out at about 12 or 13mph (according to GPS speed on my phone). I have a 24V bike too and it's not even as fast as that, so something's definitely not right. Mechanically everything is fine.

The bike is a no-thrills 36V elife voyage. I've put a multimeter across the battery and it reads 38V with about 75% charge, so I assume that's about normal.

The controller is as pictured, a G3M20211-1592-16.

Does anyone know what the issue could be please? Other than just replacing parts to see what fixes it, I'm all out of ideas.

Thanks

 

[cyclebuddy]

Charge the battery fully to 42v, put it into maximum assist, and then see what it does. These cheaper bikes mostly use a fairly crude type of speed controller that's voltage dependent and also limits top speed by assist level. Provided you're not too heavy it might just reach 15.5, or it may be that the controller is programmed to taper the assist earlier than most.

 

[DougalDogHead]

It does feel as though it might be dialing back the assistance early. The controller has a programming port as pictured here, apparently I can program it using something called an ST link V2, but that might be a little out of my depth since I can't find a lot of information on what software to use or how to do it.

Alternatively I was wondering if a different display might work with it because I know some let you change settings in the display. The only identifying marks on the current controller are "790". Someone told me that it's a king meter but I wouldn't know how to check this. So I guess the question is, how would I know if a different display would work with this controller?

Failing all that I was going to swap the controller with one that already has a display

 

[Nealh]

The controller is lishui and yes the display will be a KM.
It is usually only dealers who have access to the programming software.

Do as CB has mentioned and see what the speed is with a fully charged battery, as volatge decreases so does speed so that may be where the issue lies as well as a combo the smaller wheels and the motor rpm speed.

 

[Woosh]

The controller is as pictured, a G3M20211-1592-16.

The controller in the image is a Lishui controller. The white connector is indeed the programming port but you can't reprogram it yourself and there is no point doing that as the controller does not limit your speed but other things such as motor constants.
If you can't reach 15mph, the usual cause is you don't spin the cranks fast enough. If it is so, the remedy is usually change the crankset for something larger.

What is the tyre size and the number of gears on your bike?

 

[DougalDogHead]

The controller is lishui and yes the display will be a KM.
It is usually only dealers who have access to the programming software.

Do as CB has mentioned and see what the speed is with a fully charged battery, as volatge decreases so does speed so that may be where the issue lies as well as a combo the smaller wheels and the motor rpm speed.

The speed with a fully charged battery is 12, 13mph at a push. The reason I put the multimeter across it at 75% is because I had just got back from a ride where I'd confirmed that it wasn't going as fast as it should, and my first thought was that the battery was probably at fault, but it seemed to be outputting the right voltage, at least it was without a load on it. I'm hoping it's not the battery because replacing that will cost more than I paid for either of my ebikes

 

[DougalDogHead]

The controller in the image is a Lishui controller. The white connector is indeed the programming port but you can't reprogram it yourself and there is no point doing that as the controller does not limit your speed but other things such as motor constants.
If you can't reach 15mph, the usual cause is you don't spin the cranks fast enough. If it is so, the remedy is usually change the crankset for something larger.

What is the tyre size and the number of gears on your bike?

6 Shimano gears and 20" wheels

 

[matthewslack]

The speed with a fully charged battery is 12, 13mph at a push. The reason I put the multimeter across it at 75% is because I had just got back from a ride where I'd confirmed that it wasn't going as fast as it should, and my first thought was that the battery was probably at fault, but it seemed to be outputting the right voltage, at least it was without a load on it. I'm hoping it's not the battery because replacing that will cost more than I paid for either of my ebikes

12.5mph is 20kph, and 20 vs 24 or 26 inch wheels also have similar relationship. My guess is controller settings not right.

 

[DougalDogHead]

12.5mph is 20kph, and 20 vs 24 or 26 inch wheels also have similar relationship. My guess is controller settings not right.

This was pretty much my thinking, what I'm not sure about is whether there's anything I can do about it. Any thoughts?

 

[Woosh]

6 Shimano gears and 20" wheels

6-speed shimano freewheel on the back wheel, the smallest cog on it has 13 teeth. You have typically 40T at the chainring on folding bikes, 20" x 2" tyre circumference is 1.565m, you'll need to spin the cranks at 25000 * 13 / (1.565 * 60 * 40) = 86.5 RPM to get to maximum legal speed (15.5mph, 25kph). Too fast for most of us.
Depending on the frame, you may be able to change the 40T chainring for 50T to lower the cadence making pedaling at 15mph easier. Chinese 20" bikes are usually built for 13mph optimal speed with 280rpm motors unless you tell them that you want higher optimal speed. If it's a kit, you'll want 328rpm motor for 20" wheels.

 

[cyclebuddy]

My Chinese 20"x1.75" folder has 52T chainwheel x 13T freewheel. Unrestricted, it reaches 18mph (20NLS). To get 20mph ridden, you're spinning like a demented Hamster but it is possible.

 

[Woosh]

That's why I fitted the woosh rambletta with 8 speed cassette, smallest cog has 11 teeth instead of 13.

 

[DougalDogHead]

That's why I fitted the woosh rambletta with 8 speed cassette, smallest cog has 11 teeth instead of 13.

I think I'm going to have to do this to both of my bikes. The motor on the theoretically less powerful 24V one will assist you up to a higher speed then the 36V one which is counter intuitive and annoying, but neither of them can do much over 15.5mph regardless of how much pedaling you do without the wind behind you and going down hill. When I'm committing to work on 40 roads, 15.5mph feels too slow, and 12/13mph is just painfully slow

 

[guerney]

My Chinese 20"x1.75" folder has 52T chainwheel x 13T freewheel. Unrestricted, it reaches 18mph (20NLS). To get 20mph ridden, you're spinning like a demented Hamster but it is possible.

That's interesting, how many links are on your chain? I did a brief unrestricted test on my Dahon Helios P8 with bbs01b 52T>11T (111 links), and managed about 22mph, not quite demented. It was too fast. In my youth I'd have been looking for a 60T chainwheel...

 

[cyclebuddy]

That's interesting, how many links are on your chain? I did a brief unrestricted test on my Dahon Helios P8 with bbs01b 52T>11T (111 links), and managed about 22mph, not quite demented. It was too fast. In my youth I'd have been looking for a 60T chainwheel...

I don't know how many links are in the chain... it doesn't really matter. The chain only needs to be long enough for the derailleur to tension correctly in all gears.

To me, the standard 52/13 7-speed gearing is well judged given the (usual) 15.5mph capping, lower 35Nm torque of the rear hub motor, and purpose of the bike. I typically cycle at 12-15mph, so the unrestricted 18mph is more than enough for an assisted comfortable commute. Yes, battery life is shortened as the ride is always assisted, but the 11.6Ah battery is generous/ plenty for a small 20" wheeled folder. It's a good balance of factors, and complements my torque sensor/more powerful leisure/pleasure bikes well.

If it were my only ride, it'd probably frustrate the hell out of me: I'd want more power and a wider range of gearing to meet the challenges of a more diverse range of routes/purposes. Yes, at my age, 20mph is about as fast as I want to go on 20" wheels: 25-30mph is easily achievable and gives a more confident ride on my 9 & 10-speed 60 & 80Nm 28" wheeled bikes.