mid drive on a 20 inch BMX using the TSDZ2B

[slideNotRide]

My partner planned and built this setup for herself. Goal was to make the cheapest build on a BMX with as little deviation from the appearance of a park BMX bike as possible, and have a max speed under 20mph for legality. (We also both have regular BMX bikes. I might post them in the hobbies section later.)

She debated a hub motor for a long time but she really wanted the wheels to look cute. She also didn't want to have to figure out where to mount a controller. She thought of putting the battery in the middle of the frame, but decided that it was too bulky and hard to make a good looking mount, so it now sits on a rack behind the seat, inside a case that was designed to hold a portable printer.

The egirl bike:

She got an offset left crank to correct the asymmetry of the motor.

Tongsheng motor model TSDZ2B, Hailong charger model 20200, Viset 14000 mAh 48V battery pack, Aproca electronics case.


The odometer says she now has 1365 miles on this thing.

She weighs about 120lbs and rides in the city on roads and sidewalks with very few hills or large bumps. She averages around 15-18 miles per hour on throttle alone. Her typical trip is 4-5 miles one way, with occasional longer trips. This bike can also be thrown into the back of my hatchback with no wheel or pedal removal, so I can pick her up very easily. The only time she needed me to come get her was when she got a goat head in her tire, which is just normal for AZ.

 

[amberwolf]

Nice build.

Viset 14000 mAh 48V battery pack,

I recommend keeping an eye on the battery. Previous threads about them and posts and reviews indicate that they don't always advertise them honestly, and that makes the rest of their qualities suspect (build, cell type, etc etc).

She weighs about 120lbs and rides in the city on roads and sidewalks with very few hills or large bumps.

:lol: You must ride in an area different from mine (Metrocenter-ish). Because of the traffic (especially trucks and buses) there's lots of big bumps and holes in the right hand lanes and road edges from them warping the asphalt in the heat.... :/

Some of them have broken rims or axles when I couldn't avoid them because of traffic.

The only time she needed me to come get her was when she got a goat head in her tire, which is just normal for AZ.

Unfortunately true.

There are a few threads around here with lots of different methods for flat prevention, and some pics of how I do things in my CrazyBike2 and SB Cruiser threads.

Since it's a BMX bike, it's rims are probably compatible with moped tires, which are a lot thicker than the average bicycle tire, though significantly heavier. I don't know how they would affect her riding style, but if you ever want to try them out without buying some, and you can get to the Metrocenter area, I can loan you a couple old worn ones to mount and ride with to see how they work out. (they won't have nearly the flat protection new ones will, because of the thinned out tread area, but the feel should be about the same).

I use Shinko SR714 16x2.25" tires on the back of SB Cruiser, and have avoided many thorn flats, and even a nail (pic somewhere in the SB Cruiser thread a few years back). Did have one paloverde thorn make it all the way thru and give me a flat; another caused a slow leak but it was held in place by the tire thickness so it didn't go flat immediately and I could air it up and keep riding to somewhere with air conditioning to work on it. Haven't had a goathead make it thru them yet.


For regular bicycle tires, my best method has simply been to use thicker tires where possible (presently the CST Sensamo Control; previously the CST City and beofre that the CST General; I'd still use the last two but they discontinued them; they keep doing that with all the tires I like). Then I use the thickest tubes I can find, preferably also from CST (as I haven't had bad tubes from them yet, unlike most other brands...Sunite sometimes uses CST, so I have some of those as spares).

Then inside the tire, between the actual tube and the tread, I use an old thick tube that failed for whatever reason, and cut the valve stem out, and a little area around it, then slit the inner circumference from one edge of that valve stem hole to the other, so it can be slipped on the actual tube like a steering wheel cover. The whole thing goes inside the tire.

If there are only thin tubes availalbe, you can use several layers like this.

I used to also use tire liners, but have had too many problems with those cutting into the tire or the tube, even with my extra layer, so I don't always use liners nowadays.


The disadvantage of thicker rubber is it can increase rolling resistance, but with a motor that's not much of an issue vs a pedal-only bike.

 

[slideNotRide]

I recommend keeping an eye on the battery. Previous threads about them and posts and reviews indicate that they don't always advertise them honestly, and that makes the rest of their qualities suspect (build, cell type, etc etc).

Good to know! She had a few days of power "stuttering" after getting up to speed last year, but then it just stopped on its own. We're always wondering what's going to fail first, knowing that we chose to be cheap lol
We assume the battery will be a weak link just because of ongoing heat alone.

 

[amberwolf]

If hte battery gets hot, then it is insufficient / incapable of doing the job it's being asked to do (not a surprise with that or almost any brand of "cheap" batteries).

If it gets hot in use or in charging, either it's interconnects or more likely it's cells are heating up, and that is a problem. It can mean internal cell issues that can lead to a fire, or it can mean steel was used instead of nickle for cell interconnects, or that the interconnect methods are high resistance in some other way. If it's cells, it could be that they are literal recycled garbage cells out of packs scrapped for good reason. They could even be random cells that are not even the same chemistry much less model or brand.

They are probably, even if not recycled, cells that can't handle the demands being placed on them. Normally a BMS is chosen to protect the cells against this, but in this case they probably used a BMS that is far more capable than the cells, so it isn't doing it's job. Or the BMS is damaged or even not present, so that it can't do the job. (if the pack itself does compeltely power off when empty, then it does have a working BMS, even if it's not made to monitor current and turn off output if that exceeds what the cells can handle, or is one that can moniotr but has a limit far above what they can handle).


The stuttering, if power doesn't actually turn off (display/lights stay on, but motor stops) is normally caused by the controller seeing battery voltage sag under load down to the controller LVC. That happens when a battery can't supply the current necessary to operate the system, so it's voltage drops unitl the load goes away. As the voltage rises, the controller runs the motor again, which puts the load back on, and volage drops....etc.

If power does actually shut off (display blanks, etc) then it's not necessarily the entire battery doing it, just some of the cells (with this pack, probably all of htem to some degree but some worse than others). Then the BMS detects the cells drop below LVC, and tursn off it's output. Load ceases, voltage rises, rinse and repeat.


In the latter case, if it's happengin becuase the celsl are not all the same voltage (unbalanced****), leaving it on the charger for several hours, days, or weeks (until the chargerr stops cycling on and off) can rebalance it, so that mostly the cells will not drop low when near a full charge, at least.

In the former case, similarly, there coudl be one cell group with much less capacity that charges up to full way beofre the others, so that the entire pack is pretyt low voltage even hot off the charger, and drops even lower under load.


****balancing only fixes the cell voltages the same, it doesnt' actually fix any defects or cell differences; only replacing the entire set of cells with new matched ones does that...but it's helpful to do when a battery has varying cell voltages to be able to use whtever remainign cpacity / etc that it does have, up to a point.

 

[SlowCo]

Nice build!
I suspect placing the battery pack inside the frame triangle will improve the handling quite a bit.

 

[slideNotRide]

If hte battery gets hot, then it is insufficient / incapable of doing the job it's being asked to do (not a surprise with that or almost any brand of "cheap" batteries).

Oh sorry, it's not dangerously hot, I just meant that it gets hot from the ambient temperature/sun here. I have no idea if living in a hot climate is actually hot enough to cause the battery any issues, but when I was younger I heard that we go through hybrid car batteries faster in AZ because of the heat, so now I'm always worried about that.

If the voltage drop issue comes back I'll post about it! I think she did accidentally rebalance the cells by leaving it on the charger for a long time lol

 

[amberwolf]

Oh sorry, it's not dangerously hot, I just meant that it gets hot from the ambient temperature/sun here. I have no idea if living in a hot climate is actually hot enough to cause the battery any issues, but when I was younger I heard that we go through hybrid car batteries faster in AZ because of the heat, so now I'm always worried about that.

The heat affects anything that's an active chemistry. (it also affects passive chemsitries like plastics, etc, but usualy to a lesser degree if it doesn't melt it outright :lol: ). If the pack and cells are really good, they'll take a long time to degrade from the heat here. I've been using EIG NMC cells as packs in my bike and trike for a very long time (decade+) and they've been outside in the heat and cold thermal cycling all that time, and they're jusdt in the last year or so, maybe a bit more, beginning to seriously degrade.



If the battery doesn't heat up on it's own, druing charge or usage, then it should be ok.

Tests you can do: if you let it cool to ambient room temperature when it's inside, then charge it and keep monitoring temperature.

Let it cool to room temperature, then go ride it when it's not hot outside, or ride in this heat we have now and put it in an insulated container (inch+ thick styrofoam) that started otu at the same ambient temp as it did, and monitor temps on it and on the inside of the container during a ride. If the battery core temp goes up faster than the container temp, there may be a problem. If core goes up way slower, or even stays at ambient, it's probably good.

If the voltage drop issue comes back I'll post about it! I think she did accidentally rebalance the cells by leaving it on the charger for a long time lol

Well, that's not a bad thing, and you now "know" that it has a cell mismatch problem that is going to get worse over time (because they don't get better ).