The Minicargo Bikes (short tails)

While I have several new ideas that I'd like to try out on a new bike for myself, I've been spending time on bike/ped/EV advocacy and basic projects instead.  (This past summer I helped five friends put electric motors on their bikes.)  Because I'd really like to build myself another bike, I've decided to combine some of the other projects into one minicargo (short tail) bike project to save time.  This project isn't done, but there's enough that has happened to write an introduction.

Note added May 10, 2020:
I'm adding Short Tail to Minicargo.  My main goal for this project has been a smaller cargo bike so I've been calling it a Minicargo design, but many people are familiar with the Long tail style cargo bike, and will readily understand what Short tail means.

To start, here are the other projects:
-First, I've been wondering for a few years now which is a better motor: a direct drive hub, a geared hub, a pedelec bottom bracket, or a torque sensor bottom bracket.  No one has measured the performance, energy consumption, and ease of use on identical frames while running them in typical day to day utilitarian use over an extended period of time with several different riders.  I'd like to run one of each of the motors in a side by side comparison.
-I could also use a spare bike or two, for friends to use when they visit, or for loaning to people that I talk with who would like to try out an electric bike for a few days.  (For that matter, groups near me could really use some sturdy bikes for creating small, local bike share programs.)
-There are several people I meet with who think an electric bike could replace a car for people who don't need to travel far during the day.  We've been talking about DIY conversions as a way to make the cost of an ebike accessible to more people and could use actual data on the choices and cost
-I've also thought a lot about the "First mile / Last mile" commuter problem and what the best bike for this would look like.  A bike powerful and adaptable enough for carrying large loads and climbing hills, but that is still easily used by novice riders.  A bike that isn't too valuable, and doesn't take up much space at home.
-Finally, to inspire planners and transportation budget people.  I think that most planners and policy makers are not able to imagine bicycles being useful for anything more than one mile trips on a sunny summer day for a niche segment of the population.  The reality that I see instead (as I live without a car) is very shortsighted and poor transportation planning that is the main reason why bikes and walking aren't used more often.  After spending almost all of our highway budgets for the last 90 years on cars, there isn't any real choice when you go out - the roads are all about cars.  Bikes and pedestrians are given the nonexistent shoulder.  It's a self perpetuating cycle: "We have to spend all the budget on roads built for cars because people drive cars" but then "People drive cars because that's what they feel comfortable using because of the way the roads are built".  I honestly am not sure that many planners and legislators are able to figure out that you can't fix our transportation problems with the same thinking that created the problems.

So I hope to build at least 4 bikes with different drive systems, and see which one works the best.  Converting regular frames should result in a much better performing bike than the standard commercial bikeshare bike that has been built like a bulldozer,  but they will also be more susceptible to damage by less skilled riders so an important part of this project will be the system for loaning out the bikes.  Several people here in Vermont keep working on bikeshare and carshare programs with moderate success, so I've decided to leave that part alone for now.  Cost is also a very important part of this project, which means keeping the frame modifications simple.  I'm planning on welding on a simple heavy duty rear rack and extending the rear wheel to match it, but the rest of the bike has to stay untouched.

I've collected some frames over the last couple of years for this project.  They are all women's 17 inch (medium size) mountain bike frames, because I've found that a step through is important for many people, and also MTB frames have been better for my cargo bike purposes than other frames.

In my area rust is a problem, and 2 of the frames have splits in the bottom of the chain stays from water collecting inside them, rusting, and freezing.  The tubing has to be cut back to where it is normal thickness and replaced, and this will increase project cost.  (I'd prefer to find another frame or two and avoid the rust.)  The rest of the frame tubes are OK- although I did have to use some extreme finesse (all my weight on a 3 foot breaker bar) to remove 3 of the bottom bracket assemblies.

Simply adding an electric motor and a standard bolt on rear rack to a regular bike can result in a very nice bike, but the goal here is a little more multi use cargo ability (think of a sports utility vehicle).  It looks like a rear rack welded on is the best choice, because it would add extra capacity while making the frame stronger.  An extra feature would be shaping the back of the rack so that the bike could be stored standing up on it's rear wheel to save floor space.  The tail would have to be shorter than medium length though, because about 20% of the people who have ridden my longtail say that it is too big for them, (either it's too awkward to ride, or it won't fit in their storage spot).  There are several companies already building bikes similar to this, for example the RAD Runner, Yuba Boda Boda, Yuba Sweet Curry, Bike Friday Haul a Day, and Tern.  The goal here is to recycle old bikes into small electric cargo bikes with better specs at a comparable price.

The bike rack at one of my local Park and Rides.  Would you leave

an expensive eBike here while you took the bus to work everyday?

The problem is the surrounding roads have steep hills, and 

no one is going to ride here on a clunker bike that's hard to pedal.

(Photo credit Ariel Arwen)

The rear wheel should be extended backwards to better support the extra capacity rack or it will be harder to balance.  I've extended the chain and seat stays on my projects before and don't think it's too hard, but can it be done without adding hundreds of dollars of work time?  A jig instead of strings and squares would save time.  Also splicing the tubing mid length instead of scribing it to fit the bottom bracket or seat post, and using sleeves instead of flush welds on the tubing would help too (but be ugly).

The main question to answer was how far to extend the rear wheel.  There were 3 considerations:
-fitting in a battery pack behind the seat post (or somewhere nearby)
-size of the rear wheel
-how big is too big?

The easiest battery pack to install would be a silver fish (large aluminum case) style, placed over the rear tire and under the rack.  But this tends to be a bit top heavy in the tail and I'd like a novice level rider to feel comfortable.  A dolphin style (black plastic with angled top end) pack won't fit on the down tube of these step through frames, but it might be possible to squeeze one in behind the seat tube after modifying the frame, with the tubing for the seat stays being the main interference.  I'd like to use a dolphin style because it's plastic (corrosion resistant), commonly available (modular replacement), and 700 to 800 watt hours can be fit in without having to use premium cells (for extended running time).  It would be easier to fit a battery in this area the further the wheel is moved back.

I already know that my MXUS 750 watt direct drive hub motor in a 26" wheel will not provide acceptable power up the hills around here for someone who is out of shape.  However the same motor in a 20" wheel could possibly work.  Will a small wheel cause bad handling?  After getting in about 50 miles of test rides on bikes with 20" wheels (front and back), I've decided that a 20" front isn't a good idea on my roads, but that the back wheel has much less effect on the handling and would be worth trying.  This may turn out to be a big mistake because of the rear wheel getting caught in ruts and making the bike too squirrely, or it may turn out to be just a small problem because the tire wears out quickly, and only some riding time will tell.  I'll leave the 26" front wheel that the frames came with in place because that size is less sensitive to poor road quality.  The Yuba Sweet Curry has similar tire sizes, but with a larger rack than I'm thinking of adding.  On the plus side a 20" wheel would also give a lower rear rack height (easier to use and more stable cargo weight distribution), and add a few inches of space for the battery.  On the minus side the gearing won't be standard and I'll have to take into account motor speed.

The two hub motors arrived built up with rims, but I needed two more regular 20" wheels for the bottom bracket motors.  I wanted to use freehubs for strength and gearing reasons, a quick release (most 20" wheels have axle nuts), and 10 mm dropouts, so I got a couple Shimano FH-M525 hubs and laced up two wheels.

For the bike geeks reading this, I decided to push the builds and see how far I could take the tolerances so I got out the dial gauges.  (I know it was obsessive but it was the middle of winter.)  The best runouts that I could achieve were 0.08 mm in both directions, after that the rim surface finish and distorted bead roll around the seam messed things up.  Also I've read that lacing the leading spokes outside or the trailing spokes outside can make a difference, and since I was building two otherwise identical wheels I laced one each way to find out.  As far as I can tell it doesn't matter, but I plan on putting some torque on them and then measuring the width near the derailleur lower sprocket for another blog post.  Also the freehubs are meant for 10 speeds and I'll have to shim them down to 6 to 8 speeds (to be able to use heavier chain, however this might be useful for chain line adjustment), but the important part is that they are built for 135 mm wide MTB drop outs.

In order to find out how big is too big, I made a bike mock up and took it up to the bus bike rack in the front yard of my transit bus company for a test.

The Advanced Transit bus rack training rack

This criteria is directly related to the local poor quality bike parking, not cargo capacity.  As long as only a few bus riders use their bikes for the first mile/ last mile, they can just take their bikes on the bus with them to work (where hopefully there is better bike storage).  Then when more riders start using their bike to get to the bus, the state transportation agencies can put in more serious bike storage, and a bike this size would be more likely to fit in a future bin or rack than a larger cargo bike would.  Also, I hope to live to see commuter rail here (we have the tracks but not the service), and this size would fit in the storage area of the Budd RDC cars that are in inventory.

I took a couple of scrap kitchen shelf brackets and cut them into extensions for the drop outs drilled with many axle holes for testing.  With the largest tires that I had mounted on the rims and the wheels all the way down in the rack, I found that the axle could be moved back 3 3/8".  This is not a large amount and may not be worth the cost, but it also looks like (in combination with the 20" tire) it will give enough room for a battery pack behind the seat post so it's worth a try.

I drew up a sketch using nanoCAD to check for obvious conflicts, and it looks OK enough.  Although the seat stays will be a straight extension (sleeves would work), you can see the angle of the chainstays will have to be significantly changed to put the bottom bracket height at 10 3/4" (the lowest height that I've found OK when riding my other bikes), and it could be a challenge to keep this low cost as it's probably necessary to fit the tubing to the bottom bracket.  I'm not concerned about the small head angle change from slightly dropping the bottom bracket because the frames are all above 70 degrees and there is room within accepted practice for them to be more slack, and along with the increase in the wheel base and trail should help counter the quickness of the 20" rear wheel.  The next step is a full size drawing to double check dimensions before cutting metal.

The bikes have to meet the 750 watt On Road ebike legal limit, but because of the hills around here and rider expectations I have no interest in using smaller 250 or 350 watt motors.  Here are brief descriptions of the four motors to give you some background, you can look them up for more details.  I've listed them in order of what I think is strongest (i.e. will last the longest) to weakest, and it will be interesting to see what actually happens:

1. MAC geared hub motor with a 10T winding (25 mph with a 26" wheel) in a 20" rim, this is a deluxe motor and has been improved over many years to the point that it can take a lot more than 750 watts and survive.  This is my 3rd MAC and I've been very impressed with the first two, I expect this motor in a 20" rim will climb walls.  BMC motors are very similar, though I believe the gears are different material.  The motor controller is large and must be mounted separately in a water protected location.  Price around $550 in a kit with the rim attached, all small handlebar parts (but no display that shows speed), and includes shipping.  Weight of the motor alone is 9.4 lb, controller is 1.1 lb, small parts are 1.5 lb = total weight of 12.0 pounds for the kit.

2. Leaf Motor 1000 W direct drive hub with a 25 mph winding (with 26" wheel) in a 20" rim.  This is sold as a 1000 watt rated motor but it can be programmed to a legal 750 watts and the extra 250 watts of rating becomes a safety margin against overheating.  This motor and it's bigger 1500 W sibling can handle bursts of a couple thousand watts, and have been favorites of ebike hot rodders.  They are strong motors but very heavy compared to the other choices.  This is the only motor of the four that can regen when braking, (depending on the controller it can be set to be automatic at a certain speed), but it also is the only one with a noticeable slight drag when pedaling with the motor off (i.e. when riders have run the battery dead).  Again the motor controller is external and must be mounted in a water protected location.  Price around $425 with rim, all the little parts, and shipping.  Weight of motor alone is 13.3 lb, controller 1.3 lb, small parts 1.7 lb = total weight of 16.3 pounds for kit.

3. Bafang BBS02 with a 52 tooth chainring (for a 20" wheel).  A 3rd generation motor with improved gears and a better internal controller, probably over 8 million have been produced making it a world wide standard, It's very robust, though the MAC will most likely outlive it in high torque usage.  Being able to shift down for hills makes up for a motor that is smaller than the hub motors, but will riders power shift and break the derailleur?  Price around $425 with a nice color display and shipping.  Weight of motor with chain ring is 10.3 lb, crank arms 0.9 lb, controller is inside motor, small parts 1.4 lb = total weight of 12.6 pounds for kit.

4. Tong Sheng TSDZ2 with a 52 tooth chainring.  This is the smallest and lightest motor of the 4, and to me it feels more like a 500 watt motor than 750 watt.  The Bafang is noticeably stronger.  Being able to shift down for hills is a plus.  However this motor has a torque sensor inside the pedals instead of a cadence sensor (pedelec), and it is very smooth and easy to use.  I think this is a great motor for a novice, and the Bafang is a better choice for more experienced riders.  There have been improvements in the gears and heat resistance, but I've had a problem with the speed sensor gap adjustment being far too fussy on 2 bikes causing the motor to not run.  Price around $325 for last year's 3.6 model with a simple LCD display, the small parts, and shipping.  Weight of motor with chain ring is 9.0 lb, crank arms 0.9 lb, controller is inside motor, small parts 1.4 lb = total weight of 11.3 pounds for kit.

While I'd prefer to be trying out some of my ideas on a new bike for myself, putting these 4 bikes together will answer several basic questions that should have been answered a while ago.  I have the meters for tracking the performance of each bike, and the ergonomic outcomes will be interesting.  I've left out a bunch of details in this overview, (such as I think the chains for the bottom bracket motors could be narrower (9 or 10 speed) and still have a decent lifespan because they are running faster than normal because of the 20" wheels), so please feel free to send questions.  My plan is simply to keep working away at this as time and budget permits, so I can't guarantee when the next blog post about them will be, hopefully they will be on the road by next summer.