If you've followed some of the electric bike posts on this site, you've probably seen talk of using a trailer to carry the batteries.
My bike trailer build took a different route.
HEAT/SC v1.2
A homebuilt Hauling-Electric Assist bike Trailer/Shopping Cart.
Meet Woodrow Prime... you can call him "Woody"
I first got the idea for this and started building it in my mind 7 or 8 years ago when I first saw Mr. Sharkey's incredible pusher diesel Rabbit. I figured if it worked with a diesel Rabbit, why wouldn't it work with an electric trailer for a bike?
In those years I found a few pusher trailers on line, both gas and electric, but never did anything about it except build it in my head.
Last years spike in gas prices got me off my rear end and got me working.
To that end, I purchased 2 Currie electric folding bikes on craigslist.
The good:
The bikes were in great shape and the price was right. The bikes had 20" wheels which is what I wanted on the trailer.
The bad:
The bikes maxed out at around 13mph with the Currie Electro-Drive and their 20" wheels. Mounted on a 26" wheel, the drive train maxed out at around 15-16mph.
Now I'm sure our European friends would see nothing wrong with that speed but I'm closer to a typical American and I wanted something faster. Federal law states 750 watts/20mph... although I'm sure you could argue the fact that the bike isn't really powered, the trailer is... hmmm
After market parts are available to get to a higher speed but it was going to cost me more in parts than I had in both bikes.
I found a cheap Currie E-Ride mountain bike on craigslist toward the end of summer. The folders have the original "silver can" MAC BMC motors, the E-Ride has a newer "finned" motor but it's speed is still around 15-16mph, maybe 18mph max? Still too slow for me when used on a 20" wheel.
By now many are probably wondering "If you have all these electric bikes, why build an electric trailer."
There are a few reasons for the trailer.
1- To see if it would work, and if so, how well.
2-I still like riding my bike and the trailer set up leaves my bike in mostly stock configuration.
My bike is an old GT Outpost I found in a trash pile. I was about 2 minutes from cutting it up to use in making a recumbent when I realized it was in pretty decent shape.
I put air in the tires and spent a few minutes tuning it up and put about 25 miles on it before the rear tire's gum wall let the tube through. That was about 4 years ago and since then I've put about 1500 miles on that bike.
Last summer I bent the rear chain stays apart to fit one of the E-Folder's drives on it. This spring I took the drive off it and re-bent the chain stays back to their approximate original location and put another 100 miles or so on it. This bike has taken a lot of punishment and still gives it's all.
It's still a nice bike to ride to the store, putz around the neighborhood, or ride along the river on the bike path.
I still like riding my bike... with the electric trailer, all I have on the bike is a throttle, a rocker switch under the left brake lever, some wiring, and a hitch receiver.
3- I don't live in the worst crime area in the world but it's not the safest place either. I'm always a little leery of leaving an electric bike locked up outside a store, the post office, a park, etc. There are too many "interesting bits" on it that get people's attention.
With the electric trailer, I can take it inside or if I do leave it outside, most people don't know it's anything but a bike trailer, albeit a cute one! 
4- It's not only a trailer, it's a shopping cart.
I thought the drive train through during the winter and this spring I found a 24 volt Schwinn(Currie) S-500 electric scooter on craigslist.
The listing ended up being from a pawn shop and all they could tell me was, "It worked when we took it in maybe 8 months ago. A few weeks ago someone was interested in it and we tried charging it over night and when we turned it on, the wheel spun around about 3 times then quit. We don't know what's wrong with it."
I bought it. All that was wrong with it was the batteries and a flat rear tire. I actually got the batteries back into decent shape, the story is here.
http://www.fieldlines.com/story/2009/4/6/161139/6278
The batteries can now sit for a few weeks off charge, dropping to around 12.9 volts and seem to have close to their listed 10ah of storage.
I cobbled the scooter's motor and drive gear onto a Bell bike trailer I had found in the trash about the same time I found the GT.
The scooter's throttle used a 6 conductor mini-din connector that wasn't long enough to reach from the trailer to the bike's handle bars. I had a spare 8 conductor mini-din and used that along with a spare computer motherboard's mouse mini-din connector.
The gear on the scooter's wheel is the same size and thread as a screw on typical bike gear set, like a mountain or 10 speed bike so I laced up a spare axle to a 20" wheel and threaded the scooter's drive gear onto it. A few zip ties later and this is what I had. I call this the XHEAT.
Although this setup worked and got me on the road quickly, I had ended up changing the drive ratio too much and the range suffered.
Using a 24v, 4.8ah pack of BruceS NiCd, the trailer did only .33 miles. The top speed was 19mph, all this was without any pedaling and the pack lasted around 5 or 6 minutes.
I stuck this same battery pack on the E-Ride mountain bike and it did 3.6 no pedal miles, lasted 15 minutes, averaged 12.6mph, and reached a high of 16.8mph.
To make sure it was the drive ratio being changed too much, I put the drive gear back onto the scooter's wheels and installed them on another trailer.
It did 2.4 no pedal miles, lasted 12 minutes, averaged 12mph, and reached a high of 16mph.
I think the difference in pack output is the fact that the E-Ride mtb is a brushless motor and the scooter is a brushed.
I ended up getting another scooter and using it. This one was an IZIP, also a Currie, and also a 500, but this particular scooter was able to overvolt to 36 volts. The Schwinn would shut itself down above 30 volts.
There is a design online from a neighbor to the north of me, Bob Bidwell, that uses parts from a Currie scooter, pretty much what I ended up with in the XHEAT. It certainly was an easy project but the 12" wheels from the scooter seem to find every crack, bump, dip, rock... you get the picture... the thing would jump all over the place and that in turn would pull and push the bike and was just uncomfortable for me.
I can imagine using some sort of suspension but I wasn't ready to go that route.
I ended up purchasing one of these.
It's another Currie product, the same one used on the newer Currie, IZIP, and EZIP bikes. The included drive gear is the same size as a regular single speed bike chain. I ended up having to grind that gear down some so it would spin a thinner "multi-speed" chain so it would work with a 5 speed gear set on the 20" wheel.
Now I needed a way to shift it remotely.
I tried an old 10 speed shifter and it worked but hooking it up to the trailer was a nightmare and I could see that having some way to quickly unhook it from the bike was going to be a problem. I decided to go "electric".
I searched online for a cheap linear actuator but found nothing. I got tired of looking and ended up using an old Skill cordless drill I found in the trash.
I hooked up some limit switches and inserted a couple of diodes and had my linear acutator.
It worked well enough and I purchased a couple(1+ a spare) of DPDT rocker switches from Digi-Key to control the thing. Worked great.
Here's a shot of Woody's guts. The drill/actuator is at the top of the picture... it connects to the 5 speed derailleur and pulls it to the low gear or lets out slack in order for the derailleur's spring to pull to high.
The 'battery retention system' is two old pieces of velcro strap. The velcro straps were originally set up to hold 24 volts worth of BruceS NiCds and were crossed to try and hold the lithium pack. I added webbing from an old tool belt to help hold the lithium pack because I didn't want a ton of holes in the bottom!
Woody's frame is an old Burley I found craigslist. It was being used as a garden cart to haul paving stones and had seen better days.
I cut it down length ways about 3 inches and side to side about 3 inches. The frame is now a little over 28 inches square and will just fit through a 'two eight' door.
Performance
I only have one grocery run and a total of only about 15 miles on Woody but so far he's held up well.
With 36 volts, in 5th gear, max speed is around 26mph. At 24 volts, the max speed was around 19/20mph.
I was using BruceS's batteries at 24 volts but I haven't rewired them for 36 volts yet. The battery you see in the picture is a lithium pack from a Tres Terra(now Ultra Motors) Europa.
When it was in the Europa I think the range was around 18 miles with some pedaling. I haven't done a range test with Woody yet.
The trailer does do some bouncing around still even with the 20" wheels. I think part of the reason for the bouncing is because I shortened the whole thing up. It doesn't bounce badly but if it's under power and hits a hole or bump is will jump some.
Other than that it rides like a regular bike and I've had it going over 30mph downhill and it tracks right behind like it's not even there.
The transformation from trailer to cart or cart to trailer takes about a half a minute and it works fine as a grocery cart.
All that is required for transformation is pulling the pin from the hitch, disconnecting the wiring harness (I used a motherboard power connector), pulling a pin on the trailer arm, flipping the arm, and reinserting the pin on the brace you can see in some of the pictures.
Woody weighs about 45lbs. Here he is 'flipping his lid' with the trailer tongue in the shopping cart position.
A shot of the caster wheel in front.
A shot of the rear showing the lock/switch and the charging port. Sorry about the focus.
Cost
I had much of the things used to build Woody already laying around but here's a rundown of what I've spent so far. Most prices will include shipping and tax if applicable and if I remember...doh!
Currie (Unite 1018z?) 24v, 450w motor, allelectronics.com, $50.00
Schwinn S500 24v scooter, craigslist, $43.09
IZIP I500 24v scooter, craigslist, $30.00
Burley bike trailer, craigslist, $40.00
Alumiweld, Harbor Freight, $13.00
Dap Weldwood contact cement, Home Depot, $10.00
34 qt collapsible crate, Kmart, $8.00
2-DPDT rocker switches, Digi-Key, $2.03/each + tax + shipping, $6.13
MAAS metal polish, Walgreens, $5.00
Hapmton cam lock, Aviation Industrial Supply, $1.54
(and)
Aluminum continuous(piano) hinge, Aviation Industrial Supply, $1.45
Total: $3.23
'Woody' personalized license plate, Walmart, $0.25
Like I said before, I had a lot of the stuff for this build laying around and I tried to use up junk before buying anything. I used aluminum crutches, an old busted up jogger a friend brought by... etc.
The trailer deck and the box's bottom are pieces of hollow core door skin laminated to Tuff-R polyiso insulation. The sides of the box are from a piece of 13 ply Baltic plywood left over from a wooden recumbent bike project a few years ago(that bike didn't last long... the frame broke!) Using all this other stuff obviously kept the price down.
To be fair I should probably include the cost of the Europa since I'm using it's battery pack, it was $250.
The $30.00 scooter came with two sets of decent batteries and I could use three of those for the 36 volts the trailer needs but it would add quite a bit of weight and probably drop the range. I'm not sure how well the trailer's bottom would hold up carrying the weight of those batteries.
Total price including the Europa's battery is $458.70
Even if I were to add up all the money I've spent on electric transportation I'd still be well under a thousand dollars. Many spend more than that on their batteries alone and most of what I've bought can probably be re-sold at what I spent or more.
I'm only using one of the scooters for parts and could subtract the $43.09 from the total cost plus I'm sure I could sell it for what I have in it or more. I could probably sell the two folding electric bikes for what I have in them and if I keep the Europa's battery I could probably set it up with cheaper batteries and recoup it's cost. I've really grown to like the Currie E-Ride Mountain Bike and I don't see selling it anytime soon... it was missing it's #25 chain for the motor drive, had the wrong size seatpost, and someone had turned the handlebars backwards... that's all that was wrong with it. It was $50.00.
I need to thank a few;
The good Lord for the skills, the persistence, and a place to build things and a country in which to do it.
My girlfriend for 3 P's...
She showed a lot of patience when the trailer was taking up a lot of my time.
She got me a good price on parts.
She handed out some good praise for the trailer as it was taking shape and once it was finished.
Thanks Terri
A good friend of mine from back in my A&P school days.
He was the one who gave me the jogger he found on craigslist and offered suggestions when I was stumped on how to build something. The jogger provided many of the parts for the trailer tongue/shopping cart handle setup.
Thanks Bill.
Thanks to many on this site and the internet in general.
BruceS for nicads.
Paul Knox, (aka Knoxie on sites like endless-sphere.com and visforvoltage.com) for taking the time to answer what probably seemed to him like a stupid question from a noob on the Currie drive system.
Improvements
I should have made the trailer a little narrower and that is doable.
I would like it to be shorter also but the chain drive is already so short that it's a little out of whack in the low gears. The only way around this would be a complicated system to move the motor at the same time as you're shifting gears or install a multi-geared hub such as a Sram DualDrive, Shimano Nexus, etc. A geared hub would probably require some type of stepper motor for the remote shift.
I suppose a suspension system could be installed but the trailer really isn't bad as is... in fact...
I like it!
Comments, questions?
