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Dual hubmotor powered trailer build

ecat

10 W
Joined
Apr 8, 2009
Messages
88
The idea for this trailer came from a need to pick up and deliver bikes and scooters for my e-bike business without using a car. This is especially important here on Salt Spring Island, as going off island means getting on a ferry, which costs $$$. By using the BC Ferries "Discover" card bikes travel free, even when they are 16 feet long!

trailer drawing.jpg
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So here's the plan:

1. 8 foot long cargo bed capable of carrying three bikes, or two bikes in boxes.
2. Welded steel frame for strength and ease of fabrication.
3. Two medium power direct drive hub motors in 20" rims with matching controllers capable of regen braking.
4. On board battery capable of powering the two controllers.
5. Reasonably nice looking as it will be going to events etc.

I took a look on line, especially at Tony's Trailers, as he makes large steel bike trailers. The most important take away was how he drops the cargo bed below the level of the axles. This keeps the load low for stability. I have tipped my kiddie trailer on at least one occasion, resulting in a laptop falling out, and going skidding across the road.

I thought about using round tubes like Tony's Trailers, but ended up going with square tubing to simplify joints. The base frame is made from two bed frames that I picked up from the buy and sell, the uprights and rails are 0.75" and 1.0" square 0.063 and .0065" wall hot rolled steel tubing The tongue is 1" X 2" 0.083 wall tubing.

Welding was fairly straight forward, I am not a great welder, it just takes persistence sometimes. It is quite easy to blow through the 0.063 wall tubing, but the holes can be welded over. I thought about a fold down tail gate, but ended up just using a tube that attaches with large plastic thumb screws that thread into nuts welded to the bottom of the tube.

I also put one of these adjacent to the wheels ,between the upper rails. It is handy to secure bikes to, and reinforces the trailer structurally as well.
The most critical part of the welding was to make sure the axles were perfectly aligned, so that there would be no extra drag or tire wear. A piece of angle iron run across all four drop outs worked for this. Clamp the drop outs in place, tack them on, re check the alignment, then weld.

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I wanted to be able to adjust the power levels, and things like when the motors start relative to the towing bikes motor, so having a CA3 made sense. So the throttle signal on the towing bike is split off between the throttle and CA on the towing bike, as well as the ebrake signal. I also wired up a three way switch on the towing bike for power limiting the trailer. The signal wires as well as a 14 gauge ground wire terminate in a quick disconnect at the trailer hitch. The heavy ground wire ties the negative of the two batteries, on the bike and the trailer together, so that the the throttle and other signals have a common reference.
I used a hub motor type water proof connector that has three larger, and five smaller connections. This has proven to be not 100% reliable, and a pain to take apart, so I bought this one from Digikey, switchcraft 6282-6PG-3DC and 8282-6SG-3DC I will replace the original Ebay item with these eventually.

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sw craft conn 2.jpg

The Grinfinion 25 amp controllers are mounted on a plastic project box that contains all of the connections. A Grin shunt-CA3 connects between the battery and the controllers to get the current consumption from both controllers. There is also a DC to DC converter, a flasher circuit for the rear lights and a switch that toggles between flash and steady.

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I cut off all of the JST plugs on the CA3 and Shunt, and used a couple European style barrier strips to connect everything. This turned out to be a mistake. These strips accept only bare wires, and they have been the source of some trouble, as it is easy to make a less that reliable connection, especially when inserting two or three small wires in one hole. I plan to change these to Cinch Jones style terminal blocks, and crimp ring terminals to the wires.

barrier_strip_euro_.jpg
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For the trailer hitch I found this nice 12mm quick release ball joint on Ebay from this store http://stores.ebay.ca/illstonandrobsonltd/

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I welded up a clamp that attaches to the seat post of the towing bike, and bolted the ball to that.

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I considered doing a 5th wheel type arrangement and bolting the ball to the cargo rack on the Edgerunner. But I think the seat post is more versatile, it could be switched to another bike, even a non powered bike if you added a throttle and ebrake. The loaded trailer is heavy, so the location of the hitch is important. If it were at the axle or behind the back tire then the weight of the trailer may tend to try to kick the back of the bike out sideways if you brake or apply power while turning. As it is now if you apply power in a corner the push from the trailer wants to push the bike over, so you have to lean into the corner harder. This is something that you can adjust to.

The motors are the new Nine Continent M3007RC hubs. I had Grin lace them into 20" rims. Looking at the Grin motor simulator I chose a winding that would top out at about 35 km/h at 52 volts. This is a good speed for long distance towing, I would not want to go much faster. Towing this rig with a couple hundred pounds on board feels like driving a freight train. There is plenty of power on tap, I actually dialed the max power back from 50 amps to 38, the full power is simply not needed. The last trip I made it used 19 Wh/km which would give me a max range of about 50 km with the Prius plug-in battery.

The battery I got as an experiment from Hybrid Auto Center. This was before I started selling the Expedition batteries. Something like a 25ah 52 volt Samsung 25R pack would have been smaller and lighter. Anyways, the Prius pack is 20AH, 52 volts in a 14S 1P configuration.

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It should be capable of 200 amps, and it should last a long time. Problem is that it is big and heavy at 16 pounds. The cells are metal can Panasonic lithium with handy screw terminals on the top.

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The pack is held together in compression with riveted metal straps, and has plastic spacers with provision for air cooling between the cells.

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Hybrid Auto Center does not seem to be carrying them any more. I wanted to be able to set a lower balance point, like 4.05 volts, to extend the battery life, so I picked up a Founding Power programmable BMS. This turned out to be a pain in the ass, as programming involves interfacing to a PC and running their programming software, which is not all that intuitive.

bms.JPG

Two other problems with this BMS, it can't take a turn on surge, it goes into alarm if you try to turn it on with the controllers connected, so I had to put in a soft start button that charges the controllers caps through a resistor. The other problem is that it takes its power from the first few cells, so if you leave it on for a few weeks it unbalances the cells. A cheaper standard 14S BMS would probably have been fine, maybe with the addition of some connectors to plug in a Cell Log once in a while.

I built a battery box from 1/8" marine plywood, with two layers laminated with epoxy. its not to easy to work with the thin plywood, but with this laminating method I could make a water tight lid that fits on nicely. I secured the lid with bungee straps.

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The fenders were also made with the same method, bending two layers of thinner plywood strips over a C shaped plywood form, gluing and clamping.
The bed is 1/2" okume plywood, which I routed out in a square grid pattern on the bottom side to take some weight off. This was a lot of work, but the material seemed just too heavy. I finished it with three coats of spar varnish. I also installed a couple of light aluminum channels that bike tires can sit in. I had these sitting around. Another option would be to purchase a car carrier bike rack that holds multiple bikes on the roof or a pickup bed, and bolt this into the trailer.

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Further plans include making up coroplast side panels with my logo printed for advertising.

The biggest shortcoming of the trailer is the lack of a suspension. I thought of a way to do this, with a pivot point ahead of each wheel, and a bike shock behind the wheel. Maybe I will get around to doing the modification eventually.

Another improvement would be to use moped type rims for a heavier duty wheel. The bike rims I used are holding up fine so far.

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The trailer works as expected, and it is surprisingly powerful. If I stop on a hill with it loaded up, I can hardly move the bike and trailer, yet with a touch of the throttle, up it goes, no problem at all. The variable regen also works well, it really is essential for safety.
 
Interesting ecat! I've been to see this local guy (I'm in Toronto currently):
http://cappelcustomcarts.com/index.html

... for Manuel to build for me a rickshaw... So target weight 500 lbs of trailer with passengers, to tow behind an ebike. (All on basically flat grounds around our harbour area.) Target market? Tourists. :)

NonomiyaRickshaw0289.jpg


So one "standard" ebike but you've given me a thought about that spare hub motor I have kicking around. :wink:
 
Yup, one rickshaw goes electric assist, then everyone in going to want one!
 
ecat said:
Yup, one rickshaw goes electric assist, then everyone in going to want one!

Hehe... Anybuddy can do this. Wanna take Saltspring Island? Sneaky thing is, looking for kickbacks from local ebike retailers. "No Charge" for rickshaw rides (other than any tips given). Bucket of ice on board... "Would you like an oyster Ladies?" Caviar? Local cheese? (Pay something for food and drinks service.) "Oh. And please use the ashtray provided." Idea is to not step on any local taxi services. But cards on hand mention name of local ebike stores... hence the kickback as cards show up and translate into sales. A fleet of rickshaws and bikes operated by students, so they make some money too.

Too sneaky?

EDIT: This is ALL about promoting ebikes. (See my tag lines.)
 
Someone could probably make a go of it with rickshaws here on summer Saturday's. There is a big outdoor market, and good luck finding parking anywhere near Ganges after about 9:00am!
 
Go forth and multiply. 1) Just fabricate one, 2) Hire a student for summer employment, 3) Build #2. Repeat. SHARE any profits. I'm used to working with a team. When I was an "employee" I worked my way up/into management. On shore it's termed "Business" but read a bit about piracy and how captains and crews worked together. (Ya don't want to get marooned.) Plan is, for this biz to wire you some money each month to wattEVer sunny, warm island you choose. My suggestion? Little Exuma in the Bahamas.

BTW. Suggestion for couch was groovy, but in EV terms any/all weight bad. So go with mesh for seating passengers and add pillows that can be dried. There's a species of tall fast-growing bamboo that can be bought as a small plant from a greenhouse on the BC mainland. It's watt I plan to use to fabricate rickshaw frame in part. That Toronto Islands Guy I referenced earlier? Manuel builds these:
Screen-shot-2010-04-12-at-8.22.14-AM.png


So steel, but heavy/solid.
 
Hay rides at the fall fair!
 
Amazing Build and great food for thought! :)

I really love the idea of a powered trailer, I wasn't sure if I saw something of a stand at the front of the trailer to take the weight of the load when parked?

THIS is the kind of build I have been hoping to see where E-Bike technology is enabling you to take a much more cumbersome and or heavy load that normally

would be out of the realm of a bicycle, and making it work!



I once saw a guy with a HUGE trailer (easily as long as yours but a bit wider) towed by a pedal bike selling scrap metal at the metal recycler, but I think he stuck

Aluminum for the most part (old broken bicycle frames and such) and was thinking how an E-Bike might do that even better, you have done it!

I plan on making a much more modest version of what you are doing here as in a Semi-Recumbent Vision R-40 towing what amounts to a converted kid trailer

with plastic tubs in the back for lighter/smaller packages, and towing no more than maybe 100lbs total cargo at one time (possibly split between the cargo paniers

& trailer depending on size & weight of cargo).

Unfortunately the local "Green Grocer" store that I was hoping to do deliveries for closed down, but I still would love to do some kind of bicycle messenger service

that would handle stuff in-between what would normally be packed in a messenger bag, but not heavy/bulky enough to require a delivery truck.

(Link in my sig below if you're curious :) )
 
LockH said:
Interesting ecat! I've been to see this local guy (I'm in Toronto currently):
http://cappelcustomcarts.com/index.html

... for Manuel to build for me a rickshaw... So target weight 500 lbs of trailer with passengers, to tow behind an ebike. (All on basically flat grounds around our harbour area.) Target market? Tourists. :)

NonomiyaRickshaw0289.jpg


So one "standard" ebike but you've given me a thought about that spare hub motor I have kicking around. :wink:

E-Bike powered Rickshaw?! Love it! :mrgreen:

I'm guessing you want it to be separate so you can just use the E-Bike as is when not pulling it as opposed to a Ped-i Cab?
 
LI-ghtcycle said:
I plan on making a much more modest version of what you are doing here as in a Semi-Recumbent Vision R-40 towing what amounts to a converted kid trailer

with plastic tubs in the back for lighter/smaller packages, and towing no more than maybe 100lbs total cargo at one time (possibly split between the cargo paniers

& trailer depending on size & weight of cargo).

Unfortunately the local "Green Grocer" store that I was hoping to do deliveries for closed down, but I still would love to do some kind of bicycle messenger service

that would handle stuff in-between what would normally be packed in a messenger bag, but not heavy/bulky enough to require a delivery truck.

(Link in my sig below if you're curious :) )

I have one of these now mounted on my kid trailer that has the canvas stripped off and a couple of added rails for support, cheap, lightweight, strong, very nearly waterproof even in driving rain (found a small dribble inside after recent days of deluge) and they are designed to be stacked on top of each other or with a longer trailer frame you could put one behind the other.. About 15W x 30L x 14H. You can even put a padlock on it to keep the honest people out.

I've had cars with smaller trunks.. :mrgreen:

2820b.jpg


http://www.tractorsupply.com/tsc/product/storage-trunk
 
I like the trailer...I had planned to build a cargo bike that could do this kind of thing in it's extendable cargobed, but I never have gotten around to it. Everytime I remember the project idea (look up Barkfiet here on ES) I want to go cut metal and start welding right that instant, but there's always a reason I can't (like right now it's 2:14am and the neighbors might object :lol:).

I also have planned on a powered trailer the size of yours, too, though in my case it would be double-wide rather than double-long, simply because it's intent is to carry a couple St. Bernards side-by-side (and/or cargo larger than could be fit on a narrower bike-width trailer).

For the weights I carry in mine, I use an automotive ball-hitch behind the rear wheel at about axle-height on my present SB Cruiser trike and CrazyBike2 bike, though I have successfully used one made from bicycle stems, headtubes, and steerers, at seatpost height, on a more normal bicycle (DayGlo Avenger).


So...anyway, I might borrow some ideas from yours, when I do get around to it. ;) Thank you for posting all the detail you have...I might have questions later on, if I do get around to my own super-duty trailer. :)
 
LI-ghtcycle said:
I'm guessing you want it to be separate so you can just use the E-Bike as is when not pulling it as opposed to a Ped-i Cab?

Yes. Exactly. By config'ing design of trailer "rickshaw"-style can still do some pickups and deliveries, but also get into the tourist business. And generate revenues from sales of ebikes rather than direct from customers taxi-style. Like AW said using an auto-style ball trailer hitch to make the point it is otherwise a "normal" ebike being employed rather than those (relatively) bulky trikes with back passengers as more commonly seen in the tourist biz.
e-rickshaw-electric-tricycle-with-passenger-seat.png


That (above pic) design seems overly chunky to me.

(And yes re comment to have some sort of partial or folding leg to stand the rickshaw when not in use. Otherwise, design with passenger, etc weights more or less balanced over the rickshaw wheels axle.)
 
Cool pickup truck. I've been toying with a similar idea, more like Amberwolfs trike. But no real use in mind for it. Just something to put some useless old front hubs to work with.

I started, but never finished a project for a rolling couch thingy. Easy to convert the idea to a powered trailer able to carry 4x8 sheets of plywood, or other large cargo.

But then reality kicks in, I have a nice light trailer for my Subaru that does such work fine.
 
"Further plans include making up coroplast side panels with my logo printed for advertising."

BTW. Have been accumulating left-over election signs to maybe fabricate a shell velomobile-style re warmth and dryness and more aerodynamic.
Vetter-streamliner-side-123-web.jpg
 
Wow, that's awesome. I'd be careful about overloading those tires in the back ;)

Great show man, love it.
 
Re the tires: I am using Kenda K-924 20 X 2.125 ebike rated tires. They are fine, pretty tough and heavy. (And available soon at juicedriders.ca :D ) Is there a tire anyone would recommend for being super tough, and having a large profile to give the best ride with a heavy load?
 
Re the question about a stand to keep the trailer level: I did put a pair of rubber feet on the front, just so it can sit on the ground without making horrible scraping noises. There is only 5" of ground clearance, and with the long overhang it sits fairly level off the hitch. Curbs and big speed bumps can result in horrible scraping noises :shock:
Anyone ever see dully bike wheels?
 
Yes, several FreakBikeNation members have built various kinds. Unfortunatley the original forum that had pics of all that stuff no longer exists, and I don't have access to the backup I made before it went down (it's somewhere around here). Might be on archive.org, but I would have to remember the URL to get to it first. :/ Some people may also have pics on their own sites, if you look for FreakBike, or FBN or FreakBikeNation (with or without spaces). One was a hearse or coffin-themed trike, I think it was, if it helps.

There've also been pics I've seen of non-FBN creations, here and there over the years.

Some people laced two rims to a single hub, standard or custom-made;

Some just put two independent wheels on one double-length axle. This is one thing I have considered for my big unpowered trailer idea, and for one that would be chain-driven from powerchair motors.

Some put a frame and dropout section in the center for supporting the long axle in between the two wheels as well as the ends, most didn't, only supporting it at the ends.

One put the support *only* in the center, using a suspension fork rebuilt with both sides cut off and welded right next to each other in the center, with the single axle thru them supported only at those centered dropouts, and a wheel on either side with a washer and acorn capnut on the outer ends of the axle.

I have no idea how well any of these actually *worked*, as most of what I saw was just images of the build, and had nothing else for context. A few said they worked fine.

Probalby other variations I've forgotten.


As for tires, the moped types like Pirelli ML-75 and a Shinko that I can't remember the name of, are available in widths that will fit (many?) 20" bicycle rims. If you have wider rims, like the one I have on the back of CrazyBike2 and the rightside of SB Cruiser, which came from spare parts for old version of the Zero, you could probably get some pretty wide ones on there. I haven't been able to get the tires to test this yet, but I would really like to, as I wear out bicycle tires pretty dang fast. :(
 
Re clearance and scraping, I've seen big RV trailers with giant casters at the rear bumper. Or, just skid plate type things designed to handle the scrape without damage to frames or bumpers. Maybe just add two non swiveling casters to the rear of the trailer?
 
Jonathan in Hiram said:
LI-ghtcycle said:
I plan on making a much more modest version of what you are doing here as in a Semi-Recumbent Vision R-40 towing what amounts to a converted kid trailer

with plastic tubs in the back for lighter/smaller packages, and towing no more than maybe 100lbs total cargo at one time (possibly split between the cargo paniers

& trailer depending on size & weight of cargo).

Unfortunately the local "Green Grocer" store that I was hoping to do deliveries for closed down, but I still would love to do some kind of bicycle messenger service

that would handle stuff in-between what would normally be packed in a messenger bag, but not heavy/bulky enough to require a delivery truck.

(Link in my sig below if you're curious :) )

I have one of these now mounted on my kid trailer that has the canvas stripped off and a couple of added rails for support, cheap, lightweight, strong, very nearly waterproof even in driving rain (found a small dribble inside after recent days of deluge) and they are designed to be stacked on top of each other or with a longer trailer frame you could put one behind the other.. About 15W x 30L x 14H. You can even put a padlock on it to keep the honest people out.

I've had cars with smaller trunks.. :mrgreen:

2820b.jpg


http://www.tractorsupply.com/tsc/product/storage-trunk

Thanks for the link!

I have similar tubs for the back of my current set-up, minus the locking feature, I will have to get some! :)
 
Fantastic!

I've been working on similar general design, but you obviously put thoughts into action!


I'm still over-thinking and under-doing on my ideas, posted here on Google Docs:

https://docs.google.com/document/d/1leLktH0m-vSxp7vkphM9zr2F8Y0DVYArfpluelK7GuQ/edit?usp=sharing.

My main starting point is a high-capacity trailer that uses sensors to drive itself such that the pilot vehicle experiences near-zero forces -- only enough that the trailer knows it's time to keep up, or time to slow down. I'm hoping a finely-tuned control system could keep these control forces within ounces, or at most a 5 pounds. If I can achieve that, then the trailer can be huge, can be towed by person, bike, or car, and won't kill anyone.

My concept design has your same goose-neck hitch, and approximate cargo bed length. Was thinking to go big with 2,000 lbs capacity (for bike-based electrical contractor business) and super-calibrated throttle / regen brake controls via a linear position sensor integrated in the goose-neck hitch arm, to avoid jack-knifing and trailer push on the pilot vehicle.

I really like the look of the eCarla trailers featuring a front fork style steering front wheel (3rd wheel), and haven't decided whether to continue pursuing that, or not. I'm encouraged that you've had good results without that added complication, though I do think some built-in steer in the trailer could reduce lateral forces on the seat post.

I haven't updated the file linked above, but I'm now thinking to go your route with simpler square tube and/or prefab extruded aluminum assembly components, vs. fancy bends and near-impossible-to-find aluminum welding services for my original sleek-curving concept frame sketches.

RE: suspension
I've been looking at automotive trailer 'axleless' suspensions. There's Timbren rubber bolt-on (per side), and FlexiRide 'half axle' (ie: axle-less) torsion that bolt on to a weld-on adapter plate. I haven't looked hard, but haven't found any off-the-shelf wheel hub motors for trailer sized wheels. It seems there's a gap between bike-type ones that regular bike mechanics can lace up to bike rims with spokes, and pre-assembled automotive wheel hub motors that are much bigger heavier wheels.

RE: Ride dynamics
I'd love to get detailed reporting on how a long heavy trailer hitched to seat post affects ride safety -- especially in downhill turns, which I imagine might be terrifying. I'm less worried about the trailer over-running me while traveling straight, than about it pushing my back end out-of-line with my direction of travel, even if the forces on my bike/trailer relative position sensors aren't pushing the sensors out of 'equilibrium' position.

I hope to control basic position of bike/trailer with bike-trailer relative position sensors (look like gas shocks but with sensors instead of damping) that cause the motors to respond exponentially with throttle or braking as position varies from equilibrium. I had thought to keep the hitch-arm system simple (and easily adaptable for use by a pedestrian pulling the trailer (cart), a bike seat-post, or a car) with a single position sensor integrated in the hitch arm. But I'm now thinking to get 'stereo' reading of the relative direction of the pilot vehicle and trailer, using two linear sensors that would compress on one side and expand on the other in turns (or have a difference to each other even if accelerating or braking hard causes both sensors to extend or both to compress, when there is hard acceleration(braking) in a turn).

Anyway, if you can think of any automated throttle/e-braking that would make your ride smoother/safer/more efficient, then those are probably things I want to have my sensor/control system accomplish. This could include even differential throttle/braking on the two wheels.

Links to most of the parts above are in the linked document.

Thanks for sharing! It seems the e-trailer movement is taking off (I'm finding much more now than in 2019 when I was looking).
 
eMULE2020 said:
My main starting point is a high-capacity trailer that uses sensors to drive itself such that the pilot vehicle experiences near-zero forces -- only enough that the trailer knows it's time to keep up, or time to slow down. I'm hoping a finely-tuned control system could keep these control forces within ounces, or at most a 5 pounds. If I can achieve that, then the trailer can be huge, can be towed by person, bike, or car, and won't kill anyone.
Some thoughts on this from my Mk IV / V trailer:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=76539&hilit=trailer&start=25#p1243408
There are a number of possible variations on the ideas there, and some others, that I can discuss here with you if you like.


For organizational purposes, it might be good to put your trailer in it's own thread, however, perhaps moving your post that's already here (and my reply) to that thread, and then just linking to that thread from here. I can do that if you like, or you can keep posting here instead. :)



I really like the look of the eCarla trailers featuring a front fork style steering front wheel (3rd wheel), and haven't decided whether to continue pursuing that, or not. I'm encouraged that you've had good results without that added complication, though I do think some built-in steer in the trailer could reduce lateral forces on the seat post.

I can say that the trailers I have built for heavy cargo (especially the wiggly kind) that have their own front wheels (either a pair at the corners or single) have had a number of problems in various situations, which I did not have with the "normal" trailers with only side wheels. The first problem, of being unable to go over curbs and up driveways, etc., without the hitch forcing either the bike or the trailer to "seesaw" could be fixed by a vertically hinged and sprung interconnect bar between the trailer and the bike. But that's added complication I didn't want. The other big problem, of trailer "self steering" downhill on crowned roads and other sideways-sloped surfaces, I didn't ever think of a solution to; it was a big problem when heavily loaded.



RE: suspension
I've been looking at automotive trailer 'axleless' suspensions. There's Timbren rubber bolt-on (per side), and FlexiRide 'half axle' (ie: axle-less) torsion that bolt on to a weld-on adapter plate. I haven't looked hard, but haven't found any off-the-shelf wheel hub motors for trailer sized wheels. It seems there's a gap between bike-type ones that regular bike mechanics can lace up to bike rims with spokes, and pre-assembled automotive wheel hub motors that are much bigger heavier wheels.

I haven't used suspension on trailers yet, but have sketched a few ideas that recycle bicycle frames to do it reasonably simply. A method I was going to use on the v2 of SB Cruiser would've been something like the powerchair suspensions, or those on pedicabs,

FWIW, I highly recommend full-size (26-29") bike wheels for the trailer, as it will give a much better ride than smaller diameter wheels, even without suspension; larger wheels roll over holes and bumps and curbs much easier than smaller wheels. I'm using 26" on the big trailer now but would use 29" if I had them. I've used 20" on several trailers, and they suck on anything other than sidewalks and perfect roads. 24" wasn't quite as bad, but 26" was way better. I used some OTS wheels equivalent to 29" once, and that was even better, even though the tires were way way skinnier. If I were to ask for the "perfect" wheel it would be a 29" with 3" tires (Origin8?) on them. (that's what I was going to use for SBC mk2).

If you need a lower deck for better COG and stability with tall loads, you can just suspend the deck from the framework holding the axles, so the axles are above the deck. My Mk IV / V trailer does this with great success; to haul an upright piano I just added a second pair of wheels just outside the first (partly to distribute the load better, partly for redundancy while loaded). You can still use suspension this way as well, either by pulling the springs instead of compressing them, or by using a lever arm that converts the upward wheel motion into a downward force on the spring / shock to the frame.



Regarding hubmotors....how much power do you need, for how long? There are three basic sizes, other than the small 250w-or-less geared hubmotors. The 500-1000w geared hubs (MAC, BMC, EZee, GMAC, Bafang), the 500-1500w direct drive hubs that are all roughly the same size and mass, and the 2000-5000w direct drive hubs that tend to be used on scooters and small "motorcycles". There are some variations within those.

Most of the DD motors can be used at several times their rated power for short bursts, as long as they have enough time to cool back down between bursts of this kind of usage.

Geared hubs too, but not as far beyond their rated power nor for as long, and much longer between bursts, since it's harder for them to shed their heat.

I have a pair of small geared hubs in 26" wheels that I would someday like to use on the Mk V trailer, but I don't know when (or if, at this point) I will get around to doing it. Braking would be mechanical disk brakes, since the geared hubs freewheel in that direction and can't be used for regen (but ride easier without power).


RE: Ride dynamics
I'd love to get detailed reporting on how a long heavy trailer hitched to seat post affects ride safety -- especially in downhill turns, which I imagine might be terrifying. I'm less worried about the trailer over-running me while traveling straight, than about it pushing my back end out-of-line with my direction of travel, even if the forces on my bike/trailer relative position sensors aren't pushing the sensors out of 'equilibrium' position.
None of my trailers have been that long, but I suspect they've been more heavily loaded. ;) The problem with the seatpost-height mountings I've used (at the rear of a rear rack, or further forward) are that they tend to try to push the bike over on it's side, especially if heavily loaded and on a downhill or just if going fast enough and a sudden avoidance maneuver is needed for whatever reason.

These days I try to put the hitch at axle height. This has prevented that type of problem, and has not introduced any others I'm aware of.

For convenience and ease/strength/reliability for my heavy cargo loads, I started using automotive ball hitches, since the other types I'd made were not as reliable under some conditions and loads as I had needed them to be. So far, never a problem with the ball hitches; I use a looped cable from the bike side to the trailer side for redundancy (like the chains people often use on cars/trucks for these hitches), and I use a padlock thru the lock hole in the hitch so no one can easily remove the trailer from my bike or trike when I'm out somewhere with it and have to leave it.



I hope to control basic position of bike/trailer with bike-trailer relative position sensors (look like gas shocks but with sensors instead of damping) that cause the motors to respond exponentially with throttle or braking as position varies from equilibrium. I had thought to keep the hitch-arm system simple (and easily adaptable for use by a pedestrian pulling the trailer (cart), a bike seat-post, or a car) with a single position sensor integrated in the hitch arm. But I'm now thinking to get 'stereo' reading of the relative direction of the pilot vehicle and trailer, using two linear sensors that would compress on one side and expand on the other in turns (or have a difference to each other even if accelerating or braking hard causes both sensors to extend or both to compress, when there is hard acceleration(braking) in a turn).

Anyway, if you can think of any automated throttle/e-braking that would make your ride smoother/safer/more efficient, then those are probably things I want to have my sensor/control system accomplish. This could include even differential throttle/braking on the two wheels.

I have only worked on ideas using a single set of sensors, but it makes some sense to try the "stereo" sensor approach, if you have independently controllable motors in the wheels.

I do recommend you use a mechanical braking system in the wheels, unless you are using very overpowered controllers that can create much more regen current than ones sized for the typical 500-1000w hubmotors you might normally use on such a trailer. Without mechanical brakes, you'd also need controllers that can brake down to zero, and hold that while stopped (these actually use power to hold position, not just regenerative braking which only works above a certain speed dependent on system specifics), unless you are sure that the traction from your bike's wheels is sufficient to keep the trailer in place in all situations you'll be using it in that you must stay stopped. ;)

My present trailer has no brakes at all, and I've had a few situations with heavy loads (like the piano, or previously with a several-hundred-pound load of dog food), where I could really have used trailer brakes, because I had to go much slower than normal and be much more sure of everyone around me, and wait much longer to cross streets, etc., than I would have if I'd had the brakes on the trailer in addition to the ones on the bike / trike.
 
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