Looking for tandem conversion suggestions

[DufferDave7]

I'm looking to purchase a used tandem and then add a motor. The setup will be an adult up front and a child in the back and a tag-along attached at the rear. It will be used on a variety of terrain, but I live in the Rocky Mountains, so there will be plenty of hills. I'm also not a bike expert, but am an engineer who can usually figure things out. However, I don't want to be having to do things like machine custom parts. I want something I can do in a day.

I'm trying to figure out both a good motor setup and a tandem that will work well with that. The information I've read is all over the place, but obviously there are three general motor approaches.

Front hub - wildly varying opinions. A lot of folks comment about potential torque issues as well as front fork strain. However, it seems that most of those who have actually tried this setup have not had problems.

Rear hub - I haven't read about many folks who went this route, but it seems like it would be a reasonable approach.

Mid-drive - Seems to be the best performance option. However, install reports range from slightly tricky to "thankfully I work at a welding and metal fabrication shop". The biggest issue seems to be motor clearance with a left side timing chain. If I can find a tandem with a right side timing chain option it seems like it should be pretty straight forward. But finding a used tandem with a child height rear seat and right timing chain is difficult. I also haven't been able to find exact specs, like you need X mm distance from bottom bracket to left sprocket to fit a BBSHD. That sort of information would be very valuable.

Any thoughts on general approach as well as specific motor suggestions? Thanks.

 

[amberwolf]

Middrives often (but not always) require some adaptation or fabrication to fit them to bikes. Sometimes youc an buy specific parts ahead of time to deal with this, and soemtiems you won't know until it's actually being installed.

Hubmotors usually should have well-fitting torque arms on their axles, and good ones are almost never supplied with them (except from ebikes.ca); they are usually loose-fitting and thus not useful for the purpose. These TAs need to fit with the rest of the axle hardware at the dropouts, so anything unsual about the dropouts or their interface with the frame tubing can mean adaptation / fabrication is required to get it all setup securely.


Hubmotors, when specified properly for the application / power level /etc, and if built into a good wheel (vs what most "kits" have, using too-thick 12g spokes, etc), can be completely reliable with virtually no maintenance (maybe adding a bit of oil to the bearings now and then, no disassembly required).

Middrives will require regular drivetrain maintenance, and may require other maintenance of the internal drive mechanics, most often sprag clutch and/or gear replacements, based on posts in various middrive threads here. The bike's pedal drivetrain will have to take your power plus the drive's power, and will wear chainrings, sprockets, freewheels, and chains faster than normal.


Hubmotors will need to supply higher power for the same torque as a middrive thru the pedal drivetrain gears being properly shifted, so the battery has to be able to handle that higher power and may need higher capacity for the same range.


For the flat area I live in, I use a pair of hubmotors for the SB Cruiser trike to haul heavy cargo, groceries, and my St Bernards (presently just JellyBeanThePerfectlyNormalSchmoo), and the motors are reliable enough given the abuse I put them thru (which is why I have two, in case there is an issue..but there hasn't been with the higher-quality system on there now). .


As for specific motor suggestions, I recommend first going to the ebikes.ca motor / trip simulators, and putting in your specific riding conditions, weights, speeds, etc, and seeing what power level will be required for the worst case terrain and weights and speeds required. I recommend reading the instructions / data below it, then experimenting with different systems and options, and always using it with *no* human input power so you get a *real* motor power requirement (in case for some reason your pedal input is not available).

Knowing those, motors can be suggested that provide that reliably.

Also batteries can be suggested based on that power requirement plus whatever range you require.

Controllers can be suggested from that, once you define what features you need from the system. Depending on what you're after, there can be many feature sets (with or without displays, what you want to see on that display, programmability, customizeablity of behavior (which behaviors, etc), regen braking or not, PAS or torque sensor control (including whether you want PAS as assist levels that are just full speed/power of the assist level, or if you want continuously variable pedal control of the system), throttle, what kidn of motor drive (sine, trap, FOC,) etc etc.

 

[glennb]

I like your plan, but there's issues with it.

What's your experience with tandems?

I'll assume not much ...

How long are your kids" legs?

And are they going to contribute meaningful power?

If no to power, no point in a tandem.

If legs are short, again no to tandem.

Tandem's basically don't work with two different size riders, due to the longitudinal bracing requirement. They shimmy badly regardless, but you can't have a little step through rear end.

Also, they require concerted synchronised riding technique. Stretched out frame with wobbly novice stoker is kinda fun, for about ftwo minutes, but if your stoker doesn't get the hang of things quickly it gets frustrating rapidly.

Anyone coordinated enough to ride a tandem is capable of riding solo, so I'd suggest separate bikes, with the tow-a-long hooked yo your bike.

Food for thought

 

[DufferDave7]

Thanks amberwolf for the info. That seems to confirm my thinking that mid-drive is more hassle than it is worth.

Glennb, I do also have concerns about a tandem as I do only have limited experience and always with similar sized riders. The situation is a little different though. Its a middle school aged child who physically can and does ride a bike capably, but does not have the mental capabilities to necessarily make sound decisions once we get beyond casually riding in flat neighborhoods with little traffic. There is a good chance they will continue to ride with us as an adult and also want to be able to feel like they are participating and contributing and not just getting pulled in a trailer or something. Currently they are using a tag along but it is starting to get too small.

I’ve got a line on a CoMotion Scout which seems like it would work well for a kid or an adult in back.

 

[PaPaSteve]

Perhaps useful info . . .
IPS or Independent Pedaling System, A pedal system where the two riders of different skills or strengths can be separated and yet both can still contribute.
Originally developed by Vision recumbent company the product is still available.
This was a useful tool when the kids were small, no matter what, I could still pedal them home.
CoMotion Scout is a perfect bike for the task.

Advanced Transportation Products, Inc. Independent pedaling system for tandem bicycles.

HPVSO

www.visionrecumbentinfo.com

 

[Chalo]

Front hub - wildly varying opinions. A lot of folks comment about potential torque issues as well as front fork strain. However, it seems that most of those who have actually tried this setup have not had problems.

Tandems place a lot more of their weight (and they pack more weight overall) on the front wheel, so there won't be skidding under power. Also, tandems are made with especially heavy duty forks because their length and weight distribution allows much harder braking than a single bike, so a hub motor won't challenge them structurally either.

 

[DufferDave7]

Thanks for the input. It's looking like a front hub motor would be the best choice.

 

[glennb]

With rigid steel disc fork, front hub's an easy decision.

The only real limitation is 36 spokes, unless you get one custom drilled. It used to be possible with Aikema motors. The 100H model might suit you. They were standard 36h, but if buying from BMSBattery you could request other drillings (typically 32H, but in your case 48h).

That Comotion looks perfect for your needs, can handle a small stoker. Might eventually run out of seat post after a teenage growth spurt, and have to switch to a different frame I guess. But that's another argument in favor of front hub motor.