Highest Torque Low Speed Hub Motor for MTB Tandem?

[BigBird]

I'm looking for the highest torque low speed rear hub motor I can find for a tandem off road only mountain bike.

This motor appears to have the best characteristics - BMC V4 TT (Double Torque) Geared Front Hub Motor Kit - http://ebikessf.com/node/363 - BUT it appears to be unavailable in a rear hub application? IF anyone knows of a source for rear version PLEASE let me know.

The best available rear hub motor I've found is this one. MAC 12T 200 RPM @ 36 volt kit with 12 Fet controller and 26" Alex DX 32 rim http://em3ev.com/store/index.php?route=product/product&path=36&product_id=55
IF anyone knows of something better PLEASE let me know.

Assuming we end up with the above motor, my assumption would be that we'd want to go with the 12 fet controller and a 48 volt battery? Any guidance you can provide on this will be appreciated!

My sweetie and I are both in our 70s but reasonably fit for our age. We will be pedaling the bike pretty much all the time. We need the motor primarily to help us make the usually very short steep climbs we encounter and to help us power through rock gardens, etc. where maintaining some speed/momentum is difficult but critical for balance, etc. Thus, we would seldom/never be using large amounts of power for more than a minute at a time.

One of my concerns is the strength/durability of the mounting threads for the freewheel. With low gearing and 2 people pedaling the human torque can be quite high for very short peaks such as crossing a log or climbing a short steep ramp. We have had instances of freewheel/cassette and cog failure in the past (when we were younger and before we upgraded to top of the line components). So quality of the freewheel mount is a concern (and we would try to avoid extreme pedaling stress accordingly)

THANKS for any help or advice!

PS: We are a 300 pound team with almost 2/3rds of that being the OFG (OLD Fat Guy) captain!

 

[dogman dan]

12t Mac likely a good choice for you. A low speed direct would need to be at least slightly powered up the whole ride.

So a geared motor like the Mac as good a choice for you as it gets, in hubmotors.

Run 1500w or so max, 30 amps of 48v.

But you should look close at the Bafang mid drive motor before you settle on a hubbie. A mid drive might be a much better choice for your needs.

 

[BigBird]

I have recently found data on the latest and most powerful Bionx D500 DV system. I had dismissed Bionx based on the relatively weak power of the earlier systems BUT it appears this one is a BIG step up on the torque output and is relatively light and comes with a very wide 26" rim (which is important on an off-road tandem).

I haven't seen any independent testing, etc. BUT it does look like it could potentially be a workable system for my application.
I LOVE that it has built in torque sensing which I have found to be a very good system in technical off road use from using it in my single MTB with the FABULOUS Bosh 350W system.

I'd appreciate any comments on this system from folks that has experience with it.
Also IF anyone know of a source of independent testing that would be appreciated as well.

 

[tandemguy]

I have a DaVinci road tandem and I'm also looking for a high torque motor. The BMC 4TT front hub motor seems to check all the right boxes. Like you my stoker and I are getting "a little older" and when we head out for some long rides. After about 15 miles there doesn't seem to be as much power coming from my stoker. My only problem is that it's very pricy, so I'm still debating my choices. Let us know what your final solution was.

 

[arkmundi]

12t Mac likely a good choice for you. A low speed direct would need to be at least slightly powered up the whole ride.

So a geared motor like the Mac as good a choice for you as it gets, in hubmotors.

Run 1500w or so max, 30 amps of 48v.

++ditto that! You may want to look at my Top 10 eBike Conversion Kits & Where to Buy list for others. That list research and compiled before I went with a MAC 10T from EM3ev.com.

 

[dogman dan]

I don't doubt bionx has improved. The key question is does the bike have said torque at low rpm without low efficiency.

Two ways to get that, the best being a tiny wheel. If you must use a large wheel, then the second best way, is a low rpm winding motor, like a mac 12t, or a similarly low rpm winding direct drive.

Best of all, both. That's why I built new bike this winter, one that is trying to be like an edgerunner, 20" rear wheel. With the EBK trike kit motor, it would have a top speed under 15 mph, on 48v.

 

[friendly1uk]

Few tandems have suspension. Most carry a lot of weight, and off road use is harder on components. You need to know the motors load carrying capacity.

I have a hardtail bpm and can't give anyone a ride. The bpm has an 85kg limit. I could risk it, but one day I would pull away and 'snap' fall on the floor. I doubt you fancy that. So although the mac is a 14mm not 12mm axle like a bpm, you may still want to think about a mid drive for piece of mind.

 

[BigBird]

I've learned that the motor controller limit is 30Amps SO theoretically it is possible to get 1440 Watts at 48V.
Also the MUCH larger motor diameter dramatically increases the lever arm for the power application thus effectively like gearing it down.
Looks very promising AND it has torque sensing pedal assist at 4 levels and 9/10 speed cassette.

 

[dogman dan]

Yes, big rotors are good. I wish there was a motor as big diameter as a magic pie, but also with a wide magnet, like 35 mm. It would be a heavy bitch though.

Combine that with a smaller wheel, and yeah baby. Heavy duty rim of course, more like a skinny scooter tire.

 

[dogman dan]

But seriously, I wonder if a bafang bottom bracket 48v kit wouldn't serve your needs the best. Gear it as low as you need, for huge torque at lowest speeds.

http://ridebionx.com/products/ebike/ebike-systems/d-500-dv/#techspecs

It does look like bionx really improved the motor, Huge rotor, like the magic pie. But like all hubmotors, you put 1400 w into it, at 5 rpm, up a long enough steep hill, and it will be running inefficient, and make a lot of heat.

On the flip side, with 1400w, plus two pedalers, you will likely be flying up that long hill at 15 mph, with decent efficiency. 8) I would fear no hill less than 10% grade with that bionx. Not even hills 10 miles long.

 

[BigBird]

The Barfang COULD work but has MANY issues.

The aluminum BB with eccentric makes fitting the motor problematic and you loose the timing chain adjuster. It is also questionable if the rear cogs/cassette can handle the torque of BOTH human and motor power in the lower gears. We have folded big cogs, etc. until upgrading to the strongest stuff available as for a short time a tandem with low gears can put a tremendous amount of torque on the big rear cog with 2 people standing and pushing to cross a log or make it up a short steep bank. Also getting torque sensing pedal assist (which is not required but is desired) requires 2 parallel timing chains with 2 added chain tensioners and a Thun in the stoker position. And the biggest issue is that the motor could make very hard contact with rocks and logs in some technical situations.

It looks like cooling will not be much of an issue with the BIG diameter thin motor but in any case we mostly need the boost more to help us get through SHORT steep sections and technical stuff rather than long climbs.

 

[dogman dan]

That can work fine then. If short hills, using the hub motor intermittently in an inefficient, heating it up way won't fry it. I've never melted a motor in less than 30 min continuous use. Even when pumping stupidly high power into them.

Sometimes it's hard to give advice, I tend to give advice based on the biggest hill in my area, It's profile is almost exactly the same as the famous Alp d Huez climb in the tour de france. So I start to picture an hour or two of continuous climbing that hill until told different.

One issue, a dd motor like the bionx will put some drag on you when you have it turned off. The solution is simple though, just keep it on in level 1 assist.

For the purest motor off performance, back to the Mac 12 t recommendation. Freewheels when it's off. I'm rather surprised that bionx has not offered a geared motor to get the freewheel feature.

As for pedalec, I find my hand on the throttle 10 times smarter than pedelec. I have not tried the good kind, I just mean the cheap pedelec using hall sensors sucks. My health does not always allow me to pedal every inch of a ride, so I cannot use any pedelec myself. On a long ride, holding a throttle for 70 miles can suck too. But for off road riding, I want that ability to modulate power instantly throughout the entire ride.

You have a valid point about the chain line problems with two stout pedalers plus a motor pulling that chain. Back in my prime, I used to break chain, by never shifting out of 52-11 gear.