Tandem front hub options

[brickelltandem]

We are looking for feedback on front-hub tandem conversions- successful? problems? thoughts?
(I found some helpful comments after word searching for "tandem" and "front" but not this scenario.)

Need: power assist on hills. We are NOT looking for full time power.
A few times a year we bike carriage roads on Mt Desert Island, ME with 2-mile or 3-mile ascents of 800 vertical feet on unpacked gravel.
Basically I provide _all_ the power for about 390 pounds of bike, gear, and two riders.

bike: SUN "Brickell" Tandem, 7-speed
manufacturer link: http://www.sun.bike/product_detail.php?cl2=TANDEM&cl3=BRICKELL
modified with:
-rear-hub shifting (no derailleur) and
- modified stoker and rear toothed rings, spacers, and crank arms to make bike more usable for handicapped rear rider.

usage: 10-20 mi RT rides on 80% paved and 20% dirt/gravel/unimproved rail trails, occasional steep hills.

current plan: -(Luna Cycle) front wheel hub 1000w rating
- 48v 11ah battery pack, controller, gauge, torque arms. advanced charger.

Budget: under $1,000 US.

Alternatives: Looked hard at the drivetrain-mounted Bafang.
Like it for the ability to climb on reduced gearing, but too complex to try to mount on our bike.

Thanks in advance for your experience or advice.

 

[dogman dan]

Nothing wrong with the plan to use a front hub. But because of the increased load slowing you down, a lower rpm DD motor would be a good choice. a standard speed motor will be less efficient on hills, or during start ups, particularly with that much weight on it.

Or a lower rpm 500w rated geared motor, if you don't climb very steep hills.

 

[brickelltandem]

"a lower speed DD drive"..

thank you Dan, that is helpful feedback.
Is there a model or size you can give an example of as "lower speed", I'm not sure I know which is which.

 

[Chalo]

If I only wanted to use the motor in my bike some of the time, a direct drive hub motor would totally bum me out. It doesn't drag a lot lot, but it is like a brake dragging all the time you're not powering it. And the faster you go, the more drag torque you get. Boo.

Aaaand... Hub motors make weak wheels. Any hub motor with narrower flange spacing or thin steel flanges is going to yield a weaker wheel than a normal bike hub. And there are no hub motors drilled for 48 spokes.

For your bike and your part-time motoring, I'd consider a friction drive like the Commuter Booster. That way you can have strong wheels, no drag when not motoring, and a very light supplementary power system.

 

[LewTwo]

... And there are no hub motors drilled for 48 spokes. ...

Correct me if I am wrong but I count 36 spokes in those pictures.

 

[LewTwo]

"a lower speed DD drive"..

thank you Dan, that is helpful feedback.
Is there a model or size you can give an example of as "lower speed", I'm not sure I know which is which.

Some geared motors (i.e. Cute Q100) come with different winding for a given maximum RPM: typically 201, 260, 328.
The concept is that smaller bikes wheels (i.e 16", 20", 24") need faster hub motors that a larger bike (i.e. 26", 700c, 29er).
However people are known to have installed all the above in all of the above.

...
One problem you might have with front wheel hub and uphill gravel roads is traction. The longer wheelbase and overall load should help but consider mounting the battery on a front rack to place all the extra power system weight on the driving wheel. Something like a Sunlite Gold Tec Front Rack is cheap and fairly easy to install even on a fork without mounting points. You just need to install a couple threaded inserts or 'P' clips. It also keeps the electrical cabling to a minimum.

 

[Chalo]

... And there are no hub motors drilled for 48 spokes. ...

Correct me if I am wrong but I count 36 spokes in those pictures.

Yes, cheap crappy tandems usually have 36 spoke wheels, often no different from those on cheap single bikes. And racing tandems sometimes have as few as 16 spokes per wheel.

But the traditional tandem spoke count is 48. That makes the wheels both stronger (because the rims are better supported) and easier to keep true. Freestyle bikes have featured 48 spoke wheels for the same reasons.

Fortunately, tamdemists can now get stronger rims than ever in 32 and 36 hole drillings. A rim like the Velocity Chukker built up with only 36 spokes can make a much stronger wheel than the 48 spoke tandem wheels of 30 or 40 years ago. (But the Chukker also comes in a 48 hole version.)

 

[dogman dan]

FWIW, I'm not allowed to sell my employers kit outside the for sale section. But since you specifically asked I can skirt the rule some. Read my signature to see who I work for.

As for the DD hub, People make a big deal about the drag a dd motor creates when not powered. Yes, it does slow the bike down, particularly if you are trying to pedal faster than 10 mph unpowered. The solution is quite simple, power up the motor. Not much, just enough to eliminate the drag. that only takes about 25-50 watts, which doesn't drain the battery much. Riding at 100w, a typical 500 watt hour size battery would take five hours to drain. The vast majority of people prefer to use a very small amount of power the whole ride, once they add a 20-30 pound motor and battery to the bike. You can ride a powerful motor with very low power, just select level 1 on your power level, and save riding with level 5 power for the hills. Easy as pie.

But there are lower speed windings of 500w geared motors, such as the Mac. I'm not sure if EM3ev stocks the 12t motor or not. But,,, the geared motors cannot cool themselves if you push them hard. So I DO NOT recommend one 500w geared motor for the heavier bikes, unless they will never be ridden up a long steep hill. Long defined as more than a mile or two. steep, over 5%.

Fact is, the geared motors make more torque, but they do it with a much smaller motor, smaller magnets, less copper. And once they do heat up, they have a very hard time cooling themselves off. For this reason any DD motor tends to be a better choice when the total weight of the bike, people, cargo, motor and battery will be over 300 pounds.

My opinion is based on actual testing to destruction, that I have done on a hill 5 miles long, with a grade starting at 3% that steepens to 8% for the last mile. The 500w geared motor fails with 400 pounds. The fast wind DD motor gets alarming hot but passes. The slower wind DD motor gets to the top hot, but not hot enough to darken the windings or burn out halls.

Bottom line, it depends on the hills. If your hills will never be steeper than 4-5%, or will be short, like half a mile or less, you can go for the geared motor. A Mac in 10t winding, or E bikekits similar 10t geared motor. But if you will have steeper hills, or very long hills, then you really do need the durability of a dd motor, including the faster wind types.

I get crap here constantly for touting the slow winds. They DO NOT make more torque than a faster wind in the same motor. But they work good on heavy bikes.

So do bigger DD motors. My heaviest bikes carry a big fat 25 pound dd motor that can REALLY take the punishment. But in a front hub, that much power is a bit tricky to ride, and I doubt you want to put a 25 pound motor on the bike anyway.

 

[dogman dan]

Another post, to consider another possibility not mentioned yet. A mid drive is ideal for heavy loads up steep hills. Very popular now, with the manufacturers of electric cargo bikes.

So look at the BBs02 kit from Bafang. That might serve you better than any hub motor, but be slightly harder to install and maintain than the hub motors are.

Back reading just now, I just noticed you are going to ride gravel roads with long hills. A front wheel drive will surely have poor traction on the loose surface, so mid drive may be your only logical choice.

 

[LewTwo]

Another post, to consider another possibility not mentioned yet. A mid drive is ideal for heavy loads up steep hills. Very popular now, with the manufacturers of electric cargo bikes.

So look at the BBs02 kit from Bafang. That might serve you better than any hub motor, but be slightly harder to install and maintain than the hub motors are.

Back reading just now, I just noticed you are going to ride gravel roads with long hills. A front wheel drive will surely have poor traction on the loose surface, so mid drive may be your only logical choice.

The Captains's position has a left hand chain ring. Would that not make the BBS02 a significant installation challenge ?

 

[dogman dan]

Ohh, it would. I thought something looked funny in the picture. Looking at it, something seemed odd, but I couldnt' say what.

You couldn't put it on the back riders position either.

 

[Chalo]

A front wheel drive will surely have poor traction on the loose surface, so mid drive may be your only logical choice.

Tandems always have to have a rider in the front seat, most of whose weight is on the front wheel. The wheelbase is long enough that there isn't a dramatic weight shift on climbing or braking like with a single bike. So traction is rarely an issue on either wheel.

A crank drive like Bafang BBS02 is not workable, as you noticed. Hub motor wheels are likely a structural problem for a tandem unless the riders are lightweight.

That leaves a Stokemonkey type setup driving the captain's right side crank, or a friction roller drive, or a left side drive sprocket on the rear wheel. Of these, it seems to me that friction drive is likely to be the best match to the OP's sometimes-not-always motor usage pattern.

A front hub motor like the Q128, with overrunning clutch, symmetrical spacing, and aluminum spoke flanges, would have fewer drawbacks in this application than most hub motors.

 

[LewTwo]

I have no experience with Tandem bicycles but Chalo's comments about weight distribution mirror my own thoughts in that regard. However I note that the bike is also equipped with fenders and the OP's comments on modifications do not mention their removal. Any form of friction drive would require the removal of at least one of those.

The OP did mention the combined weight of 390 pounds. The stock bike has a listed weight of 50 pounds leaving 340 pounds for riders and equipment. I am going to make the unfounded, wild assumptions: 40 pounds of equipment in rear mounted painters, 125 pound rear rider, 175 pound front rider. According to the Sun Brickell's manual it has a maximum load capacity of 575 pounds. The average American adult male is reported to tip the scales at 80.7 kg (177.9 lb). 'Light weight' is sometimes a matter of perspective. I am 150 pounds so that seems 'normal' to me. The OP's comments would indicate riders in the 'average' weight range from my perspective.

The Sun Brickell's specifications indicate 26x1.95 tires on aluminum rims with 12 gauge SST spokes mounted via a aluminum hub and 3/8 inch steel axle in a steel fork. If the fork width is a standard 100mm then this does not sound that far removed from what one might find on a average beach cruiser. As most 'ready made' e-bike wheels (that I have seen) come with double wall aluminum rims and 12 gauge steel spokes I suspect that there would not be a significant difference between strength of the existing wheel and a kit wheel from Ebike Kit or Luna Cycle or most any other reputable E-bike vendor. Given all the above I do not feel a standard E-Bike wheel would be a significant risk ... (in my opinion bearing in mind that opinion worth exactly what it cost). If it did turn out that the rim was not strong enough then the wheel could be relaced with a stronger rim.

Note:
If the fork width is not 100mm then a finding a hub motor kit wheel is going to be a significant challenge.

Reference:
Body weight https://en.wikipedia.org/wiki/Human_body_weight

 

[Mundo]

Chalo wrote

And there are no hub motors drilled for 48 spokes.

I hope some manufactures come out with 48 hole hubs, this would be great for tandems and cargo bikes

 

[brickelltandem]

Wow, thank you all for very informative and reasoned debate!
The bike shop mods did away with the left captain chain ring, both rings and rear internal hub are driven with single chain on right side. 42 or 48 tooth in front i think.

Bafang mid drive is certainly preferable, could create many challenges getting clearances and parts if DIY. Will ask shop for an estimate,
Otherwise am leaning to front hub drive. From these posts will pick a lower speed build. We probably never exceed 20 mph anyway.

Appreciate overheating advice, its my main concern. If the controller cuts out we will just stop and cool down. For our usual paved rides we should be fine, carriage roads a challenge.

Will post our build.
Are specialized tools needed to install the bafang mid drive?
Built and sold bikes from salvaged parts as a kid, but am sure my tools and experience are outdated.

 

[Rassy]

Been following this thread with interest. I'm a big fan of BBS02s and was about to respond to installation issues when I noticed your bike apparently has a rear coaster brake. The coaster brake is probably not compatible with the BBS02 because of the built in freewheel on the BBS02 which would prevent the captain from pushing backwards on the pedals to activate the coaster brake. The stoker may still be able to activate the coaster brake, unless a freewheel was installed there during the modifications that were made to facilitate the stoker's handicap.

As to the installation of a BBS02, it is my opinion that it is easier than installing a hub motor and the necessary torque arms, let alone the lacing or having to use different rims that don't match. Tool wise, you need to be able to remove the original crank which requires a standard tool. There is also a special tool to install the Bafang lock ring, but I have also just tapped it on with a large screw driver and a hammer:

https://california-ebike.com/product/bct-1/

 

[LewTwo]

The coaster brake is probably not compatible with the BBS02 because of the built in freewheel on the BBS02 ...

Good catch

 

[dogman dan]

We really need to stop looking at the wrong picture.

He said he has the 7 speed. So he can use a rear hub motor, or a BBso2. He's got a rear rim brake.

Now that we know he's got the sprocket on the right, go for the mid drive.

I guess the modification, was removing pedals on the rear, correct?

 

[LewTwo]

actually what he said was
"modified with:
-rear-hub shifting (no derailleur) and"
I underlined the relative part ... I missed the detail about "seven speed".
Sturmey-Archer made several 7 speed internally geared hubs. That includes at least two models with coaster brakes.
There are also Shimano and Sram but I do not know if they were available with coaster brakes.

The more relative question might be how a 7 speed IGH would fair under the added power of a Bafang mid drive. I believe several people have used Rohloff and Nuvinci with the bafang mid drive but I seem to remember a thread indicating the the motor was too powerful for a SA 7 speed IGH.

 

[spinningmagnets]

Is it possible to put two chainrings on the rear BB, with both on the right side?

 

[brickelltandem]

Whoops, I misspoke about the chain rings. As pictured, riders connected on the left, and rear chainring to 7-speed hub are on the right. Both riders can pedal, or both can freewheel together, but two pedalers are not independently freewheeled.

What specifically about the left chainrings would interfere with a Bafang drive?

 

[Chalo]

The Sun Brickell's specifications indicate 26x1.95 tires on aluminum rims with 12 gauge SST spokes mounted via a aluminum hub and 3/8 inch steel axle in a steel fork. If the fork width is a standard 100mm then this does not sound that far removed from what one might find on a average beach cruiser. As most 'ready made' e-bike wheels (that I have seen) come with double wall aluminum rims and 12 gauge steel spokes I suspect that there would not be a significant difference between strength of the existing wheel and a kit wheel from Ebike Kit or Luna Cycle or most any other reputable E-bike vendor.

There are differences. The hub motor's flanges are likely to be much closer together (about 40mm versus roughly 70mm), reducing side bracing angle and thus reducing the strength and stiffness of the wheel. Hub motor flanges are often steel and/or thin, both of which exacerbate spoke breakages. And the large diameter of a hub motor causes the spoke to insert to the rim at a goofy angle, which promotes breakage at the threaded end.

The failure of rim support caused by broken spokes is a more serious matter on a tandem where the gross weight is higher than that of a normal bike. It's more likely to lead to collapse of the wheel, especially if the bracing angle is already marginal.

 

[dogman dan]

ARGGh,,, The BBS02 has the chain ring on the right, you can't just put it on the left. You said right in the first post, it would be too complicated, but I did not get it why at first.

The brakes still not a problem, he's got at least a mount for a rear rim brake, even if the hub was changed. If it had the brakes originally, the mounts are there. they are not there, on the other picture. So we were looking at the wrong model of the bike at first.

You could still use a BBs02, but more modification, Only the front rider having pedals. Take the rear crank off, and replace with some kind of foot peg.

And you'd have to put the derailleur shifter back on to use a rear hub.

Deja Vu all over again, We're back to deciding which front hub kit will work good for you. I'm back to recommending the front version Heavy Duty DD kit from E-Bikekit. Just ride it set to level one power to eliminate the drag from the motor when you don't want much assist. A couple touches to the display puts you in full power for the hills, on the fly.

I still cannot recommend a geared hub. We've had people overload and burn out geared motors. But in the three years I've worked for, nobody has burned out and returned the low rpm DD motor. Not one. And we sell more of that motor, than we do of the geared one.

Run a semi knobby front tire, to improve traction when you are on the gravel, and it gets steep.