LWB recumbent mid-drive motor choice

[bambuko]

I am building LWB recumbent and would like to add power assist.
So far I got the frame:

It has LH front single chainwheel crankset driving intermediate bottom bracket with two freewheels (one for the mid-drive motor):

RH side of intermediate bottom bracket has the usual arrangement of 3 speed "front" derailleur and 8 speed rear derailleur.
Both the motor and human power through the bicycle gears.
My intention is to have proper power assist with minimum power and torque (rather than "no pedalling" moped), not only for legal reasons but also because it makes sense to me - I only need motor when it comes to hills.

It seems to me (after some research and advice) that the best option is brushless, planetary geared, hub motor converted to mid-drive use.
The choice is between Q85 and Q75 (both 24V and 201 rpm).
The difference between the two is:
-weight Q75 is 1.2kg and Q85 is ?
-Q85 is both sensor and sensorless, whereas Q75 is only sensorless
Neither of the two motors has disc brake version (which would make fitting of the sprocket easier) - this is only available from Q100, but I guess it will not be impossible to make something?
-Q75 is 200 watts, which afaic should be perfectly enough for mid-drive with power going through all 24 bicycle gears
-Q85 is supposed to be 200-250 watts, but is it necessary?
-Q75 quotes "reduction ratio: 13.4", but no info about Q85?

So... which one would be better? am I making the right choices?

 

[trex]

that does not seem like good advice.
Crank drive needs a sprag clutch built into the bottom bracket.
How are you going to do that with your bottom bracket?

 

[bambuko]

Sprag Clutch is a free-wheel device...
There are two freewheels on the LH side of the bottom bracket.

 

[trex]

indeed. I should have gone to specsavers.
I think a front hub is more stealthy and just as powerful.
This tandem is roughly about the same size fitted with the woosh Big Bear kit and their15AH rackbattery:

 

[bambuko]

I smell another misunderstanding "...front hub is more stealthy and just as powerful..."
I am not proposing to fit hub motor to either front or rear wheel.

Hub motor (with added sprocket) will drive one of the freewheels on LH side of intermediate bottom bracket.
Second freewheel will be driven by myself from the crankset fitted to the front bottom bracket.

Tandem pic you have btw is very fortuitous - my drive arrangement is very similar, just the stoker is replaced by a motor.

Hope it's clearer now

 

[trex]

so you want something like this?

IIRC, d8veh converted this Diamonback with an 8-fun SWX in 2012 before the arrival of 8-Fun BBS01

 

[bambuko]

Yes
Yes, it is mid-drive and it uses hub motor driving the crankset, but because it is DF it only has one bottom bracket with all the complications it causes.

In my bike I have intermediate bottom bracket, which makes everything much simpler.

 

[D8ve]

The mid drive is usually done with a bbs... Drive from China or woosh.
These are easily fitted to the front bottom bracket. You could also fit to the rear one.
It's possible to do what you want but it's a lash up construction rather than mature technology.
I don't see any rake or trail in your build, is it deliberate?

 

[bambuko]

...bbs... Drive from China or woosh...

acronyms, abbreviations, foreign lingo... not sure

...easily fitted to the front bottom bracket...

that wouldn't make sense in my drive arrangement

...could also fit to the rear one...

that's what I am doing i.e. connecting mid-drive to the rear/intermediate bottom bracket

...It's possible to do what you want but it's a lash up construction...

you are entitled to an opinion

...I don't see any rake or trail in your build, is it deliberate?...

specsavers time

Yes, the design is deliberate.
See: Experiments with rake and trail variations (Tony Foale)

 

[trex]

regarless of the choice of motor, it is likely that you will have to construct a mounting kit for the motor with 135mm dropout to go onto the seat tube to take a rear hub motor. The simplest solution is to ask the advertisers on this forum to send you on sale or return the motor or kit they think is most suitable for your project.
I can think of 3 or 4 that may be interested:
cyclezee with their Ezee kit
eclipse with their cyclone kit
woosh with their Big Bear kit
panda with their rear xiongda XD (145mm dropout) kit
All these motors are heavy.
The Ezee kit is for 100mm dropout, also difficult to add a freewheel.
The best for your project may be the front xiongda XD. Woosh may sell you their front XD second hand cheaply, they have it on one of their test bikes gathering dust.
They may even sponsor your project!

 

[bambuko]

OK, just notice adverts bar to the right
to start with first few:
Whoosh - uses Bafang 8-Fun motors
Eclipsebikes - has either Bafang-8 or Cyclone kits

Both of these are at least twice as heavy as the one I suggest in my first post (Q75 or slightly heavier Q85).

There is an interesting thread on German Pedelec forum Der Q75 ist da
This motor is closest to my idea of fun (which I accept is not everybody's idea what power assist is about )
Has anybody here used it? and have any experience of it?

BTW mounting kit is the least of my worries - have both kit and abilities to make anything I need.

 

[trex]

you mean like this kit?

http://www.alibaba.com/product-detail/8-fun-2014-the-world-lightest_1762755436.html

 

[bambuko]

I've been too many times to China to trust anything on alibaba (or any info on Chinese supplier's webpage).
Most other (reputable) sources quote the weight of both Bafang and Cyclone motors around 3kg
Maybe you got something new I am not familiar with... but unless somebody I trust confirms it (hands on, not just by looking at webpage) I reserve judgement

I must admit - if true, this motor would be another candidate

ps
This is manufacturer's website
Looks remarkably like Q75 from BMS

So, the answer to your question is yes - this is what we are looking at.

 

[trex]

compared to front or rear hub wheel, the extra weight of the mounting kit, chain and sprocket in your design adds about 1.5kgs to the finished bike.
Another factor that you may not have taken into account is noise. The crank drives are very quiet because they use helical gear and have low spindle speed. Their motors are generally inrunners, much more thermally efficient. By using a hub motor as a crank drive, you'll negate all the gains.
If you have the ability, think about making your own crank drive for tandems with parts canibalised from a TCM crank drive kit.

 

[bambuko]

Yes, noise is the factor, but Q75 is reported as very quiet, no doubt helped by gearbox design:

 

[trex]

seeing that you have made up your mind, the Q85 weighs 1.76kgs
http://www.avdweb.nl/solar-bike/hub-motor/cute-q-85sx-motor-test.html#h2-weight

I bought stuff from bmsbattery before, they are bang up to date with the trend but the quality is not as good as bafang.

 

[bambuko]

All these mid-drive kits are designed for DF upright bikes:
mid-drive kits and are limited to using only rear derailleur.
A lot better than having no gears at all (hub motor driving wheels directly) but not as good as in my design that can use both rear and front derailleurs giving me the full range of 3x7(or 8, 9) gears.

Understand your suggestions, but prefer to discuss best motor for what I am doing

 

[bambuko]

...I bought stuff from bmsbattery before, they are bang up to date with the trend but the quality is not as good as bafang...

Sorry to hear about quality
I wonder if Bafang has similar small motor?
Q75 weighs 1.2kg
BTW - I am familiar with the test you are linking. Very useful indeed!

 

[trex]

I do take your point about the front chainrings. However, the usefulness of the front derailleur is only evident when you climb unpowered very, very steep hills, I can't see that happening.
bafang do not have lightweight rear motors.
BTW, all the lightweight motors are front motors. How are you going to add a sprocket?

 

[Deleted member 4366]

I have a Q75, but I must admit that I, ve not used it. My understanding is that it still packs a fair punch because it runs at higher speed than a Q100 and uses a higher reduction ratio, so that it can get similar power to the Q100 at the same wheel speed. I guess that its smaller size means that 15 amps would be the maximum. The Keyde also uses high speed and high reduction, and it has ample torque for normal riding, although it soon heats up when hill-climbing to the point that the thermal control cuts down its power. The Q75 doesn't have any thermal controls, so you'd have to test how it behaves.

The Q100H is not much heavier at 2.3kg and it's performance is much better known. I think that one would be my choice. The weight saving of the Q75 is only an advantage if you're using it as a wheel hub motor or if you have a really light bike.