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Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Thu Jul 10, 2014 2:08 pm
by noob_for_life
Forgive me for asking a basic question. I'm trying to figure out how to help my son with his electric bike problems.

He has fallen in love with the Bionx pl350. But, it's a budget buster for him. I'm trying to figure out if there's a way for us to build something ourselves which would help him. Here is the situation:

We live in San Francisco. He starts high school this year and wants to bike there. He weighs something like 130 now, maybe less. He'll probably get up to 180 by the time school ends. The commute is 4.2 miles each way. He has one OK hill on the way there. But, he has two good hills on the way back. The bigger of the two hills has, I believe, a 300 foot climb over 0.8 miles. He is only interested in pedal assist to help climb hills. He's not interested in a motor powering the bike on the flats. And he wants to set a wide range of support so he can gradually build the strength to manage the hills himself. A torque sensor would be a great help.

We could afford a Bionx PL 250. But, our local retailer thinks it's underpowered for SF. Can anyone point me either to a kit or to plans which might help us do the job?


Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Thu Jul 10, 2014 3:45 pm
by Wheels_78
I'm very happy with my THUN/cycle analyst V3 set up so far. Way more flexible and powerful than the bionx for the same money.

If I'm being captain obvious here, you're aware of all that and what you're looking for is a more home grown torque sensor/control system then I can't offer much, but I look forward to your build thread.

Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Thu Jul 10, 2014 4:04 pm
by Drunkskunk
There are only 2 real ways to go for this. 1 is a budget buster and the other is a bit technical, but both will work better than the Bionx

I do agree the 250 would be underpowered SF. I think the vastly 350 would be underpowered as well. SF demands a 500 watt motor, in part because a larger motor has greater capacity for not melting down, even if he only uses the same assist as a 250w motor would provide.

Lets start with the easy. A Bafang 500W CST will handle his needs now and in the future. Power that with a 48 volt 10Ah battery, though a common 6 FET 20 amp controller and you have the start of a good bike.

Now the hard parts. The Bionx works so well because it uses a strain gauge to sense the true torque applied to the chain. That's tricky tech to replicate. Using a THUN crank set will do the same thing, but more accurately. But it's not cheap. it needs to be combined with a Cycle Analyst, and together, that runs around $400.
The other method involves using the Cycle Analyst, but needs you to build a chain idler for the top chain of the bike, then attach a strain gauge to the arm the idler is mounted to, and program the CA to work off it's input range. This works because as you push on the peddle, you cause more tension on the top chain, which presses harder on the idler. the strain gauge can sense how much pressure is on the chain, and interpret that as how much pressure is on the peddle.
Cheaper to make, but it might take a while to build and then tune it to work well.

Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Thu Jul 10, 2014 5:23 pm
by r3volved
What a mid drive needs is a torque sensor in the rear hub. Sensing the strain at the rear sprocket/cassette, not the cranks or the chainring.

Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Thu Jul 10, 2014 6:09 pm
by Wheels_78
Never been to SF, but I used to play a lot of Crazy Taxi and I've seen Mrs. Doubtfire too many times.... Based on that and my limited experience I'd want all of 1000watts for those hills... I live in the prairies and find spots where my set up bursts 2000 all the time. (I'm pedaling with smaller muscle groups on a HEAVY bike running very squishy tires... YMMV).

Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Thu Jul 10, 2014 10:57 pm
by jslabonte
Looking at nycewheels, I'm guessing your budget is around 1000$ right?
I'm afraid that is a little too low for a real torq sensored setup. A quality thun sensor will set you back 250$, you will then need a cycle analyst to control it(150$). Then you need the motor, a quality direct drive if you want to keep it silent like a bionx (crystalite hs3540?) that would be around 300$ laced in a wheel,plus a controller (maybe 125$ for a lyen). Add a battery, and shipping cost for all thoses bits, and you are in the 1500$ range, at the very least. Also you are then pretty far from a simple setup IMHO.

Maybe you could consider a simulated torq sensor setup with a sinewave controller from ?
They are cadence based, and built on crappy wheels, but you may be a able to get him his first setup for around 800$...

As a current BionX owner, I would go for the pl350, or my first option.

Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Fri Jul 11, 2014 3:48 am
by d8veh
IMHO, torque sensors are over-hyped. If you get the right controller, a normal PAS sensor is perfectly adequate. I have them on all my bikes, and they work perfectly.

For what you've described, a really good kit would be the 500W Bafang CST (cassette gears) or BPM (free-wheel gears) with the S12S sinewave torque simulation controller from BMSBattery. The controller has a lovely LCD, where you can set the level of pedal assistance that you want. Both motors free-wheel very well without power, are not as heavy as the Bionx, yet they give considerably more torque for climbing. The CST gives about 24mph max (22mph on the road) at 36v in a 26" wheel. The BPM comes in low-speed (17mph) or high-speed (25mph) versions.

BMSB have a good range of batteries. You need to choose one that has a continuous discharge rate of at least 20 amps.

Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Fri Jul 11, 2014 4:51 am
by silentflight
BionX 250 and 350 are both nominal wattage ratings. Those who have ridden them know that they give much more power when climbing hills. I have measured well above 1000 watts with a Watt's Up meter spliced between the battery and motor of my BionX 350 when pedaling hard on a reasonably steep hill.

Will an SF dealer let your son try the 250 and 350 on his route? Perhaps he will want to ride steeper hills in other parts of the city, though. In any case, test riding is critical to making a decision.

A torque sensor is far superior to a cadence sensor for anyone who is an avid cyclist, especially on hills. Don't even consider a cadence sensor bike until your son has ridden both enough to know the difference.

Here is a BIonX 350 on sale for the price of a 250, $1199. I haven't read the small print, is likely last year's model with a freewheel instead of cassette, possibly other minor changes, but may be a very good deal. ... -2410.html

The older I2C BionX kits (about five years old or more) could be powered with higher voltages. My PL250 has run at 36V and my PL350 at 48V.

Search craigslist, ebay, etc for any possible used BionX kits for sale, it happens from time to time.

Re: Noob: DIY Torque Sensor Pedal Assist?

PostPosted: Fri Jul 11, 2014 11:46 am
by teklektik
Since the rider is looking to pedal unassisted and you folks have hills - I'm guessing a gear motor or mid-drive are the best coices because of the motor freewheel.

You should consider stopping by Ebikes SF. Ilia is a respected long time ES member. His site does not always reflect what he has to offer and since he's handy - worth the stop. His mainstay product is the somewhat pricey BMC motor but he has sold MACs and recently has been building with the Bafang crank drives. At the very least, you may be able to ride some other kit alternatives.

For a full Bafang kit, there is also EM3EV. Paul has a selection of batteries - it looks like you might get away with a modest size.

As posted above, the CA V3 is a nice addition and supports either the Thun or less expensive TDCM torque sensors. It can also instead use a simple PAS wheel with adjustable assist and/or RPM scaled PAS assist. This can be used with any motor (the Bafang mid-drive has PAS already).

Without any idea what money you have to spend it's a little hard to do more than just make shotgun suggestions - but there you go! :D