Power / Torque Sensor Roundup

[Alex W]

I tuned AGain and WGain up a lot and it is much more responsive. I think the default settings were so low that I was often accelerating as fast as the bike would (especially with a low torque multiplier). I can verify that it now starts in under one crank revolution if I start pedaling with the left crank.

I don't think any perceived lag on start has to do with torque sensing at all. If you apply torque and pedal at the same time the response will be just the same as a PAS mode configured for the same power output. Most likely it was your slow up-ramp more than anything

What power level is it ramping up to in torque mode if it hasn't measured any torque yet?

 

[justin_le]

Not all mid drive builds use a single chain for both motor and pedals. A common and simple example is connecting the motor to the left side of the rear wheel with a dedicated chain.
Avner.

True, I guess it comes down to nomenclature but that's not really what I would refer to as a "mid-drive". That to me is just a fixed ratio geared drive, no different in operational principle than geared hub motor or a friction drive, ie. motor spins at x ratio to the tire. A "mid-drive" should imply that the motor goes through the bicycle transmission and takes advantage of the varying bicycle gearing, so that the motor has variable gear ratios to the wheel.

 

[ilikepancakez]

I'm looking to try and turn my cyclocross into a pedelec. Would this be good?

http://www.ebikes.ca/shop/ebike-parts/cycle-analysts/ca3-dps.html (CA3 DPS)
http://www.ebikes.ca/tdcm-128.html (TDCM torque sensor) [Torque sensor has different spindle length options, what's that about?]
http://em3ev.com/store/index.php?route=product/product&product_id=166 (BBS02 Mid drive)

 

[justin_le]

I'm looking to try and turn my cyclocross into a pedelec. Would this be good?

http://www.ebikes.ca/shop/ebike-parts/cycle-analysts/ca3-dps.html (CA3 DPS)
http://www.ebikes.ca/tdcm-128.html (TDCM torque sensor) [Torque sensor has different spindle length options, what's that about?]
http://em3ev.com/store/index.php?route=product/product&product_id=166 (BBS02 Mid drive)

No that won't work at all. The BBS02 system already contains a bottom bracket and spindle system integral to the drive mechanism, there is no way you can take that out and replace it with a torque sensing bottom bracket. And even if in some miracle you could, the TDCM sensor doesn't measure spindle torque anyways, it measures the backwards pull of the chain tensions, and so it wouldn't be able to differentiate between human pedal effort and the motor drive.

If you want a torque sensing pedalec, then the easiest approach by at the moment is to use a front or rear hub motor so that the motor isn't part of your pedal drive chain, and then any of the numerous torque sensing systems (Thun BB, TDCM BB, BeamTS deflection sensor etc). We may yet see systems like the BBS02 with built in torque sensor available to the kit industry as they are in many oem mid-drives, but AFAIK it hasn't happened yet.

 

[ilikepancakez]

I'm looking to try and turn my cyclocross into a pedelec. Would this be good?

http://www.ebikes.ca/shop/ebike-parts/cycle-analysts/ca3-dps.html (CA3 DPS)
http://www.ebikes.ca/tdcm-128.html (TDCM torque sensor) [Torque sensor has different spindle length options, what's that about?]
http://em3ev.com/store/index.php?route=product/product&product_id=166 (BBS02 Mid drive)

No that won't work at all. The BBS02 system already contains a bottom bracket and spindle system integral to the drive mechanism, there is no way you can take that out and replace it with a torque sensing bottom bracket. And even if in some miracle you could, the TDCM sensor doesn't measure spindle torque anyways, it measures the backwards pull of the chain tensions, and so it wouldn't be able to differentiate between human pedal effort and the motor drive.

If you want a torque sensing pedalec, then the easiest approach by at the moment is to use a front or rear hub motor so that the motor isn't part of your pedal drive chain, and then any of the numerous torque sensing systems (Thun BB, TDCM BB, BeamTS deflection sensor etc). We may yet see systems like the BBS02 with built in torque sensor available to the kit industry as they are in many oem mid-drives, but AFAIK it hasn't happened yet.

Thanks for saving me $500 haha. Do you have any recommendations for any rear hub motors that are light weight but still provide a good amount of torque? So far I've found the Q100H which looks pretty good, but I don't know if there are any better options.

Also, to make a fully built pedelec what do I need specifically. Is all I need a motor, battery, torque sensor, and cycle analyst?

 

[Alex W]

Battery (I don't find a big one to be valuable), controller, cav3, torque sensing bb and motor. Try to ride the tdcm before buying, it has a little play that is disconcerting and it's quite heavy.

A throttle is cheap and useful for debugging, but I don't have one installed. A 3 position aux switch is very nice.

I'm using the same 8fun front hub as the Faraday, it's a great hub with proper disk brake specs (q100 is off by 2 or 3mm), but it isn't easy to find. It weighs around 2kg, which is on the lighter side. My bike has an igh rear hub, so putting the motor up front made the most sense.

You'll probably need to do some easy custom wiring, I had to replace the motor and hall sensor wires on the controller to match the plug on my hub.

My battery is 10+ ah and I'm getting 50-70 miles of range in typical riding, 100 miles if I turn off the system except for on major hills. Pedelec bikes are quite efficient if they are built as nice bikes first and electric bikes second. I'm still hoping to find a great 6ah turnkey battery to save weight.

 

[Kodin]

What sort of pressure are we talking about when looking at chain tension? 5 PSI? 50? What stops you from simply buying a cheap pressure transducer, mounting it to a piston with a roller, and placing it under your chain on a hub-drive? I may have to do some tests to see how much force it takes to deflect the chain while standing on the pedals...

EDIT: Derp, I totally said "mid-drive" when I meant hubmotor config.

 

[justin_le]

What sort of pressure are we talking about when looking at chain tension? 5 PSI? 50?

Pressure makes no sense at all in the discussion of chain tension. You are talking about tension in pounds, not pressure in pounds per square inch. This is trivial to calculate. Take a 180lb person standing on a 170mm crank with a 44t chainring (~89mm radius):

Tension = Force * Crank Length / Chainring radius = 180lb * 170mm / 89mm = 340 lb. With a small 28T chainring it would be 540 lb of tension and so forth.
If this chain is being deflected by a pulley connected to a transducer (like the beamTS device), then the downwards force on the transducers is to a first order the tension times the sine of the deflection angle. So if the idler pulley causes a 10 degree 'kink' in the chain, then deflection force on the idlers with 340lb of chain tension would be 340lb * sin(10 degrees) = 59 lb etc.

What stops you from simply buying a cheap pressure transducer, mounting it to a piston with a roller, and placing it under your chain on a mid-drive?

Nothing at all, except that electronic pressure transducers aren't cheap, and as mentioned before installing a system like this on a mid-drive would be of very limited use since it would be measuring both the chain tension caused from the motor output as well as from the cranks. Ideally you want your PAS sensing system to only measure the human power output and not the combined human+motor output.

I may have to do some tests to see how much force it takes to deflect the chain while standing on the pedals...

I'm a fan of tests in general, but in this case you don't need to do that. You just need to use a paper and pencil and brush up on middle-school physics!

Thanks for saving me $500 haha. Do you have any recommendations for any rear hub motors that are light weight but still provide a good amount of torque? So far I've found the Q100H which looks pretty good, but I don't know if there are any better options.
Also, to make a fully built pedelec what do I need specifically. Is all I need a motor, battery, torque sensor, and cycle analyst?

You forgot the motor controller, but add that to the list and that is it. As far as hub motor recommendations go, if you want light weight but with decent torque then just about any rear geared hub motor will do the job just fine, there are literally hundreds of models and brands out there which fit this bill.

 

[Kodin]

Let me clarify. I'm talking about a hubmotor configuration. The transducers out there now are ~$25 USD. How much do the BeamTS sensors run?

Forgive my lack of middle school physics. My school district cut pretty much all programs except those required for state testing.

 

[justin_le]

The transducers out there now are ~$25 USD. How much do the BeamTS sensors run?

Definitely more than that, but you are right that the automotive pressure transducers are pretty inexpensive and seem to be mostly scaled to a 0.5V - 4.5V output range, which would definitely play nice with most torque sensing ebike electronics. It's a little more roundabout than measuring force more directly from the deflection on a beam, but you could put an idler pulley on top of a small diameter hydraulic piston and have the chain tension cause a downwards force on the piston putting pressure on the transducer.

In the above example I showed for a 10 degree 'kink' in the chain you'd be dealing with on the order of ~50lb of downwards force. So with a 0-100 PSI transducer, you'd want a hydraulic piston with a ~0.75" bore for roughly a 0.5 square inch area, so that a 50 lb force results in ~100 PSI in the fluid.

 

[Kodin]

Then the only problem I'll have to come up with is how to integrate it with a front or rear dérailleur so it follows the chain... Still have to think about that one.

 

[judson]

A throttle is cheap and useful for debugging, but I don't have one installed. A 3 position aux switch is very nice.

What does this switch do? I see that it is offered with many PAS kits (for example, http://www.ebikes.ca/shop/conversion-kits/direct-drive/crystalyte-front-hs-kit-advanced-pas.html) Not sure what is gained.

I also have heard that the TDCM BB sensor has some 'play to it'. Is this a widely accepted experience, or just with some units? Can we hear more details, play in the BB seems like a deal killer.

Thanks!

 

[Triketech]

I really like the Thun connected to a Cycle Analyst 3. A lot. Unlike any of the torque sensing hubs its dead on at any speed. Coupled with the 1 turn pot it goes from 0X to 10X with the twist of fingers. Normally I ride under 1X.

 

[Alex W]

A throttle is cheap and useful for debugging, but I don't have one installed. A 3 position aux switch is very nice.

What does this switch do? I see that it is offered with many PAS kits (for example, http://www.ebikes.ca/shop/conversion-kits/direct-drive/crystalyte-front-hs-kit-advanced-pas.html) Not sure what is gained.

I also have heard that the TDCM BB sensor has some 'play to it'. Is this a widely accepted experience, or just with some units? Can we hear more details, play in the BB seems like a deal killer.

Thanks!

The switch is input to the generic AUX input (a analog input) on the Cycle Analyst. On my bike I have that configured to the "PAS Level" in 3 steps, 0, 0.45, and 0.9. At 0 there is no assist, at 0.45 the motor is throttled to try and match my measured human wattage * 0.45 (so lower assist), and then 0.9 is the higher assist. You can also wire up a potentiometer, but I prefer the ease of switching between 3 discrete steps. I often turn my assist off on flats and going down hills and usually leave it on the medium setting on flats and climbs. Sometimes when it's really dumping rain and I just want to be home I switch to the high setting.

I've only ridden one bike with a TDCM bottom bracket. It was brand new and the play was noticeable to me.

It looks like you are in Seattle. G&O Cycles is a local shop on Greenwood which is a Grin dealer and Tyler (one of the owners/employees) knows a lot about these products. One of his personal bikes has the TDCM, and many bikes in the shop have the 3-position aux switch. They sell stuff for the same price as ebikes.ca, so you might as well shop local and get to benefit from their expertise too.

 

[teklektik]

What does this switch do? I see that it is offered with many PAS kits...

The switch is input to the generic AUX input (a analog input) on the Cycle Analyst.

On my bike I have that configured to the "PAS Level" in 3 steps, 0, 0.45, and 0.9. At 0 there is no assist, at 0.45 the motor is throttled to try and match my measured human wattage * 0.45 (so lower assist), and then 0.9 is the higher assist.

You can also wire up a potentiometer, but I prefer the ease of switching between 3 discrete steps.

This doesn't need to be an 'either-or' situation - you can have both.

Here's an example of combining a pot and a standard controller 3-speed switch (NOT a CA 3-position AuxPot switch). This gives quick switch access to 0% and 100% assist while the middle setting tracks the pot so you can adjust the assist 0-100% on the fly to better match terrain.

 

[fourbanger]

At the risk of sounding daft, what's stopping people from using a torque sensing bb in a freewheeling chainring setup like cyclone, gng, etc?

Also, I wonder what would happen if one took a thing such as the beam ts and mounted it vertically to the left hand side of a stokemonkey type of setup, measuring the chain flex from the pedal to the motor rather than vice versa?

 

[flangefrog]

I'll just post this link to some other options here: https://endless-sphere.com/forums/viewtopic.php?f=2&t=79332

Note there is a wireless pedal torque sensor system that might work with Bafang mid drives.

I will be buying an Enoeco and Sempu BB torque sensor to evaluate soon.

Edit: here's a very expensive universal power meter I just came across: https://www.kickstarter.com/projects/poweraxlcycling/the-worlds-first-universal-bicycle-power-meter

 

[ElectricGod]

I want to use a torque sensor to just output motor torque and then display it on an LCD. I don't care about assisting throttle output. I only want to see what my motor output is in real newton meters or foot pounds.

Does anyone know of an option like this?

 

[amberwolf]

Where is the motor mounted? hub, BB, etc?

BeamTS can ride on any chain to do this (probably belt, too), so any motor driving a chain could be read by it.

Similary the BB types (Thun, tdcm) could do that, even if not mounted in your actual BB, if you build a BB-type mount of rhtem and use them to transfer the motor power like a jackshaft.

Probably other ones too.

Hubmotors you'd need to do smething that senses axle twist vs frame, I think. Nothing off the shelf for that that Ican think of.