Bottom Bracket Torque Sensors

[twentysix by twentyfive]

I'm wondering if a knowledgeable CC'er or 3 can help please.

I'm wondering how the mechanical torque is actually measured and turned to an electrical signal. Here's my (probably) faulty thinking.

The pedalling torque is transferred (on the drive side at least) from pedal to crank and then via chainring to chain and onward to the wheel. The BB axle is just a support structure and doesn't appear to experience torque unless the non drive foot is forcibly restricting rotation. Also if indeed the axle does experience torque (which it might when pedalling on the non drive side) how is the rotating BB axle coupled electrically or otherwise to the outside world (the bike wiring) without added extra complication and resistance to rotation with slip rings or the like. I assume that resistive strain gauges are involved but maybe some magnetic or other methods may be employed. I assume strain gauges as these would be quite cheap although magnets are cheap too.

Puzzled is I and my tiny brain might explode

TIA

 

[Espresso vecchio]

An interesting question to which I too would like to know the answer provided it's put in the very simplest terms for me.

All I know is that my middrive Bosch-motored ebike is a delight to ride

 

[twentysix by twentyfive]

This sort of thing

 

[Dadam]

So, I don't know how they actually work in practice (I'm interested too). How they could conceivably work is a strain gauge or piezo load cell in the axle/shaft. Drive side or non drive side doesn't really matter. Push on the pedals causes a torque on the axle that is resisted by the drive train and the inertia of the bike. If you're holding the brakes firmly there's no movement but there is still a torque (twisting force). When you pedal, this torque will go up and down as your foot moves from the top down so the controller needs to average out the torque. I guess a well programmed controller should be able to use the initial torque on the first downstroke for quick assistance on setting off, then settle to an averaged value over the last couple of seconds or rotations.

Just thinking about this and it's pretty much how a power meter would work. https://en.wikipedia.org/wiki/Cycling_power_meter

 

[presta]

Drive side or non drive side doesn't really matter. Push on the pedals causes a torque on the axle..

When you step on the right pedal the force is transferred directly to the chainring via the spider, no torque on the BB shaft, just a shear load between the fulcrum force and its reaction. From your own reference:

Bottom bracket power meters......detect when torque is applied to the left pedal and then doubled.

 

[twentysix by twentyfive]

That's my analysis @presta

Yes strain gauges must be involved @Dadam but I can't get passed the fact that these must rotate with the axle so how is the electrical signal picked up without messy slip rings and the like? The strain gauge voltages are pretty low I think, requiring amplification. I'm missing something I think.

Anybody got one they would take apart to find out? Indeed post it to me so I can disassemble and I'll reimburse postage. I'll assume it's broken no need to send one that is still useable.

 

[the snail]

When you step on the right pedal the force is transferred directly to the chainring via the spider, no torque on the BB shaft, just a shear load between the fulcrum force and its reaction. From your own reference:

Bottom bracket power meters......detect when torque is applied to the left pedal and then doubled.

On my tongsheng, the pedals aren't directly connected to the chainring, there's a clutch inbetween, and both cranks attach to the bb spindle. I don't know how the torque sensor works though, my impression is that it is quite low-tech, with a spring and sensor that measures the extension, but I could be completely wrong.

 

[CXRAndy]

On my tongsheng, the pedals aren't directly connected to the chainring, there's a clutch inbetween, and both cranks attach to the bb spindle. I don't know how the torque sensor works though, my impression is that it is quite low-tech, with a spring and sensor that measures the extension, but I could be completely wrong.

No it's a proper strain gauge, albeit a cheap one

https://endless-sphere.com/sphere/threads/tsdz2-torque-sensor-broken.98665/