Pedal Assist Sensors

[Warren]

Adrian,

You are discovering one of the huge frustrations of working on new bikes. Try mounting a "universal" bike rack on the back of almost any new bike.

Up until the 1990's it was pretty simple. In the last ten years, bottom bracket design has gone crazy. The last good idea was the cartridge bottom bracket to replace the old adjustable bearing design. Actually, this was done by the French decades ago, but didn't catch on, until the Japanese started doing it. Many fine quality bikes are still done this way. Then someone got the bright idea of getting rid of the square taper, crank attachment. This was a huge mess. Trendy, bike-shop, bikes offer a new system about ever two years now. No shop can stock all the different ones. All are inferior to the square taper, cartridge bottom bracket in one way or another. But they keep coming out with them. "New and improved" sells new bikes.

Warren, in the trenches at your local bike shop

[full-throttle]

Just a thought - could you say glue strong 4mm magnets inside the chainring bolts and use an off the shelf pickup, mounted using a couple of zip ties around one of the tubes perhaps..

[justin_le]

What we need is a magnet PAS ring that attaches inside the inner chainring, and is split in 2 semi-circular pieces so that you can insert it in place without even removing the crank arm.

But what does it mount to? Modern cranks don't have any exposed axle, so it would have to be the chainring.

Yes, I was thinking something that would pinch the inner chainring. So long as the pinching device was on an arm of sorts that could slide in and out to accommodate different chainring ID's then this would be fairly universal. The only annoyance is that it seems like there is equal distribution of 4 and 5 bolt chainrings, so you'd have to consider the spacings between these pinching arms to make it work on both.

Hang on why make it two pieces, just make it one piece with a cutout.

I thought of that too. I guess it depends on how tight of a resolution you want with the magnetic poles, since some of the splined spindles have large OD's and would require a decently wide cutout. Riding with the THUN sensor currently using 16 pulses per revolution, it really helps to facilitate an instant response from the PAS control that you can't get from say a 5 magnet ring.

Obvious would be to use the chain ring mounts, as they are standardised. But that would still probably require using longer chainring nut/bolt things.

Not to mention that even with the chainring mounts there are still quite a few "standards"
http://sheldonbrown.com/cribsheet-bcd.html

Hang-on, most of those nuts/bolts are magnetic (at least the ones I have). How about just get some 6mm diameter rare earth magnet, stick them on the inside of the chain ring bolts, then mount the sensor. That could work. Wonder if the magnets would stay on?

Most likely they would. Another approach would be to use magnets on the chainring bolts as a method of holding the PAS ring itself in place, and then use a smaller diameter but higher resolution set of magnets for the PAS pickup. With this approach you could have your plastic PAS piece include several sets of mounting magnet holes to account for the different 4 and 5 bolt hole standards, and then the user would simply pop and glue a set of magnets into the appropriate holes for their crankarm.

Down tube size and shape vary drastically, I was thinking the seat tube would be better, ie. standardised in size and round most of the time. But some front derailuers brackets sit really low (like on my Gaint MTB dura ace) and would get in the way, so it may not be a universal solution. The sensor mount would need some degree of adjustment if an existing bracket is in the way. Maybe back to mounting it off the bottom bracket nut, not sure.

Hmm, if there is one thing that is largely standardized it's the length and diameter of a bottom bracket shell. So if you had a pickup that uses two zip ties or narrow hose clamps going around either end of the bottom bracket tube, I think that would do it. It also forces a consistent radial distance from the axis so that there won't be alignment issues if you have a double set of magnets for quadrature encoding. Just need make sure there is a channel and space for the cable routing that often passes under the BB device.

-Justin

[Warren]

Justin,

"if there is one thing that is largely standardized it's the length and diameter of a bottom bracket shell."

You are a couple years behind. This is what is showing up on all the new bikes.

http://www.bb30standard.com/

Manufacturers love it because they don't have to thread the two sides of the shell. A huge cost savings. Most don't even have it bored for a bearing press fit. They use a plastic sleeve between the bearing and the bottom bracket shell. Cheap, simple, and prone to working loose in days.

Nothing to attach to. Nothing to put a ring under. Sorry.

Warren

[kfong]

That got me thinking, why not put a magnet on the spindle as a reference and a magnet on the extreme end of the cranks. You could weaken the cranks so they deflect and measure the difference in angles as torque if the hall sensors can detect such a shift. I'm not sure they can though, but it would solve the wiring issue. Resolution would be poor though.

I once thought about a homemade torque sensor, one which detects the actual torque you're providing.
What I came up with is a thick steel plate (or other material) which attached between the crank and the chainrings.
Normally the crank and chainrings are connected together using 4 or 5 bolts (let's assume 4 for the sake
of discussion)

DSC00778.jpg

The cranks attach to the red holes, the chain rings to the green holes. The plate has slots in it
such that pressure on the cranks will bend the plate (in the planar direction) and change the
angle between the red and green holes. The plate has magnets (little blue squares) glued to it
such that the torque can be measured with hall sensors (torque changes the angle between the
magnets, a sensitive microprocessors can measure this based on speed and timing etc)

what do you guys think, should be simple, no ?

[MattyCiii]

I wonder how hard it would be to put wireless strain gage into pedals? No modification to crank arm or Bb required. Some room in the pedal platform. Bolt on. Nice.

[kfong]

With bluetooth it would be very possible to do. Torque will only be seen on the down stroke unless you wear cleats. Milling out a custom crank would be easy to do. Should be room enough for a single lithium cell as well. Maybe both cranks to get a more consistent torque reading. I'll have to look into this when I have the time.

[justin_le]

I wonder how hard it would be to put wireless strain gage into pedals? No modification to crank arm or Bb required. Some room in the pedal platform. Bolt on. Nice.

Coming soon:
http://sites.garmin.com/vector/#power

I was going to do this as my senior design project at university a few years ago and it's good to this approach finally show up in the market. It make the most sense for modular fit with with any bicycle, and the relative motion between the pedal and crank can easily spin a micro generator that keeps the sensor circuit powered up and transmits the load data wirelessly to a computer on the bike.

Not quite so easy as a DIY project though.

[rkosiorek]

I wonder how hard it would be to put wireless strain gage into pedals? No modification to crank arm or Bb required. Some room in the pedal platform. Bolt on. Nice.

Coming soon:
http://sites.garmin.com/vector/#power

I was going to do this as my senior design project at university a few years ago and it's good to this approach finally show up in the market. It make the most sense for modular fit with with any bicycle, and the relative motion between the pedal and crank can easily spin a micro generator that keeps the sensor circuit powered up and transmits the load data wirelessly to a computer on the bike.

Not quite so easy as a DIY project though.

check out the price though, only $1499USD. but it is a neat idea.

not very difficult to install the strain inside the pedal. issue is how to transfer the measurement wirelessly and not have it look like those big chunky "Block" pedals on kids first trikes.

rick

[Lebowski]

you can also seek the solution in a totally different direction.
When you're doing normal light cycling your cadence and rotation
of the crank is very smooth, the rotation speed is nearly constant.
When you do heavy cycling the speed of the crank varies greatly
over one rotation of the cranks, when you push down with one foot
crank speed goes up, while you transition force from one leg
to the other crank speed goes down.

It's not fool proof of course but might work...