Chiasmata
1 µW
I installed an ERider T-17 torque sensing pedal assist system (PAS) and GMAC-10 kit from Grin Technologies on a Motiv Duet tandem bike that previously had a BionX ebike kit installed. I’m very happy with the results. Since I’m a novice when it comes to bike mechanics there were various questions that I had to sort out. These notes are meant to help others who might be interested in setting up torque sensing PAS on a tandem. Experts out there please chime in with corrections or clarifications.
The basic approach was to install the ERider in the rear bottom bracket where it would be oriented with the spider on the right side of the bike, same as on a one person bike. On the right side I installed the Miranda crank that came with the ERider 68mm four-arm spider kit. On the left side I reused the existing crank with its tandem bike timing chain gear. In my case it turned out to be necessary to connect the ground of the Cycle Analyst to the bike frame. See discussion of that below.
I liked the torque sensing PAS that the previous BionX kit had, but when it came to installing one of the bottom bracket torque sensors available from Grin Technology I wasn’t sure where to start. Which model torque sensor should I use? Should it be installed in the front or rear bottom bracket? How would I deal with the reversed rotation direction if installed in the front? Would the sensor pick up the pedal effort of both riders? Would it work if the bike was being ridden solo?
From the product information for the ERider T-17 it seemed like installing that model in the rear bottom bracket might be a viable approach. The good folks at Grin confirmed that idea. The ERider senses torque on the chainring spider which is not directly connected to either crank so it does the trick of sensing pedal effort of both riders.
More details …
An initial question was whether the bottom bracket shell on my bike was compatible with the ERider. There are a variety of bottom bracket shell designs, but it’s my understanding that one design is very common. I think it is “BSA” and is often simply referred to a “threaded bottom bracket”. That’s what my bike has and that made it compatible with the ERider T-17. The length of the shell which I could measure was 68mm.
My bike had three chainrings, but I typically only used the middle one. To simplify the new setup I installed a single chainring. The chainline specified for the ERider was within a couple of millimeters of the chainline on one of the existing chainrings so no issue there.
Since I was going to install a chainring of about 38 teeth I chose a four-arm spider which is good for chainrings with 44 teeth or less.
If you are going to install the ERider T17 yourself you’ll need a wrench for the bottom bracket cup and a tool for the locking nut. There are probably many brands that will do the job, but here are two specific examples that will work:
ParkTool BBT-9 fits the 16-tooth 44mm bottom bracket cup.
ParkTool BBT-22 fits the 20-tooth locking nut.
In my case it was possible to reuse the existing left stoker crank with its timing chainring because (a) it was compatible with the square taper shaft of the the ERider, (b) the orientation of the square hole in the crank had the same (90 vs 45 degree) orientation as the Miranda crank, and (c) it was about the same length as the Miranda crank.
The ERider needs to be cabled to the Cycle Analyst. To accomplish that on the tandem I purchased two PAS Extension Cables (SKU: CAP-PAS_Ext). Don’t make the mistake I initially made of connecting the ERider to the 6 Pin HiGo Mini-B plug of the Baserunner L10. The full product information page for the Baserunner explains that that plug “is not used or required when PAS or torque assist is handled via a Cycle Analyst, but it will provide a future option for using the native PAS feature of the controller without a CA device attached.”
I had the ERider installed by a local bike shop. I cabled it to the Cycle Analyst and made appropriate adjustments to the PAS DEVICE and PAS CONFIG settings following the recommendations in the Grin video “How to Setup a Torque Sensor with a V3.1 Cycle Analyst”
At first the assist did not work properly, it only gave power in momentary bursts and the Cycle Analyst reported erratic human-watt values including negative 200-600W. The solution turned out to be making a ground connection between the Cycle Analyst and the frame of the bike. Grin customer support said. “It seems like quite a few folks especially with long frame bikes and Erider torque sensors have had success with this now.”
Grounding the frame —
Grin customer support advised me, “one easy spot to get Ground signal would be from the Cycle Analyst's 5.5x2.1mm DC Power plug's outside terminal (inside is B+) if you're not using that connector for anything else.
That or any of the un-used JST connector available on the Cycle Analyst. Ground is Pin #2 on all except the 2-pin Thermistor input.” [Another suggestion I heard about later was picking up the Ground signal from the 6 pin PAS plug of the controller. See below.]
I purchased a “DC Power Cord, 2.1x55mm Jack to Bare Leads, 6ft/22awg” from a local store, we-supply.com. I had read in a discussion on Endless Sphere, subject “Erider torque voltage output consistency issue,” about grounding the bike frame and that Grin Technologies recommended including a 100mA fuse in the ground wire. What I found available at We-Supply was an Inline Fuseholder 3AB/3AG, 18AWG and a 125mA fuse of the 3AG form factor. I soldered the fuse holder into the ground wire of the above mentioned power cord. I left the positive wire of the cord intact for mechanical strength, but trimmed back and taped the bare end to make sure it doesn’t come in contact with anything. I put a crimp-on spade terminal on the bare end of the ground wire and attached it to a water bottle holder lug on the bike frame. That grounding made all the difference in the world. The torque sensing assist started working perfectly.
[In a later communication from Justin at Grin Technologies he provided this clarification about the need for grounding in a small percentage of cases, “… the reason for grounding the frame is that in some setups there seems to be electrical interference between the ERider bottom bracket and the PWM output of the motor controller which can cause the ERider's signal voltage to freeze. In about 90% of installations this isn't an issue at all but somewhere around 10% of setups seem to exhibit that, and I imagine that having the long PAS extension cable gives more opportunity for electrical noise pickup.
We will probably be making an off the shelf cable that goes in the 6 pin PAS plug of the controller with a ring terminal for grounding to the frame at any nearby eyelet so that those affected by this have a more straightforward plug and play fix.”]
I’m using a digital auxiliary switch (CA3_MFSwitch for CA3-WPx) to set the pedal assist level in six steps from zero to 4xHuman-Watts.
A couple of minor points. The ERider T17 installation instructions mentioned a wire fixing ring which wasn’t present. Grin said it is not needed. I take it the fixing ring is no longer part of the kit. The instructions also refer to the locking nut as 8-tooth, it’s 20-tooth.
I’m very happy with the components I purchased from Grin Technology. I particularly like the fact that the components are generally interoperable with components from a variety of sources. I had to retire the whole BionX system when the proprietary-design battery unit lost capacity and there was no replacement available, at least none that I had confidence in. The company went out of business in 2018.
I hope these notes turn out to be helpful to others. Enjoy the build.
The basic approach was to install the ERider in the rear bottom bracket where it would be oriented with the spider on the right side of the bike, same as on a one person bike. On the right side I installed the Miranda crank that came with the ERider 68mm four-arm spider kit. On the left side I reused the existing crank with its tandem bike timing chain gear. In my case it turned out to be necessary to connect the ground of the Cycle Analyst to the bike frame. See discussion of that below.
I liked the torque sensing PAS that the previous BionX kit had, but when it came to installing one of the bottom bracket torque sensors available from Grin Technology I wasn’t sure where to start. Which model torque sensor should I use? Should it be installed in the front or rear bottom bracket? How would I deal with the reversed rotation direction if installed in the front? Would the sensor pick up the pedal effort of both riders? Would it work if the bike was being ridden solo?
From the product information for the ERider T-17 it seemed like installing that model in the rear bottom bracket might be a viable approach. The good folks at Grin confirmed that idea. The ERider senses torque on the chainring spider which is not directly connected to either crank so it does the trick of sensing pedal effort of both riders.
More details …
An initial question was whether the bottom bracket shell on my bike was compatible with the ERider. There are a variety of bottom bracket shell designs, but it’s my understanding that one design is very common. I think it is “BSA” and is often simply referred to a “threaded bottom bracket”. That’s what my bike has and that made it compatible with the ERider T-17. The length of the shell which I could measure was 68mm.
My bike had three chainrings, but I typically only used the middle one. To simplify the new setup I installed a single chainring. The chainline specified for the ERider was within a couple of millimeters of the chainline on one of the existing chainrings so no issue there.
Since I was going to install a chainring of about 38 teeth I chose a four-arm spider which is good for chainrings with 44 teeth or less.
If you are going to install the ERider T17 yourself you’ll need a wrench for the bottom bracket cup and a tool for the locking nut. There are probably many brands that will do the job, but here are two specific examples that will work:
ParkTool BBT-9 fits the 16-tooth 44mm bottom bracket cup.
ParkTool BBT-22 fits the 20-tooth locking nut.
In my case it was possible to reuse the existing left stoker crank with its timing chainring because (a) it was compatible with the square taper shaft of the the ERider, (b) the orientation of the square hole in the crank had the same (90 vs 45 degree) orientation as the Miranda crank, and (c) it was about the same length as the Miranda crank.
The ERider needs to be cabled to the Cycle Analyst. To accomplish that on the tandem I purchased two PAS Extension Cables (SKU: CAP-PAS_Ext). Don’t make the mistake I initially made of connecting the ERider to the 6 Pin HiGo Mini-B plug of the Baserunner L10. The full product information page for the Baserunner explains that that plug “is not used or required when PAS or torque assist is handled via a Cycle Analyst, but it will provide a future option for using the native PAS feature of the controller without a CA device attached.”
I had the ERider installed by a local bike shop. I cabled it to the Cycle Analyst and made appropriate adjustments to the PAS DEVICE and PAS CONFIG settings following the recommendations in the Grin video “How to Setup a Torque Sensor with a V3.1 Cycle Analyst”
At first the assist did not work properly, it only gave power in momentary bursts and the Cycle Analyst reported erratic human-watt values including negative 200-600W. The solution turned out to be making a ground connection between the Cycle Analyst and the frame of the bike. Grin customer support said. “It seems like quite a few folks especially with long frame bikes and Erider torque sensors have had success with this now.”
Grounding the frame —
Grin customer support advised me, “one easy spot to get Ground signal would be from the Cycle Analyst's 5.5x2.1mm DC Power plug's outside terminal (inside is B+) if you're not using that connector for anything else.
That or any of the un-used JST connector available on the Cycle Analyst. Ground is Pin #2 on all except the 2-pin Thermistor input.” [Another suggestion I heard about later was picking up the Ground signal from the 6 pin PAS plug of the controller. See below.]
I purchased a “DC Power Cord, 2.1x55mm Jack to Bare Leads, 6ft/22awg” from a local store, we-supply.com. I had read in a discussion on Endless Sphere, subject “Erider torque voltage output consistency issue,” about grounding the bike frame and that Grin Technologies recommended including a 100mA fuse in the ground wire. What I found available at We-Supply was an Inline Fuseholder 3AB/3AG, 18AWG and a 125mA fuse of the 3AG form factor. I soldered the fuse holder into the ground wire of the above mentioned power cord. I left the positive wire of the cord intact for mechanical strength, but trimmed back and taped the bare end to make sure it doesn’t come in contact with anything. I put a crimp-on spade terminal on the bare end of the ground wire and attached it to a water bottle holder lug on the bike frame. That grounding made all the difference in the world. The torque sensing assist started working perfectly.
[In a later communication from Justin at Grin Technologies he provided this clarification about the need for grounding in a small percentage of cases, “… the reason for grounding the frame is that in some setups there seems to be electrical interference between the ERider bottom bracket and the PWM output of the motor controller which can cause the ERider's signal voltage to freeze. In about 90% of installations this isn't an issue at all but somewhere around 10% of setups seem to exhibit that, and I imagine that having the long PAS extension cable gives more opportunity for electrical noise pickup.
We will probably be making an off the shelf cable that goes in the 6 pin PAS plug of the controller with a ring terminal for grounding to the frame at any nearby eyelet so that those affected by this have a more straightforward plug and play fix.”]
I’m using a digital auxiliary switch (CA3_MFSwitch for CA3-WPx) to set the pedal assist level in six steps from zero to 4xHuman-Watts.
A couple of minor points. The ERider T17 installation instructions mentioned a wire fixing ring which wasn’t present. Grin said it is not needed. I take it the fixing ring is no longer part of the kit. The instructions also refer to the locking nut as 8-tooth, it’s 20-tooth.
I’m very happy with the components I purchased from Grin Technology. I particularly like the fact that the components are generally interoperable with components from a variety of sources. I had to retire the whole BionX system when the proprietary-design battery unit lost capacity and there was no replacement available, at least none that I had confidence in. The company went out of business in 2018.
I hope these notes turn out to be helpful to others. Enjoy the build.
