Favero Assioma DUO-Shi Power Meters in Shimano PD-GR500 Flat Pedals

There is a variety of commercially available power meter pedals available on the market, but nearly all of them are specifically for clipless type pedals. One product out there is a power meter in a platform pedal, but as of this writing it is ~US$1500, which is...a lot of money. An intriguing option seemed to present itself with the Favero Assioma DUO-Shi power meter spindles. While they are officially listed as being compatible only with a small number of Shimano clipless pedal bodies, I had a suspicion that the physical geometry of many more Shimano pedal bodies would be the same or very similar. This suspicion was further encouraged by looking at exploded parts diagrams from various Shimano service manuals, many of which seemed to show the same axle/spindle assemblies.

This Instructable article will go over my little project to adapt the Assioma spindles to some Shimano PD-GR500 flat pedals.

"Dude, it is pretty odd to be into cycling enough to want power meters, but still riding flats like some casual weekend rider!"

Yes, I am in fact aware of this. Call me crazy!

1. Favero Assioma DUO-Shi power meter spindles
2. Shimano PD-GR500 flat pedals
3. Right-handed M15x1 tap
4. Required. Under US$20 on Amazon or eBay.
5. Left-handed M15x1 tap
6. Required. Under US$20 on Amazon or eBay.
7. Basic machining tools
8. Ideally, you would have a milling machine and associated tooling, which makes this trivially easy
9. If you are without a milling machine, you can do what is needed with a hack saw and file if you are careful and set up some guides to file the spindle entry of the pedal body perpendicular to the axis of the spindle.

This is not really a "step" that you need to do, but rather it is one of the steps that I went through when trying to determine for certain if I would be able to fit the DUO-Shi's into platform pedals. The first thing that I did was buy the cheapest set of used clipless pedals that I could find on eBay (~US$22 shipped), which were also listed as officially compatible with the power meters, so that I could make measurements on them. The sacrificial items here are a set of PD-R540's.

I did not have the proper Shimano tool for removing the spindles, but since I was not planning to use these pedals, I just forced them out by using pliers as a wrench. Just for fun, I took apart one of the spindles. It turns out that all of the pedaling force is carried by the metal sleeve and bearing balls at the end. The black plastic sleeve bears no load, and only serves to press the sleeve into place during installation.

A key measurement that I needed was the inside diameter of the region that the metal sleeve inserts into, and the outside diameter of the sleeve. The "right" way to do this is to use telescoping bore gages and a micrometer. Calipers are OK for diameter measurements, but micrometers are better. In any case, the inside diameter that I was interested in was too far inside for calipers anyway.

The ID of the sleeve receptacle came out to ~13.83mm, and the sleeve OD was ~13.845mm. So, this is a light press fit, which makes sense since you really cannot remove or install the spindle without a wrench.

To really get a look at things, I decided to do a cross section of the pedals. You can make OK sections with a hack saw, followed up with wet-sanding. After using the saw, I made some passes with 150 and 320 grit wet sanding paper on a hard sanding block. This is not "lab-grade" sectioning here, but it is more than suitable for this purpose. From this I was able to get reliable measurements of thread depth, and other feature distances. Also, this shed some light on how the spindle actually locate themselves in the pedal bodies.

The sleeve is a press fit, and it also bottoms-out on the second shoulder from the bottom. The black plastic sleeve simply presses it in, and keeps it pressed in. ALL pedal force and torque are carried by the spindle through the sleeve and bearing balls.

Next, I needed to make similar measurements on the candidate flat pedals. I had already looked through the service manuals for the PD-R540's and PD-GR500's, and felt pretty confident that the spindles were mostly or completely identical.

So, with the pedals in my possession, I started taking them apart. The insides of these were a very greasy mess, so I needed to clean them out well by twisting a paper towel into them with some 99% isopropyl alcohol. You can see in one of the images that the spindles themselves are effectively identical, other than the threaded sleeve being metal in the GR500. From the outside, it was a little tough to tell what was going on inside with the threads, but some fit checks indicated that I would need more thread depth.

So, being that the Assioma power meters are ~10X the cost of these pedals (I got the first pair at a significant discount with a coupon), I chopped them up. Risking damaging the power meters was not worth saving the cost of a discounted pair of flat pedals! Ultimately, I am glad that I chopped them up because I did in fact realize that modifications would be needed to make the power meters work optimally in the GR500's.

The dimensions related to the sleeves were identical to the R540's, so that much was great. The big difference was in the threads of the GR500's. They both started further into the bore, and ended sooner than in the R540's. In addition to that, the GR500's had an outer "collar" which would prevent me from being able to properly tighten the power meter spindles, and it would need to be removed. Beyond that, even with the collar gone, the threads stopped a little than 2mm short of where they would need to extend to in order to allow the Assioma spindles to thread in fully.

At a minimum I would need to cut off the outermost collar, and tap the threads 2mm deeper. The issue I saw with this was that the start of the threads was ~2.5mm further in on the GR500's, leaving that much more of the spindle improperly supported. While it may not seem like much of a difference, it could end up causing premature failures of the spindles' outer sleeves, and if that happens during a full effort pedaling session, it could be really bad, So, I decided that I would cut the spindle bore entrance down another 2mm.

While I wanted to go 2.5mm, I decided against it since that would cause the end of the Assioma spindle sleeve to bottom out in the end of the 13.83mm bore. It could have been extended deeper with a reamer, but it did not seem worth the effort.

Note that in the CAD images, the red surfaces indicate places where modifications of the original GR500 pedal bodies were needed. Also, the lengh-wise cross sections of the GR500 bodies look different from the CAD because I had actually tested out a 13.82mm reamer in there to see if it could cleanly extend the bore, but it seemed to catch on the anodizing and enlarge the whole section which was not acceptable to me.

CAD modeling of the Assioma spindle/meters was done by importing the PDF datasheet for them into Inkscape (the part drawings are in vector format), and then exporting a DXF into CAD where I was able to scale and trace the outline for a revolved feature.

The easiest part of this was making the measurements on the DUO-Shi spindles. The OD of the nose of the sleeves came out to ~13.81mm. This means that it was intended to be a slip fit in the pedal bodies, definitely not an interference fit. Clearance around the OD would be ~0.01mm (~0.0004"). Other than that, there were no surprises. Same M15x1 threads, no clearance concerns for the nut at the end.

Obviously, relatively few people have access to machine tools, so you would need to substitute a hack saw and file for the setup that I used. It is entirely doable, but you would need to put care into clamping the pedal body and setting up guides for the saw and file to keep things as perpendicular as possible to the axis of the spindle. In my case, I gripped the spindle threads in a collet and clamped the pedal body into place since this seemed to get it nice and perpendicular.

1. Mill off the outermost collar section.
2. I slowly "snuck up" on the flat at the bottom of the collar so that I could zero my depth there.
3. With the collar gone and a "zero" depth established at that first inner flat face, take an additional 2mm off.
4. Do NOT exceed this or you risk having the Assioma spindle nose bottom out. You could salvage things by adding a metal washer between the Assiomas and pedal body, but it is best to avoid this. With 2mm removed, there is ~0.5mm of clearance left for the sleeve nose.
5. Extend M15x1 threads by ~5mm.
6. Even without taking the extra 2mm off, you would need to extend the threads by a little more than 2mm. So, adding 5 more full-depth threads is needed. In my case, this came out to ~8 full turns of the taps until I could get the Assioma spindles to fully thread in by hand. This will vary depending on how many lead-in threads your tap has.
7. The pedal bodies are some casting alloy of aluminum, and threading is super easy. Some aluminum alloys (6063, MIC6) are more of a pain to tap since they "gum up", but this stuff makes brittle little chips that clear out easily. Cutting fluid/oil is not required here, but I used a little Tap Magic to help things along.
8. File down sharp edges.
9. Cutting off the collar and a little of the body left a really sharp circular edge. I used a hand file to break this so that I would not cut myself when handling them.
10. Clean out pedal bodies.
11. Wash out all of the metal chips and any cutting oil. Degreaser, alcohol, acetone, or anything else like that is fine. If you have an old toothbrush, you can stick that in there to help the cleaner along.
12. Assemble
13. The DUO-Shi's come with some blue thread locker on them, and you should keep it on there. If it is used up / gone from a prior installation, add some new thread locker on. This is a safety issue.
14. Although the installation manual specifies 8-10Nm installation torque, you would need a special torque wrench or attachment to do it. So, get a 19mm wrench and carefully snug the DUO-Shi's down into the pedal bodies. If you over-tighten them, I found that the pedal spin with a little bit of roughness / clicking. I loosened them up and retightened them, but a little bit less, and the clicking was gone.
15. Activate, calibrate, enjoy!
16. Follow the rest of the setup instructions from Assioma, and go for a ride!

This modification is done entirely at your own risk. Risks include, but are not limited to, damage to the pedals, damage to DUO-Shi power meters, and serious bodily injury if there is a mechanical failure anywhere in the pedal assembly while riding. Modifying the pedal assembly as shown in this article contains inherent risks which you are subject to, and willingly accept, if doing this project.