Flatpack Q85 Drive Assembly

After a bit of testing and a couple of prototypes, I’m ready to release the flatpack drive files to the rest of the community. The idea behind these was to make a budget option for using the Q85 brushless motors, and I think I’ve succeeded. The whole drive system is cheaper than the motors themselves, which means you can come in well under £400 for a drive system if a bunch of people club together to get the plates done at the same time.


Material thickness can vary depending on what you use but no more than 5mm thick. If you use steel you will probably be fine with 2mm or 3mm. If you use aluminium then you’ll probably want 4mm or 5mm. Exact thickness doesn’t matter, you put it into the calculator to figure out what spacers to use.

Bill of Materials (approx cost at time of writing)

  • Set of plates cut (£110) – 4 of each plate, much cheaper if you can club together to order multiple sets at once.
  • 10 * 100mm M8 Countersunk bolts (£20)
  • Spacers (8mm inner diameter)
  • 4 x 4 inch Vex Omni wheels (£80)
  • 2 x 100mm 1/2 inch threaded rod (omni axle)
  • 4 x flanged bearings (£20)
  • 4 x 1/2 inch nyloc nuts (or normal nuts and spring washers)
  • 2 x Q85 Motors (£200) or equivalent. More info on other options coming later.

Tools Required (not including those needed to mod the Q85)

  • Allen Keys
  • 8mm Drill
  • Counter Sink bit
  • 8mm Tap
  • Files (for any adjustments)
  • Hacksaw

If you’re going to cut your own plates then you’ll need a bandsaw or a hacksaw and a lot of patience!

All these are adjustable. Different omni wheels can be used for example, but you may need to adjust the rear axle, etc. Theres a fair bit of tolerance in the plates too if you are cutting by hand. As long as the holes are accurate to each other, which is easy to do by clamping all plates together and drilling as one, then you should be fine.

Step 1: Cutting the plates

The easiest way is to send the files off to a company to do, such as SendCutSend or OSHCut in the US, or Fractory in the UK (and possibly EU). If you have access to a CNC, then you can cut them that way too. If you send them off to get cut, I recommend clubbing together with other people as the more you get cut at once, the cheaper it is. For example, a single set of plates may cost £200, but if four people want to build drives then that price can be down to just £40 a set. (rough prices from Fractory)

You can cut the plates by hand too, using sheet aluminium and a bandsaw. Thankfully the actual shape of the plate isn’t critical. Just clamp all the plates together once cut to make the holes all at once. This will allow everything to line up.

Don’t use thicker than 5mm aluminium, as otherwise you may run out of room in the box due to the width of the Q85 motors. 3mm steel is another option and may be cheaper. Steel will be harder to cut manually tho, so its a bit of give and take.

The holes are intentionally undersized on the plans, as one plate on each foot is to be tapped at m8 size. The other holes need to be drilled out to be 8mm for the bolts to pass through.

For the two plates that will be on the outside of the foot, use an m8 tap in all the holes except the ones for the omni axle, and the pivot point. There will be 5 to tap on two plates for 10 in total.

The opposite plate (the one that will go next to the battery boxes) will need the holes countersinking to allow the bolts to sit flush. Take care to countersink the correct side of the plate. Easiest way to make sure is to take the two plates (they’re the shorter ones, that don’t have the tapped holes) and lay them out so they are a mirror of each other, with the slot for the motor axles at opposite ends. Then add the countersink.

Step 2: Spacers

To make sure that the plates are all spaced correctly, you need to get some spacers specifically for your foot shell and motor. A calculator can be found here: https://r2djp.co.uk/brushless/calc.php

  • a = gap between foot channel and inside of shell
  • b = foot channel width (internal)
  • t = total width of motor (not including axle)
  • thickness = thickness of plates

You will need 10 of each size. These don’t need to be too exact, within a mm is fine. I’ve used plastic ones with no issues yet, but use whatever you are comfortable with.

Tip: if two of the sizes end up being very close. Make sure to store them well labelled.

Spacers aren’t the only way to do this. You may also have luck using m8 nuts on each side of the plate and manually adjusting until you have the correct distancing. I’ve not tried that myself, as the spacers seem to work well.

Step 3: Ankle joint

The ankle pivot point is intentionally cut small on the plans to accommodate different designs of ankles. Better to have too small a hole that can be drilled out than too big a one that can’t be used.

To this end, you will need to drill out the pivot hole to the size required for the ankle bolt to pass through.

Step 4: Omni wheels

The omniwheels I use are from Vex Robotics, but any 4 inch one will do. Other sizes can be used too, but you will need to modify the plates to get the wheel riding at the right height.

I use two omnis, joined together with a 3d printed coupler and two flanged bearings that fit into the vex robotics wheels. You may also need some washers or spacers to keep the wheels aligned in the center, depending on the width of the motor you use.

Step 5: Building

Now its time to assemble everything.

Take two of each plate, of the smaller ones you want one with tapped holes and the other with countersunk holes. Put the bolts through the one with countersunk holes and lay it on its back so the bolts are sticking up. Then you can add the first set of spacers, followed by one of the larger plates, the second set of spacers, and the second large plate.

At this point you’ll want to add the axle and omnis. Then you can put the last set of spacers on the bolts.

Once assembled like this, carefully place the last plate with threaded holes on top and stand the assembly up. This bit is much easier done with a second pair of hands, but is doable on your own. Get the bolts lined up with the threaded holes and start screwing them in. Better to do each one a little so the bolt bites on the thread and gets the plate in place, then tighten them up. Depending on the type of spacers, don’t over tighten them. Plastic spacers may crack.

After you’ve built the drive, you may want to slacken off a bolt at a time and apply some threadlock.

You should now have a drive assembly, minus the motor.

Step 6: Installation

You may think its strange to install the drives before the motors, but it is actually easier this way for most builds (your mileage may vary of course, thats the fun of droid building!).

Slide the drive assembly into the foot shell, the two larger plates should sit on either side of the center channel, and the inside plate should be more or less against the inside of the shell.

You can then attach the drive and shell to your ankle.

Step 7: Motors

And finally….

One of the benefits of this type of drive assembly is that the motors can be swapped very easily. Installation is as simple as sliding the motor into the slots for the axle, then tightening the nuts on each side. Run the cable out of the motor and up into the ankle however you decide to do that.


You now have a nice simple brushless drive system. This can then be hooked up to your speed controller of choice (roboteq if you take out another mortgage, or vesc/odrive if you want a more budget friendly option).

Note: My preference is to have the drive wheel at the front, so its closer to the center line of the droid. And I have the omni just hovering off the floor, barely touching it.

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