US units have practically been standardized in FRC. Working with imperial units is painful and is not consistent with the metric units used in the rest of the world.
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You could use metric but when motor mounting holes are 2 inches apart and all stock aluminum is 2x1 converting that would be a nightmare and teams would much rather use hole numbers.
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Unfortunately FRC , and FIRST in general, are US centric, so the units they use follow that mindset.
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I think it mostly just comes down to the fact that most FRC teams are from the US, so most students and mentors have more experience with imperial measurements. It doesn’t help that a lot of COTS parts, fasteners, extrusions, etc use imperial by default.
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Because the vast majority of teams are based in the United States, and if you order from [insert any US industrial supply vendor here] they will be very well stocked in material based on imperial measurements.
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I had this question a bit ago, and some of the answers I got were:
- The majority of First is located in America
- Most of the COTS parts from vendors are based in America, so they naturally use imperial units
- and it takes a lot of time and money to change standards at this big of a scale.
I love metric personally, imperial makes no sense in comparison.
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As far as the rules go, it really comes down to FIRST being based in the US, and the majority of teams being US teams. The dimensions used in the rules reflect the ease of availability of parts here in the US - If I go to the local hardware store, I can find tape measures in inches easily, I can buy 3/4" thick plywood easily. I can’t find the equivalent metric stuff nearly as easily, if I can find it at all locally.
When it comes to FRC-specific vendors… well, they’re all US-based as well. A lot of the measurements we take for granted, like the 2-inch bolt pattern for motor mounting, have a long history. Your modern brushless motors all have that because that’s what the gear boxes that they work with have, and the gear boxes have it because that’s what the CIM has always had. It’s a self-perpetuating standard at this point, and the relative cost to change it is pretty high for very little gain.
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We always used metric units and will be using metric since my team is located in Turkey and probably most of the US teams will be using imperial if rest of the US keeps using imperial units.
We also modify some of our COTS parts to metric units to easly adapt or use them with our robot. Most of the time only challinging part for us is to find the correct imperial bolts and nuts for the COTS parts.
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The bigger question is why hasn’t the US as a whole not switched. We’ve been talking about it for 60+ years. 70’s, 80’s, 90’s pick a decade and if we had switched we would have been mostly converted by now.
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Because switching everything over costs so much for minimal gain. So many things are already tooled in imperial in the U.S. that switching everything out would cost trillions to overall end up not changing anything.
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I grew up using Imperial. Over time, I adapted to also using Metric. If I had to choose just one standard, I must admit, Metric would be my choice, it just makes better sense.
The thing really bugs me though is when a vendor makes a product with both standards right next to each other. For example, take a look at the slightly older PDB. The battery connections use M6, yet the cover that goes over them is held in place with 6-32!
Only in Murica!
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The only piece of hardware I wish FRC vendors would switch to metric are bearings. Good quality bearings are mostly manufactured in Europe and Asia, and they’re all standardized metric sizes.
Imperial sized radial bearings (R series) are harder to find*, generally more expensive, and universally worse quality. I would like to swear off them completely, but there are a bunch in every one of our swerve modules. 
*I’m giving a pass to 1/2" hex and 3.5" ID x-contact bearings, since those don’t have a metric equivalent.
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Low quality meme is low quality.
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I dunno why this thread’s gotten this long. I see a solution in the very first reply.
<patriotism> YEEHAW MURICA EAGLE BANG BANG FREEDOM </patriotism>
It would be really nice if by 2100 we used metric units… but at this point we’re so locked into imperial units that either a) we’d half-switch or b) fail entirely or c) end up with awful decimals and none of those are any fun.
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I have a feeling it was not a light decision to end up with M6 lugs on the PDP, likely a supply issue or something similar. It’s the only metric fastener we’ve ever ended up with on a robot I believe.
also worth mentioning
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I wasn’t aware Liberia and Myanmar had landed on the moon.
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NEO550s use M3 screws, 775pros use M4. I’d be surprised if you haven’t used them somewhere. 
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