Hello, we are experiencing a persistent issue with our chassis. Although there are no visible signs of friction on either side, the robot pulls to the right when we operate it. What do you think might be causing this, and how can we resolve it?
I don’t see anything that would be a super easy fix. However I’ve had a similar problem happen a few times. Some things to check;
~Are all of your motors functioning (all are spinning, same speed, not overheating or malfunctioned. Check the individual motors via the devices section in the brain, and check the code for anything wrong)
~If all of your motors seem fine, check for friction. I know you said you couldn’t find any, but it can still be there. It’s very sneaky sometimes. Make sure that your spacers aren’t pressing against the metal/bearings/wheels too much. Also make sure that you have bearing flags on every single axle possible, and you could potentially drill them, but that’s more of a last resort.
~While it is unlikely, make sure nothing is dragging on the field or hitting the wheels.
~Make sure your axles aren’t bent. Even a tiny amount of bending can adds lot of friction.
~If you’re using screw drive, make sure that the inserts are functional and aren’t adding any friction either.
Lastly, if all else fails you can make the side that’s going too fast slower so that they both are equal. This can be done by adding friction or slowing it down slightly in the code. This is a last resort effort though, and I would strongly advise against it. Sorry for the whole essay, drivetrains can be hard. Good luck with it though, may the odds be ever in your favor. 
Before I go on to try to help, I used to be a builder. I am now a designer, and thus, I may have some knowledge gaps, I thought I could give at least a little bit of a help here.
First of all, I would like to thank you for the photos, they can help out here a little bit. I don’t see anything wrong here, but gere are some things to check if you haven’t already;
- Double check the spacers one more time. Make sure that there is a little bit of space where the spacers can move around a little bit on each axel. Not very much, but still where it is noticable. If there is none of that on some axels, that puts pressure on the gears and/or wheels of that axel, increasing the friction. Also, check for bent axels and screws in your drive. One bent screw or axel can cause an issue like that. I have a VEX kit at home, and I remember that one bent screw caused a similar issues with my own drivetrain.
- Check your motors. From what I see, you are running a 44 watt drive (perfectly fine drive by the way). Make sure all of your motors are running in the correct direction. I had a similar issue with a drivetrain last season, and that was the cause. Also, while you’re there, check to see if there is any overheating motors. Depending how old your motors are, they may make a beeping noise after a while, which a temperture sensor going off in the motor. Either way, when a motor overheats, it will often slow itself down to try to cooldown, or it may even stop moving entirely, causing the robot to pull to a side. Checking for errors on the brain can help with that.
- Check the code. If you are using something else other than a 4 motor drive (like a 55 watt drive for example) or your are just using custom code for your drivetrain, make sure it is actually correct. Very occasionally, custom code can do stuff like that.
- There is the potencial for a severe weight balance on your robot. This is very unlikely, but I still feel it is important to bring up. If you notice on side of your robot is noticeably heavier on the left side compared to the right side when you pick it up, that weight inbalance can cause issues. Note if that there is only a small balance difference (eg: one side having 2 motors and the other not having any at all on your intake), it usually would not cause an issue like that.
Sorry for the huge amount of text there, but those are the things I would personally check. Good luck!
First I would remove the standoffs in your drivetrain, as they are prolly bending it slightly, as the distance between the flat faces of the c channel is actually around 2.43 not the 2.5 length of the standoffs. Additionally you want more crossbraces on your robot. I would run a minimum of 2 full length braces on you robot. Additionally your lift attachment looks sloppy. Typical to attach upright c channels to drivetrain, you would use 0.5 in long u channels to “hug” the bottom 2 holes of the c channel.
Onto your friction problem. If you are using rubber bands to secure your motors to motor caps, make sure the rubber bands didn’t slip and get wound on the axle, slowing it down. Additionally chekc to make sure all axles are straight. After that if it still doesn’t help, remove the most central wheel/gear and see if one half of the drivetrain is causing the issue. If that still doesn’t help, try rebuilding and paying closer attention to friction.