My team is trying to make test chassis this off season. We have two sets of different gearboxes (a single speed and a double speed) meaning we are building two different chassis. While brainstorming what these chassis would look like we started considering if there is a point in making a chassis that is smaller than our max perimeters. So, that brings me to my question: are there any reasons to build a test chassis that is smaller than the max perimeters?
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My team just made a new test chassis small enough that it can fit through doors with a set of bumpers on.
Makes it easy to move from room to room or load into a car.
(Edit) CAD included
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Doors. Having to take bumpers on an off is a pain in the neck. Standard doors are 32 or 36. Some are smaller.
If you go with 26 inches to a side, you can have 3 inches of bumper on a side and you’ll usually still be able to squeeze through a 32 inch door (using the squish of the bumpers).
That assumes you are in a place where you can open the door all the way, and not just 90 degrees.
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Of course if you can tip it like that you’re home free anyway.
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OP: Your robot would have to be decently short for above to work, so just keep that in mind. Also if you robot is decently heavy you’ll have to carry it an awkward sideways angle because bumpers aren’t great for sliding on the ground.
we never build a comp bot to max dimensions so why build a practice bot that way?
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There ALWAYS is a point. There might in fact be several, depending on how much smaller you’re talking.
-1" under: Standard bolt-head/tape measure/“oops” allowance. Also, for the old-timers: the “Box” allowance. (Bolt heads that didn’t go into the box needed to be cut off.)
-As noted, doors with bumpers. Probably about 6" under.
-Strategic/small-space robots. Usually used when it’s going to be tricky to get 3 robots in the same place for max points, occasionally for navigating through a crowded field. These may only be smaller in one direction.
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This year our team decided to go for 28x28 for our practice drivetrain, the reasons are:
- it’s square
- it’s even so there is no offset mounting
- it fits the rule book with room to spare
I have warned teams when doing inspection that they are squeaking in under the frame perimeter limit by less than 1/4 inch or 1/8". I told them about a photo somewhere on this forum where several tape measures were laid side by side, starting at the same point. By the time they reached 120 inches, there was at least an inch difference between the longest and shortest ones. This means if a different tape measure was used, they could have failed inspection.
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I see, however, wouldn’t that mean that my team would want to build it with max dimensions since my team has been building the actual robot around the max dimensions. Setting aside that smaller test chassis can be more convenient.
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There are plenty of reasons to build small robots, light weight, more space in climb zones, easier to avoid defense, etc. If you want to just test a new drivetrain, build it as close to the size and weight of your usual competition robot, including CG. If you want a demo bot or simple code test mule, then small bots are more convenient to transport or store. If you are planning on tearing down the bot before season to harvest materials, you could build it oversized so as not to cut down stock. Although depending on how oversized this might ruin any test results.
It would mean that you want to leave yourself at least 1 inch of frame perimeter in case the inspector’s tape measure is wrong (or in case your tape measure was wrong when building).
Speaking from experience, smaller robots are much easier to deal with (up to a point). They’re easier to move and lighter weight. If you’re making an offseason test chassis then you probably don’t need to mount any big complex mechanisms to it that would require a large footprint, which means there’s no good reason to build a big bot.
From my recent experience: 1678 went 26x26 last season and were very happy with it. We have an offseason project at 29.5x29.5, and can’t get it out the front door of our building or into the machine shop without removing the bumpers. It’s a pain. 2102 did a 22x22 robot in the 2018 offseason, which was small enough that working on the machine was a pain. 26x26 will probably be our jumping off point next season unless strategy (or a crazy mechanism) dictate otherwise.
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Why is your team doing this? Do you gain any strategic or performance advantage from having the extra inch of frame perimeter? In my previous post, I mentioned teams that almost failed inspection because they were doing what your team has been doing. Looking at their scoring mechanisms, it is my opinion that they got any advantage by making their chassis at the maximum size.
Well the idea of building max perimeters is that why would we give ourselves less space to work with.
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You should calculate how much extra space a chassis with the maximum frame perimeter will give you vs a chassis with a frame perimeter that is 1 inch less. Then ask yourself whether your mechanisms can fit in the smaller space. Next, ask yourself what you would do if you if you fail inspection because it is determined your frame perimeter is over the limit. After that, ask yourself what you would do if you had also designed your mechanisms so that they are right up to the frame perimeter. Lastly, ask yourself how long it would take to make the frame perimeter of your current robot 1 inch less than it currently is and how much extra time it would have taken to design and build your mechanisms to fit into the smaller perimeter inthefirstplace.
Remember, design limits should not be design goals. Those who treat design limits as design goals will eventually run into trouble because they did so. Risk management is a valuable skill to learn even though it may not appear relevant to STEM education.
Because if you’re over size, the sawzall makes your robot frame smaller.
No exceptions.
And because there can be strategic value in building smaller.
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Smaller robots are lighter, lighter robots are faster. A lighter robot also gives you room to beef up any delicate mechanisms, and to add new stuff on later in the season.
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Adding a .5" chamfer on all 4 corners buys you about 1.1" of frame perimeter in a pinch! But definitely better do design undersized from the start.
That can work unless the robot has some structure above the chassis/bumper zone that protrudes into those corners. Several of the ones I inspected that were right at the limit had such structure. I don’t recall enough about their design to be able to say what the effect of extending those chamfers upwards.
Thanks so much for the tip. I just want to ask a question that one of my teammates brought up about this whole thing. If the problem is the inspectors measuring tape wouldn’t we be able give the inspectors the measuring tape we used to make sure we were under perimeter?