"Modular" Chassis
 

After seeing the intensity of the competition this year and having to repair damage from bent frames due to impacts on the field, as well as having to change out wheels from competition to competition, I decided I'd try my hand at designing a new chassis. This design is based very much off of this year KOP chassis but the side panels are all custom made. The cross-brace is the same part that AndyMark sells, and the frame is designed to fit a Mini Toughbox. The point of the design is to make it easier to change out wheels, even possibly have the side modules on the side ready for a quick change when needed. The outside plates are intended to be made of steel rather than aluminum to provide extra strength, but all of the other parts are aluminum. It isn't easy to see but there's a spacer plate between the main body of the frame and the side modules to give the bolt heads room to sit. Also, the large cylinders are just placeholders for 4" wheels. The design can easily be modified to fit larger size wheels. This was my first attempt at designing a chassis, and I'd like to hear some of your feedback.

Re: "Modular" Chassis
 

I don't see an image or a file to look at. Sounds pretty neat in terms of concept.

Re: "Modular" Chassis
 

What material/size is plate the gearboxes are attached to?

This chassis concept is pretty similar to what 3467 ran this year. We liked the kitbot concept but disliked the side plates because most of the hole patterns were pretty useless to us. Additionally we weren't huge fans of the slim drive channels as it made it hard to integrate Colsons. So we CNCed our own plates out of 1/8in Aluminum and placed a lot of evenly spaced .188in holes every inch to give the design groups plenty of options for mounting and the ability to choose what size hole they needed. We mainly drilled the holes out to .25in for 1/4-20 hardware which we have unofficially standardized to. I know its overkill but every team has their bad habits. :rolleyes:

Overall it was probably the best and most reliable drivebase I have ever worked on in 8+ seasons. Evenly spaced chain runs, color coded spacers, and identical wheel + hardware on every wheel made assembling two robots a breeze with every last part interchangeable. I think we had to swap the center wheels once during 6 events but other than that it ran like a dream all year. Still have good chain spacing.

In terms of strength it held up very well but we went with an 8 sided concept so bolted on to the left and right module was a large piece of U-channel that the bumpers attached to and gave us 3 faces on each side (as seen in the bottom picture). This provided a decent amount of extra strength on our sides but if you design your bumpers right you can make your drivebase frame stronger without using the robot weight for it.

If you want I can send you the .step file for ours.

The side plate for the main body module is made of 1/4 inch aluminum to allow for a 1/8 inch counter-bore for the bolt heads to begin to rest in, minimizing the size requirement for the spacer in between the modules.

Re: "Modular" Chassis
 

See if you can view them at their links:
http://i.gyazo.com/c4f53fa4f98f8137beaf770d52faef82.png
http://i.gyazo.com/9d88494ff802356b32acf0d9608c1a98.png

Re: "Modular" Chassis
 

I see. When I saw your exploded picture I didn't notice the two plates separating from each other. I was mistaken, this is similar to what we ran this year but you guys are taking it to another level.

Part of me is curious if you are gaining much in terms of weight you are "spending" on the additional plates/parts to easily remove a module compared to how much that actually pays off. Modular designs are awesome! When we've done them they work out amazing. When we overlook one it usually comes back to bite us. This year we had both sides. Right before the UNH eliminations we noticed our extremely hard to access shooter gearbox was coming loose and it was impossible to access those two bolts with it on the robot. With just a few minutes before our alliance went on the field our pit crew threw a small bracket to hold it in place to survive the rest of the tournament. Before Northeastern we removed the gearbox, addressed the underlying issue of why the gearbox was coming loose, and then changed the bolts for ones we could access while on the robot.

Overall, I would focus on what have been the fail points in your drivebases in the past and work to on designing using reliable mechanisms and then consider how you would make it easy to access/take apart. When it comes to your drivebase you usually have to consider this on a part by part basis and the reason being is that most of your superstructure is mounting to that base. As a standalone robot, our drivebase modules come off as a whole like yours with 8 bolts. Once we add in everything else that we use to accomplish the game it turns into a lot of bolts and a lot of time to remove a module. Instead we focused on the individual parts to get them out of a completed robot faster.

This was the first year we really stepped back to look at our few successes at making good and bad drivebases to build one that didn't need maintenance and if it did we could easily remove any part.

Re: "Modular" Chassis
 

I like this line of thinking: "It didn't work so I'm gonna try to fix it" Kudos on that one.


The bigger question: Why did frames fail due to impacts? I ran much thinner material this year and took a massive beating but had 0 failures in my frame. I'm curious what the difference was. So I'm going to ask a few questions since I assume you had a frame fail hence looking at this...

1) Did you have 100% bumper coverage?
2) What was your frame like this year?
3) Where did your frame fail?


In the bumper era I'm always confused when I see teams having frames fail. With proper bumper coverage the KOP frame should be more than adequate. Heck, we used thinner material on our intake and never had a material failure in it (we DID have failures of VP belts though). So I'm really curious what sort of failures you had.



Also, RE changing wheels: Colson wheels. We're well north of 100 matches and hours of drive practice and not even thinking of changing wheels.

Re: "Modular" Chassis
 

Horizontal wise, we had full coverage of the areas that failed. Vertically, the 2-inch requirement caused about 3/4 inch of the frame open underneath the bumper.
We used the KOP frame in a square configuration.
The lower corners of the outer rails from the KOP frame were bending in to the point where they would begin to strip the plastic from our hubs.
The team is strongly planning on Colsons next year.

Re: "Modular" Chassis
 

The next step that I'm going to take is going to be weight reduction on the spacer. I'll be cutting out as much material as I can while leaving room for the holes required for assembly, and since the spacer isn't as much of a structural support, there should be plenty of room for removal.

Also, here's a rendered image of the chassis fully assembled:

Re: "Modular" Chassis
 

This is a good start. Your design process is fairly sound and it seems like you're focusing on making improvements to an existing design where it makes sense to and keeping the rest. Not bad.

I think you probably don't need the entire module to come out as one piece to do this. You could save some weight by removing the inner plate and just using the tubing for that purpose. Just put the robot on blocks and remove the outer plate when you want to change wheels. Making the outer plate itself removable is a bit of a challenge but shouldn't be too hard. Hint: Drop some riv-nuts in your tube for the outer plate standoffs to thread into rather than making them coaxial with the inner frame standoffs.

Alternately, for the outer wheels you could do a "tube axle" - a 1/2" OD, 1/4" ID piece of round with a 1/4-20 bolt running through it. This lets you assemble the wheel, pulley, and spacers onto the axle on the bench, then you "just" slide the entire assembly in place from underneath or above. Pop the 1/4-20 bolt through once it's aligned and you're done. There isn't really an elegant way to do this with the direct driven wheel though, unless you want to slide the gearboxes out to change this wheel!

As a general rule, you want to get your strength from your cross section, not your material choice. Rather than upgrade the flat plate from one material to another, make the plate not flat anymore. Can you use aluminum C-channel here? C-channel is much stiffer than flat plate because of the flanges, and thus is less prone to bending. It is also only marginally heavier than a flat plate of aluminum, while a flat plate of steel is much heavier. It's more strength and weight efficient to change your geometry than it is to change your material.

Re: "Modular" Chassis
 

Thanks!

The reason for making a module that can come off is because this year's KOP frame had a removable outer plate, but we had to put nuts on the ends of the axles to hold them in, and we could only change out the wheels between matches. By making a removable module, you can have a module set aside and prepped so that when the time comes you do a quick change of the module rather than each of the wheels.

Right, and I did add the cross-brace on the front ends of each side module to add that extra strength so it can be made of flat aluminum rather than flat steel.

Re: "Modular" Chassis
 

We had similar feelings after 2013. We used the Vexpro Versa wheels with W tread pattern and were constantly changing wheels at competitions and it wasn't pretty. We ran a few matches with only 5 wheels at champs because a swap took too long. We heavily looked at WCD style bases since taking wheels on an off is a breeze but realized it wasn't the best solution for us. Changing to a more reliable wheel like a Colson is great solution if you feel you are swapping wheels too much. We wouldn't have needed to do our one time swap this year if we had designed it for a drop center (oops). If you design in an 1/8in-3/16in drop you will never have to change a wheel.

This design has a 1/8 inch drop center and can be used with Colson wheels so if it does ever get used that would probably be the best configuration. Another use could be to be able to swap modules that have various wheels on them between matches to be able to adapt the drivetrain to your opponents/teammates.

Re: "Modular" Chassis
 

If you do this then all configurations combined must be under 120 lbs unfortunately.

Some inspectors (incorrectly in my mind) even consider modular swaps of identical components to be something that requires re-inspection.

Oh. Well, just an idea haha.

And yeah that doesn't seem right to me either. If it's identical then it's identical, it should only require re-inspection if it's a major change, like the type of wheels change or the way it's attached, or the size. Otherwise it shouldn't trigger anything more than an "oh we did this" "oh okay thanks for telling us you're all set"