pic: 303 Prototype Sheet Metal Drivebase

Prototype drive-base CAD for an off-season project since we got a new sheet metal sponsor.

I just wanted to start a discussion to see what others had to say about this design, particularly on the geartrain. We had bad experiences with belts this year, and in MAR several other teams have done all gear drives particularly well (341 and 25 just to name a few). Any responses would be helpful.

Step 1 when starting sheet metal - talk to your sheetmetal shop about best practices with bending, material selection, and structural integrity.

We got that down, and the sponsor has approved all the stuff on that render. Any criticism on the geartrain or anything else is much appreciated.

Can you show an image of your geartrain and provide details on what gears/ratio you are using ?

This looks really robust and cool. I especially like the aesthetics of the gears.
A few questions I have:
How do you intend to incorporate bumpers into this design?
Is the bellypan aluminum as well? If so, how will you mitigate electronic component isolation issues?
Why did you make the right-left sides congruent rather than mirror images?
What issues did you have with belts that led to your dislike?

Looks good,

Exposing a portion of wheels on the front/back will allow the drive train to climb a obstacle better. Consider a angled bottom front and aft.

A staggered perf pattern allows for more weight reduction and will help prevent oil canning.

Without seeing an image of the gears more exposed it’s difficult to give a good review of the system overall. Just looking at what is visible though it looks like the system would certainly run.

I am curious about a few things:

  1. What size gears are used on the wheels and idlers?
  2. What is the distance, center to center, between the wheels?
  3. Are the wheels on a flat plane or angled?
  4. What type of gears do you plan on using (Vexpro? AndyMark? Steel? Aluminum?)
  5. What are the overall frame dimensions
  6. How wide are the drive modules?

The shifters are just WCP 3 cim dog shifters, we haven’t decided on the ratio with that yet. The gears in the drive modules are all the same so it’s just 1:1.

Thanks! Honestly thought I’d get a lot more criticism for the design then I have so far.

We are thinking of using one piece bumpers that are attached with thumb screws to the front and back. That idea hasn’t been fully flushed out or CADed yet so it’s subject to change.

Yes it’s aluminum, all of the control system parts are electrically isolated themselves if we don’t have issues with shorts in our testing in the pre-season we probably won’t do anything special to isolate them.

Not sure what you mean by congruent vs. mirror image.

We have had issues with tensioning in the past and rarely belt snaps. We figured if we could sacrifice the weight then we might as well do away with them.

Definitely going to do this with this design at some point. Optimally I’d like it to be able to support wheels up to 6in in diameter.

Something else that will be done :slight_smile:


Not sure off the top of my head, will have to check the CAD tonight

Center two wheels are dropped 1/8in since we have had success with that drop using 6 wheels

Vexpro 40t aluminum with the hex bore.

Dimension wise all I can remember off the top of my head is that the perimeter comes out to 111.2in

For the hole pattern, could you stagger the hexagons so that there are only thin lines of metal between the hexagons? Like this:

__ /
/ \ _/

Perhaps I’m looking at the render incorrectly, but it looks like the gearboxes are offset from each other (the one on the robot’s left is more forward, the one on the robot’s right is more aft) rather than being directly across from each other. I assume this was designed so that the right and left sides would be congruent to each other and easier to manufacture, but I would think if they were lined up, the bellypan would be more uniformly accessible. I was hoping you would speak about why you decided to design it as you did.

I would be seriously worried about frame rigidity, I would over-design and avoid any type of bend.

Where is the CG?

This frame looks similar to 971’s 2013 drive base. We just released CAD of that frame. Take a look and see if it gives you any ideas. It has treated us very well, so you are off to a good start.

Off the top of my head, your front and back frame rails aren’t beefy enough. Make the top flange like 1.5 - 2" long, and add a bend off the back of it for strength.

Any reason why you didn’t leave the wheel cutouts as a hole with metal all around it rather than cutting it out to the edge like you did? It will be stronger, especially in the corners.

I like to leave some holes in the top of the frame rail so I can see things like tread wear, and belt (or gear in this case) health.

Oh yes you’re correct, I decided to do that to keep the CG dead center.

I actually have been looking at that CAD among others for inspiration since this is only my second attempt at an FRC drive-base.

That’s a good point and I’m going to change that before we send it off to be manufactured.

Another good idea that I’ll probably add. Thanks for all the suggestions, this is the reason I posted here so we don’t have to waste our sponsors time and moeny.

I am not certain, but I think daisy only had 2/4 wheels per side directly driven this year. Dustin may have to correct me on that.

Anyway, one thing to be super careful about is efficiency. 341 puts a ton of effort into really precise machining, speccing parts/materials, and maintenance, and I would assume 25 does the same. Lots of idler gears means not only a lot of weight and a lot of money, but it means a lot of inefficiency.

If you are willing as a team to put this much effort into improving drivetrain performance, IMHO the effort would be better placed learning how to correctly implement timing belts (hint; dead space center to center, no tensioners). They are efficient, light, and far more durable than they are often given credit for.

I second this. As scary as belt drives sound, they’re actually easy to plan out and put together. In our case this season, they were nearly maintenance and worry free. Plus your wallet will thank you.

I’m a programming/electrical type, so take what I say with a grain of salt. From the images, this chassis seems like its going to make maintenance difficult.

Some things to consider:

  1. Make it easy to replace any part. In the event something breaks in your DT (gears, wheels, bearings), you don’t want to have to sit out a match because it takes a long time to do maintenance/repairs. Consider making openings in the top of the covers above your parallel plates, and some in the bottom of the belly pan. openings in your belly pan directly below your gear boxes can allow you to drop the entire assembly without having to have access from the top of the robot (that’s how we’ve done it at least). This requires some attention when designing the superstructure though to provide the rigidity you’ll be losing by cutting up the belly pan.
  2. Maybe it’s just that I can’t see it in the images but, it doesn’t look like there’s a way to access the screws that mount the motors to the gear box from the outside of the chassis. At a minimum this is going to make replacing a bad motor very difficult. The same goes for holes to access the screws that attach the gear box to the chassis (those ones are pretty critical if you want to build this).

Make all of your idler gears dead axles that double as standoffs. It makes your frame much, much more rigid, saves a little weight, and is generally a “free” way to add rigidity.

Be sure to consult with your sponsor to see what kinds of tolerances can be held around two bends. Axle holes may not line up if they can’t bend vary precisely.

Why do you want to have a gear powertrain? Saying that 341 and 25 have them is not a sufficient answer. What advantages does it you? What issues with other powertrain solutions you’ve used does it solve?

Not to mention it will save on hex bearings, which are notoriously hard to procure for FRC teams. Based on the picture, it cuts a minimum of 18 hex bearings out of the design, and presumably 36 if you were to use round bearings in their place on the idler gears. Using Vex prices, that would also result in over $120 in savings, as well.

There are only two hex bearings outside of the one’s in the gearboxes. The rest at circular

This is simply an off-season project to look at the efficiency and feasibility of such a design. Whether or not we use it in the official season is to be seen. The center to center for all the wheels are compatible with .25 chain with no idlers so we will be testing it with chain as well as gears.