Drive Train Frame Construction

While trying to mount bumpers on a drivetrain I was cad’ing, I started searching different frame construction of robots over the years.

I’ve noticed that generally the 3 most common ways to construct the frame are:
A “single stage frame” where the bumpers are mounted with the help of supporting tubes sticking out between the wheels.

A “single stage frame” where the outside of the side frame is covered with an additional tube on which the bumpers are mounted on, so every side of the wheels is “protected” by a tube, similar to andymark’s base kit.

The third version i noticed is a “two stage frame” where above the frame the wheels are mounted on there is another frame, similar to 254’s drivetrains over the years.

I was wondering if the great frc community could help me understand the advantages of every different frame construction so my team can make a better decision on which is best for us.
I came to this topic when looking for the best places to mount my bumpers, but i guess there are more things to consider other than easy bumper mounting when designing a drivetrain frame.
Thanks in advance for your time.

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Another option for supporting bumpers is one that my team has used in the last 3 years. Basically, you have a bracket in the place where boxtube goes in the first design. It serves the same purpose and is perfectly legal but saves a bit of weight.

Here is an example from our 2017 robot:

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The primary reason for the various items between the wheels is support. Bumpers have to be supported (backed by) the frame of the robot every 8" per the rules. However, what that support is is left up to teams, and could be as simple/light as what 865 uses. It really doesn’t have to be much.

My team usually uses the third option, paired with wraparound bumpers that are clamped down to that high rail. Lots of teams use the kit drivetrain; if a team doesn’t use cantilevered wheels then they will probably run the second option by default.

One thing I will say is that if teams use the first option and don’t use full bumpers on the sides, it’s a lot less obvious where the Frame Perimeter is, thus harder to draw a yellow card for contact inside the Frame Perimeter when somebody hits them. However, the first option is one of the lightest. Second and third options are slightly (very slightly) stronger than the first option when it comes to bumper reinforcement.

By the way, teams using the third option will often use angle aluminum for the upper rail. It’s lighter than tube.

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One thing to note:

Option 2 is a standard tank drive, which necessitates an outside frame rail.

Options 1 & 3 are West Coast drivetrains, which utilize cantilevered shafts and special blocks to hold bearings inside of the tube and thus have no need for an outside frame rail.

Since the first option is lighter and probably easier to build, if I don’t have any use for the upper rail for support of other mechanisms in my robot, isn’t it better for me to use the first option?

It depends on how you mount the bumpers. Due to how my team likes to mount bumpers, we tend to use style 3. We’ve found it easier to mounting sub assemblies and other things like that too. We are not the best team to always gauge things when it comes to weight efficiency though. We tend to way over build things.

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Is type 3 better at preventing damage to the side rails and possibly affecting your drivetrain?

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Our team gets our base welded, so type 3 isn’t too hard. I’d imagine that pulling off a type 3 without welding would probably require a lot of gussets.

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Type already answered for me, and I noted in my response, that we clamp our bumpers to that rail. This is a very critical application for us–my team once won a regional partly because one of our opponents lost a section of bumpers and was disabled while playing defense, and we’ve basically said “That shall never be us”.

I wouldn’t necessarily say any particular option is the best. For example, if weight is a primary driver, I’d aim for 1, 3, 2. But if weight isn’t an issue, OR if I need stiffness against bending more, then it’d be 3, 2, 1 or 2, 3, 1. Ease of building is going to be highly dependent on your specific design but I’d be going more 2, 1, 3 on that, because locating and attaching those blocks is a pain anyways. My team welds, so we locate, weld, and we’re all set.

As far as damage prevention, especially to the drive… 2, 3, 1 by a very close margin. Obviously the bumpers help out a lot, but 2 blocks most direct drivetrain contact while 3 allows protrusions but not large ones through. That said, 254 and others used to run 1 before there were bumpers without too many issues. [Insert “EricH old man rant” here.]

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