pic: Sheet Metal Drivetrain


This is my first attempt at creating a sheet metal drivetrain.

8 wheels with 6" VEX Pro wheels
All material is .09" thick Aluminum
Weight: 35 lbs

Since this is my first attempt I am looking for comments and critiques about how I did.

Are there any flanges on those cross members or spacer plates in between the blue drivetrain plates? That can easily add a lot more rigidity.

This. If you’re curios about how much strength proper sheet metal flanging and bending can have, make a simple square out of cardboard, and try to bend or twist it. Then take a sheet of the same cardboard, and “flange” the edges (you might need a bit of tape to get to stay…). Try the same amount of bending and twisting, and you’ll notice a large amount of extra rigidity. Even more so if you can stay away from rectangles and stick to triangles.

Looks like a good start, but you might want to take it easy on the cheese holing until your design is structurally sound…

I can’t tell but I would like to assume the center 4 are dropped? That’ important.

Yeah, The center four are dropped. Looks around 1/4th or 3/16th… I may be wrong though.

Assuming this isn’t Breakaway-specific (e.g. normal bumper height and no bumps), we have used a sheet metal drivetrain since 2006. In 06 and 07 it had a slight angle to the front to allow it to climb ramps (and 8" IFI wheels, 6wd drop-center), in 2008 it was a flat square, in 09 we had a crab-drive that was still mostly sheet metal, and this year we had a rear chassis pan of sheet metal with a welded skeleton.

The 06-08 one is probably of most interest to you. It’s really simple. there are four main pieces that run the length of the frame, each one is aprox. 4-6" tall ( can’t remember the exact dimension) and is the length of the frame. They are rectangular. They have a flange on the top and bottom. There are three axle holes in each for a 3/8" chrom-moly pipe, taped for a screw (I don’t know the exact size). The two bumpers are the same height as the side pieces, but folded to go around them. There is one chassis divider on the middle of the chassis, going between the two inner sides. There are also two plates in between the outer and inner sides, in between the wheels on the top and bottom. That’s it. No lightening holes. The outer plates are designed to be easily removable for maintenance. Each chassis had a different drive (2 fatcim through DeWalt on 06, 4 CIM through DeWalt in 07, 4 CIM through AM Shifter in 08) but that doesn’t matter much, it only changes the holes in the inner plates for transmission mounting. I don’t know any more specs or weight, but it worked well.

It appears you dropped the middle wheels. That’s good. Then it will probably turn. If you don’t drop the center wheels or use omnis on the corners, it won’t turn. And that would suck.

Before you commit to a particular fabrication technology, make sure what you are working on can be manufactured. Not all sheet metal shops have laser/waterjet cutters; some only have a turret/punch machine that create sheet metal parts by stamping out all the holes with punches. When designing parts for these shops, you are limited to much simpler designs (e.g. standard punch sizes or some multiplication thereof). Any complex curves or triangular lightening patterns can be fabricated, but only by nibbling the turret punch. This can make the part, but all your nibbled edges will have scalloping to them.

Get rid of the outer corves edges, and make all the outside edges straight. Then put a flange onto each of these edges. When working with sheet metal, it’s all about the flanges.

You’ll need to further increase the corner strength; triangular/angled corner gussets is an easy way to accomplish this.

How do you plan to join all these pieces together? Rivets are great in shear, but can only be removed by drilling out the holes. If you need something that may need to be removed, use PEM nuts. On 228, we use tons of PEM nuts (#8-32 and #10-32) to hold almost all of our sheet metal parts together, as you can use countersunk flat head screw on your outer frame perimeter to comply with the ridiculous frame perimeter rules that 1/16" high rivet heads would fail.

Well that’s the understatement of the century.

To further on the “not turning” subject though, I can give an example in which shows you that wheel placement and such matters on your turning. I was talking to team 1561, apparently their students had a laps of brain function and took the kit bot and put 4 (Yes that’s right, 4) AndyMark Roughtop 8" Plaction wheels. They were shocked when it didn’t turn. What was the reason?

When the wheels are that far apart, the inside wheels rotate slow than the outside wheels. <Blah blah blah> That’s the whole reason that cars have differentials. Other wise the inside wheel would have to skip. Your wheels have a better chance on slipping when you have a lower coefficient of friction. Well… Roughtop have a coefficient of friction that is rather high. So it will not slip very easily. See the issue with this?

Simple rule of thumb, the closer together your primary driving wheels are, the better your turning. <-Only applies to high friction wheels.

EDIT: Okay, I have to admit; you did a GREAT job with the CAD. How long have you been doing this? IT looks like you are using Inventor though; such a pity. :stuck_out_tongue:


I cheesed everything because I wanted to experiment with different types of lightening.

Yes, there dropped a 1/4"

3/16 rivets

Thanks, I took me about two weeks to do(but I didnt spend all of the two weeks on it, I had to go school some of that time:mad:)
Ya, I am using Inventor because thats what we learned at school, but I’m going to try to learn Solidworks when I go to college next fall.

I meant there should be flanges perpendicular to the horizontal cross members, not just flanges to mount the crossmembers. Either its the picture and I can’t tell you have them, or you misunderstood and should add those. (All the gray parts)

good job paikoff

Ok, ya I misunderstood, but I got it now

There is another strengthening method used for sheet metal, and that is beading. Basically you put a 3/8" wide x 1/8" deep (can be larger or smaller) “bump” along the length of a straight member. This adds significant rigidity to the part; however it complicates the flat geometry (what needs to be fabricated/cut when the metal is still flat).

Here is one type; they also make beading dies for presses that are better for beading like you’d need for such a chassis.

Nice CAD work; The 0.090 material would be plenty think. With some creativity, you might be able to go down a gauge or two. One thing to consider is the long unsupported members: These tend to be very weak in compression (they fold) and, unless you are certain they are only in tension, you need to avoid making them a “slender column”. This is where flanges and beading and other deformations will help most.

This looks an awful lot like the 148/217 chassis from this year. You may want to study those.

Beautiful cad-work. a couple more pointers. Add in somethig to represent your chain paths. In order to get 0.090 as light as 0.060, you have to remove 30% of the surface area. For a 12" x 12" piece, that would be 61 1" diameter holes. Experiment with Poster board and glue to learn how sections create strength. You can print out your designes and glue this to posterboard. You can then literally cut and paste a scale chassis together. A good example to think about is: A piece of 1x1 box with 1/16" wall is lighter and stronger than a piece of 1x1" U-channel with 1/8" wall. You can do this “test” at Home Depot. Grab a couple pieces and give them a twist (not enough to permanently deform them, but enough to feel how rigid they are). Think about why that is, and how that might apply to your chassis design. Keep up the good work!