pic: ABS-122, shifter-in-tube chassis



After seeing Chak’s and PwngNick’s take on the gearbox in tube, I started wondering if a shifter could be fit in there. At first after laying it out I thought it was impossible, but once I put the versasprockets in the back inline with the CIMs it became (sort of) easy. See the other pic for the view of the geartrain.
Free speeds are 15.4fps high, 7.2fps low. Wheels are 4" colsons with 1/2" hex bore from Vex. Weight is 24.7lbs not including bellypan, which would probably add another 2lbs or so to the weight. Chassis dimensions are 32x26" (LxW).

The main feature of this is the low weight and saved inside chassis space, but it also has a low part count compared to normal gearbox/chassis combos. The only annoying part to make it the dog shifter shaft, but that can be done in a manual mill by starting with a 5/8" hex bar.

It doesn’t look like you have any form of chain tensioners on this setup. Although you can do without them, I would still really recommend putting them on even in the case of using large tooth count sprockets. We used a setup similar to this in terms of chain, and really wished we had designed a way to tension the chain during the season.

We used 16t #25 sprockets all season with only a single jump at SVR (and that was after another competition in Utah) and that was after the chain got very loose. It’s possible to add tensioners in the form of versablocks or even WCP sliding bearing blocks, but I like the set-and-forget of direct C-C. Are you certain that your sprockets were correctly aligned? I’m guessing that they were and you guys just did much harder driving than us, but it never hurts to check. :smiley:
With massive sprockets such as these I’m not as worried about the chain coming off as much as I am about the chain picking up tape. :stuck_out_tongue: I could swap these out for #35 chain sprockets, but the extra weight isn’t worth it to me. In-season if it became a problem switching to #35 chain would be my first choice.

I’m really impressed with how compact this is. What size are those sprockets, by the way? I think you’ll be fine leaving it at exact C-C, but since the chain is outside of your tube, it’d be easy to add a static tensioner after your first or second competition. Just for peace of mind you might want to print something that fits around or on the top of your motors to ensure the chain doesn’t rub against the motors in the worst situations.

Thank you!
The sprockets are 38 tooth versasprockets from Vex with a minor diameter of just over 2.8". They are inline with the CIMs, so I’m not too worried about the chain hitting. In the past 115 has used polycarbonate chain guards, so if there was a problem I would stick those in.

I like this. I think it would also be a neat way to do a single speed drive with a PTO, not super clunky like most PTO setups.

I was thinking about making a PTO option of this. With the Versadog it becomes a lot easier too.

We never had issues with it slipping or jumping or breaking or ever coming off. We just find it very hard to write fast, accurate and repeatable autonomous modes with so much slack in our chain. It gives semi un repeatable results in our vision code when only the front 2 wheels move while the back 4 don’t having to make up the distance in chain slack. It would have been much better to just run tensionors from the start.

I see, that’s a good reason to run proper tensioning.
How did it cause errors/to what extent? If two wheels are moving, theoretically the whole bot should move anyway. Would direct driving the center wheel help?

So this year our robot had about 60% of our weight on the front wheels. Direct driving the center wheel or any wheel might have helped. We ran a single gear reduction 12-66t and then a 12-22 #35 chain reduction to the wheel. (We later switched from 12-22t to 12-28t and is much better) In our case, direct driving any wheel may have helped but we really enjoyed this setup. We would get no movement on our robot for about 1/8th of a turn on the back wheel to catch up in chain slack. The whole robot wouldn’t move because we had tons of turning scrub also so I guess a few things would have helped. But in any case, the chain slack and slop in the hex shaft really was a pain when trying to tune our code. I guess that’s why 971 goes through so much to get all of the slack out of everything and makes custom hex. :rolleyes:

If 60% of your weight was on the front wheels, I would recommend (other things equal) that you drive the front wheels directly; this is ultimately the same reason that West Coast drives the center wheels directly - to make the drive to the most dependable wheels invulnerable to a broken chain/belt.

On the other hand, if 60% of my robot’s weight were on the front wheels in a static configuration (and presuming I had at least six wheels), I would be VERY worried about the robot falling on its face in a braking maneuver.

That is completely dependent on CG height and, given 4587’s short robot, probably not a major risk.

We had #35 chain direct driving the first wheels. Given the strength of 35 chain, and the fact that we put the chains in line, as well as calculated the direct c-c we had no issue other than chain stretch which is to be expected. We had all 6 drivetrain motors above those wheels accounting for the rest of the weight minus the catapult and battery.

With this being said. After 2 regionals, Champs, and an offseason already we never replaced 35 chain, or 25 chain. We were never worried about it breaking either. It is just something that doesn’t occur that often if chain is used correctly, which we did, just without tensioners. Which in our case worked very, very, very well, and in many cases would be fine for most every other team in FRC not wanting to do some of the precision complex things things we need to get our code really dialed in.

Looking at the size of those tubes, would you be able to put the wheels inside them to maximize space used in the tubes? That would also allow you to move your side rails out more to create more space in the center of the frame.

The two wheels directly driven by the gearbox have to be outside the tube as the gearbox is inside the tube already there. The other wheels could be put inside the tube, but that doesn’t really do anything for you.

You know, now that you have the gearbox AND the wheels in that tube . . . you could probably get the RoboRio and the VRM in there if you get creative . . .

How to Give Electrical a Heart Attack, part 3

I do electrical, +10000000000 :eek:

I know it sounds strange, but if this is the case, I would seriously consider not driving the back wheels at all – save the chain and sprockets and heartache. I might even consider skipping the back wheels and putting some pegs or furniture gliders. Here’s why: If 60% of the weight is on the front wheels, the CoG is closer to the front wheels than the middle wheels. If the CoG is low enough that you are “not at major risk” of braking hard enough to tip forward, you are probably “at minimal risk” of ever needing to put weight on the rear wheels, and at “fuggetaboudit” for need to drive those rear wheels.

Interesting idea. From what I can tell, unpowered wheels used to be more common on early FRC drivetrains, but have fallen out of style in the past several years. It would probably work well in this case, but I do see one thing that might be a concern. In a six-wheel drop center (which might or might not be what we’re talking about, I don’t know 4587’s robot), doesn’t the robot tip backwards onto the back four wheels when driving forwards? Now I know that the position of the center of gravity affects the tilting of the chassis, but this would be on a case by case basis. If the chassis DOES tilt backwards when driving forwards, not powering the back wheels may make acceleration quite slow.