Now that I have a pair of 3-motor gearbox plates from ThriftyBot, and AndyMark is selling lengths of its gearbox spacer extrusion, I’m again thinking about building a flippable robot to play STRONGHOLD as an example for workshops this fall. I aim my workshops at teams whose best tools are a drill press and horizontal band saw, or cold saw, or at least a chop saw. Does anyone have suggestions for driving 13 to 14 inch diameter wheels from the FRC ecosystem? I’m expecting it will be a belt-or-chain drive, but am open to other possibilities.
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13-14" wheels? Can’t say I have experience with those… but 12", I have some. Chain drive, reasonably sized sprocket, use a hub. Whatever else you do don’t hog the sprocket unless you absolutely have to.
The context is that back some many years, my team at the time ran 6" in the front and 12" in the back. 35 chain, same sprockets off the motor but the 12" had a rather larger sprocket than the 6" (I want to say about 2x the tooth count for some odd reason). Custom hubs, dead axles (this was before AndyMark really got going, let alone the rest of the FRC ecosystem), steel sprockets with large holes cut to lose weight. We took off just a little bit much and those sprockets didn’t survive the second offseason (2 regionals, Champs, 1.5 offseason events)–admittedly they really only saw use for about 30 seconds/match. I don’t have a good picture, or I’d post one–I’ve got some potato-quality that don’t show much if you’re interested.
If I was doing it today I’d make sure the FRC ecosystem hubs were available and chain 'em up. I’m not sure I’d trust FRC-sized belts in that application but some heavier-duty belts would probably work OK.
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#25 chain is pretty common on Razor Electric Scooters, and you can find wheel assemblies like this one with a sprocket already installed (a bit smaller than what you’re looking for):
https://www.monsterscooterparts.com/rear-wheel-assembly-razor-pocket-mod-ver-31.html
Alternatively, I think some bike wheels/hubs have a 48 mm bolt circle (almost but not quite 1.875") for brake disks.
I would discourage a team with the stated resources from investing time and resources into a 14in wheel drive train and instead suggest picking a tried and tested kitbot upgrade kit and put the rest of their time and resources into a scoring mech and driver practice.
Given you are aiming the workshop at low resource teams, id suggest just doing a kitbot with an 8in pneumatic wheel upgrade kit. (given the 2016 focus)
My team matched the resource level you’ve stated and I would have had the team avoid the creation of a drive train/ wheel usage like you’ve stated
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It’s not entirely clear to me what the goal of his workshop is. I’d agree that a custom drivetrain workshop shouldn’t be the highest priority given the audience, but what if he intends to design this drivetrain himself and then use it for a general assembly skills workshop? If it’s made clear that the design is atypical, I don’t see a huge issue with assembling a “fun bot” for training purposes.
Alternatively, it could be interesting for him to walk the teams through the process of adapting a non-FRC wheel to FRC-specific hardware. Some day there might be an obstacle-oriented game that 1) bleeds FRC suppliers dry of easy-to-use pneumatic wheels and/or 2) lends itself to using wheels larger than 8" (the biggest FRC COTS wheels around). In that scenario 14" is probably a stretch, but I could imagine an obstacle game where 10-12" wheels are the brute force solution.
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We ran 14-inch omnis in 2016 with a chain drive. There’s some information / specs / photos on our website here.
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The reason I’m looking at 13 to 14 inches is “flippable”. II want the thing to be within the bumper rules whichever side is up. The STRONGHOLD bumper zone was 4-12", and bumpers are ~5" tall, so the axles must be at least 6-1/2" off the carpet. The low bar sets the maximum.
In the workshop, I go into a wide variety of possible kit mods, including those available as a full COTS kit. However, to inspire creativity and point out not-so-obvious limitations, I like to bring a demo that’s near the edge of what can be done with a shop that costs under $1000. The first demo I did was less than 16" square, with a horizontal battery. The next was a mecanum with a usable frame opening. The rear wheels were driven by a TB-micro as in the standard mecanum upgrade, but the front wheels were belt-driven from the mid-frame gearbox. I did this with only COTS parts and square cuts and making a few holes larger, and a bit of sandpaper.
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@EricH, What do you mean by hogging a sprocket?
Added: I certainly wasn’t planning to have sprockets with built-in chain guards like Harley sprockets, if that’s what you meant.
This, perhaps?
I don’t understand, what problem are we solving in the 2016 game by being invertable?
Seems like you’re just greatly complicating your scoring mechanisms an making and already heavy drivetrain heavier for no gain.
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If you are a “drill press and band saw” team, I would strongly recommend against using any drive train wheels larger than 8 inches.
Using 12+ inch wheels will require an extra stage of gear reduction beyond what pretty much any COTS drivetrain gearbox can offer, which were all optimized for 3-8 inch wheels.
“Drill press and band saw” teams should strive to use as many COTS parts as possible in their drive train to ensure that is rock solid and reliable, and instead focus their limited other resources towards mechanisms and manipulators.
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Among other things, saving a lot of effort trying to keep it from getting inverted.
I only had one scoring mechanism in mind; I’d just have to make sure it’s symmetrical.
And THAT is exactly why I started this thread - to see if such a solution was available but I hadn’t seen it.
And the extra stage of reduction would be from chain or possibly belt, not a third stage of gears.
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Was this a common problem in 2016?
But that requires pretty much no effort. Flipping a robot that fit under the low bar should already be extremely difficult.
Maybe you’re thinking of 2010? Those bumps were big enough and steep enough to flip even short robots if you didn’t navigate them right.
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Well actually… It’s not a drop-in kitbot mod, but the WCP SS gearbox has a few options between 15.3:1 - 19.48:1. My general feeling is that an ideal drill press/chopsaw team should be able to design/put together a VersaFrame chassis w/ clamping blocks anyway. I don’t recommend it, but you could also run a CIM/NEO/Falcon through a planetary gearbox to a get a bit more reduction before a kitbot-friendly gearbox if you really want to go the AM14U route.
While I hopefully have a VEX engineer’s attention:
Did VEX/CTRE consider having a lower KV/free speed for the Falcon 500 during development? This could reduce the amount of reduction needed for both larger wheel drivebases and manipulators. I figure that even if it was considered, the having a speed closer to CIMs just makes the Falcon seem more like a drop-in replacement and addendum shifted gears already exist as a way to reduce # of stages.
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Nope, not what I meant.
In this case, “hog out” is slang for “remove lots of material to save weight”. See also: many teams hog out their bellypans with a diamond or similar pattern.
We started with a solid steel sprocket. What’s on the robot now is solid steel. But the originals were placed on a mill and large thru-pockets were machined. Somewhere along the line, the remaining hub weld failed.
Wasn’t that I saw. Some teams flipped on occasion, but certainly not enough to justify a decision to use 14" wheels.
That said, 2481 did a huge shooter wheel this year. I think they tried a 14" at some point. @mdraear, any advice?
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Don’t use bike wheels - but if you must, they do make a 16" kids bike wheel (which I believe is actually 12"). We never had a 16" bike wheel for testing.
Useful tidbits if you must use bike wheels.
- You can’t just look at the “inch” size. See the following article: https://www.westernbikeworks.com/tire-sizes-article#:~:text=There%20are%20five%20tire%20diameter,used%20by%20many%20hybrid%20bikes
- A versa hub can be mounted to the standard disk brake mounting pattern by threading metric bolts through the versa slots. This isn’t a perfect solution; but, we did this prototyping and it seemed to work.
- If you can’t find a rim with a disk brake mount, you can alternatively look for a rim with a removable cassette. You will need to have a cassette removal tool or take it to a bike shop to have it removed. You can then screw on a disk brake adapter like this in it’s place. Note that you will need to come up with a way to stop if from un-threading itself if you want drive it in two directions. https://www.amazon.com/Bicycle-Freewheel-Threaded-Flange-Adapter/dp/B06Y4PSCYF/ref=sxts_sxwds-bia-wc-p13n1_0?cv_ct_cx=bike+brake+disc+adapter&dchild=1&keywords=bike+brake+disc+adapter&pd_rd_i=B06Y4PSCYF&pd_rd_r=bad735d0-74c6-4563-9079-51758dd07939&pd_rd_w=amiE6&pd_rd_wg=kUIlW&pf_rd_p=13bf9bc7-d68d-44c3-9d2e-647020f56802&pf_rd_r=4NP68XAC278GA6PXJE5H&psc=1&qid=1598288010&sr=1-1-791c2399-d602-4248-afbb-8a79de2d236f
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We’ve done something similar, mounting a disc brake to an AndyMark hub. We’ve used that solution on several robots at this point (as a friction brake for elevators and arms), and never had it fail.