As hard as it would be to control, would it be possible to gear a normal 120lb FRC robot to travel at 20 feet per second while still having decent acceleration? It wouldn’t be practical but it sure would be a fun driving experience.
Well, some of the lapbots in 2008 were reportedly geared near 25-30 FPS. Admittedly, they weren’t 120 lb.
So, yes, it is theoretically possible. Now comes the fun part: calculating the gearing. I would suggest having a 2+ speed gearbox, one for starting motion and one for flying.
I think you also want at least 6 motors in the drive. That should help quite a bit with acceleration in addition to a multiple speed gearbox. 254 routinely gears for about 17.5 fps in their high speed but they are also usually underweight and have their west coast drive down to a science.
As an actual competetive robot it would be pretty crazy for most games, but just as a offseason project it could be pretty cool.
It is definitely possible. I can’t quite remember where I read it (it may have been on their website), but I believe Slipstream (Team 254’s 2011 robot) had a max speed of 20fps in one of their gears. I believe CD-Media has a few pictures of their gearbox around it. You may be interested in looking around for those and see if you can’t get a good look at their gearing. When watching 254, I rarely saw them use it aside from the moments when they had to make quick runs across the field, and with 5 other robots on the field, that was not often. But it it would still be a nice feature to have in case of emergencies.
It is definitely possible.
Hope this helps!
-Leeland
Edit: Okay, 19.8. This robot also weighs only 100lbs, but it should still be possible.
This looks like a job for the JVN calculator!
…which I haven’t spent enough time studying to learn what all the numbers mean. :\
Most people find 12-14 feet per second to be the upper limit of what is controllable on a regulation field. (And even that is game-dependent; while it was desirable in a game like FIRST Overdrive or LogoMotion, you wouldn’t want to think about that in a game like Breakaway with serious obstacles slowing you.)
Now, COULD you build an FRC-legal drivetrain? It’s plausible with the right math (I echo the endorsements of JVN’s mechanical design spreadsheet) and from parts available on AndyMark’s website. As an exercise, try to figure it out before you hit the spoiler:
How I reached it
AndyMark Shifters (standard gearing) with two CIMs per side, fed through a 12:22 sprocket reduction to an 8" FIRST wheel gives a top-gear speed of 19.64 FPS after accounting for losses–close enough. With a total weight (including battery and bumpers) of 100 pounds on the driven wheels, you’d be facing a worst-case drivetrain current of 148 amps…but once you got in motion, you’d likely dip back below the all-important 120A of the breaker before it tripped. If you wanted to be super-paranoid–nothing wrong with that–then keep the weight on the driven wheels at 80 pounds or below.
It would be extremely difficult for a driver to control, which would certainly take some of the fun out of the driving experience (but it might be a good learning exercise anyhow). We learned our lesson the hard way in 2008. The plan was to have a closed-loop control system which in theory would have mitigated the problem, but our wheel encoders were unreliable and we had to fall back on an open-loop approach with disastrous results. YMMV…approach with caution.
How I reached it AndyMark Shifters (standard gearing) with two CIMs per side, fed through a 12:22 sprocket reduction to an 8" FIRST wheel gives a top-gear speed of 19.64 FPS after accounting for losses–close enough. With a total weight (including battery and bumpers) of 100 pounds on the driven wheels, you’d be facing a worst-case drivetrain current of 148 amps…but once you got in motion, you’d likely dip back below the all-important 120A of the breaker before it tripped. If you wanted to be super-paranoid–nothing wrong with that–then keep the weight on the driven wheels at 80 pounds or below.
I’m no physics buff, and could be wrong, but wouldn’t this have a pretty slow acceleration?
Not if you design it correctly. 973 was geared for 17/7ftps and excelerated faster in high gear. And while we did use 6 motors in drive for half the season, it was designed to work with only the 4 cims. This is why the gearbox calculater is such an essential tool for custom drivetrains because it allows you to know for sure if it will work or not.
Sidenote: 148 in 2008 was originaly designed for 30fps, but later reduced because it was too uncontrolable.
I didn’t run it that far through–he just wanted 20 FPS, he never asked if he could reach it on a regulation field! 
Someone want to channel their inner Mark Leon and do the math?
Is this actually true? John? Other 148 members? That seems like something even 148 wouldn’t attempt.
Our drive this year was geared for 18.5 ft/s in high gear after accounting for losses. Before losses, the theoretical free speed was something like 22+ft/s.
I discussed this in another thread, but controlling the robot at that high rate of speed is not something to take lightly. Here is the other thread- Post #s 8, 10 and 12 were my comments on driving at that speed: http://www.chiefdelphi.com/forums/showthread.php?p=1085342#post1085342
Some teams routinely can handle the high speed, but it is certainly not for everyone. My main point is don’t assume you can drive that quickly and be able to just control it like you could a slower robot. Make sure you have good reasons to go for it, you’ve done the math, have a solid design and a good strategy for control and you will be ok.
-Brando
Thanks!
Now as I’m remembering back, I do vaguely recall that. That would have been something to see that little nine-sided robot cranking around the field at 30 ft/s.
-Brando
Something tells me that it may have been underwhelming. (Really fast in a straight line but hard as heck to handle making it painful to watch)
Considering how that swerve was driven (RC Car controller) if any robot was going to do 30FPS well it would’ve been 148… But with John saying that it would’ve been foolish, it makes me believe that 30FPS is just too fast to be successful on an FRC field.
I remember seeing a few 25fps-ish robots in 2008 and most of them seemed to be under driven or barely controllable.
(Though, at 30 FPS, they’d have been running laps in what? 5 seconds?)
Yeah I agree with that. If any robot in any game was going to move that quickly, a round swerve drive robot driven with an RC controller definitely seems like the one to do it. I imagine taking non-banked turns at that speed is what started causing some issues (not to mention just the shear reaction time required at that speed).
-Brando
You’re dead on Brando. Check out their 2008 video: http://www.youtube.com/watch?v=6fLf71xlVhE
Looks like 148 could’ve risked rolling over at 30fps, they were getting up on two wheels (Maybe one? Don’t know how a 3WD swerve handles) during hard cornering.
You don’t need 30 FPS for a slightly longer than 5 second lap without traffic.
Speed Racer, still my favorite robot. A perfect example of KISS. Don’t even touch the trackball and score more points than 90% of hurdlers.
Our 2008 lapbot was geared for 23 fps and had very good acceleration. 4 cims, 70 pounds, 6:1 ratio from CIMs to 6" wheels.
I can vouch for that, speedy little bugger