My hot take is that shifting is basically obsolete with the introduction of brushless motors. All of the conventional wisdom posted above was built on the backs of CIM drivetrains.
And quite honestly, you don’t even need six Falcons or NEOs in the drive - and I think people only go to six because that’s the number of miniCIMs they used, not because of anything empirical. Four brushless motors at a single speed will handle more than enough FRC use cases.
We (973) selected 6 NEO 2019 midway through the season because we could afford to do so, and at that time didn’t have enough data on it’s robustness… so it was a risk mitigation.
I advocated for 973 to go to 6 Falcon for 2020 for the same reason.
Now that we have two seasons worth of data, I’d agree with your point. For the teams really pushing the limits of performance, there is some upside to 6 over 4 if the cost and weight aren’t an issue just to reduce heat losses in wiring some.
If the field has obstacles, such as bumps or ramps, you’re likely going to have at least 4" diameter wheels, maybe even 6 or 8" wheels. Such large wheels have higher surface speeds and therefore need more gear reduction to slow down output shaft RPM to have a reasonable (13-19 ft/sec) driving speed.
You can get away with just a single stage of reduction for 4" wheels, but it will require a very large gear that is quite heavy and hurts robot’s ground clearance locally at the spot, which may be fine if obstacles can always be driven over totally straight so nothing can high-center in the middle of the robot. It can actually be just as light to have 2 stages of reduction because it lets you use smaller gears and make the gearbox plates much smaller too.
For larger wheels like 6" or greater, you pretty much need 2 stages of reduction, and now it become quite easy to put the shifting (Dog or Ball) on that 1st to 2nd reduction.
(8-10):84 with 6" wheels was very viable with CIMs/Falcons/NEOs this year.
Agree for shifting to get faster sprints. Even if you’re crossing the whole field, the half a second you’ll save isn’t worth the added weight/complexity/driver training.
For defense though, I’ve come around to see the benefit of shifters for certain strategies. At the beginning of last season I agreed with you that there was never a need to shift nowadays. But my students showed me how in 2019 there were times when we would get caught on the other side of the rocket and not be able to get back to the loading zone because of a defender blocking us that we couldn’t push through. They anticipated a similar problem this year in and around the rendezvous zone, so in addition to our normal driving gear (~18 ft/s) we decided to include a “super-low” gear (~6 ft/s IIRC) that’s only used for playing through defense. We ended up using it once or twice each match to help get out of jams.
I find this comment weird. I agree with a lot of the comments saying shifting isn’t really needed anymore, but if you’re going to shift regularly, autoshifting seems like the best way to do it. Why spend your driver’s cognitive load on thinking about gears? The last time I spent much time thinking about this (2017), we didn’t find it too hard to get autoshifting working well.
We had a really high top gear (IIRC 21 fps free speed, with 6 miniCIMs). We autoshifted at the lowest speed where the high gear pulled an acceptable level of current, and put some hysteresis on it (+ a time lockout, IIRC). It honestly was pretty easy to tune to feel natural. Asking the driver to shift all the time would’ve been absurd, and they probably wouldn’t be up-/down-shifting at precisely the right times to maximise acceleration anyway.
So sure, if you’re going to shift 3 times a season, make it manual. But auto-shifting enabled us to have a really high top speed without burdening the driver (or killing our battery).
EDIT: One of my students and I talked through the details with another team at the time. They found it surprisingly non-difficult as well
Not to pull the classic “we’ve always done it this way”, but, well, we never had an issue shifting manually. The way we set up our robots usually limits the driver to driving (sticks and shifting) and releasing the game piece, occasionally pickup as well if the operator has too many buttons. It’s very rhythmic: pick up game piece in low gear for increased precision, shift to high while positioning to move to goal, move to scoring location, shift to low for better positioning and/or pushing power, release game piece, shift back to high, etc.
Most of those shifts are happening during turns or powerslides, so I’m not exactly sure how you could appropriately replicate that. Of course this is FRC, so you can never follow a set pattern. There’s also a lot of counter-defense movement that requires very precisely timed shifting you’d want manual anyway. I don’t think we ever really saw shifting as acceleration maximization for long distances. Long sprint means high gear, small movement means low gear. Autoshifting gains during a longer sprint seem rather negligible in most “normal” use cases.
Ninja edit: Not sure how much this is noted, but there’s a significant difference between having a button to toggle shift states and a dedicated shift high and shift low button.
ofc this is all null in the era where you can throw an additional 600w of motor on the dt for sub 3lbs and basically no space.
In an attempt to convince my team (but mostly just myself) to switch to a three Falcon SS setup, I ended up writing this whitepaper:
Considering the fact that it was written at 3 AM on a whim, in response to this very thread, actually, it’s not incredibly well done, in my own opinion, but it exists, I suppose.
Now that you’ve explained it it seems obvious. I’m not sure what I was confused about earlier. Thanks!
I will add that I made a number of absolute statements, and despite their probable and/or definite accuracy, this was primarily a result of the papers function as a persuasive essay. No definitive claims were intended, although many claims were made.
I definitely agree that it matters exactly what your control scheme is for your shifters. Now that I’m thinking about it, I’m not sure that I know of any other team that has used my preferred setup:
If button ‘A’ is held down, then you are in low gear
If button ‘B’ is held down and ‘A’ is not, then you are in high gear
If neither is held down, you get auto shifting
My problem with that is some of the fancy trickery you can start getting into by playing with shifting, like punching out of a varying radius turn, reversing on the spot at high speed to fake out a defender (without browing out), or powersliding, though you can get that with raw power now. The holding down to force high or low starts to get in the way.
To clarify, #ballshiftsohard. You just can’t do this stuff with dogs.
Oh no. This is starting to turn into a manual vs automatic transmission debate.
Manual vs. Automatic vs Single Speed Transmission
Emphatic Squinting Begins to Intensify
Is it really a transmission if it’s single speed now?
Apparently yes if you go by google's vauge definitionI hereby declare versaplanetary gearboxes are transmissions!
Or my favorite kind of transmission, the Inflexible planetary:
A truly Ground Breaking new Gearbox, if I may say so myself.
For 2017, 45 used a two speed. I think it was an AM Evoshifter, forget the ration but with 3 CIMs each. We decided it would be helpful for making full field sprints to the loading station, but being able to go slow for getting the gear on the peg and of course having more power for pushing matches. For the controls scheme, I believe I just pressed a trigger to shift.
I was driver and shifting was an experience. We realized we needed to shift while the bot was moving already in low gear as to not brown out too much or put too much stress on the dog gears (I still have the remnants of several of those dog gears…)
I would 100% use a 2-speed again if the game called for them and we thought we could use them well.
Did you have any issues with current draw or motor heat when geared that fast? My team was geared to about 15fps and the motors were cold after each match so we are planing to shoot for 17fps on future Falcon gearboxes but if the Falcons can handle 22fps we might have to aim for even faster gearing.
The Falcon 500s can probably handle going 22 ft/s, but I would recommend against doing so. Play around with a sprint distance calculator and I think you’ll find that gearing faster saves negligible amounts of time for long sprints, and actually loses you time for shorter ones.
Alongside that, faster drivetrains are generally harder to control. I think you hit a good sweet spot around the ~15 ft/s range.
We tend to gear faster with no shifting. I’d rather move around opponents than try to win a pushing war and waste time
While you need to be a good offensive bot for that strategy (and we’re not quite there yet), it teaches good driving behavior. It’s easy to forget how much time pushing loses you
I’m not the lead strategy guy on our team, but I believe this is why we geared higher this year
Why three? What does that get you over two?