One speed vs Two speed gearboxes

[metalthorn]

My team is trying to decide whether we want to go with a two speed gearbox or one speed gearbox. For the last two years we have been running 2 speed gearboxes, this year our mentors are talking about possibly shifting(no pun intended) to a one speed. The main argument for the two speed is the flexibility it offers. The main argument against it is that they are expensive and they may not be worth the price. We are discussing this now among the team but I want some other perspectives on the issue.

[tindleroot]

Other than the actual cost, shifting gearboxes require more space on the robot, more weight, and more air.

Team 135 used a two-speed shifter last year, and I think it paid off since our drivers knew when to shift for the optimal performance. If your drivers know when to go fast vs. slow, then two-speed should be used despite the additional costs. Otherwise, if you really want to save money and weight, then you can eliminate the shifter.

[Tungrus]

Other things you should consider:
- the game and strategy, what's it gonna be?
- field layout and obstacles?
- is your robot going to be primarily defensive or offensive?
- if your primary design is offensive, will you be able to score points yourself or expecting minimal help from alliance partners?
- if its defensive robot, will you be able to successfully defend, are your drivers ready to defend irrespective of match result? If your partners are not able to score would your be ok?
- do teams in your region generally have a well designed offensive robots?

We have been using shifters and its a challenge for our drive team to switch in to defense mode, its more of mental block and hesitation.

[JesseK]

We too have gone between shifting and non-shifting gearboxes over the years. The major argument I'm enforcing this year for shifters is getting through defense. I've never played on such an open field before*, and man was it eye-opening.

In recent years it wasn't so much about pushing others around, but rather having a high "sprint" speed combined with a low, more controllable speed. 2013/2014 were more about beating defense to a point on the field rather than outright pushing them. To this end, we've considered gearboxes which have multiple gear ratio options by swapping out internal gearing.

*I did mentor software for a small team in 2005/2006, but didn't go to an actual competition until 2007.

[asid61]

I generally see shifters as a good thing. Autoshifting code meanns that you can go 6 cim drive and have peak speeds of 18+ fps. You can get around defense.
Of course, if there's tons of obstacles, you could easily go to one speed and save some time and money.

To save money, you might consider switching to an in-house shifter.

[the.miler]

I believe you can be highly competitive going single speed. If you accelerate quickly and manuever well, you can slip by defenders. You can also easily get in the way of others with those same attributes. You can achieve high acceleration, good turning, and sufficient pushing power with a single speed drivetrain.

That isn't to say that shifting isn't useful, but I distinguish between useful and absolutely mission critical. If you would rather reallocate your limited resources to making the stuff on top of your drive base better (I believe this is probably more important), I don't believe that is a bad move. 846 did better single speed than any year we ran shifting.

Your mileage might vary.

[rick.oliver]

Big fan of two speed gearboxes and prefer to use the maximum differential ratio between high and low gears.

I value the ability to get the robot where you want it to be; either in a hurry or under duress. That is to say, I value the ability to either out run or out push an opponent to achieve the desired field position.

I also value the ability to maintain a desired field position once achieved.

[Caleb Sykes]

Originally Posted by rick.oliver

I also value the ability to maintain a desired field position once achieved.

Are you saying that a two-speed robot would generally be better at this than a single-speed robot?

[Abhishek R]

Originally Posted by rick.oliver

Big fan of two speed gearboxes and prefer to use the maximum differential ratio between high and low gears.

I value the ability to get the robot where you want it to be; either in a hurry or under duress. That is to say, I value the ability to either out run or out push an opponent to achieve the desired field position.

I also value the ability to maintain a desired field position once achieved.

I think it really comes down to preference and what you design for. There were plenty of teams who used single speed gearboxes to outmaneuver their opponents as well as play effective defense successfully, 118, 610, and 1986 off the top of my head.

[asid61]

Originally Posted by inkling16

Are you saying that a two-speed robot would generally be better at this than a single-speed robot?

More pushing power, so you can force them away to stop them from slipping past.
You can choose speed or pushing power, not both. A single speed drivetrain can't have both.

[Lil' Lavery]

First and foremost, ask your drivers. How often did they shift in prior seasons? How useful did they feel it was?

Based on my experience with shifting drivetrains, I'd argue that for most teams it's really not worth the cost and effort. For teams looking to get from the 80th percentile up to the 90th and who finish with plenty of time for driver training, they could be worth it. For teams who are building up to the last minute or are in the middle of the pack currently, it's probably worth focusing your resources elsewhere (such as finishing earlier or your manipulator).

Especially with COTS 6-CIM options and high traction wheels, single-speed drivetrains can execute most defensive and offensive strategies. Holding position is more a function of your mass, bumper design, and traction than the maximum torque you can transmit. Geared properly, single-speed drivetrains can often give all the accelration and velocity most drivers can handle accurately. A true "pushing match" is relatively rare. Simply getting in a position before the opposing team often suffices. If an offensive team has to bulldoze their way through your robot, you're playing great defense already. If you beat the defensive robot to your scoring position, it's better to invest the rest of your resources into a manipulator that can score quickly and reliably than building a drivetrain that can mask a poor manipulator.

Finish early. A practiced driver on a single-speed will almost always best an inexperienced driver with a dual-speed.

[metalthorn]

I appreciate all the input and different POV.

Thank you

[stufflikethat]

There have been many posts on this topic and there is no overlying correct choice. It all depends on your team.

If it is a problem with cash then, imo, I would go with a single speed drivetrain. They are not necessarily a bad thing due to them offering a mix of push power and speed all in the same gear. Also a SS gearbox will make your robot lighter meaning you can put weight towards other things. I truly believe both are perfectly good options and if you are struggling with other things it is best to take the cheaper not to mention easier of the two options so that you can put your time elsewhere.

[Jared]

It really depends on your strategy. That said, a reliable shifting gearbox will never hurt.

If you look at some very successful teams, you'll see that some teams like 254 have run two speed transmissions for over 10 years. On the other hand, team 118 didn't have a two speed gearbox in 2011, 2012, 2013, or 2014, and they were very successful. 118 has also put brakes on their gearbox in the recent past too.

Originally Posted by asid61

Autoshifting code meanns that you can go 6 cim drive and have peak speeds of 18+ fps. .

I've seen you say this a few times. How would you set up an auto-shift system? When do you shift up and shift down?

[asid61]

Originally Posted by Jared

It really depends on your strategy. That said, a reliable shifting gearbox will never hurt.

If you look at some very successful teams, you'll see that some teams like 254 have run two speed transmissions for over 10 years. On the other hand, team 118 didn't have a two speed gearbox in 2011, 2012, 2013, or 2014, and they were very successful. 118 has also put brakes on their gearbox in the recent past too.




I've seen you say this a few times. How would you set up an auto-shift system? When do you shift up and shift down?

Autoshifting is done by either current sensing or heat sensing on the main breaker. When the current spikes (or the heat) for an extended period of time, you shift down. You can find a curve fit for the graph of the main breaker current limit versus time, or just have hard limits and times. This prevents breaker blows.
Generally you only need to utilize this in a pushing match, and the driver gets distracted. You could potentially also use collision detection, where the driver is pushing on the joystick but the robot is not moving. Then the program shifts down after a couple seconds. Shifting up is manual.
There is a number of ways to accomplish autoshifting.