2-Speed vs 1-Speed drivetrain
 

I searched some old threads but couldn't find a clear answer.

What is it about a single speed drive train that allows it to accelerate faster than a two gear one? I don't see why it does, but it seems like there should be a technical reason that I'm missing as to why it does accelerate faster.

Re: 2-Speed vs 1-Speed drivetrain
 

It doesn't. Not necessarily, at any rate.

All else being equal, the acceleration of a drive depends only on the speed to which it's geared - the faster the top speed, the lower the acceleration. A two-speed drive has two possible gear ratios, and thus the acceleration depends on which gear you're in, but there's no fundamental difference in the way it works aside from (maybe) some minor frictional losses in whatever shifting gearbox you have that you wouldn't have in a single-speed.

Re: 2-Speed vs 1-Speed drivetrain
 

Maybe mechanical efficiency. Just because of the gearing required for shifting. Their is not much difference in watching a 9 fps and a 12 fps robot accelerating just one will reach the top speed and the other will continue to accelerate. Robot weight might also be a factor.

A 1 speed will usually run something like a 12 fps. When 2 speed might have something like 8/15 you could be watching it accelerate on the high gear alone?

Re: 2-Speed vs 1-Speed drivetrain
 

Assuming the same amount of motors on a robot of the same weight with the same wheels on the same surface (include same voltage, all the other variables except speeds), acceleration will be dependent on what speed you are geared for. Lower gear ratios provide greater torque, meaning greater acceleration. So when in contest between a shifter and a single speed with all the assumptions made above, the lowest speed (highest gear reduction) will accelerate quicker. So if the shifter robot is in high gear (low reduction) and the single speed is geared for a more moderate speed slower than the shifter's high gear (more reduction), the single speed will accelerate faster. If the shifter is in a low gear that is slower than the single speed (more reduction), the shifter will accelerate faster. That being said, it may not cover as much distance per unit of time than the single speed because the single speed is (naturally) faster, and while the low gear's acceleration is higher, it peaks at a low top speed.

I think what you are referring to is a 6 CIM Single Speed vs. a 4 CIM shifter. By adding motors, you increase the amount of torque put into the drive system, thereby increasing the acceleration. With a single speed, you can optimize your robot to accelerate quicker than a 4 CIM robot at a higher speed. This means that a 6 CIM single speed at 16 ft/s will cover more ground per unit of time than a 4 CIM shifter at 16 ft/s. While this is one use, one of the more popular uses is to use 6 CIMs in a single speed transmission geared lower than your traditional shifter speed, optimized to cover a set amount of distance in the same time as a faster shifter would. Why purposely make your robot slower with more acceleration? With more reduction in your transmission, you will have more torque to push opponents with, yet still be able to get from point A to point B in the same amount of time as a faster robot with less torque, or a faster robot in high gear. However Once a shifter robot goes into low gear, it may have more pushing power than you (though you will still travel farther and faster than it).

Now if you combine the two, and make a 6 CIM shifter, you can accelerate faster and push more in both high gear and low gear, but be wary of tripping your 120A breaker. That being said, the effect of increased acceleration works best within a magic window. The differences in acceleration becomes most noticeable between the speeds of 8 and 12 ft/s, and dwindle down to near negligible as you get further away from the "magic window" (that includes being slower than the range and faster than the range). This means if you want to abuse the acceleration increase, a single speed at ~10 ft/s (or a low gear around that speed) is the best way to get the most out of your extra motors.

Re: 2-Speed vs 1-Speed drivetrain
 

The correct answer is "nothing, really", but the real answer is that it is because of the way people like to gear single speed drivetrains. In my mind, having a low traction limited gear "lets" you gear your top gear really fast, like 16+ fps. If you're doing a single speed drivetrain, you're likely going to gear less aggressively, as you likely don't want to be stuck playing defense with a robot geared at 19 fps. Some teams like to gear their single speeds in the 12 fps range, which do accelerate faster than the 18+ fps two speed teams.

Of course, a lot of it depends on how you like your drivetrains, here on the West Coast most good teams just gear in the 16+ fps range no matter if they're single speed or shifter. And a lot of butterflyesque drives (33, 2K14 drive in a day for example) are geared in the 18 fps range so they can do the reduction to their wheels in only one ratio.

Re: 2-Speed vs 1-Speed drivetrain
 

Ok, thanks, the "nothing really" was what I was mainly looking for. I knew many teams use a 6 CIM single gearing drive geared for about ~15 fps or so, but I was wondering if there was something inherently better about a non-shifting drive in terms of mechanical efficiency.

So as a follow-up, if you were to gear a 6 CIM drive for say 5 fps, you would probably throw the main breaker in no time if you got into a pushing match, right?

Re: 2-Speed vs 1-Speed drivetrain
 

Just be careful if you pursue a 6CIM single-speed drive; they push the capabilities of the FRC electrical system to the limit. You will drain batteries very fast, and you must be extremely careful when selecting your gear ratio because if you stall 6 CIMS you will trip your breaker and that is no fun.

Re: 2-Speed vs 1-Speed drivetrain
 

Try this - find a friend with a manual shift car. Try to start the car from a complete stop in 2nd gear. Now try it in 1st gear but shift quickly to 2nd gear. This is kind of what the robot is doing.

With a lower gear ratio the top speed is less but the torque is better at lower speeds thus you accelerate faster. Then you shift into 2nd gear at the correct time and enjoy power in another band and a higher top speed. It would be faster (than a single speed) unless it takes too long to shift.

In FRC it seems most teams with 2 speeds run around in 2nd gear most of the time and only shift into low gear when they need more torque for low speed maneuvers or when pushing against something.

Our teams gears for 4.5 and 14.5 fps when using 2 speed transmissions and 10-12 fps when using single speed transmissions. We use either setup depending on the game requirements. For example this year there were no obstacles on the field so a high top speed was desirable and low speed was desirable due to the brutal play - so dual speed transmissions helped. Some years the field is divided into sections so high top speeds are less desirable, maneuverability is more important.

HTH

Re: 2-Speed vs 1-Speed drivetrain
 

No, the opposite. Gearing down below 7 fps or so doesn't add any pushing power to your drivetrain, as you're traction limited. Basically, if you push with more than a certain force, your wheels will slip no matter what. If you're pushing with 6 CIMs at 5 fps, your wheels will likely slip, and your main breaker will likely be OK (for a while, at least). On the other hand, if you're pushing in an 18 fps gear, your wheels won't slip, your motors will stall, and you will likely trip your main breaker quite quickly.

Re: 2-Speed vs 1-Speed drivetrain
 

You'd actually be fine. Assuming a full weight robot, rough top wheels, etc, you'd be traction limited at about 30 amps per CIM. 180A through the main breaker won't trip it for at least 20 seconds or so.

Re: 2-Speed vs 1-Speed drivetrain
 

I've found that 10-12 fps for single speed drive train's is just too slow. Whenever we went single speed, we ended up going 15 fps actual speed, and we never had a problem.

Re: 2-Speed vs 1-Speed drivetrain
 

33 is geared 12:72 - about 14fps real.

When you people talk about drivetrain speeds would you all mind mentioning if you are talking theoretical or after efficiency losses(and at what efficiency)? IE: In my experience a robot's geared for 16fps theoretical is around 14.5fps real.

In general, I don't understand gearing above 15fps real with a full weight robot. The optimal time to distance numbers get over a half field away and the robot gets slower overall. Of course, if the robot is lighter than 120lbs teams can get away with a higher gearing while maintaining a similar acceleration. This is the real trick to getting around the field faster.

Cheers, Bryan

Re: 2-Speed vs 1-Speed drivetrain
 

They don't.

Assuming they're under identical conditions acceleration rate is dictated by gear ratio. Lower gear ratio = faster acceleration, higher gear ratio = slower acceleration. A single speed drivetrain does accelerate faster, a lower gearing does.

Re: 2-Speed vs 1-Speed drivetrain
 

Actually at Tech Valley (Finals 1 if I remember), We blew our main breaker in a pushing match with just 4 CIMs. Though we did have 40A fuses on each of them.

Re: 2-Speed vs 1-Speed drivetrain
 

Were you geared for 5 fps?

Re: 2-Speed vs 1-Speed drivetrain
 

The ideal situation for us several years ago was to go with a 2 speed transmission, and use code to do "auto-shifting" depending on the speed of the robot.
It proved short lived when we started having roll pin issues with our shifter. Although there are now fixes to this issue, I still dont think that shifting often in every match is a reliably good thing to do.

I say run as quick as your drivers can handle comfortably and give yourself the option to switch into a ~5fps low gear range in pushing situations. There are many more options available that you can buy compared to just a couple of years ago. Our robot this season accelerated much better vs. our 2013 robot.
In 2015, we plan to run faster (add additional motors also), with the intent that our 5th year driver will be able to handle it and he wants to.

Re: 2-Speed vs 1-Speed drivetrain
 

Here is some data on 3 gear ratios and acceleration / top speed tests.

http://www.chiefdelphi.com/media/papers/2911

Re: 2-Speed vs 1-Speed drivetrain
 

So this year we ran a two-speed 4 CIM drive with auto-shifting, geared about ~11 in low and 16 or 17 in high, on 4 inch wheels. With the current auto shifting setup, would it be beneficial to make the low gear lower to accelerate faster?

So what was your 2014 setup like? I didn't see it to much in the pits, but it was definitely one of the fastest robots I saw this year.

So would an ideal system (barring weight and other limiting factors) be a 6 CIM two-gear drive, the low being only used for pushing, and high gear for everything else? I think this is what 254 used this year.

Re: 2-Speed vs 1-Speed drivetrain
 

boy this is a mess

2 speed= Lets push stuff!!. yaya robot

1 speed 6 cim (13-14fps) = We don't have time to push, bum us and watch us run. (see 610/talk to Mr.Lim he is the MAN)

Re: 2-Speed vs 1-Speed drivetrain
 

Everyone should try datalogging acceleration if they have encoders on their drivetrain, particularly with 2 speed transmissions. Depending on the ratios, there really is no difference in acceleration.

Re: 2-Speed vs 1-Speed drivetrain
 

Quick aside question, am I right in assuming that the wheels slipping is the only thing that prevents the breaker from tripping at low fps?

For instance, if a weird situation occurred where a gearing of 5 fps did NOT slip the wheels in a pushing match, the breaker would still trip in about the same amount of time as if the gearing was 16 fps correct? Basically, the CIMs draw the same amount of current at stall regardless of the gearing so any stalling of a 6 CIM drive is just as dangerous.

Am I right in that assessment?

Re: 2-Speed vs 1-Speed drivetrain
 

If the motors are stalled, they will draw their stall current.

That said, there's pretty much no traction material available that won't slip drive wheels with 6 CIMS geared for 5 fps.

Drive trains are "traction limited" (wheels slip) for the most part. The amount of current drawn per motor while the wheels are slipping depends on your gear ratio. Going well above 40A per motor can trip the resetting breakers. Going well above 200A per system risks tripping your main breaker a bit too quickly.

Re: 2-Speed vs 1-Speed drivetrain
 

Yes, stall current is stall current. Ideally your low speed gear ratio in your 2 speed drivetrain should be as fast as possible while still being traction limited. That gives you a higher speed in low gear without sacrificing any pushing power.

Re: 2-Speed vs 1-Speed drivetrain
 

Well....stall current is kinda stall current. The stall current per motor for 4 motors is slightly different than the stall current for 6 motors. Stall current for a 6 CIM system is about 75 Amps/motor and the stall current for a 4 CIM system is about 87.5 Amps/motor. This is because when you add more CIMs, the motor voltage will decrease. This voltage can drop to about 7.5-8 volts when stalling. The stall current cited in the CIM motor documentation (~131 Amps) is assuming the voltage is a constant 12 volts (not what we see in robots). It is correct that friction is what limits the stall current. Also, note that there's a difference between the static friction and kinetic coefficient of friction. In fact, you can even have just as much pushing power with a 4 CIM drive as a 6 CIM drive (if geared properly).

Re: 2-Speed vs 1-Speed drivetrain
 

I am aware that the voltage can drop down to 8 volts for brief instants in time, but does it stay down this low for extended periods during a pushing match? I will have to remember to check the driver station data logs next time I have access to the driving laptop.

I had always thought that, for things like pushing matches, voltage briefly dropped down to ~8 volts but then jumped back up to some value around 12. The battery should be applying a relatively constant voltage, so I don't see why the voltage would remain notably lower for any extended period of time.

So yes, the stall current might be ~90 amps for short periods of time, but I am thinking that the stall current is much closer to 130 amps for most of the time in a pushing match.

Re: 2-Speed vs 1-Speed drivetrain
 

As most people have said, it's the gear ratio (or number of motors).
However there is one other thing that makes a bigger difference on acceleration than most realize. That would be the number of rotating components (gears) in the gearbox. Typically a good 1-speed gearbox will have 1 or two stages and have 3-5 gears (2 small pinions). Most shifters have 2-3 stages with 6-9 gears (3-4 small pinions). Not only do the motors have to get the robot moving they also have to get the gears spinning. And in the early stages of the gearbox they don't have the mechanical advantage of multiple gear reductions to increase torque. This is made worse when the gears are steel not aluminum.
Now whether or not that actually creates a noticeable difference, maybe not. However it certainly is something to keep in mind when designing a gearbox.

Re: 2-Speed vs 1-Speed drivetrain
 

There is a lot of talk about speed in this thread.

There is no talk (yet) about designing for speed and still being able to turn well. There are extra variables in play here would make the fastest robots worthless without special consideration:

• Weight distribution (want it slightly off-center from track center-of-area)
• Wheel Base (want it wider than track length)
• Track Length
• Wheel Tread Composition
• Drive Train Frame Rigidity (want it STIFF)
• Center of Mass (want it LOW)

Re: 2-Speed vs 1-Speed drivetrain
 

This is false.

This year we were having problems tripping our main breaker, and did a fair number of tests pushing against a wall. With a not-quite-full battery and the compressor running (i.e. pretty reasonable in-match conditions), we were only drawing ~50 amps per motor on a 6CIM drive when stalled.

Re: 2-Speed vs 1-Speed drivetrain
 

The voltage the battery outputs is proportional to the amperage being drawn, and the resistance of the circuit. This is also known as ohm's law.

V=IR

Although batteries are not always perfect voltage sources, you can use ohm's law to determine the voltage drop in your robot fairly accurately. If you were drawing 300 amps, and the total circuit resistance was .020 ohms, you would lose 6 volts. This does not fluctuate over time, it is directly related to the current being drawn. (ignoring temperature and other factors). Hopefully this clears things up.

Re: 2-Speed vs 1-Speed drivetrain
 

I'm not an engineer (yet), but doesn't this calculation have to be related back to the capacity of the battery somehow? When drawing, say, 5 amps, intuition tells me some tiny lead-acid battery would have a much larger drop in voltage than a huge deep-cell marine battery would. Maybe I'm rusty with my basic electrical physics, but with V=IR, if you had a 300 amp draw with .02 ohms, that would mean that you must be drawing from a 6 volt source (or be in series such to draw 6 volts), not "losing" 6 volts. Our applications are of course done in parallel however on the robot, so series is not an option. If we know the current draw and resistance of the whole circuit like you said, then we solve for our voltage across the whole circuit. I'm thinking the theoretical voltage drop of the battery under load is another calc entirely. I'm simply raising a concern, not saying this it absolutely correct. I'd love to be educated otherwise if someone has a good explanation.



Here, they relate a voltage drop on current draw to a capacity fraction.

Re: 2-Speed vs 1-Speed drivetrain
 

A larger battery will have a lower resistance, causing less of a voltage drop.

Re: 2-Speed vs 1-Speed drivetrain
 

Are you sure you were geared for that speed, or were you geared slightly faster? If you used your drive under load in high gear, it's possible you were applying a lot of load to it constantly over the match and one last peak load tripped it. It's also possible that your main breaker was faulty and tripped early. Lots of variables here.

Re: 2-Speed vs 1-Speed drivetrain
 

Interesting. I will have to have the kids run some tests like this when we get the chance.

This makes perfect sense. As we were talking about the drive train and gearboxes, I had forgotten about the electrical system itself, and was (incorrectly) assuming negligible resistance.

Re: 2-Speed vs 1-Speed drivetrain
 

So, theoretically, we should be designing to maximize power out of our battery as well as out motors? IE load the battery such that V^2/R is maximized when you think you'll need the most power as possible (pushing matches).

Re: 2-Speed vs 1-Speed drivetrain
 

Well, I wouldn't say WORTHLESS. There are times when you don't want to turn and that might be a feature.

But it is important to remember that your gearboxes/motors are merely part of your drive system. It is comprised of several parts (incomplete list):

• Main Breaker
• Motor Breakers
• Battery
• Motors
• Gearboxes
• Wheels
• Drive Surface
• Frame
• Controller

Each of these systems plays a HUGE role in the overall effectiveness of the system. While the gearbox and wheels may be big parts of it please make sure you analyze the whole system instead of simply saying "we wanna go fast".

Ricky Bobby didn't design his car, he just drove it.

Re: 2-Speed vs 1-Speed drivetrain
 

I'm not exactly sure what you mean here, as the electrical power output of the battery increases as more current is drawn. Regardless of the power though, if you are in a pushing match, your main concern would be amperage. With DC brushed motors, torque is proportional to current. Gearing slower gives you more torque for the same current, or less current for the same torque.

IMO, it would be better to focus on reducing resistance on the robot. At 300 amps, each extra milli-ohm is .3 volts. Just a few extra milli-ohms to the robot can make the difference between brownout and not. Also relevant here is acceleration in high speed robots. Since increased resistance means less current for a given voltage and current=torque, a few milli-ohms can mean a decent drop in the available torque for acceleration.

Re: 2-Speed vs 1-Speed drivetrain
 

It may be worth mentioning that the acceleration increase between, say, 8 fps and 16fps is negligible, in the area of several hundreths of a second. We had an excel doc made by one of our physics-oriented parents that graphed speed vs. time for different gear ratios.
The calculations did not take into account friction, but I've heard that it still doesn't help to change gears.

Re: 2-Speed vs 1-Speed drivetrain
 

I can understand why you asked about moving to a lower gear to accelerate faster. Much like a car, if I was seeing how fast I could go from dead stop starting in 1st gear vs. 3rd gear, you start with 1st gear due to the torque advantage.
I'm saying that instead, ideally you have the 1 speed that you are comfortable with and use an adequate amount of motors to get the power you need to achieve that speed you gear it for.
Shifting into a real low gear was an option for us to try and get through pushing matches when the traffic was bad and we were busy trying to acquire and position ourselves to shoot the ball. We would have been perfectly happy with a 1 speed setup if it wasnt for this scenario.
All of our defensive maneuvers were using the higher 1 speed.
Our drive was much more effective adding just 1 Banebot 775 to each side. We took AM SuperShifters and modified one end of the plates, added the BB, and the right gears to match the 2 CIMs.
We had none of the 6 CIM issues that teams complained about during the season. Next season, we plan to add....