# 2-Speed vs 1-Speed drivetrain

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.

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.

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?

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.

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.

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?

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.

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

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.

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.

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.

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

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.

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.

Were you geared for 5 fps?

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.

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

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

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.

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)

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.