2-Speed vs 1-Speed drivetrain

[waialua359]

Originally Posted by Abhishek R

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.

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....

[colin340]

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)

[sdcantrell56]

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.

[Orion.DeYoe]

Originally Posted by Abhishek R

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.

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.

[JesseK]

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)

[Andrew Schreiber]

Originally Posted by JesseK

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)

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.