Supershifters and Mechanum wheels

[Jeffy]

Building a competitive drive train is about building an efficient, stiff and robust base with the correct math done regarding gearing. There are resources a-plenty for learning to do this.

At 159, we had such incredible success with our traction drive last year, that it's unlikely we will ever use a drive without traction wheels.

As per lining up...
I build bots with a philosophy that if you can't line it up with a tank drive and a few minutes of practice then your mechanism is broken and not your drive train.

[z_beeblebrox]

Don't underestimate the power of the kitbot. 2013's new and improved kitbot performs better than many, if not most, custom drivetrains and, possibly most importantly, is quick and cheap, so you have more time to spend on developing other systems for the robot.

Additionally, with simple modifications, such as building 1114's kitbot on steroids, adding higher traction wheels or perhaps shifting transmissions.

Personally, I haven't played enough with Mecanums enough to give a verdict on their performance, but from what I've seen, a decently constructed, well driven tank drive can often outperform all but the best Mecanum drives in everything but going sideways. When considering Mecanum, you have to decide if going sideways is worth being slower and having less traction.

Post #254! Go Poofs!

[Woolly]

Originally Posted by z_beeblebrox

Don't underestimate the power of the kitbot. 2013's new and improved kitbot performs better than many, if not most, custom drivetrains and, possibly most importantly, is quick and cheap, so you have more time to spend on developing other systems for the robot.

Additionally, with simple modifications, such as building 1114's kitbot on steroids, adding higher traction wheels or perhaps shifting transmissions.

Personally, I haven't played enough with Mecanums enough to give a verdict on their performance, but from what I've seen, a decently constructed, well driven tank drive can often outperform all but the best Mecanum drives in everything but going sideways. When considering Mecanum, you have to decide if going sideways is worth being slower and having less traction.

Post #254! Go Poofs!

Mecanum is good for when you need an omnidirectional drive system but don't have the time/resources/money/programming team to devote to swerve or butterfly drive. For example, we didn't make the decision to go mecanum until just before the start of week 2 during the build season, after realizing that an omnidirectional drive could be very advantageous.
I'd take a mecanum kitbot with basic WPI code over a hacked together and poorly programmed swerve drive any day.

If you really, and truly need an omnidirectional drive train, I'd start with mecanum and plan to go to swerve or butterfly later.

If you don't, look at some of the tank drives with traction (and sometimes omni) wheels around FRC, such as Team Titanium's Tremendous Turning Twinspeed Tank Transportation in use on this year's robot.

[bs7280]

Ok, say a team decides to go with a drive base (nothing specific for the sake of helping other teams who view this) for each base, how does one select the "best" gear ratio to be chosen on a bot? Looking at different gearboxes for different drive base types, there are alot of choices and it is quite intimidating to choose considering how expensive they are.

[Anupam Goli]

There is no single blanket "best ratio" to use. It all depends on your strategy and what you want to do. Do you want to be quick? Do you want to have pushing power, but don't want to be necessarily quick? If you are using a single speed gearbox, you can look between 4.67:1 to 13:1. We ran an 8.46:1 ratio in a Toughbox mini. We got about 9 fps and we had decent pushing power. The toughbox might be a good starting point. The good thing about the toughbox is that you can just buy the gears and change the ratio based on your results. You can also adjust the ratio in the sprockets. Remember, the higher the ratio, the less speed you have, but the more torque you will have behind those wheels. Striking the balance is something you and your team have to figure out. If you search in the CD papers, you can find JVN's drivetrain calculator, which allows you to learn everything you want to about a drivetrain before you build it.

[Jeffy]

Originally Posted by bs7280

Ok, say a team decides to go with a drive base (nothing specific for the sake of helping other teams who view this) for each base, how does one select the "best" gear ratio to be chosen on a bot? Looking at different gearboxes for different drive base types, there are alot of choices and it is quite intimidating to choose considering how expensive they are.

We use a two speed and mostly because:
We can set a gear used for acceleration and pushing matches to be traction limited (Torque created at ~40 amps times wheel radius > force of static friction of wheels on carpet)
We can set a gear to get us where we want to go as fast as possible. (balance of acceleration from top speed of gear 1 with top speed)

If you choose a single speed, a balance of top speed and pushing force is ideal. Usually teams set there bot to be right on the edge of traction limited. This is not inherently ideal, but is simple and can give a successful result.

JVN design calc as suggested above is the way to go.

[jbsmithtx]

As above said, there is no "best ratio". It should be fit into your strategy. So, with a wide open field like this year, you might want a higher ratio if you were going to be running cycles to and from the pyramid, so you can cross the court swiftly. Whereas if you were a team that wished to block a cycler, you might have a much lower ratio, so that you could at least push them around and slow them down. However, with a divided field like last year, you would be looking at a totally different setup, with a lower "speed" ratio (because of acceleration up to your top speed). It all depends on strategy and how you think matches will end up being played out.

[craigboez]

Originally Posted by Jeffy

I build bots with a philosophy that if you can't line it up with a tank drive and a few minutes of practice then your mechanism is broken and not your drive train.

Solid philosophy.

In the past few years we have used both AWD robots with traction wheels and robots with mecanum drives. The team has tended to favor mecanum for its alleged maneuverability strengths. I think this is a bit of a fallacy, as we haven't been able to drive or align the mecanum robot any better than the AWD versions.

Building a mecanum robot is a great learning experience from a build and a programming standpoint but don't expect a huge boost in driver performance. It isn't as maneuverable as it looks on paper.