Gear Ratio to Maximize Chassis Speed?

[cdizzle]

Hi Everybody,
So, we want to maximize our chassis speed but still be able to carry at least 120 pounds, does anybody know the gear ratio we'd need to use?
Our prototype chassis is basically the kitbot, it has a 6 wheel drive, and is driven by 2 CIMs with 2 toughbox minis, but replacing gearboxes is an option.
Adding motors is also an option, but we would like to stray away from that.
Thank you for your help!
-Team 2367

[Ether]

Originally Posted by cdizzle

Hi Everybody,
So, we want to maximize our chassis speed but still be able to carry at least 120 pounds, does anybody know the gear ratio we'd need to use?
Our prototype chassis is basically the kitbot, it has a 6 wheel drive, and is driven by 2 CIMs with 2 toughbox minis, but replacing gearboxes is an option.
Adding motors is also an option, but we would like to stray away from that.
Thank you for your help!
-Team 2367

http://www.chiefdelphi.com/media/papers/2755

http://www.chiefdelphi.com/media/papers/2059

http://www.chiefdelphi.com/media/papers/1469

[Gregor]

The kitbot has a wonderful ratio for the needs you described. Add 2 more CIMS to your gearboxes though.

[nathan_hui]

Need to know wheels and total weight of current bot.

Also, your current setup will do okay. Add two more CIMS and you'll do as good as a lot of other teams. If you don't, your acceleration will be kinda bad (it might take half the field to get up to speed). Think of motors as torque generators (with max RPM limits).

Also, search around on the forum first. There exists at least one other thread that discussed this very topic (with some more generalities), and you're not the only team with this question, nor are you the only one that needs answers. See http://www.chiefdelphi.com/forums/search.php.

[cdizzle]

Thanks for the input! We're planning to have 1 or 2 DC motors on other places on our robot, but one concern of ours is the battery draining really quickly if we put 5 or 6 DC motors on there. Do you think we should worry about that?

[nathan_hui]

Probably not. We've run a 4 CIM drivetrain for several years now, and those robots have had multiple other motors on there. Just bewarned that driving and doing other stuff at the same time might cause brownouts. If you use only new batteries for competition, and keep them topped off, you should be okay, but run the calcs for brownout scenarios just to be safe.

[F22Rapture]

Have teams noticed much difference between 2 CIM and 4 CIM drivetrains, outside of pushing power? We might cut it down to 2 to save weight if it doesn't substantially affect speed and maneuverability.

[Nuttyman54]

Originally Posted by F22Rapture

Have teams noticed much difference between 2 CIM and 4 CIM drivetrains, outside of pushing power? We might cut it down to 2 to save weight if it doesn't substantially affect speed and maneuverability.

I would not recommend this, you will notice a significant decrease in acceleration, and your robot will feel sluggish. In addition, you would have to change your gearing or you would likely not be "traction limited". That is to say, when your robot hits a wall or gets in a pushing match, your wheels will not spin.

You might think this is a good thing, but it means that your motors are stalled and drawing maximum current, which will very quickly blow your drivetrain breakers (and could possibly blow your 120A main breaker).

[thefro526]

Originally Posted by Nuttyman54

I would not recommend this, you will notice a significant decrease in acceleration, and your robot will feel sluggish. In addition, you would have to change your gearing or you would likely not be "traction limited". That is to say, when your robot hits a wall or gets in a pushing match, your wheels will not spin.

You might think this is a good thing, but it means that your motors are stalled and drawing maximum current, which will very quickly blow your drivetrain breakers (and could possibly blow your 120A main breaker).

This, all of this.

There's another thing to think about as well. If you're removing CIMs from your drive train, odds are they're down low in the robot. Do you really want to remove ~5lbs from some area that is less than 12" off the floor? My experience tells me that this is a horrible idea, even if we're only looking at Center of Gravity and ignoring everything Evan said above.

There is a reason that the '4 CIM' drive train has become the standard in FRC. When properly geared, you shouldn't see any issue with brownouts or lessened battery life...

If you're looking for a good resource for doing the calculations that go into designing just about any FRC mechanism, check out this white paper: http://www.chiefdelphi.com/media/papers/2755