Crab drive question

hello, my team and i are trying to put together a crab drive in the off season, and i just had a quick question:

for crab drives with 2 wheels per housing (8 wheels total) what is the average size of each wheel. from what I’ve noticed they look small and wide.

any help is appriciated

I haven’t seen many, if any, crab drives with 2 wheels per module.

Really, the best answer I can give you is it’s up to you. The wheels that come in the kit are 6" in diameter. Some teams like 118 use 3 or 4" wheels that they make themselves. You can purchase 4" wheels at or I believe AndyMark will be selling a 4" Plaction wheel soon.

Any pictures you have that your inquiring about would help.

Some advice:

A crab drive will only be advantageous if it can change direction faster than a conventional “tank” style drive train. If a tank drive can turn 90* faster than your crab can swivel sideways… why bother?

The speed with which you can change direction will be proportional to how fast you can turn your modules, which will depend on the gearing on the rotation motor.

The gearing on the “module orientation motor” will depend on the torque of the modules. Torque=ForcexRadius, so if your wheels are very wide (or far apart, in this case) and have a high CoF, it will take a lot of torque to turn them. This will mean you will have to gear your rotation motor down, resulting in slower maneuvering.

So to answer your question:
-The dual wheels ought to be very close together and have little friction.
-Radius is entirely dependent on your gearing and how fast you want to go. But I might suggest you go with very small wheels (3-4”), since this will put less torque on the vertical bearings.

Hope this helps.

Dual wheeled crab might not buy you much. One of the disadvantages of crab is that the wheels can no longer be near the outside edge of the robot frame. You sacrifice stability for manueverability. In order to make a two wheel module the turning radius of each module would bring the wheels even closer to the center of the robot. If this fits with your design goals then give it a try.
I always need to add here that teams should try new things but with a caveat. Don’t dedicate yourself to crab drive for your next season. You don’t know if crab will be a good choice for the game. We don’t make that decision until our brain storming sessions show that crab will have an advantage.

The advantage to crab is not lost if a tank style drive can turn faster. Crab allows you to change direction without changing orientation. If you need to move sideways and keep an actuator pointed forward then crab will allow that while tank will need two turns to accomplish the same thing.

Wouldn’t a Mecanum drive also be a reasonable consideration? As far as I can tell, the KOP just doesn’t allow us enough motors (or motors of the right type) AND not severely limit the possibilities as far as ball manipulation goes.

You haven’t seen 118, have you? Or 111? Or 1625?

Mecanum is reasonable, but has its own set of tradeoffs.

All drive systems should be considered once the game is known and understood. Each drive system has it’s strengths and weaknesses, and it’s up to each team to evaluate the game, their teams capabilities, and each drive system and determine which one is best for the strategy they’ve chosen.

A Mecanum or Omni drive would provide some of the same (or more) maneuverability options that a Swerve drive provides. However a Crab Drive will (if designed correctly) provide more pushing force than a Mecanum or Omni drive. So if you want your robot to push AND be able to move sideways, a Swerve Drive is the right option. If you JUST want the ability for omni-directional motion a Mecanum or Omni-drive might be right for you.

I’m typically a believer that there is ALWAYS a game situation where you want to be able to push (even in games like last year). We’ll see what the future brings.

What is the motivation behind 2 wheels?, because that’d be a beast to steer those modules. We used 1 FP this year to steer four 4"dia 1.75"wide nitrile covered tires, it did it fine but a FP should be overkill

Yes, I have realized this. I suppose in years like 2005 and 2003 where most robots were designed to pick up the game pieces from a certain angle it would have been beneficial.