Wheel/Tread Suggestions for Crab Modules

[Akash Rastogi]

In class I began prototyping a few crab modules just for a bit of fun, but I couldn't really come up with the perfect wheel solution. I'm inclined to use something small and durable like 4" colson performa wheels. Suggestions for wheels from experienced crab/swerve builders would be greatly appreciated. Other note: Does it really matter what's used?

*Base suggestions on assumed use of carpet next year*

[Alan Anderson]

Crab/swerve drives tend to see a lot of side loads on the wheels. Take that into account.

[RMS11]

Make your own with blue nitrile tread from mcmaster might be a good option...

[Jimmy Cao]

In my past experiences with swerve/crab drives, I have only used the KoP wheels (AndyMark). The only issue we've had with those is that the spokes or hubs do not hold well to large loads, thus, they'll crack, and then things easily go bad (broken wheel, broken hub, plenty of possibilities). Even the 2nd generation AndyMark wheel (2008) broke a few times.

This year, the low CoF makes the AndyMark wheels hold up extremely well (that, and I believe the hard outside makes it more durable).

I think team 118 has had a good deal of experience with crab drive systems using small wheels with grippier tread than the KoP stuff. Maybe someone from there can offer some tips about using small wheels.

[Akash Rastogi]

My original idea is either the colsons or custom 4"OD and 2" wide wheels with blue nitrile on them. Any thoughts? And Alan, I'm not familiar with how side loads and other forces affect the wheel (cracking, breaking?), could you please enlighten me on this?

[Jimmy Cao]

The side loads are all transferred through the spokes, into the hub/axle. This can often cause cracks either in the hub itself or in the spokes. For example, http://www.chiefdelphi.com/media/photos/31155 can happen. In 2007, with the first iteration of the AndyMark KoP wheels, we found that it was necessary to replace the wheels in our swerve modules at the rate of about 2/competition, due to either 1) cracking of the hub, or 2) cracking of the spokes. I recall that in one of the elimination matches at WMR that year, we took a solid hit from another robot, and a chunk of the wheel literally just broke off, so the wheel resembled a pac-man.

However, this problem (I believe) should become less significant as the wheels get smaller in diameter.

[Aren_Hill]

Our first swerve prototype had 4"od by 2" wide custom aluminum tube wheels with blue nitrile,

2007 was 4" od by 1.75" wide
2008 was 4" od by 1.50" wide
future we'd probably go for 4" x 2" colson performas due to the nature of how we control the drive, simply for tread durability

[AdamHeard]

Originally Posted by Akash Rastogi

My original idea is either the colsons or custom 4"OD and 2" wide wheels with blue nitrile on them. Any thoughts? And Alan, I'm not familiar with how side loads and other forces affect the wheel (cracking, breaking?), could you please enlighten me on this?

The tractive force of the tread will exert forces on the wheel, that exert forces on the wheel module, that exert forces on the wheel module shaft. These are the forces he is referring to.

We used 2" wide nitrile roughtop on our prototype kingkrab, worked great. This does not mean it will necessarily work for you, the higher the friction of your wheels, the higher the forces on your modules. Also keep in mind that the higher the friction, the more torque required to turn the wheels.