pic: FRC 4532 Drive Base Frame

[Clinton Bolinger]

Are you trolling and actually going to use 8-inch "Mechanums"?

I assume this by your username.

-Clinton-

[mechanumrules]

Originally Posted by Clinton Bolinger

Are you trolling and actually going to use 8-inch "Mechanums"?

I assume this by your username.

-Clinton-

Unfortunately, my team outvoted me on this. There is a catch to being a software guy!

[Akash Rastogi]

Originally Posted by Clinton Bolinger

Are you trolling and actually going to use 8-inch "Mechanums"?

I assume this by your username.

-Clinton-

I was wondering the same thing, this would actually make sense.

On the other hand, your frame looks nice and solid.

[Siri]

Originally Posted by mechanumrules

I'm telling you, this drivetrain is money! We're using AM Toughboxes with an external sprocket reduction of somekind, (I don't know, I'm a software guy!) to give us a final speed of 6.2xx fps. Should be enough torque to overcome the carpet scrub, right? We don't need to be fast this year, absolute precision is key!

Does it drive yet? If so, weight it up and turn it on carpet (and videotape it). If it doesn't, your main concern should be getting it there. It looks easy enough to switch in omnis if you decide you need them. Either way, I'd suggest you stop extolling the hypothetical performance without recording the actual one. That's plenty to do itself, good luck!

[IraJason]

I have to say, congrats to you as a rookie team for being able to design your own drive base. It's also good to see how enthusiastic about it. It's definitely an achievement!

However, I do need to echo some of the sentiments posted here. You really need to test this drive configuration on carpet, and never assume it's going to work perfectly. On my team's rookie year, we did not have carpet to test our drivetrain on that was identical to the field's. We got to competition, and we found, after several matches of frustration, that our drivetrain was not outputting enough torque to move on the carpet, and we were tripping our breaker. Our second year robot had a 4 wheel drivetrain, and while I don't remember it being impossible to steer, I do remember it having a bit of trouble trying to turn under high speed.

So just as others have suggested, really try to test this in real world conditions, and don't assume anything. Believe me, 20 minutes of testing now can save you a day of frustration and frantic work during competition.

[SteveGPage]

Originally Posted by mechanumrules

Unfortunately, my team outvoted me on this. There is a catch to being a software guy!

What regionals will your team be attending? Many experienced teams are more than willing to assist you in the build season, and at the regional. If you are going to be at any of the regionals we will be attending (Chesapeake and DC) we would be happy to give your team any assistance needed before and during the regional.

Also, double check the number of your team. Last I knew, we were only up to FRC 4411 (The team from China).

[jason701802]

Originally Posted by SteveGPage

I'm going to echo all the posts above, and share our horror story. We built something very similar many years ago. We tested it on tile and smooth concrete surfaces, and it turned like a dream. We didn't have any carpet, and didn't think it would matter much. When we got to our first regional and took it out on to the field for practice, it skipped, shook, shimmied and bounced every time we tried to move. We couldn't control it at all. We had to pull two wheels off and replaced them with castors to even get some control. It wasn't pretty.
Steve

We did almost the exact same thing our first year (2001), except we (kind of) solved it by covering two of the wheels with duct tape. Admittedly, it did work well enough to win us a regional, although that had more to do with our pushing ability.

[RoboMaster]

Please watch this.

http://www.youtube.com/watch?v=u0X_5iQXbZw

From http://www.chiefdelphi.com/forums/sh...ad.php?t=97744

[fox46]

I see tie wraps and tape in your future.

Excellent workmanship but I too will have to chime in that you guys need to add another set of wheels in the center, lower than the other four or you need to reorient your drivetrain. Another option is replacing two of your wheels with omnidirectional wheels.

[John]

Looks like a nice, simple frame. Good job, it should be very durable.

As others have mentioned, you really need to test the turning ASAP so you can fix any problems that come up. More practice time never hurts, either, so even if it comes out great you'll want to get it up and running soon.

Originally Posted by mechanumrules

The wheels are 8" AndyMark Pneumatic ones!

We did all the math! It might be a little slow turning, but at 150lbs it should turn fine!

Would you mind posting this math? Since unless I am missing something key to your drivetrain, this should not be able to turn without jumping.

To turn:
max turning force >= max turning scrub

Assuming center of mass in center of robot:
max turning scrub = 4 * (length/2) * (weight/4) * mu = length*mu*weight/2
max turning force = 4 * (width/2) * (weight/4)* mu = width*mu*weight/2

width*mu*weight/2 >= length*mu*weight/2
width>=length

Since this design is longer than it is wide, you will not be able to turn with all four wheels on the ground*. What will result is one or more wheels skipping off the ground and the drivetrain quickly turning due to the dramatically reduced turning scrub. This motion is extremely difficult for the driver to control because it involves rapid, unpredictable changes in speed.

*Note: I assumed two major things while making this analysis:
1) Your center of mass is close to you center of area. Without knowing your full design I do not know if this is a good assumption, however, if you center of mass is too far to one end you may tip while crossing the bump.
2) You wheels have similar coefficients of friction in the forward and lateral directions. This should be a good assumption for pneumatic wheels, but if you decide to go with omniwheels on one end this no longer holds true and you will have a much easier time turning.

Your best bet to alleviate turning problems is to either:
A) add additional dropped wheels (probably one per side, in dead center)
B) add wheels with lower lateral friction (like omniwheels)

[Mr. Rogers]

Put in 2 more 8'' pneumatic tires in the center, drop them a 1/4'', and you'll be gold (for this configuration) Otherwise you are placing about 35lbs on each wheel (assuming you build about max weight) and these wheels are very grippy on carpet. With a wheel base that long, your drivers will be very unhappy. We almost did something like that last year (our rookie year) but our mentor team helped us impliment a much better 8wd with the 2 center pair of wheels dropped about 1/8'' to 1/4'' (I would go 1/4'' using pneumatics, they squish alot more) And find time to test whatever you have on carpet before your regional!!!!

[KrazyCarl92]

Another luxury of using pneumatic wheels in a 6 wheel drop center drive is that you have another degree of freedom beyond simply a 1/4" drop of the axle. You can slightly adjust the pneumatic wheel pressure in the outer wheels versus the center wheels. Maybe you think the drop isn't enough? Pump the center wheels up to 35 psi, and only pump the corners to 25 psi. The drop is too much? Pump the center wheels to 30 psi and the corners to 35 psi. These numbers are arbitrary and are in no way exact, but just pointing out that you would have another way to fine tune your turning ability and find what works best for you.

[Jeffy]

Originally Posted by mechanumrules

The wheels are 8" AndyMark Pneumatic ones!

We did all the math!

I'm sure many would like to know what you came up with for coefficient of friction. If you wouldn't mind...

[JamesCH95]

Any drive testing yet?