[Tristan Lall]

Originally Posted by kajeevan

ok specs on this design are:

being an omnibot i believe speed is very much important so it goes 16f/s.
wieght is about 40lbs but i can bring it down to about 35-36 i believe.
the gearbox is custom but is really simple im using many toughbox gears and axles but the ratio is changing in order to get up to speed.
yes im trying to keep it square but if you say moving the wheels out to the edge wont change the performance im willing to do that.
aslo yes everything on the base is bolted im following my mentors advice when i was in grade 9 saying that if it can be bolted bolt it because if a weld brakes during competition theres no way to fix it

if we don't need total length i do plan on keeping the base 26'' by 26''.

Just a few things:

• With the 4-wheeled omni-drive you've got there, you're capable of getting at most 4 × P_CIM ÷ √2 watts of output power in a straight line, because of the geometry (when 4 wheels are at 45° to the instantaneous direction of travel). But you'll still consume the full amount (4 × P_CIM). You can only get full output power when spinning (which is probably useless). That's a big enough performance penalty that you should weigh the increased maneouverability against the decreased efficiency. (It might not be the best for a game with a lot of pushing of things, for example.)
• Ground clearance and stability are of concern when the sides overhang like that. Note also that if you use casters to support the corners, whenever they're exerting force on the ground, that normal force is no longer available at the drive wheels, and consequently you have less traction. That might be important if you need to push something.
• If you do go with an asymmetrical design (like Andy and others had mentioned), the math is a tiny bit harder because the wheels are no longer equidistant from the geometric centre. This is OK, because for fancy motion (other than straight lines and spins), your centre of rotation is going to move around (i.e. it won't necessarily coincide with the geometric centre). Seriously consider a way of implementing that fancy motion, because it allows an additional degree of freedom to be used at any given time. (And the controls guys need to be thinking about an interface for it ASAP....)
• That's going to weigh too much...the 40 lb estimate seems optimistic. Consider a design where the bumpers form part of the structure (though it means the bumpers have to be precise, strong and rigidly attached, it also means that you're not duplicating structure). Also look at supporting the wheel axles on plates that mount to the back of the bumpers—I'd say you can eliminate six lengths of box beam that way. (Maybe the support plates can be birch plywood, too, for weight reasons.)
• Can the box beam be replaced with C-channel? It's generally easier to install and remove things from a piece of channel, because there's no groping around inside. (Especially important when using bolts and nuts.) And there's no need to machine out clearance holes. The tall direction of the beam is the strongest, anyway. As a result, there's likely little need for the second vertical web.
• When using bolts at the corners, you often need to put more than one at each joint. Otherwise, it acts like a pinned connection, and doesn't resist angular motion (so the frame might parallelogram*).
• Consider the tradeoff between the weight of the Toughbox parts, and their margin of safety versus the loads expected on this gearbox. (They're too strong!) You might consider smaller gears, especially for the first reduction. Of course the Toughbox gears are very easy to assemble, given a shaft with a hex section. Either way, plan to lighten the gears, as a matter of good practice.
• Depending on the flatness of the event floor, and the design of the field, you may run into issues where three wheels touch the ground, but the fourth doesn't. That will make control troublesome with a 4-wheeled omnibot, or a Mecanum system. (At GTR and Waterloo, the floors are very flat, even with the masonite. At the Championship, this is absolutely not the case, because of the plastic tiles laid down underneath.)

If you want to discuss any more of this, I'll be in Toronto over the weekend, and maybe early next week.

*Karthik, I checked: this is sometimes used as a verb in engineering literature. I'm not making things up.