Speed 900s

Robo Challenge and uh... um...

The 2 Dutch arenas , the well used Feather arena and the rarely used heavy arena both use OSB floor.

The German arena used in the past melamine covered particleboard, but has changed to OSB or plywood.

Well im guessing the robot being talked about is being built in England...so will be used in England for competitions.... the biggest competition for fw's in England is the UK champs.... which uses a steel floor.

I'm gonna use proper speed 900s, 1 per side, stick with reliability, and it's not a very high gear ratio, so it's gonna be quiet fast, but if its not powerful enough, with slow acceleration, then I will fit another, and steal the magnet idea for the robo chalange arena... Hopefully this robot is going to be more competetive than the last one...

What kind of gear ratio do I need to run a 1-2kg spinning bar from one of the fake eBay speed 900 like motors? 1:1, 2:1, 3:1, 4:1?

Depends on how many teeth you want, and how wide it is.

But the fakes, don't overvolt them to much, and use a large reduction gearing. (4/1 @12-18V sounds nice)
3000 rpm is a nice rpm for a disk.

It's a bar so 2 teeth- I will most probably run at 12-15v. I think 1:4 sounds good

Also- it says that the max rpm is 6700 so with a 4:1 ratio that would be 1675 not 3000?

Yes, 6700 rpm/4 = 1670 rpm

Maybe a 3 to 1 could work.

Sorry for all the questions- how many mah do I need for my battery pack when running one of the motors continuously for 3 mins to power the bar (so will sop and start and stall a fair bit)
I tried to work it out for myself but it came out as 60mah which sounds wrong so the correct equation to figure it out would be useful to!

Also, could I use this relay to control one of those motors? Could I control it directly from the receiver (connect the two terminals on the relay to the positive and negative terminals on the receiver without a battle switch?