Bite/RPM on spinning weapons

Currently attempting to design a bar for Archangel and its beetle counterpart, going for something high RPM (around the 10k mark) and single toothed to try and get plenty of power out of the hit - obviously higher RPM means less bite, has anyone got a sort of figure for mm of bite that a spinning weapon would need to get a big hit in rather than just creating sparks?

I do realise that bite also takes into account speed of the machine as well, it's only a rough figure to make sure I'm not going to end up with something that will only ever generate sparks!

Thanks in advance!

The bite figure is a combination of weapon rpm, # of teeth and collision speed. And the resulting bite's effectiveness depends on tooth shape, material you're hitting, etc. You need to know all the variables before you can pull up a number and as a result the bite depth you get is at best a ballpark figure. A 10,000 single tooth weapon doing normal feather drive speeds will get not-bad-bite #technicalterms

The way I worked the bite for Conker and Binky's new weapons was to work out how far forwards the robot moved in one revolution of the disc at any given speed, this giving me the bite attainable on a stationary object. It is probably not perfect, not taking into account the position of the weapon and the 'width' of the weapon in the plane of rotation but it should get pretty close.


For example:

Weapon Dia: 160mm or 0.16M

Weapon RPM: 520Kv on 12S 2:1 ratio = (520*(3.7*12))/2 = 11544 rpm. With the weapon dia, calculate the tip speed at 96.49mps

Robot speed: 1083Kv on 4S 16:1 ratio = (1083*(3.7*4))/16 = 1001.775rpm

I then plugged the speed into the Technobots robot speed calc with 73mm wheels to get 3.82mps maximum theoretical speed.

From there you take your Robot speed in mps and divide it by your weapon rps to get: 3.82mps/96.49 = 0.0395m OR 39.5mm.


Of course that is all theoretical speeds. For example, I know that Conkers weapon max'd out at exactly 11'000 rpm and that if a robot is charging at Conker that the combined robot speed is higher. So, with that in mind:


Weapon speed drops to 11'000 and a tip speed of 91.94m/s, but if we say Conker was driving straight at Binky which has a theoretical top speed of 3.09mps, the two added together make 6.91mps. So that gives us 0.0751m or 75.1mm.


Both those values are for single toothed weapons, if its double tooth, its halved as you have half the time before the next tooth comes round. Unfortunately its not as simple as that either. As you increase the bite on s single tooth the counterweight has to get heaver to offset the change, and it reduces the RMI-to weight ratio, making your pay for more bite with additional unused weapon mass. You have to strike a balance.

All that said, that is if you are being really picky. Go with 40mm on your bar and you will be fine, unless you want to trawl through numbers.

No point using max forward speed. Use 1 or 2 m/s. Most of the hits in the arena are at slow speed.

Ah great, thanks! I know that any figure is only going to be theoretical and just a roundabout estimation, but it's more a case of trying to make sure I'm not building something that will never get a bite!

Going to try and start designing my own stuff at some point in the future (already sorted Archangel's bar out) but my theory on it all is genuinely not up to spec haha. I suppose some of it is a bit of trial and error to strike that ideal balance so I'll get designing some to see how that goes!

You might find this Excel spreadsheet handy: http://www.nswrfc.org/Nick/RoboCalc.xlsx, It will help calculate a robot's driving speed and the maximum bite depth.

Excellent, cheers Nick! Will get running some of the stats - got my bar sorted now or close enough anyways, so now I'll be able to tweak everything nicely, all ready for the world champs in April!

(also, that pun was totally unintentional, but I'll take credit for it anyway)