Help with gear ratios

[daveimi]

A lot of the mags seem to be in the 5000 - 6000 rpm range. Assuming 300mm wheels what sort of gear ratio would I be looking at for the normal ~10mph?

Have seen calculators on the web but can't find them now I need them.

[Rapidrory]

10mph = 4.47 m/s
0.3m Diameter (300mm) * PI = 0.94m circumference
0.94*4.47=4.75

Therefore the wheel needs to be turning 4.75 times per second to get 10 mph

Taking the motor speed at 5500 rpm
5500/60 = 91.7 revolutions per second

91.7/4.75 = 19.3

Therefore you need a gear ratio of about 20:1

[MikeNCR]

The quick shorthand calc is-

output rpm*wheel diameter in inches*0.003=speed in mph

so, rearranging that-

speed in mph / (wheel diameter in inches * 0.003)

It's not perfectly accurate, but it's close.

[Revsmin]

I just use this because I'm too lazy to do the maths myself

[daveimi]

Thanks for the help as always, Rory you've done that before!! And thanks for the info on the Calculator, knew I'd seen it one of the sites.

20:1 is higher than I was expecting but it would be easy enough I suppose with a two stage reduction. Something like 5:1 then 4:1 to the output shaft.

[Rapidrory]

Yeah, it's the only bit of maths I can do really :P

You're using pretty big wheels, so a high gear ratio isn't too surprising; I think the more common wheel diameter for a heavy is closer to 100mm, so they're more likely to get away with single stage. But as you say, 5:1 and 4:1 are fairly easy gear ratios to setup.

[Revsmin]

Most heavyweights now are geared for 15mph rather than 10; 10's a bit slow for a heavyweight. Plus most heavys have smaller wheels, for example Dystopia has 6" wheels. A good rule is the wheels diameter in inches:1 as a gear ratio when dealing with Mags or Bosch 750s

[maddox10]

Funny how the Dutch/Belgian heavies get by with much larger gearratios.

20-1 ain't uncommon. Bullfrog and TAN are in that range. 10" to 16" wheels.

Gravity is around the 15-1 range. 10" cart or solid HDPE wheels.

On the other hand, those machines use 2 stage gearboxes.

[craig_colliass]

With regards to gear ratios and top speeds I have found they are disconnected with the real world of fighting robots.

Saint is not slow by any means and that works out (In theory) at about 9.25mph in fact is is probably closer to 8.25mph.

Gabriel is a little faster but probably only just into the 10mph in reality. These robots have very little problem with the arena surface so they can in practice reach these speeds.

Most people drive tin snail flippers. yes the wheels could with no resistance rotate to an expected speed of 15mph none has managed this in practice as they get stuck on the arena floor, experience roll resistance and other factors that slow them down, There are exceptions to this like Storm 2, and now Brutus. these are designed to use kinetic energy to knock and push there opponents into the arena barrier or pit. both are exceptionally expensive machines to build and run.

so remember in the robot scene as with most things in life

High speed costs!!! If you want a robot to be cheap and fun keep the speed down, the quality high, and the concept clever

[daveimi]

"High speed costs!!! If you want a robot to be cheap and fun keep the speed down, the quality high, and the concept clever"

This is exactly where I'm coming from. I think I'm going to stick with the wheel chair motors as I know I can get replacements for £20. I know they aren't a daft wattage (about 250W), but as you say "real world" torque was incredible. I could sit in the wheel chair (12stone) with my cage fighting brother in my lap (16stone) and the torque of both motors happily pushed us up the drive at full chat and up the kerb etc. The other thing was get some mags, get gearboxes built, but then trying to source replacements is my big worry. I know home built gearboxes will be much higher quality that the wheel chair items but I live a few doors down from a mobility shop so it's been incredibly easy to get hold of everything. Don't know what to do! And they fit nicely into my body shell.

[adz600]

Just fit 4 off them a wotty or roboteq will run twin wheelchair motors with ease also would look good and should have lots of grip,still loads of space in the shell for a linear drive motor and a lifter could be cool

[daveimi]

Yeah been thinking it needs a weapon rather than be just an invert-able wedge.

[Max]

Yeah been thinking it needs a weapon rather than be just an invert-able wedge.

I'd take it one step at a time, start with just an invertible wedge but leave space to add a weapon at a later stage after you've taken it to an event or two.

[terry]

Lol @ Craig ... Tin snail flippers !

[craig_colliass]

Glad you liked that one Terry, I hear you are building a new robot.
I'm looking forward to your next creation don't forget to put up a build thread.

OK targets to aim for if you want to build a Heavyweight.
1/ Get the robot through the first Tech Check.
2/ Drive robot into the arena without it breaking.
3/ after 3 minutes drive robot out of the arena (this one can take a few attempts!)
4/ get the robot to fight in all the shows that weekend.
5/ your first fluke win,
6/ congrats you've got as far as me after
7 years.