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.
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
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.
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.
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.
Last edited by craig_colliass; 7th December 2014 at 20:20.
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.
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
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.
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
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