gliding wheels....

right, i am wondering how you make something that allows the robot to freewheel…
if you have ridden a bicycle, then its like that… ie when you are up to speed the wheels continue without having to put any more power in (so you can get some speed up then glide along)…
the reason is that i’m worried about our motors and speed controllers…
currently we think the reason the speed controllers keep blowing up is because we push the robot around by hand sometimes and its putting currents through in the wrong direction… and by doing this we could push the robot along without generating power … (if we could work out how…)

i don’t think that would fry your speed controllers but here are 2 options for you. what you are talking about is called a coaster and you can buy basically hubs and have them re-spoked for small wheels, this will work but don;t expect to be able to have back wards on your robot because there is no clutch in them that you can engage and disengage.

a better option in my mind is if you need to push the robot around design a quick release somewhere in your drive train and disengage the wheels from the motor. the quickest way i can think of is a decent cotter pin that you can pull in and out.

but even better then that is make a cart. if you design a light cart there is no reason you should push your robot around, even though i doubt that spinning the motor back wards would cause enough current to cause any damage.

our mentor suggested using the base of our crate for a cart… i really disagree though … as i dont think its practical as we have to mess around rebuilding the crate etc at the regional. . . .
i know we generate a bit of power as when we move the robot, it powers up the control board :ahh: …
for some reason the fets keep blowing up on the speed controllers and we really can’t afford £50 a time…

its called a Sprag clutch. its basicaly a rim with inner teeth. and a shaft with spring-loaded arms that grab those teeth in one direction but are depressed in th other direction.

i can’t seem to find anything like that at rs (

the idea is to let the chain move the wheels but the wheels not to require the motors to be moving … .

The only thing that I can think of that would allow this is a clutch like in a car with a manual transmission. There are one-way clutches that would allow you to free-wheel, but only in one direction, and I am not exactly sure how a bike hub works to allow it to free-wheel in both directions.

One thing that might work is a centrifugal clutch. We have one on the mini-chopper that my friend and I built. It is mounted on the output shaft of the engine and lets the engine idle up to a certain speed. When the engine speed increases, there are clutch shoes that move outwards and grab the drum around them to turn it. There is a sprocket already attached to the drum. I do not know if there is a cenrifugal clutch that would work on a FIRST robot due to the speed and torque required to engage it, but it is something to look into.

Are your speed controllers in brake or coast?

Just go to the store and buy a little red wagon. That should take care of your problems for now at least.

We put our cart in the crate with the robot.


Good question!

I’d recommend contacting IFI directly and discussing your problem with them…ie frying Vics. They are a very patient and talented bunch.

Technical Support Hotline - 903-454-1978, hours are 9am to noon, 1pm to 5pm CST. (…so I’d guess you ~5hours ahead)

(The EE’s on these forums are also top-notch. A PM might yield an answer to you vexing problem)


I concur. Team 57 went with the little red wagon cart this year. It had a custom built wood frame to support the robot and a really hot paintjob. We never quite managed to get the spinner rims installed, though…

Anyways, there’s a large number of things you can use for a cart. If a little red wagon is still too big, get or make a 4-wheeled furniture mover. it’s a square of flat 2x4’s with four casters on it. Your cart can be as simple or fancy as you require it to be.

If you’re still wanting to just push the robot around, get a couple of quick release connectors. Run the outputs of your speed controllers through these before they go to your motors and just pull them whenever you’re going to push the robot around.

What you’re describing on a bicycle is a one-way or Sprag clutch as Tytus already answered. These are a bad idea for your robot because they would only allow it to be powered in one direction; that is, you’d never be able to go in reverse.

I think you’ll find it’s much easier to build or buy a small cart than it is to engineer “neutral” into your drivetrain so that you may push it around.