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Hi,

I am making a School Project. The goal is to build a vehicle. I have assembled everything as shown in the picture. It has a 40000 rpm motor, 12v dc battery, 6 inch lawn mower wheels for back. For the front wheels and steering - I made a small contraption using swivel wheels and pvc pipe. I connected the wires neatly, and put a start/stop switch. The motors are turning very fast, so are the wheels.

But the car isn't moving even an inch on the floor. When I lift the back wheels up, they are spinning very well and fast. Also if I push the vehicle manually without battery, it moves. So there is no friction issue. But when I turn on the battery, the car doesn't move. I tried on all surfaces - wooden floor, carpeted floor, stone floor. Still won't move. I also nudged it forwards, still won't move. What is that is preventing the wheels to not move, especially when the motor is spinning at 40000 rpm.

Someone suggested that I should get a gear system. But I get negative points if I put a gear system, as the car needs to be done under $200. Plus a gear system requires a lot of other work.

Any idea or help as to how to.make this vehicle move. I don't expect a fast car, but I want it to move with my current design. Much appreciated.

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  • Are the motors actually spinning when it's on the floor and not moving, as your question states? If the motor is spinning - you can see the motor shaft moving - and the wheels are not, how are the wheels attached to the motor? Is the motor shaft slipping within the wheel hub? – 3Dave Mar 3 '19 at 16:23
  • The shaft isn't moving when it is on the ground. – ThatBoyCoder Mar 3 '19 at 22:27
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You need a lot of gearing.

When you switch the power on you are expecting that cart to go from 0 to max speed instantly... The motor does not have sufficient power for that.

You need to work out the distance travelled for 1 wheel revolution and think about what happens when full motor speed gets to the wheels - can you run that fast?

So a cheap possibility is to use bicycle chain sprockets - smallest on the motor and largest on the wheels - you may need more than one stage.

For gear ratios it is driven / driver for rpm or you can consider diameters. Checking calculations is easy - rotate the input so the output does 1 full turn - count how many times the input rotated.

I am assuming that the motor will have sufficient power...

I will leave you to analyse your school project now as I have given you lots of hints. Note this question may well get closed.

Edit: consider a car wiper motor - lots of torque and has in-built gearing already...

And you need to improve the connections to the motor, going on what you show in your picture...

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    Agree with your assessment, however a very cheap/easy alternative to using bicycle chain sprockets and a way to get exactly what is needed is additive manufacturing (i.e.: 3d printing). You can design what you need to give the gear reduction with exactly the combination needed. Also, you talk about "sufficient power" ... the OP isn't missing power, they're missing torque. They have very little torque. Using gearing will improve the torque needed (torque multiplication) to motivate the "car". – Pᴀᴜʟsᴛᴇʀ2 Mar 3 '19 at 16:01
  • @Pᴀᴜʟsᴛᴇʀ2 Just thinking if the motor is mounted under the cart then it has to be fairly small , given 40k rpm ... it's not a starter motor which would drive that directly... – Solar Mike Mar 3 '19 at 16:06
  • @Pᴀᴜʟsᴛᴇʀ2 Perhaps I'm misunderstanding this, or the question is incorrectly phrased, but the statement "What is that is preventing the wheels to not move, especially when the motor is spinning at 40000 rpm." implies that the motors are spinning, but the wheels are not. That would indicate an issue with the link between the wheel and the motor shaft, not an issue with torque. ?? – 3Dave Mar 3 '19 at 16:24
  • @3Dave did you miss this bit : "The motors are turning very fast, so are the wheels" – Solar Mike Mar 3 '19 at 16:28
  • @SolarMike I didn't miss it, but the other statement (quoted above) says something very different. – 3Dave Mar 3 '19 at 16:29
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That motors will not have enough torque at 40,000 rpm to motivate a six inch wheel with direct drive.

Also, your vehicle (even if it worked) is going to spin counterclockwise, with only one rear wheel driven. I can't tell if you have one motor or two.

Also consider what Solar Mike suggested: a 6 inch wheel spun at 40,000 rpm, in ideal theoretical conditions, would propel your vehicle roughly 714 MPH, approaching the sound barrier! You should do the math yourself to see if I'm right.

Without a doubt you need some sort of reduction gearing.

I suggest a different motor. You could get a great motor/reduction gear combination from a used automotive power window regulator from a junkyard for about $25. Such a motor will have a very large output torque, at a reasonable output shaft RPM. It will also consume more battery capacity as it will require a higher current.

You will probably still need to connect the rear wheels with an axle to prevent the vehicle constantly torque-steering to the left. You then have the option of driving the axle from a gear, and center mounting the motor.

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