Scioly.org

iwonder wrote:Alright, fine.

One axle has a wingnut brake, the other has a motor, that's what we're talking about, right? There's already some sort of switch to turn off the motor. Using that switch, remove the battery and put say a 3-5 ohm resistor across it instead. The motor wouldn't 'slam on' the brakes, because of the resistor, but it would help.

This seems like it would be the best solution. Might need to play around with different resistors to get the right braking strength.

Be mindful that you'll be putting a large amount of current through that resistor (although for a short period of time) - so you'll probably want to go with ceramic resistors, wirewound resistors, etc - something rated to a decently high wattage. You'd likely burn out 1/4 watt carbon film resistors pretty easily.

But that's probably the best way to decrease the braking when using the motor as an eddy current brake. Doesn't need to be complicated - could be as simple as throwing a SPDT or DPDT relay on the motor - connect one terminal of the motor to the common pin, the NO pin to your power source, and put the resistor across the NC and common pins.

Bazinga+ wrote:

windu34 wrote:Braking with both axles should help because there will be more contact with the floor thus the coefficient of friction should be increased

No thats not why. The coefficient of friction depends only on the material, not contact area. But friction force is friction coefficient times normal force so yes breaking with both axles should double your force from traction. The problem is you need to gradually slow down and not just slam a wingnut in and skid to a stop, which is difficult to implement on both axles.

Contact area does matter in friction, though, just we don't learn about it in early physics. I do see what you mean, though.

One way that was popular to slow down braking when I did electric vehicle last time was that it was suggested to put a spring around the axle between the fixed surface and the moving wingnut. One that was strong enough to decelerate the vehicle but not enough to slow it completely before it hits the fixed surface that a wingnut usually hits. The compression of the spring would slow down the car because of friction and force to compress it.

A Person wrote:

Bazinga+ wrote:

windu34 wrote:Braking with both axles should help because there will be more contact with the floor thus the coefficient of friction should be increased

No thats not why. The coefficient of friction depends only on the material, not contact area. But friction force is friction coefficient times normal force so yes breaking with both axles should double your force from traction. The problem is you need to gradually slow down and not just slam a wingnut in and skid to a stop, which is difficult to implement on both axles.

Contact area does matter in friction, though, just we don't learn about it in early physics. I do see what you mean, though.

One way that was popular to slow down braking when I did electric vehicle last time was that it was suggested to put a spring around the axle between the fixed surface and the moving wingnut. One that was strong enough to decelerate the vehicle but not enough to slow it completely before it hits the fixed surface that a wingnut usually hits. The compression of the spring would slow down the car because of friction and force to compress it.

A better way that I have recently experimented with is instead of having the spring between the wingnut and the fixed surface, have the spring on the opposite side of the wingnut so that it is stretched rather than compressed. This way, the wingnut will hit a flat, uniform fixed surface the same way every time rather than having a spring in its way which could cause variations in the braking. As recommended previously, you will need a very strong spring with a high "k" value for it to be effective