# Is it possible to know if a vehicle is braking?

1. Feb 5, 2018

### physea

Hello!
How can we identify that a car is braking, just by accessing its wheels?
I think dynos work like that? Ie they can tell if the torque is negative or positive by touching the spinning wheel?
Any hint?

2. Feb 5, 2018

### Alloymouse

If I am interpreting your question correctly, you're asking if a device (e.g. a dynamo) can detect if the car is slowing down.

In fact, a dynamo is a great way of detecting a decrease in the speed of the wheels - a decrease in rpm (due to braking) will result in a lower rate of magnetic flux cutting, resulting in a lower power output of the dynamo. If we track the direction of current, we can then classify it as either increasing or decreasing in speed, based on a tested convention. Hope this answers your question :)

3. Feb 5, 2018

### CWatters

Any measure of the wheel RPM will do.

4. Feb 5, 2018

### Staff: Mentor

I don't think he meant dynamo -- the term dyno is short for dynamometer, which is a device used to measure engine power at the wheels of a vehicle:

https://en.wikipedia.org/wiki/Dynamometer

5. Feb 5, 2018

### Staff: Mentor

See the link in my post above. Dynamometers work by applying a retarding torque, and measuring the angular acceleration of the wheels in contact with the dyno drum. There is a neat, inexpensive version of a dyno that works using just an accelerometer in the car. The car's weight and other factors are input to the device, and the output is the power of the vehicle's engine under acceleration.

https://www.gtechpro.com/

6. Feb 5, 2018

### jrmichler

On a level road, yes. On a downhill, you could have the brakes on and still speed up. Conversely, you could be stepping on gas while slowing down on an uphill grade.

You could monitor the temperature of the brake disk/drum. If braking, it's getting warmer. And here is a way to do just that: https://www.omega.com/pptst/OS212.html

7. Feb 5, 2018

### Alloymouse

I see, thanks for the heads-up!

8. Feb 5, 2018

### Baluncore

You can identify braking by measuring the hydraulic pressure in the brake line. Or sensing if the brake light is on.
ABS usually senses rotation rate of the wheel with an inductive sensor and a toothed disk on the hub.
Going progressively faster while braking is possible, if you are going downhill or if accelerating at the same time.
So, what exactly do you mean by braking?

9. Feb 10, 2018

### jim hardy

I really like the suggestion of brake fluid pressure. My 1953 Ford used a pressure switch to control brake lights.

Study up on anti-dive braking. Should be detectable from forces on upper control arm.