Wow, quite a lot of answers. Somehow my email notifications got lost since I did not receive any after my last post. Sorry for taking so long to get back to you guys...
Your scenario isn't nearly as simple as it's being portrayed, and your end goal is going to dictate the best method of reading/detecting braking to accomplish that.
I never imagined it to be easy, that's why I turned to you folks
1.0-1.2g braking is likely max braking for most vehicles under ideal conditions. Dirt roads, dirt on the road, rain, snow, will all have a significant impact on max braking forces.
Sorry I did not make that any clearer: I am talking about emergency brakings under normal (or best-case) conditions. Snow, dirt, rain, are not going to be considered in this very 1st prototype (or proof of concept).
Most normal driving will also never see anywhere near 1.0-1.2g of braking. More likely to be around 0.5g for normal daily driving (maybe even less for some people). So are you looking to detect any normal braking event, or are you looking to detect some sort of emergency stop situation (in which it is assumed that you are braking as hard as possible) Are you going to be doing anything when this is detected that could be problematic if it's just a case of the person stopping really hard just for the fun of it?
As I just said above: I am interested in just those emergency stop situations. This projects aims at documenting why this particular situation required hitting the brakes so intensively. One part will be a front bumper mounted wide angle camera that takes a picture of what is going on in front of the car. To be able to capture the actual source of the situation lag should be as low as possible. Besides the camera thing we will also trigger an audio recording in the passenger compartment of the car. Moreover, drivers will be instructed to articulate the reason for their braking. Therefore, if a driver made an emergency braking just for the fun of it (who does that for fun?) all it would produce is a false positive recording. Not an issue for my scenario.
According to simple law of physics, the static friction is only a small portion of gravity of an object, the factor is called static frictional coefficient. Unless you have some tank tracks or sticky surfaces (spider man), you will not have a static friction that exceeds gravity. So you will not have a deceleration greater than g.
Somebody has something to say about this coefficient for tire on dry road:http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html
is pretty interesting. I haven't worked through the article, yet. That's probably why I am going to make a fool out of myself now: my emergency braking is around 1g comes from a site which explains how airbags in cars work (I'd provide the link but unfortunately it is in german). However, I found a Wikipedia article about Formula 1 cars that are supposed to decelerate with up to 5.0 g. This is quite remarkable since the best street legal car, the Bugatti Veyron, is -and I quote- "claimed to be able to brake at 1.3 g" [SOURCE]
Another interesting section is to be found under Deceleration
As for the braking thing, I think you should ride with an accelerometer and record the entire process of a number of hard breaks, soft breaks and what not so you have data to look at. Without data, I could argue 0.5g is trigger and needs to be at 0.5g+ for 0.1 second and you say different and neither has proof this is what actually happens.
Generating braking samples myself is what I consider my very last option since this is going to cost me quite some money (gas, tyres, brake pads and discs, depreciation, etc.).
Two options that I can think of.
1)Put a pressure sensor on the brake master cylinder to detect brake pressure and send to Arduino. They make a simple on-off style switch or you could have a gauge pressure sensor if you wanted precision.
Sorry, not a solution for me. This thing is going to be inserted into actual customer cars and should as easy to install as possible
2)If you still have your brake light switch that opens and closes by the movement of the brake pedal, you can send that signal through a voltage divider to the Arduino letting it know that you have applied the brakes.
If the Arduino knows when the brakes are applied then, you will know when you have data that is related to braking.
Not really. This would allow me to detect that there is some braking while not being able to determine its severity.
Thanks for all your feedback.