Brake pressure sensors OR longitudinal G forces?

[dsmithwc04]

I'm on the fence on purchasing a couple brake pressure sensors for my CDS copilot 2+ or just using the on-board long G forces to review my braking points.

I reviewed a data file that comes with the CDS software and noticed that the longitudinal G force graph matched pretty darn close to the front brake pressure sensor graph. Sure, having the two brake pressure sensors would allow some additional info such as observing if a problem with the brake system is in the rear or the front by reading the pressures but not sure if there is much more to gain from adding brake pressure sensors when you can clearly see where you might be able to brake a little more efficiently by watching the g forces. Am I wrong? Perhaps using the longitudinal g force with a plunge type sensor on the brake pedal together would be good enough? I already have an unused plunge sensor that could work on the brake pedal.

[mousecatcher]

I consider brake pressure sensors to be almost mandatory.

With brake pressures, you can show the numerical bias on the dash. This is really great for repeatability and driver confidence out the gate. This is also really great for initial setup at different tracks (downhill vs uphill braking). Also really great for repeating an initial wet setup (and switching back to dry).

You can also see the exact point where you lift the throttle and apply the brakes. You can also see the initial brake application force and the taper off, without the noise from the g sensor or the Z axis error. This is great for showing lazy braking.

You can also see trail braking in action.

You can also see the bias variance, static and dynamic. This is great for seeing slop in the system.

You can also differentiate between light braking and just throttle lift.

[romoman]

I think mousecatcher hit all the main points. If you can afford them, their is certainly good data to be had from using them. If you don't have the budget, Long G is a very good substitute.

[S Lathrop]

Before I would make the decision of brake pressure vs. longitudinal acceleration, I would learn something about the algorithm that calculates longitudinal acceleration.

Commonly the acceleration numbers are arrived at by differentiating speed over time. If you are only looking at deceleration rates over some distance, or how hard is the car braking this is not a problem. But if you want to look at the deceleration rate as it relates to some other variable, that calculation might be totally erroneous. I have been frustrated by this problem.

An example would be a combined G graph. It is way more accurate with when calculated with longitudinal and lateral g sensors. The same issue would be true if you were trying to build a friction circle graph.

Brake pressure sensors are not very useful unless you have both front and rear brakes.

The one thing brake sensors are very good for is to show if you driver is riding or dragging the brakes. It is also very valuable in working out brake function issues such as brake release problem. It is also a great way to set brake bias prior to hitting the track.

I would recommend that if possible keep the option of having either long acceleration or brake pressures.

[dsmithwc04]

Thanks for the great info guys. I can't imagine where I would be without the help from everybody on Apex. I gentleman is selling 2 used CDS brake pressure sensors and I was debating on purchasing them and if their usefullness outweighed using longitudinal g's alone. It appears they are definitely worth the investment. The only questions I have is one of the sensors is a 0-2500psi but the second is only a 0-1000 psi. Would I be okay with putting the 0-1000psi on the rear brakes and the 0-2500psi on the front brakes? The data file that came with the CDS software had a few laps at mid ohio and the rear brake psi never got above roughly 450psi with the fronts reaching 700psi or so. What kind of pressures do we see in the FE?

[romoman]

I don't know in the FE specifically, but I doubt you'll max out a 1,000 psi sensor on the rear. Buy 'em up and then take John Block's Autoware seminars to learn how to use the data from them.

[TimW]

Would I be okay with putting the 0-1000psi on the rear brakes and the 0-2500psi on the front brakes?

Do the math of if you had to do a panic stop and you were able to put all your weight on the brake pedal, what that pressure would be. The sensor has to handle the normal operating range as well as the boundaries. The difference in the 1000psi sensor is that its burst pressure is likely only 40% of the 2500psi sensor. The burst pressure is where the sensor can fail catastrophically, so given that its plumbed in your brake system you need to ensure it is at a high enough level above the extreme boundary conditions. My experience is with 2000psi sensors, with a 4000psi burst rating or 100 bar sensors with a 5x burst rating.

[dsmithwc04]

I have no idea what the "panic" pressure would be on the rear system but after a little search I found most never reached 500 psi according to their data. I would much prefer the sensor be a 2500psi on the rear brakes but from what I've heard 1000 psi should be plenty for the rear. I need to contact CDS to see what the burst rating is. Good call on bringing that up! I would never put it on the front MC though and your right about implementing a good safety rating in anything that is connected to a system where your life is on the line.

[romoman]

I don't know whose sensors CDS uses, but I think the burst pressure of 2x rated pressure is wrong. For most KA sensors, it's 3x and for Ashcroft G2 it would be 5x. MSI is 5x rated pressure or 20k psi, whichever is lower.

You don't need to worry about using a 1000 psi sensor.

[dsmithwc04]

I appreciate the info romoman. I sent an email to Peter at CDS to see what the burst pressure is earlier and will let you guys know when I get a response. Thanks all!

[jcorsico]

I use a Honeywell MLH series sensor and the burst pressure is 10x (it's a 2,000 psi sensor with a 20,000 psi burst).

[R. Pare]

I have no idea what the "panic" pressure would be on the rear system but after a little search I found most never reached 500 psi according to their data. I would much prefer the sensor be a 2500psi on the rear brakes but from what I've heard 1000 psi should be plenty for the rear. I need to contact CDS to see what the burst rating is. Good call on bringing that up! I would never put it on the front MC though and your right about implementing a good safety rating in anything that is connected to a system where your life is on the line.

Panic stop pressure can easily reach 1500 - 2500 psi.

[Modo]

As a Bill Scott student, ultimate braking begins with testing, for the driver, brake here closer, then here closer, then here closer, providing he is doing his job, applying a geometrical amount of pressure, ultimate is 15% lockup as quick as possible (yea touchy feely sort of thing), it will grab ur nads, make sure your anti-submarine belts are not directly over them!!!!!
..
I previously thought the ideal training for a grand prix driver was to jog-run while doing long division in his head ........ ouch!!!!! actually u have to add in immediate eye "what's in front of you'

[dsmithwc04]

As a Bill Scott student, ultimate braking begins with testing, for the driver, brake here closer, then here closer, then here closer, providing he is doing his job, applying a geometrical amount of pressure, ultimate is 15% lockup as quick as possible (yea touchy feely sort of thing), it will grab ur nads, make sure your anti-submarine belts are not directly over them!!!!!

What would be nice is compare the rate of time the driver took to get from no brake to full brake using wheel speed to factor % of lockup as the determining factor of full brake. Unfortunately, I'm out of channels on my CDS

[romoman]

This is what most folks I know use for a basis of braking performance http://www.optimumg.com/docs/Brake_tech_tip.pdf

[BLS]

Don't confuse "burst" pressure with over pressure rating. Most common sensors are rated to 1.5 max pressure without failing electrically. In other words, they will take 1.5 of their max rating and work correctly when they return to normal pressures. Above that level and they will no longer read correctly. Burst pressure, where the device would leak, is usually much higher.

[Modo]

1.5 the rating, sounds likes a yankee device and a 'Gift' GO US, we be good, yea

[jcorsico]

Don't confuse "burst" pressure with over pressure rating. Most common sensors are rated to 1.5 max pressure without failing electrically. In other words, they will take 1.5 of their max rating and work correctly when they return to normal pressures. Above that level and they will no longer read correctly. Burst pressure, where the device would leak, is usually much higher.

Good point. The "proof" pressure on the Honeywell MLH series sensors (which is the pressure above which the sensor element will fail, but not leak) that I use is 2x the rated range. So for a 2,000 psi sensor, it can withstand up to 4,000 psi without damage. It bursts at 20,000 psi.

[dsmithwc04]

I posted this last night but perhaps my phone played a trick on me and never submitted it?

Anyways, Peter from CDS told me the burst pressure is 4x the nominal max rating and if he remembers correctly the element failure point is 2x the nominal max rating.

The 2500 psi sensor will obviously work fine for the front system. I can't imaging the 1000 psi sensor having any issues on the rear of the car in any condition imaginable. After examining the brake system a little more thoroughly I think it is impossible under any reasonable extreme condition to put more than 1000-1200 psi on the rear brakes. With a burst pressure of 4000 psi I know I will never get close to a dangerous situation because of a fault with the sensor and that is my main priority. If I ever get to a point where I put more than 2000 psi of pressure on my rear brakes and broke the element and kept the car in one piece while doing it I will be the first to pat myself on the back for such a feat.

[jcorsico]

For what it's worth, the calipers on your FE are limited to 1015 psi "working pressure" according to the AP Racing data sheet. I assume they can withstand higher pressure peaks.

[CDS]

Before I would make the decision of brake pressure vs. longitudinal acceleration, I would learn something about the algorithm that calculates longitudinal acceleration.

Commonly the acceleration numbers are arrived at by differentiating speed over time..

Our systems all use actual accelerometers for long g. Always have. Calculating it is instead of measuring is inferior.

We have probably done 15 FE's with brake pressures, everyone uses 1000 psi sensors and never pegs them. Steve Stadel @ One Formula makes a nice manifold for installing the sensors.

[George Main]

I'm on the fence on purchasing a couple brake pressure sensors for my CDS copilot 2+ or just using the on-board long G forces to review my braking points.

I reviewed a data file that comes with the CDS software and noticed that the longitudinal G force graph matched pretty darn close to the front brake pressure sensor graph. Sure, having the two brake pressure sensors would allow some additional info such as observing if a problem with the brake system is in the rear or the front by reading the pressures but not sure if there is much more to gain from adding brake pressure sensors when you can clearly see where you might be able to brake a little more efficiently by watching the g forces. Am I wrong? Perhaps using the longitudinal g force with a plunge type sensor on the brake pedal together would be good enough? I already have an unused plunge sensor that could work on the brake pedal.

DS,
The only difference between a FRONT brake measurement and long G (measured), is the understanding that engine braking, aero drag, rolling drag is involved in G measurement. As far as analysis of your braking performance, it can be done quite well through a G number, one that is not calculated through math, but an actual measurement of Long G. If you put a long g sensor and a brake pressure sensor, say the front one.. you will see, when the brakes are applied they are exactly the same in terms of the shape of the trace. Modulation of the brake will model each other (Brake sensor, Long G sensor). The difference is the off throttle/no brake area.. the engine brake, rolling chassis drag and aero drag that will cause long G in the braking direction. This before the application of brake.
A brake pressure sensor added to a Long G can be redundant, if you are purely after your own braking ability.
As was said before, two brake sensors have the ability to measure the brake bias on the car and you can create a starting percentage number that can be read on the dash, that you know is a good balance. A number that you can alter according to conditions of the track, car and driving style. But you have a familiar starting place.
If I read your post correctly , your concern about measuring your braking performance for your driving performance , you have what you need already.. the Long G is a perfect sensor for that, as long you understand that there is measurement taking place, when you lift the throttle and not applying brake yet.