The tech answer to the question of how your zed abs system will react to larger calipers / discs is explained a little lower down , it is a bit of a read but well worth it if you want know the definitive answer to the question.
The real world solution
From the tech answer below it is clear that the abs system can be effected and not in a good way, however and this is crucial to remember when fitting a brake upgrade kit "just because it works on car xxx, it must work on your zed " approach, is way wrong. What you need to ensure is the brake upgrade kits is either designed with the characteristics of the original braking system taken into account to minimise the differences, or the donor car brake system has the same or very similar characteristics and in that way the ABS system WILL work as originally designed.
To this end upgrades such as the Nissan Skyline GTR 33/34 Brembo calipers and discs will work with the 300zx set up as the Pressure-Torque And Pressure-Volume Relationships ( explained below ) is compatible, however the Nissan 350z is not, after market kits designed as vehicle multifit are also rarely compatible.
One final word of warning, disconnecting or disabling ABS safety systems WILL effect your insurance and could in the event of a fatal accident leave you carrying the can!
Jeff TT
Definitions
ABS – Anti-lock Braking System – An electromechanical control system designed to monitor and influence wheel dynamics, and ultimately vehicle dynamics during braking manoeuvres. In order of priority, these systems are intended to enhance vehicle 1) stability, or the prevention of over steer 2) steer ability, or the prevention of under steer and 3) stopping distance. Typical systems consist of 3-4 wheel speed sensors, an ECU containing the algorithm processing the wheel speed information, a series of solenoid-driven valves, and a pump-motor subsystem which can be actuated to interrupt and release brake fluid pressure from the wheel-end brake components (calipers and such).
ABS Control In Super-Slow-Motion
In order to best explain how the ABS "depends" on the base braking system, let's have a look at a typical ABS event at the micro level – from the processing algorithm's perspective.
Say you are driving down the motorway at 75 MPH ( tut , tut ) when all of a sudden a lorry in front of you spills its load of water across all three lanes of traffic. Now, this alone would not be so bad, except the water is still sealed in large drums anyone of which would certainly make a mess of your zed, time to take evasive action.
Being the trained high-speed individual that you are, you immediately lift off the accelerator , dip the clutch (if your zed is a manual), and at the same time hit the brake pedal...but in the heat of the moment you hit it a little too hard!!
Previously the ABS ecu was hanging back watching the world go by, seeing a constant stream of 75 MPH signals from its four wheel speed sensors. Let’s call this "observation mode." Upon your application of the brake, however, the ABS snaps to attention, its antenna up, ready for action.
You have just hit the brake pedal after all, and who know what’s coming next.
After 50 milliseconds (it’s actually much faster than that – 7 to 10 milliseconds is typical – but it easier to work out) the ABS takes another snapshot of the wheel speed information in an attempt to figure out what's going on. This time the wheel speed sensors are all reporting a speed of 74 MPH. Doing a quick calculation, the ABS determines that in order to have slowed 1 MPH in a 50ms period the wheels must be decelerating at a rate of 0.91g’s. Because you are driving a sports car, the engineer who calibrated the system ‘taught’ the ABS that your car is capable of decelerating at this rate, so the ABS continues to hang back and watch the event from the spectator’s booth. No problem so far.
The next 50ms, however, are a little more interesting. This time around, the wheels are reporting 72.5 MPH. Now, it may not seem like a big jump, but to slow 1.5 MPH in a 50ms window equates to a deceleration of 1.36g’s. Not alarming, but the ABS ‘knows’ that based on this deceleration level, the wheels are probably beginning to slip a little more than they should – after all, your car is probably not decelerating at quite 1.36g’s..and any error between the two indicates slip.
ABS is now in "ready mode." It’s probably too soon to jump in, as the wheels might spin back up on their own in the next 50ms loop, but things are definitely looking bad!
As the first drum of water bounce left and right, missing your car by inches, you stay on the brake pedal but push even harder. This time around, the left front wheel speed sensor is registering 68 MPH – a 4.5 MPH drop in the last 50ms, or a deceleration level of 4.1g’s. Doing the math faster than you can (after all, you are busy dodging barrels of spring water), the ABS quickly comes to the conclusion that, unlike the left front wheel at this moment, the car cannot possibly be decelerating at 4.1g’s. Best case is that the car was decelerating at 1.0g (or thereabouts) over the last 50ms, so the ‘real’ vehicle speed is still somewhere around 71.5 MPH, even though the left front wheel speed is reading 68 MPH – a 3.5 MPH error.
So, based on a wheel deceleration of 4.1g’s, a slip level of 5% (3.5 MPH¸ 71.5 MPH), and a couple other factors not listed here, the ABS jumps in and enters "isolation mode." (Note that the wheels are nowhere even near "wheel lock" – the 100% slip point.) The first thing the ABS does is shut off the hydraulic line from the master cylinder to the left front caliper, isolating the driver from applying more pressure – after all, it was the driver that got us into this mess in the first place.
Next, the ABS starts work in "decrease mode," releasing the excess pressure from the left front caliper in order to allow the left front wheel to re-accelerate back up to the vehicle’s actual speed – 71.5 MPH in this case. Since the ABS knows how quickly the wheel is decelerating (4.1g), how fast the car is actually going (71.5 MPH), and the pressure-torque characteristics of the left front caliper/pad/disc assembly (we’ll come back to this one in just a second), it can precisely calculate how long to open its release valve to vent that extra pressure, leaving just enough pressure in the caliper to maintain 1.0g of deceleration (or thereabouts).
Let’s say that calculated time turned out to be 10 milliseconds. Bang! Valve opens, pressure is released, and 10ms later it closes, leaving just the right amount of pressure in the caliper so that the wheel spins back up to exactly 71.5 MPH, but continues to decelerate at 1.0g. Everything is going as planned.
Time to close the loop and enter "increase mode." Once the ABS sees that the left front wheel has returned to near the ‘real’ vehicle speed, it slowly reapplies pressure from the master cylinder to make sure that maximum sustainable brake force is being utilised. To this end, the ABS calculates precisely how long to pulse open the isolation valve, slowly building pressure at the left front caliper until once again the left front wheel begins to slip. It performs this calculation based on – you guessed it – how quickly the wheel is re-accelerating, how fast the car is actually going, and the pressure-torque characteristics of the caliper/pad/disc assembly.
In our hypothetical little world, the ABS calculated that four pulses of 5ms each were necessary to build the wheel pressure back up to the point that the wheel began to slip again, returning to "isolation mode."
The cycle is repeated on all four wheels simultaneously until either the driver gets out of the brake pedal, or until the car has come to a stop. Hopefully, this did not include punting a drum of water or two along the way as the ABS kept all four wheels slips in the 5%-10% range, allowing you to turn and swerve to your heart’s content as the drums bounced out of your path. Happy car, happy driver.
The Potential Impacts Of "Big Brakes"
Let’s now take the exact same scenario, but add a twist: you are returning home from having that long-sought-after big brake kit installed. You know, the one that required new 18" wheels to clear the 8-piston calipers and 16" brake discs. Driving around the car park you couldn’t believe the improvement in pedal feel and initial bite they displayed. These things must really throw a boat anchor behind the car at high speeds, right?
Well, let's see.
Resisting the temptation to run in the fast lane at triple-digit speeds, you once again find yourself behind the lorry at 75 MPH. drums fly and you again stamp on the brakes, but with the increased confidence of your new hardware to slow you down in time. Plus, you now know how the ABS works, so you hit the pedal hard confident that you will have both deceleration and steer ability. It couldn’t get any better.
Like scenario 1, after the initial 50, 100, and 150 milliseconds the ABS takes snapshots of the wheel speed information and registers 0.91g’s, 1.36g’s, and 4.1g’s on the left front wheel. Again the ABS quickly comes to the conclusion that, unlike the left front wheel at this moment, the car cannot possibly be decelerating at 4.1g’s. Best case is that the car was decelerating at 1.0g (or thereabouts) over the last 50ms, so the ‘real’ vehicle speed is still somewhere around 71.5 MPH, even though the left front wheel speed is reading 68 MPH – a 3.5 MPH error. So far, so good – just like last time.
Here’s where things start to get interesting, though. ABS enters "isolation mode" and shuts off the hydraulic line from the master cylinder to the left front caliper, isolating the driver from applying more pressure. Next, the ABS starts work in "decrease mode," and once again calculates that 10ms are required to the excess pressure from the left front caliper in order to allow the left front wheel to re-accelerate back up to the vehicle’s actual speed – 71.5 MPH in this case. Unfortunately, this calculation was based on the standard vehicle’s pressure-torque characteristics of the left front caliper/pad/disc assembly. Let’s talk about this briefly while the drums roll in closer.
Pressure-Torque And Pressure-Volume Relationships
When a braking system is designed and installed, the components are chosen to provide a certain deceleration level for a certain amount of force applied by the driver to the brake pedal. While the overall relationship is critical, there are many ways to achieve the same end…but fundamentally the parts are chosen to work together as a system.
One of the most important relationships for the ABS engineer is the pressure-torque (P-T) relationship of the caliper/pad/disc assembly. In so many words, for a given brake fluid pressure, X, the caliper/pad/disc assembly will build up a certain amount of torque, Y. For the sake of argument, let's assume that adding 100 PSI of brake pressure to the stock caliper in our example vehicle generates 100 ft-lb. of torque.
Another important relationship is the pressure-volume (P-V) characteristic of the system. This relationship defines the swelling or expansion of the brake system for a given increase in pressure. Let’s also say that our stock vehicle brake system ‘swells’ 1cc for every 100 PSI.
Unfortunately, there are several big-brake systems available today which pay no regard to the original P-T or P-V relationships of the original vehicle…and in fact many make it a point to affect drastic changes in these relationships in order to give the consumer that feeling of ‘increased bite.’ While the upside is certainly a firmer pedal and higher partial-braking deceleration for the same pedal force, the trade-off can be ABS confusion.
Back To The Water Drums
So, back to our example – the ABS has just calculated that a 10ms pressure reduction pulse was necessary to vent that extra pressure, leaving just enough pressure in the caliper to maintain 1.0g of deceleration (or thereabouts)…but the new system with its decreased P-V characteristics (increased stiffness!) releases twice as much pressure as the stock system in the same 10ms window (the equivalent of a 20ms pulse with the stock system)! Of course, the increased P-T characteristics (bigger discs! bigger pistons!) don’t help either, as now three to four times as much torque has been removed from the wheel as with the stock system, leaving only enough torque to decelerate the wheel at, say, 0.3g. In ABS land this is known as a ‘decel hole’ and feels just like you momentarily took your foot off the brake pedal.
Now, given that huge pressure decrease, the ABS quickly enters "increase mode," trying to correct and build the pressure back up near the vehicle’s maximum sustainable brake force. This takes time and time equals lost stopping distance.
The ABS calculates precisely how long to pulse open the isolation valve and determines that four pulses of 5ms each are necessary, just like before. Because of the new P-T and P-V characteristics however, after only two pulses the wheel is again being forced into slip, leaving the ABS scratching its head and wondering what’s going on. Not expecting wheel slip so soon, the ABS quickly releases pressure in an attempt to recover, but the damage has already been done.
The cycle is repeated on all four wheels simultaneously until either the driver gets out of the brake pedal, or until the car has come to a stop…but this time the ABS is always one step behind. In some cases the ABS is robust to modest changes in the base brake system, but in extreme cases there can be a significant negative impact to the vehicle’s steer ability (increased front wheel slip due to poor control) and a measurable increase in stopping distance (multiple ‘make up’ decrease pulses).
So, your chances of stopping in time or swerving to avoid one of the bouncing drums have been decreased. In this game, inches count and you sure need every one.
So Big Brakes Are A Bad Idea?
Well yes and no in fact, if designed and chosen properly, these upgrades can make the most of these control technologies while providing all of the cooling and thermal robustness advantages these kits have to offer.
In many cases fabricated adapter brackets to mount them to a variety of different cars, to market the kit as a ‘one-size-fits-all’ without first determining if the system will be compatible with the remaining foundation braking system.
:dance::dance::dance::dance::dance::dance::dance: Think it looks good now, just wait and see. WELL DONE THAT MAN Jeff TT