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/rotor assembly. In so many words, for a given brake fluid pressure, X, the caliper/pad/rotor 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 Barrels

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 rotor! 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 steerability (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 barrels have been decreased. In this game, inches count and you sure need every one.

TCS/ESP/EBD Impacts


The analogy above translates directly to the TCS/ESP/EBD subsystems without exception. Like the ABS, these three technologies rely heavily on the P-T and P-V characteristics of the OEM system, and any changes can manifest themselves under braking, accelerating, or dynamic maneuvers.

Are You Telling Me That Big Brakes Are A Bad Idea?

So, will all big brake upgrades wreak havoc on the chassis control systems found on your favorite ride? Not necessarily. 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.

The "secret" to brake system compatibility is that there is no secret – it just requires fundamental engineering expertise and design know-how.

As mentioned earlier, far too many of the big brake upgrade kits on the market today pay no attention to the P-T or P-V characteristics that the car originally possessed. In fact, there are kits available today which have P-T characteristics which more than double the output (PÞ 2T) of the stock systems they replace – "200% More Stopping Power" must be better than stock, right?

In most cases, these vendors procure large quantities of big rotors and red calipers, fabricate an adapter bracket to mount them to a variety of different suspensions, and market the kit as a ‘one-size-fits-all’ without first determining if the system will be compatible with the remaining foundation braking system, let alone the electronic chassis controls. Sure, it’s quick, cost-effective, and looks like a million bucks through your 18" wheels, but what about ultimate performance?

The Solution

Unlike the "if it works on brand P, it must work on your car" approach, at STOPTECH all brake upgrade kits are designed with the characteristics of the original braking system taken into account to minimize these differences. This is the reason that when you order a STOPTECH big rotor upgrade kit the new caliper bores may actually be smaller than the units you are replacing to "balance the equation." This is just one way in which our engineers attempt to retain the original system’s P-T and P-V integrity. Sure, it's not one-size-fits-all, but neither is your car or your driving style. Why should you expect any less from your brake upgrade?

In closing, next time you think about bolting on those 16" rotors and 8-piston calipers remember that there are a number of chassis control systems out there just waiting to be confused. Select wisely and reap the benefits.