Xcceleration

Brakes 101

Slotted verses Cross Drilled disc rotors.

There is a great deal of confusion and misinformation on the subject of which is better, Slotted or Cross Drilled. It’s a lot like comparing chalk to cheese. Both have very different uses and if used in the correct application provide the results you expect.

Slots or grooves are machined into a disc rotor to extract dust and debris from the disc and pad friction coupling. Loose pad material is caught in the grooves and exits after the pad passes over. This reduces the glazing effect on your pads when dust is trapped between the disc and pad union. Glazed pads reduce braking efficiency (friction) and therefore more effort is required to slow the vehicle. More effort means more heat generation which then increases wear. This is the beginning of the cycle of degraded braking performance. The number of slots in a disc rotor is important too. A properly design slotted disc rotor will have enough grooves to efficiently remove dust and debris but not too many grooves which weaken the disc and increase pad wear.

Cross Drilled and Slotted disc rotors have the benefits of slots and drilled holes. The slots or grooves help extract loose pad material and the drilled holes are designed to quickly and more efficiently evacuate excess dust, water, and even dirt or sand in 4WD applications. In wet weather and off-road environments water, sand, and dirt can get trapped between the disc and pad, which greatly increases stopping distances. The holes in a properly engineered cross drilled disc rotor are precisely positioned to ensure that each hole cleans a different part of the pad face and as a group of holes will evacuate loose material from to whole pad surface. In addition to efficient cleaning these drilled holes increase the mechanical braking torque “pad bite” of the disc rotor by reducing the disc surface area in contact with the pad. This increases the initial bite and responsiveness of the brake system in an emergency-braking situation. That extra few feet could avoid an accident.

O.K now we know the difference, which is best for you?

We all like to talk about how much power our performance vehicles have, 300, 400, 500+hp but do you know that it can take in excess of 1000kW of energy to stop that same vehicle? Where does it go? Into the brakes!

The disc brake acts like a sponge, absorbing massive amounts of kinetic energy in seconds. This energy transfer generates intense spikes in temperature and stress on the cast iron disc. This is the key to making the right decision.

Now Cross Drilled rotors are undoubtedly efficient at cleaning dust and debris from the disc and pad union. But the more holes and grooves you machine into the disc rotor the smaller you make that important sponge which is trying to absorb massive amounts of braking energy. This increases the amount of temperature and stress the remaining disc rotor material must process and in extreme circumstances may lead to cracked discs. In normal driving conditions where disc temperatures do not exceed 500 deg C, Cross drilled range of discs provide excellent braking performance in a variety of environments or conditions.

Please note: In 4WD applications drilled rotors are not recommended for use in heavy mud or wet clay where the ventilation system can become blocked and greatly effect disc cooling.

Slotted disc rotors are the heavy duty braking solution for applications where temperatures may exceed 500 Deg C. The slotted ranges of disc rotors maximize that essential rotor mass to absorb extreme braking energy while maintaining a clean braking surface. Typical applications for slotted disc rotor range would be in high performance road vehicles, heavy vehicles such as 4WD’s, or vehicles carrying/towing heavy loads that need the security of heavy duty brakes that can handle extreme loads.

What to do about the Rear Brakes?

One of the most common questions received from new owners of front brake upgrade kits is "Do I now need to upgrade my rear brakes too?" To answer this, we need to look at the role of the rear braking system from a few different perspectives. The answer may surprise you, especially hearing this from a company that sells big brake upgrades!

Rear Brake 101

One of the many design factors that goes into the development of a base braking system is the mysterious "bias" or "balance." Truth be told, it's a pretty simple concept to grasp: for vehicle stability under braking, it is required that the rear brakes do NOT lock before the front brakes. Simple, right? Most of you probably knew that already.

OK, so what governs the 'lock up' point of the rear brakes? Drum roll, please:

1. tire traction capability (friction) 2. tire normal force (weight on the tire)

This can be proven from looking that the fundamental relationship for maximum sustainable tire force: F=µN, where:

F = the lock up point, or peak force µ = tire-road coefficient of friction N = normal load sitting on the tire

So, when the OEM is designing a brake system, they 'size' the system components (calipers, master cylinder, rotor OD, etc.) to generate the proper amount of torque at both ends of the vehicle so that the front brake force ('F' above) exceeds its peak traction first. At this point, the front brakes lock and the car slides in a nice, stable straight line.

Potential Impacts of Big Front Brakes

Fortunately (from a safety standpoint anyway), when most big-brake suppliers adapt a mondo rotor and caliper package to a vehicle, they end up actually increasing the FRONT bias. How? By increasing the effective caliper piston area and the rotor effective radius, these two factors work together to increase the 'mechanical gain' of the front brakes, building more torque for the same pressure, everything else being equal. So, from a bias perspective we are not pushing the vehicle toward instability, but rather just the opposite - we are under braking the rear axle! The obvious impact would be an increase in stopping distance - probably the one thing the new owner was actually hoping to reduce. Ironic. So, say you chose to install these big brakes on the front axle but want to maintain the OEM bias. What's the answer? Well, one way would be to invest in big rear brakes too, which increase the rear mechanical gain to the point that the system is balanced once again.

So, What's The Harm In Doing That?

Well, let's look at why we upgraded the front brakes in the first place. Contrary to popular belief, the real reason sports- and racing cars use big brakes is to deal with heat. Period. There has been a bunch of stuff published which will disclaim this, but when you look at the braking system from a design standpoint, making them 'bigger' doesn't fundamentally do anything for stopping distance. It's all about the heat. So, you upgraded the front brakes because of thermal concerns but as a hidden surprise got a shift in brake bias. As a band-aid to this condition, you now spend thousands more on a rear brake upgrade because the front system was not sized correctly in the first place. Sure, it looks great, but there is another option...

Which Is?

When upgrading your front brakes, it is possible to size the caliper pistons and rotor effective radius to maintain the original brake system's pressure-torque relationship. Yea, it takes more engineering know-how and you can't sell the same part to everyone anymore, but you are not altering the base brake balance from what the OEM intended. Now, if you sized the front brakes correctly, why would you need to change the rear brakes? Good question. If there are no thermal concerns with the rear brakes (and on a front-engine street car there rarely are) then by installing a rear big-brake kit all you are doing is (a) spending money and (b) adding unsprung weight. This is not usually viewed as favorable, unless you like driving a heavy, expensive car.

Oh Yea - One More Thing…

Finally, under an OEM bias condition, the rear brakes only contribute about 15-20% of all the braking force the vehicle generates, and when you install sticky tires you actually DECREASE the amount of work they need to do. Why? Because at the higher deceleration levels afforded by race tires, there is more weight transfer taking place, reducing the normal force on the rear tires and increasing it on the front (remember F=µN from above?). If anything, we now want to decrease the rear effectiveness. Ironic once again. Of course, if you decide to upsize your rear brake system components you can also impact the front-rear torque relationship, and consequently you can "bias" the "balance" more toward the rear. Go too far, and the rear brakes could lock before the fronts. Again, not the end result you were expecting, right?