|Brakes: Brakepad Issues
What are performance brakes and how do they differ from OEM? What are the issues for drivers who want more than brakes that don't squeak, don't change, don't just lie there as appliances?
The braking effect of pedal pressure and clamping force is a function of the size of the pad, master cylinder, caliper piston ratio, heat and time -- and the tradeoffs, once you decide what your main objective is. I think effective braking is mostly about feel, about the control and release feedback you get from the mechanical system. If you can feel the brakes, you can control the pressure and release. The goal is to maintain your braking force just at the threshold. Not about the pressure; but about how quickly and smoothly you can release.
When someone tells me he wants better performance from the brakes, the first question I ask is, What are you doing with the car? Are you talking racing or club events? Track or street? Do you need your brakes for stopping the grocery-getter, which means long periods where they're not being used at all and are only collecting street grime? Or are you talking about high temperatures (the extreme is a place like Martinsville), or high-heat endurance runs like Daytona or LeMans? Each environment encompasses different problems that need to be addressed by the brakes.
All of the performance brake manufacturers, Wilwood, SBS, Alcon, Brembo, PFC, talk about their components in the context of the application.
Brake systems work by using hydraulics to transform the pedal pressure of the driver?s foot into caliper clamping force by the pads on the rotors. OEM brake systems make various assumptions about velocity, driver pedal feel and stopping distance expectations to reach a lowest common denominator performance feel. Change the assumptions and you change the requirements. To accomplish the results, the feel you want to get a controlled stop in the expected amount of time, you may need to go back and change some or all of the pieces that contribute to the job:
The pedal design, position and mechanical advantage
The hydraulic effectiveness of the master cylinder and caliper piston(s)
The design of the caliper
The size and number of pistons
The size and material of the brake pads and the profile of their Mu (coefficient of friction over heat)
The brake rotors and their interaction with the pads (do the pads rub directly on the metal of the rotor, or on thin layer of like material as is the case with film transfer pads.
In this topic, we'll take a quick overview and then look specifically at brakepads. Next time we'll examine calipers and why many designers think multiple, non-equal size, piston calipers are the hot (sic) set-up. Other topics will look at rotors and the debate on gas-slotting; at brakelines, fluid and ancillary bits like drybreaks; at the pedal/master cylinder/biasing system relationship; and at the process of bedding your brakes once you?ve chosen the application.
But first, the basics. Brakes work on the principle of friction: they reduce vehicle velocity by converting the velocity in the system to heat. Brake calipers clamp the brake pads onto the rotor to reduce the rotational rate of the rotor and, by extension the hub, bolts and wheel. If you clamped the pads to the rotor with unanchored Visegrips, they'd just spin with the rotor. No friction. No braking. At the other extreme, if you lock up the brakes, stopping the wheels before the velocity has been converted, you stop generating rotating friction to convert the remaining velocity into heat. What you need is a system that applies enough controlled friction to slow the rotation as fast as possible without locking the wheels. It takes a complicated system to do this job.
Defeating the gremlins of chatter, judder and grab
The faces of the brake pads and the faces of the rotors really need to be perfectly parallel, so the pads apply uniform pressure, consistently and simultaneously across both pad faces, on the rotor. Anything that changes the parallelism of the pad to the rotor, including the drag from the application of the leading edge of the pad to the turning rotor, causes inconsistencies in the pressure of the pad on the rotor and can result in the characteristic brake problems of chatter, judder and grabiness, as well as the well-known pad taper.
Chatter, a high frequency rattle, results when the brake pad flutters on the rotor. This can be caused by the slop inherent in the system or because the rotor is not (or is no longer) balanced or is excessively rough. Judder, a lower frequency noise, usually results from improper alignment of the pad and the rotor, where the faces are no longer in parallel, for example, when the pad material is starting to shear off the backing plate and flutters when pressed against the rotor. (Wilwood and SBS are both now using the unique Nucap retention system that uses tiny hooks all over the face of the backing plate to grip the friction material that is molded on the plate with great pressure.) Grab is the effect when the pad grabs the rotor unevenly. (This is still a major problem with carbon/carbon set-ups.)
Where do chatter, judder and grab come from, and how can brakepads help avoid them? The biggest problem with caliper brakes is that the braking face is out in the wind. Road scuzz mixed with abraded brake pad material ends up plating the rotor with uneven blotches. In the extreme, these blotches can cause local overheating of the pad, destroying it, which, in turn, causes local overheating of the rotor, which causes crazing, which leads to junked rotors. Brake pads that transfer a film of friction material to the rotor help reduce the problem for high temperature, track brake systems. But transfer film pads don?t work for street applications because you just can't get street brakes hot enough to bed them in properly and get the film to transfer. Even if you can bed them properly the pads and rotors don't get hot enough in regular street use for the filmed rotor to demonstrate the advantages of the process.
While transfer film brake pads become a better solution for high temperature track braking, heat is a real enemy to effective braking in this application. Brake fluid boils at 600 F and aluminum (as in brake calipers) melts way below 1000 F. Yet it's common to find brakepads that see 800-900 F. (Consider this, the next time you see glowing rotors on F1 brakes: it takes temperatures of 1600-1700 F for carbon to glow yellow.) Appropriate heat management is therefore more than somewhat important.
Many drivers are surprised to find out just how much heat the brakes are generating. I painted a set of track day brake pads with temperature sensor paint (from PFC) and asked the driver what he expected to see when we looked at the pads after the session. He said he expected to see a gradient in the colors, showing the gradual heating of the pad. Instead the paint was a consistent pale grey, showing that the entire pad, now junk, had soaked up heat to 1200 F.
If you're concerned about track event performance, you need to be concerned about the heat. The design of the piston can help. For example, the Wilwood thin wall stainless piston transfers less heat from the pad to the caliper because it has less surface area than pistons with thicker piston cup lip. But your brake pads need to do more in heat management than just help shield the caliper from the rotor. Obviously the pad material is important.
Why pads perform differently
There is an almost infinite number of combinations of metallic and organic pad materials: carbon and ceramic, carbon and Kevlar, carbon and brass, carbon and carbon. Friction material chemistry is an art form and because it's largely organic chemistry, changing even one atom of the compound can significantly change the resulting material.
As you can't make a pad with infinite gradient, you have to pick the ranges for your applications and match the material's thermal characteristics to the thermal characteristics of what you're doing. You have to make choices somewhere between the material that's soft enough to grip when cold, and the material that grips at 1200 but just slides at ambient. And you have to pick a material that gives you the braking feel you want. When choosing pads, you need to consider the hydraulics of the pedal push, the stiffness of the calipers in transferring that pressure in order to compress the pads on the rotor, the grippiness of the tire, and how the system releases when you let up on the pedal.
To convert speed you must convert stored kinetic energy to heat, over a desired timeframe. Brake people are continuously trying to shorten that timeframe, with the result that the spike of the bite in performance pads , the conversion of kinetic energy to heat over time, gets higher.
Manufacturers define the performance -- the grippiness -- of their brakepads by measuring the Mu, a function of the material coefficient of friction over temperature. Pads are said to have rising, linear or digressive rates, based on the Mu. A digressive rate pad means you have to push harder to maintain the friction of the brakepad on the rotor. A rising rate means you have to push less as the braking effect increases. A linear rate means you maintain the same pressure. The friction increases with temperature in a rising rate pad. Linear Mu means the friction value is maintained as the temperature increases. Many performance pads show rising friction values as the temperature goes up, then drop off as the temperature reaches a point where the pad outgasses, and powders, reducing the friction with the breakdown film. A very high Mu pad can spike so quickly that the wheels lock, which means the brakes then aren't working at all, as we've said.
Cheap pads tend to be digressive, which is counter to the way most people brake. They tend to let up as the braking takes effect. SBS pads are linear and sustain well. The carbon ceramic ProTrack pads from SBS have an initial linear rate and then a rising rate to counteract lessening pressure as the driver eases off in the latter portion of the braking. The new ceramic Polymatrix Q pad, designed for medium temperature have a rising rate, then drop off when the material gets hot enough to outgas.
As we said before, when pads outgas and start to dust they create a film of lubrication between the pad and the rotor that decreases its ability to stop. In addition, outgassing causes the characteristic juddering as the pads release and grab, release and grab, as the outgassing forces the pads apart. SBS puts its pads through a patented Dynamic Energy Surface Treatment (DEST) that promotes the outgassing in the final step of production (rather than on the car).
Putting gas slots in the brake rotors or cross-drilling is intended to address the problem of outgassing, as well as reducing the weight, but as I?m fond of saying, the coefficient of friction of air sucks. The tradeoff is that slotting or drilling the rotor increases the chance of uneven thermal stress. Where there are slots or holes, there is no material to grip, plus as the rotor wears those slot edges get sharper and you wind up with a cheese grater working on your brakepads.
Many of the problems resulting from heat, uneven wear and outgassing go away with transfer film pads because you?re not running on the iron of the rotor but on a layer of carbon. The transfer film has the mechanical effect of smoother engagement, less chatter, and less grabbiness because the pad and rotor are working on homogenous compatible surfaces.
The advantages of transfer film pads
At high temperature (over 800-900 degrees F) we believe the transfer film approach that Scandinavian Brake Systems (SBS), Wilwood and Performance Friction take in their track event and racing pads is a better solution. Our preference for transfer film pads comes from the benefits that accrue when the brake rotor is meeting the pad with a more continuous surface of the same material. Because of the compatibility in the interface of the two homogenous surfaces, there is a smoother engagement and release. That is, the mechanical effect of the film is to reduce chatter, reduce the tendencies to judder caused by the imbalances produced by long-term chatter, and less grab. There may even be some chemical bonding because of the homogeneity of the materials.
Once it's bedded the transfer film provides a consistently smooth layer at the interface of the pad surface and the rotor. But unless you heat your brakes to that extent with hard braking from the kinds of speeds you can generally only achieve on the track, the film does not transfer, and other advantages to materials like ceramics come into play.
Some of the street advantages to ceramics -- such Wilwood's Polymatrix Q pad and the ProTouring pads SBS is in the process of introducing -- include even heat transfer across the face of the pad and a cleaner surface on the rotor. Disc brakes are exposed to the elements and collect crud, which is not abraded without occasional heavy braking (something to keep in mind, and practice.) Ceramic pads leave less build-up.
It all comes down to choosing the brake system that best matches what you're trying to do with the car. If you're enhancing street performance, you need a system that optimizes the cold brake scenario and mitigates the problems that result from never being able to heat up the rotors. Being able to lock up the brakes doesn't mean you're getting ultimate brake performance; in fact it means you're generating even less working friction and heat.
If you're working at the other end of the spectrum, you want a braking system that gives you the control and release feel you want even as the temperatures increase, while helping you manage unwanted heat and the problems of outgassing and breakdown. Understanding a little about the mechanics of the pad and rotor interface will help you get the performance you want.
In future LEMD University topics, we'll talk about bedding performance brake systems and other elements of design. If you have questions, we welcome your emails at firstname.lastname@example.org
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