When Changing Brake Pads

Although it is possibly the most important (safety wise) and most frequently used application in getting from A to B, a driver will and should notice the slightest change to their driving comfort and vehicle performance with their brakes.

Unfortunately as any reduction in performance is often gradual, the change becomes more noticeable often only in an emergency situation, or when a component gives out it‘s own warning signs of pending failure.

 

Unfortunately in Australia anybody can work on a braking system. As brake repair specialists, we need to understand and spend a lot more time, in giving a professional and safe service to our customers.
The most misunderstood complaint from the person behind the wheel when operating under brakes is “steering wheel shimmy or shudder” through the car.
The two most common reasons/causes of this are:
  • 1.BRAKE ROTORS WARPING
  • 2.BRAKE PAD MATERIAL BUILD UP.

 BRAKE ROTORS WARPING

 If on the first couple of brake applications during road test, shudder or shimmy is experienced, then there can only be a few reasons/causes:
  • A) Something is lodged between hub and rotor, ie dislodged surface rust or foreign substance is stopping the rotor and hub sitting “true” to each other.
  • B) The Brake Rotor has been machined during manufacture incorrectly or damaged during transport prior to fitment, (highly unlikely it will be both).
lf the road test is good with no shudder or shimmy and that appears at a later date, then there is only one way this issue can be caused = heat.
The misconception here is we are not referring to just excessive heat, but too greater change in heat, from moderate/high down to cold, (river crossings and car washing etc).
Braking involves friction between the brake pads and rotors. When the friction is excessive, so is the heat. Average around town driving will not create excessive heat, even in the early days of bedding in, but braking for sustained periods will. A small or average hill will not cause issue, but coming down a mountain or lesser hill with a trailer certainly will. Riding the brakes for long periods of time will cause considerable heat build up. Sustained braking in these circumstances will cause the rotor material and pad material to get very hot and with that, softer, than at a lower operating temperature. If at the end of this braking, you are at a complete stop with brakes applied, the majority of the rotor is cooling at a quicker rate than that section of the brake rotor that is squeezed between the hot brake pads under hydraulic pressure.
 

BRAKE PAD MATERIAL BUILD-UP 

The least understood, but most common cause for experiencing brake shudder or shimmy, is due to the build-up of brake material on the brake rotor face. Transferring pad material onto the rotor face is an important part of bedding in the pad and rotor faces, to ensure optimum performance, over the life of the brake components. Poor bedding in, or not bedding the material surfaces to each other at all, will lead to greater driver dissatisfaction. We are all well aware of the importance of not having finger prints or grease on the rotor surface, as it creates high spots, which is a build-up of material due to one area reaching higher temperatures than the material around it.

Many drivers overheat their brakes without even knowing it.

In fact you can overheat standard pads and rotors without ever feeling brake fade. Long gradual stopping, hill descents, and general bad driving habits are all causes of overheating.
When brake pads reach temperatures outside of their designed operating temperature range (this is the same for performance & standard pads) the compounds they are comprised of, act unpredictably by depositing themselves onto brake rotor surfaces, unevenly.
  • Example;- good driver (person who doesn't abuse their brakes) but due to brakes recently being changed is; driving on a multiple lane road 80 kms limit in moderate traffic, braking lightly well before traffic comes to a stop, (reading the lights well ahead). Once the pads contact the rotors with initial braking pressure, temperatures begin to rise. The gradual even braking is applying heat to the system causing the heat to keep increasing. By the time you get to a full stop, odds are that there has been no lift from the brake pedal or, at least not for long and heat has been maintained in the rotors over the entire time. The brakes are now very hot and border-line overheating.
Now at a stop, your foot is holding the hot pads in one place with pressure and most drivers use more pressure when stationary, than is required. Now with everything high in temperature and in constant contact, brake pad material is transferring to one spot on the rotor surface. If this continues, that area is heating up quicker and quicker, as the uneven pad material is increasing and eventually a shudder will develop. If stationary for an extended time or parked, explained earlier warping will occur and shudder (shimmy) will appear on the next braking application, in the front end of the vehicle.

 

A correct bedding in procedure is under valued by many repairers (some think it is a waste of time), but if done correctly, it will eliminate these issues and complete a professional brake and rotor service .All brake pads must be bedded-in with the rotor they will be used against to maximize brake performance. The bedding-in process involves a gradual build up of heat in the rotors and pad compound. This process will lay down a thin layer of transfer film onto the brake rotor surface. See technical bulletins 5 & 8 on our website for the correct and recommended bedding in procedure. When bedding in a car try and do it without stopping completely or braking longer than a 3-5 seconds, allow a non braking period between applications for cooling, and before parking at conclusion drive for a longer time without braking, to maximise cooling before brake components become stationary.

In summary when installing new brake rotors and brake pads:

  • When installing new pads, the rotors should be new or at least resurfaced to remove any transfer film from the previous set of brake pads.
  • If not, incompatible brake materials will cause an uneven build up of material and can cause glazing of pad and rotor, leading to poor stopping performance and brake shudder (shimmy).
  • It is critical that the installer clean any rust, scale, or debris from the hub mounting surface thoroughly and clean all preservatives from the rotor itself.

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