Are you sure that the judder you feel under your brake pedal or coming back through your steering is actually warped brake disks? We have quite some experience with this and specialised equipment used to measure brake disk “run-out”, the technical name for a warp in your brake disk and in most cases the disks are as true as the day they were made. Read on for our top-tips on reliable judder free brake disk installation.
One of the DIY jobs that we see many of our customers tackle is brake pad replacement. Quite often this is a job which is rolled in with a disk brake rotor change though in our experience, not quite often enough. We have seen some spectacular examples! That said, it is a very simple task in fact and most people who have the confidence, knowledge and experience to change the brake pads can change the brake disks also. If you are changing your brake disks because your old ones were warped then pay attention very carefully. If you have never done this job before, then pay attention very carefully!
Our guys have been to see the production facilities in Ate where the finest quality brake disk rotors are manufactured and where the tolerances used in production are so small that the unit of measure is microns. The disks that you will find on your production car are also engineered so thoroughly that only a serious bashing on a racing circuit or a seized brake caliper is likely to produce the heat necessary to buckle or warp a brake disk. This is especially true of modern “ventilated” brake disks which are structurally very strong indeed. We have seen brakes tested to destruction on a dynomometer and it takes a truly incredible amount of abuse to cause permanent damage!
So if your brakes are not warped or buckled why do you feel a judder in the pedal or steering? In most cases this is a very simple installation issue and when we say most cases actually we mean all cases bar the very unusual outlier. The issue is 95% of the time a simple case of poor preparation and cleanliness during assembly. This is normally a build up of corrosion, contamination, brake dust and often crud from wheel cleaning which finds its way between the brake disk and the hub flange. The new brake rotor installed on top of even a few microns of grit or corrosion will run out of true and because of the distance from the hub to the braking surface of the disk, this error is multiplied. Have a look at this simple illustration below that shows the brake disk and caliper all parallel with no run-out.
In a typical installation with a “fist caliper” where a caliper on sliding pins with a single hydraulic piston automatically centres itself on the brake disk every time the brake is applied. This system is brilliant in its simplicity but suffers badly when the disk is fitted over contamination. The “out of true” braking surface will nudge the brake caliper inwards, then outwards every 180º of wheel rotation. This means that the inside and outside surface of the disk will push the caliper in and out typically 900 times every kilometre you drive. Have a look at these two illustrations below to see the way the caliper is nudged by alternate sides of the brake disk rotor every 180º.
You are correct if you are thinking that this “nudge” that the disk gives the caliper is only a small fraction of a millimetre. You are not correct in thinking that the high spot will wear down and it will stop, in fact every time you press the brakes the caliper just finds the average centre and the wear continues. From our experience the issue is not apparent for some months, typically 5,000km or more depending on the vehicle and it’s suspension and steering design. This image below might give you an idea of where the wear would be on one brake surface, of course it will be similar on the reverse side of the disk 180º apart. What appears to be a warped brake disk is actually the thickness of the brake disk changing every 90º of wheel rotation as the pads drop into and climb out of these thin sections of disk. The difference is usually only a small fraction of a millimetre but it is enough to cause quite a judder through the car!
In cases where a brake rotor is not returned within a week this is almost always the case. Are there other causes? Well yes, an impact to the wheel or tyre with a kerb, pot-hole or during an accident can damage the wheel hub and this has the same effect but we see this very very seldom. Most cases however when tested on our comparator show these distinct thin areas of brake disk opposite each other and on the opposite bake disk surfaces and no other fault suggesting poor installation practice.
Is there a way that we can be sure our installation is sound? Yes, there is and it is as simple as cleaning carefully and a little corrosion protection. Have a look at these two cleaning tools from our Hazet range, both using an abrasive Scotch Brite® pad to polish the hub face.
In the case of a hub flange using wheel studs we have the self centring Hazet 4960-45/14 which you place over each stud to clean the hub face.
For hubs that use bolts then the Hazet 4960-160 centres on the hub/bearing cover and polishes the seating area for the brake disk.
We recommend a very light film of “Ate Plasti Lube” brake grease is applied on the clean metal surface to keep it corrosion free and of course once clean, a little of this lubricant on the caliper slide pins and the caliper bracket where the brake pads bear will keep things working smoothly, quietly and reliably.
Be sure to ask for cleaning and lubricating products when you buy your brake disks from OTTO, our trained staff are always happy to help!