Common Brake Disc Information

Brake Disc Run Out


Discs do not warp or run out of their own volition. When run out occurs, it is invariably caused by incorrect fitting of the disc, or inconsistencies in the calliper/piston.

When fitting or refitting discs at any time it is vital that the mating surfaces of disc and hub are scrupulously clean. The tiniest speck of rust, swarf, or dirt will cause run out after 3 - 4,000k’s.

Similarly securing bolts/nuts should be torqued correctly and equally. Calliper pistons should always be checked for equal performance/movement. Sticking pistons are sure to cause distortion and poor brake performance.

All the discs we sell are carefully boxed and each box carries detailed fitting instructions and checking procedures – PLEASE READ

  • Justifiable warranty claims on our discs are very rare indeed!

Brake Disc Problems and Cures

After many years experience in selling and fitting automotive brakes, we have accumulated some interesting statistics and information about why brakes shudder. First of all, eliminate the reasons

not connected with the pad and disc combination, that cause shudder.

These are imbalanced tyres and wheels, loose steering linkages, sticking caliper sliders or hydraulics, sticking master cylinder will give rise to brake shudder.

The remaining reasons for brake judder will be down to two disc related problems, which are DISC DISTORTION or LACK OF PARALLEL.

Disc Rotor Distortion [Run out when mounted on stub axle]

First of all we have to assume that when the disc is mounted to the hub that it is measured with a dial gauge and runs out perfectly true for the first day of installation. This is a critical element to how long the disc will last before problems arise, far more critical that most mechanics realise. The maximum run out acceptable on a disc is 0.15MM.
If run out above this figure is detected, remove the disc clean the hub again of any rust scale or grit and rotate the disc one bolt hole and re-inspect. This procedure of checking for run out has a critical effect on other disc problems, which will be described later.
  • Even torqueing of the nuts is absolutely essential. Uneven torque can twist or distort a rotor by a considerable amount and can result in disc run out and eventual thickness variation.
In today’s commercial world very few manufacturers sell heat-treated discs. These discs, even if mounted and checked for run out within the above limits can distort during use over a period of kilometres and give rise to vibration. This has an incident rate of about 1 in 140 discs sold.
It is sufficient for just one disc of the pair to become distorted before vibration is noticed.
  • It is interesting to note that rear wheel brake vibrations are normally felt through the brake pedal on application of the brakes and front disc distortion is shown up as steering wheel flutter.
With certain vehicles, using “wide bank” brake pads, which have a tall profile, this design of taller pads promotes a condition of “dynamic distortions”. Brake shudder is detected under heavy braking but at low speeds the shudder goes away. This is because of differential heating of the disc between the outer and inner due to the differential rubbing speeds.

The only way to avoid or minimise this problem is to use a pad with a higher thermal conductivity, i.e. a semi metallic (EBC Red grade) or EBC latest V4 (Green) brake pad with high copper content. The effect of the higher metallic content stabilises temperatures by drawing heat away from the disc, which gives rise to the fact that many German manufacturers which use these wider band pads use semi metallic pads (in spite of their huge dust problems) for original equipment. 

Lack Of Parallel [Disc thickness variation]

Lack of parallelism of the brake disc occurs, when the discs are fitted with excessive run out or that generate run out, during their lifetime. Because the pad is always touching (or first touches), the disc at the highest point maximum deviation of the run out, it gradually wears the disc thinner at the point where the pad is most often contacting. This has the effect of causing a lack of parallelism (thickness variation) of the brake disc of very small dimensions, which are sufficient to show up as violent brake shudder. Again either front or rear, depending on whether the vibration is detected on the steering wheel or the brake pedal respectively. More abrasive pads will accelerate this phenomenon, such that the lack of parallel and shudder, are detected at around 1000-15000 kilometres. Less abrasive pads may prevent the shudder being detectable for up to 6000 kilometres

Lack of parallel or thickness variation will also cause one set of pads on the axle to wear faster than others and promote dust generation. The constant rubbing of the pad on the disc even at the lightest or zero pressure will cause on set of pads to be constantly heated, surface carbonisation occurs and dust is generated.
  • Our findings re that there are no way of avoiding disc thickness variation and brake shudder unless rotors are mounted perfectly true to begin with.
It is sad to say, that in all the instances that we have inspected and monitored mechanics fitting brake discs, that hardly a single one bothers to clean the abutment face of the brake hub free from rust, scale and dirt adequately and that is quite common for mechanics to allow 0.1-0.25 mm of run out to be present, when the vehicle leaves the workshop.

This is a recipe for disaster and will almost guarantee that violent brake shudder will be the outcome within a short number of miles even if, from the workshop, the shudder was not noticed (as can often be the case) due to the run out.

Conclusions

  1.     Discs must be mounted perfectly true within 0.04mm
  1.     All ancillary parts must be checked, calliper, piston, slider, wheel balance etc
  1.     Lack of disc parallel due to poor set up run out is not covered by warranty
  1.     Disc thickness variation is the major cause of brake judder
  1.     Bad initial fitting is the cause of disc thickness variation
  1.     4 & 5 cause uneven pad wear and dusting
  1.     Disc thickness variation of the smallest values has a critical effect on judder

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