During my run along the straight recently during the ‘Le Mans Classic’ I was doing over 125mph ‘four up’ when suddenly the chicane came up, some very hard braking was called for, this hard braking continued throughout the run, but very quickly a judder was evident during braking. This had also appeared during the Castle Combe runs. Now I know you don’t always use such aggressive braking but it did show up a deficiency in my brakes and you never know when good brakes are required.

My investigations led me to PowerStop Brakes Ltd (01608 646837) who came up with the following: -

“After twenty-two years experience selling automotive brakes, EBC have accumulated some interesting statistics and information about why brakes judder. First of all let us eliminate the reasons not connected with the pad and disc combination that can cause judder".

These are imbalanced tyres and wheels, loose steering linkages, sticking calliper sliders, hydraulics or sticking master cylinder. These are mentioned in order of likelihood of their occurrence and it is fair to say that about 3 out of 10 brake judder complaints will arise from the above 4 effects.

The remaining 7 out of 10 reasons for brake judder will be down to disc related problems which are: -

•  Disc Distortion or Lack of Parallel;

•  Disc 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 from the first day of installation. This is a critical element to how long the disc will last before problems arise, far more critical than most mechanics realise. Some disc manufacturers suggest a tolerance of 5 /1000ths of an inch, which is way too large for this measurement. The maximum run out acceptable on a disc is 2/1000ths of an inch. If run out above this figure is detected, remove the disc, clean the hub once again of any rust, scale or grit and rotate the disc one bolthole 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 wheel nut is absolutely essential. Uneven torque can twist or distort a disc by a considerable amount and can result in disc run out and eventual thickness variation as described later on.

It is interesting to note that the rear wheel disc vibrations are normally felt through the brake pedal on application of the brakes and front disc distortion is shown up a steering wheel flutter. The only cure for a distorted brake disc is to have the disc re-ground or replaced.

With certain vehicles, using ‘wide band’ brake pads, which have a tall profile, promotes a condition of  ‘dynamic distortion’. Brake judder is detected under heavy breaking but at low speeds the judder 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’s latest V4 (Green) brake pad with high copper content. The effect of the higher metallic content stabilises temperatures by drawing heat quickly 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)

We start off by assuming that all discs are perfectly parallel when they are produced. Lack of parallelism of the brake disc, however, can develop with discs which 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 judder. Our findings are that there is no way of avoiding disc thickness variation and brake judder 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 it is quite common for mechanics to allow 5-10/1000ths 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 judder will be the outcome within a short number of miles, even if, from the workshop the judder was not noticed due to the run out.

 Conclusions

1. Discs must be mounted perfectly true within 2/1000ths of an inch.

2. All ancillary parts must be checked, calliper, piston, slider, wheel balance etc.

3. Lack of disc parallel due to poor set up run out is not covered by warranty.

4. Disc thickness variation is the major cause of brake judder.

5. Bad initial fitting is the cause of disc thickness variation.

6. Items 4. and 5. cause uneven pad wear and dusting.

Disc thickness variation of the smallest value has a critical effect on judder.