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Technical Bulletin - Friction Forum

2014-03-10 09:28:40

If you’ve ever seen a block of ice in a glass crack suddenly when a warmer liquid is poured into the glass, then you’ve witnessed a perfect demonstration of stored internal stresses being relieved suddenly through cracking – the same process that happens when a brake disc becomes warped.

 

Stresses in the ice are formed from the rapid cooling of the water in a freezer into a solid form. If ice were to be warmed slowly it would not crack. It’s the sudden change of temperature (or thermal shock), caused by ice coming into contact with the warmer liquid, that causes the internal stresses to relieve themselves by cracking.

 

Similarly, during the casting process used when making a brake disc, molten iron is poured into moulds at temperatures in excess of 1350°c and allowed to cool rapidly into solid form. This rapid cooling causes internal stresses to be stored within the disc. Gradual and moderate thermal cycling (as is experienced when bedding-in discs properly after fitment) can effectively relieve these stresses but sudden, dramatic temperature changes can lead to distortion or cracking of the disc.

 

Regardless of the brand, it is vital to communicate a moderate and progressive ‘bedding in’ process to your customers who, in turn, should advise the consumer.

 

Fact File:

-   All brake discs have the capacity to warp if the ‘bedding in’ process is ignored.

-   Following a moderate and progressive ‘bedding in’ process is the only effective method of avoiding the thermal shock that leads to disc distortion and cracking.

-   New discs should be ‘run-in’ just like you would a new car - no excessive braking for the first 200 miles.

 

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