All drilled brake discs develop cracks around the holes.
It does not matter if you only drive on the road, drilled brake discs will develop cracks. If you drive on track or if you put a lot of heat into the discs the cracks will be bigger.
In short, all drilled brake discs develop cracks the only difference is the size of the cracks which depends on how you use the discs.
The best way to put a stress point in something is to drill a hole in it and brake discs are no different.
So why do drilled brake discs exist?
Drilled brake discs is an old technology from the 70s and 80s (I am speculating on the years so do not quote me but you get the idea)
Drilled brake discs were developed for 2 main reasons.
1. To evacuate the gas that was produced by the brake pads when they were hot. While brake pads gassing still happens, modern brake pads have reduced this problem massively.
2. To save weight. Drill brakes discs are not a lot lighter than normal brake discs but the weight is being taken from the most important part of the car. ie brake discs are unsprung weight and it is also rotating mass. Just like fitting lighter wheels, lighter brake discs bring benefits in acceleration, cornering and braking.
So why are things different now?
Two main reasons all related to brake pad technology.
1. Modern brake pads are able to operate at much higher temperatures than brake pads of a decade ago let alone 20-30 years ago. This has put more heat stress into modern brake discs and has exasperated the fundamental of problem of drilled brake discs ie they crack.
2. Gassing. Modern brake pads does not produce so much gas when they are hot as they once did. This has negated the need to drill a hole through the brake disc.
So what are the alternatives to drilled brake discs?
1. High quality standard replacement brake discs. Brake pads technology is so advanced now that for everything except the most extreme conditions ie on track, plain brake discs will not cause the brake system to experience fade, assuming the brake system is of appropriate power for the car (and if it is not, no brake disc of the the same diameter is going to help).
2. Grooved brake discs. Because brake pad gassing has been reduced so much with modern performance brake pads, simply grooving the brake disc is enough to expel the excess gas and maintain braking performance. Modern racing cars all use grooved brake discs now as far as I am aware. What is different, is the design/shape of the grooves used by different brake disc manufacturers.
Chris Harris has done a video comparing the M235i to the Golf R. In the video he refers to the dimpled brake discs fitted to the BMW as faux drilled brake discs implying that the dimples (just another form of grooving a brake disc) are for show only.
This is not true. The dimples in the brake disc do look like holes when they have been used but they also give the gasses from the brake pad somewhere to go. As soon as the pads passes over the dimples the gas is free to leave the brake disc. The benefits are only noticeable in extreme circumstances but there is a benefit nevertheless.
Drilled Brake Discs- Summary
The two reasons not to fit drilled brake discs are as follows:
1. Looks – You can get the same appearance of drilled brake discs with brake discs that have dimples. As the dimpled brake discs are used the surface of the dimples are coated with the black brake dust giving the appearance of the brake discs being drilled.
2. Performance – The performance of grooved brake discs is just as good if not better that drilled brake discs with none of the associated cracking problems that come with drilling a hole through the surface. Grooved brake discs will last longer and perform just as well if not better.
Or to put in another way, there is no reason to fit drilled brake discs to a car that is using modern brake pads.
The opinions in this article are the opinions of StrikeEngine and the information may or may not be correct.