I made mention of this in some other threads and now here's the full explanation. I had seen or read somewhere on the web several years ago that so as not to lose surface area, it was best you kept the hole size no bigger than the disc is thick. To prove that to myself, I needed to do a bunch of math calculations. I found a nice, simple to use calculator for cylinder surface area on-line .....
https://www.calculatorsoup.com/calculators/geometry-solids/cylinder.php
All I had to do was plug a couple numbers in (hole radius, cylinder height) and it gave me all the area values I needed (area of the holes, area of the sides of the holes). With these, I was able to determine the ideal hole size for the disc thickness I was drilling. So, it turns out that using holes bigger than the thickness of the disc won't lose surface area as opposed to no holes until you get very big, like around 3/8" or 10mm. But, you will create the most additional surface area if you use holes the same size as the disc thickness. So, I drew up a chart, highlighting the ideal hole sizes in red. Go above or below those sizes and the amount of additional surface area you generate begins to drop off .....
Drilling your disc has several advantages. Probably the biggest and best known is improved wet weather braking. On a solid disc, in wet weather the pads will hydroplane on the disc when you 1st apply the brake, until the water is forced out. You will actually feel and notice this when you first squeeze the brake lever. You'll have very little, if any, braking force momentarily until the water is dispersed. Drill holes and you give the water someplace to go. You will also save some weight but the other big advantage is creating more surface area for better cooling. So, you want to create as much as possible.
https://www.calculatorsoup.com/calculators/geometry-solids/cylinder.php
All I had to do was plug a couple numbers in (hole radius, cylinder height) and it gave me all the area values I needed (area of the holes, area of the sides of the holes). With these, I was able to determine the ideal hole size for the disc thickness I was drilling. So, it turns out that using holes bigger than the thickness of the disc won't lose surface area as opposed to no holes until you get very big, like around 3/8" or 10mm. But, you will create the most additional surface area if you use holes the same size as the disc thickness. So, I drew up a chart, highlighting the ideal hole sizes in red. Go above or below those sizes and the amount of additional surface area you generate begins to drop off .....
Drilling your disc has several advantages. Probably the biggest and best known is improved wet weather braking. On a solid disc, in wet weather the pads will hydroplane on the disc when you 1st apply the brake, until the water is forced out. You will actually feel and notice this when you first squeeze the brake lever. You'll have very little, if any, braking force momentarily until the water is dispersed. Drill holes and you give the water someplace to go. You will also save some weight but the other big advantage is creating more surface area for better cooling. So, you want to create as much as possible.