Do drilled rotors measurably improve braking?

[fredintoon]

Drilling is good.
The hole edges scrub the pads clean and if the brake disks look like gramaphone records they'll improve after the bike has done a few hundred miles.
There's a weight loss too, but honestly, it ain't much.
The bores add more surface area than the holes remove.
Smaller the hole, the more you gain.
0.42" holes are break even, larger than that and you lose.
And yeah, it looks neat.

 

[arcticXS]

This is a great thread, some really good in-depth technical discussion, mixed with personal preferences and experience. Most modern discs are drilled, but are also thinner, mostly 4 or 5 mm thick. This changes the relationship between surface area removed vs. suface area gained. I assume Fred based his numbers on the stock "boat anchors", which are 7 mm thick.
One aspect I did not see mentioned, was the friction properties of the disc itself. These definitely vary a lot, from stock stainless discs to cast iron and other steel alloys. The XS discs are poor, just like all other from that era. Newer cast iron or steel ally discs will have better friction properties, especially when matched with a suitable pad material.
When changing from OEM stainless Brembo disc to a steel alloy ISR disc on my 1995 Ducati, the brake improved significantly.
So, for any real improvement in braking power, as well ar reduced weight, a modern, lightweight disc made of a suitable material is the way to go.
(drilling the stock disc, and painting the centre portion gold still does not make it a Brembo...)

 

[jetmechmarty]

If you fit the bike with a nearly 2 lb rotor, you'll save over 4 lbs unsprung weight. That should make a noticeable seat-of-the-pants improvement.

 

[XSLeo]

There area few points that weren't explained well. On cooling drilling holes smaller in diameter than the rotor is thick increases the cooling by increasing the surface area. Holes bigger than the thickness decrease surface area thus decrease cooling.
You often see rotors with holes too large and too many. This makes the surface area much smaller thus increasing heat and decreasing effective braking.
Many new sport bikes use rotors with what appears to be not much surface area on them. These rotor are larger in diameter, this increase the leverage of the rotor to wheel, increasing braking. They also use very strong calipers, 4 or 6 pot. Thus make the pads squeeze the rotors harder. They also use much different pad compounds than our pads.
I don't think these high tech compounds will work well on our low power brakes. They take more pressure to work.
On cleaning, as the brakes are used the rotors wear, these bit of metal imbed in the pads. This decreases the effectiveness of the pads by preventing the entire pad surface from contacting the rotor. The edges of the holes or slots should be sharp. These sharp edges clean the bits of metal off the pads.
Leo

 

[kevski]

The main problem with our bikes that have original equipment is the rotors themselves, and that problem is the fact that they are stainless steel which although nice and shiny it contains chrome, so if we all put modern non stainless rotors on and refurbish our braking systems our braking would be better, also modern drilled and wavy rotors have increased surface areas which aid cooling when put under harsh use.

 

[zmaxmotorsports]

Drilling is good.
The hole edges scrub the pads clean and if the brake disks look like gramaphone records they'll improve after the bike has done a few hundred miles.
There's a weight loss too, but honestly, it ain't much.
The bores add more surface area than the holes remove.
Smaller the hole, the more you gain.
0.42" holes are break even, larger than that and you lose.
And yeah, it looks neat.

You completely lost me with the you gain more than you lose part,You lose a little surface area for the pads to grab.

 

[zmaxmotorsports]

The main problem with our bikes that have original equipment is the rotors themselves, and that problem is the fact that they are stainless steel which although nice and shiny it contains chrome, so if we all put modern non stainless rotors on and refurbish our braking systems our braking would be better, also modern drilled and wavy rotors have increased surface areas which aid cooling when put under harsh use.

Ive seen a whole lot more warped sport bike rotors than the older heavy factory rotors.

 

[kevski]

Ive seen a whole lot more warped sport bike rotors than the older heavy factory rotors.

That is the nature of building to a cost, hence why a lot of sport bike riders change to higher spec brake rotors and calipers, and our old rotors are so thick they are difficult to warp, I still have the old rotors on mine, my calipers and MC have been fully rebuilt and braided hoses used, and naturally I ride and brake according to the braking ability of the fitted components.

 

[jabcb]

Another point worth noting in the stainless vs. non-stainless debate…

Cars need a lot more stopping power because they are a lot heavier.
Car brake rotors rust if you leave the car sit for a while.