Why are discs cross-drilled?

[Skunk]

I've had my disc made for my Bucket. I got curious as to the actual purpose of the cross drilling and I found this. So for anyone else looking or curious I give you:

Many higher performance brakes have holes drilled through them. This is known as cross-drilling and was originally done in the 1960s on racing cars. Brake pads will outgas and under use may create boundary layer of gas between the pad and the disc hurting braking performance. Cross-drilling was created to provide the gas someplace to escape.

Although modern brake pads seldom suffer from outgassing problems, water residue may build up after a vehicle passes through a puddle and impede braking performance. For this reason, and for heat dissipation purposes, Cross Drilling is still used on some braking components, but is not favored for racing or other hard use as the holes are a source of stress cracks under severe conditions.

Discs may also be slotted, where shallow channels are machined into the disc to aid in removing dust and gas. Slotting is the preferred method in most racing environments to remove gas, water, and de-glaze brake pads. Some discs are both drilled and slotted. Slotted discs are generally not used on standard vehicles because they quickly wear down brake pads; however, this removal of material is beneficial to race vehicles since it keeps the pads soft and avoids vitrification of their surfaces.
On the road, drilled or slotted discs still have a positive effect in wet conditions because the holes or slots prevent a film of water building up between the disc and the pads. Cross drilled discs will eventually crack at the holes due to metal fatigue. Cross-drilled brakes that are manufactured poorly or subjected to high stresses will crack much sooner and more severely.

Disc brake discs are commonly manufactured out of a material called grey iron. The SAE maintains a specification for the manufacture of grey iron for various applications. For normal car and light truck applications, the SAE specification is J431 G3000 (superseded to G10). This specification dictates the correct range of hardness, chemical composition, tensile strength, and other properties that are necessary for the intended use.

[Disco Dan]

Great post! Thats very interesting to know!

...shame I have the "stress cracks" on both my front disks from high speed braking.... heheh

[twinkle]

...shame I have the "stress cracks" on both my front disks

What comes after the stress cracks? Seized front wheel?

[Disco Dan]

What comes after the stress cracks? Seized front wheel?

Yeah.... if they go they bang they will go bang under braking due to the heat.... which means all of a sudden the front disks start to pull themselves apart... locking against the caliper and seizing the front wheel.. solid. then its superman time....

[Freakshow]

Yeah.... if they go they bang they will go bang under braking due to the heat.... which means all of a sudden the front disks start to pull themselves apart... locking against the caliper and seizing the front wheel.. solid. then its superman time....

I am guessing you are looking forward to this???

[Coldrider]

I have never heard of a disc actually breaking up, but maybe someone has a story they can share with us.

[cowboyz]

Have seen a disc fall apart on a XR400 farm bike. But then it was a farm bike and maintance is an evil word.Didnt lock anything up. Just fell to pieces.

But we are talking about speeding along at probably close to 40-50-60km/hr on a farm compared to speeding along at 110-120-220 on the road.

[yod]

....compared to speeding along at 110-120-220 on the road.

ahem...sorry...what?

[Pancakes]

Had it in an old FWD rally car, rear right disc popped after a fast section then into a water crossing. It made a kindof spiral shape so half the disc width was out each side (if that makes sense). That just locked instantly. I was in the passengers seat and the driver was on to it and just locked all the other wheels so we just carried on onto some open grass. Wouldn't reccomend it on a bike!

[tri boy]

I think it is very rare for modern discs to fracture to the point of structural failure. Most likely to happen in extreme race conditions, (any racers out there ever experience this?).
Stainless of high grade is also a common disc material. (The Scrambler did manage to warp one though).

[imdying]

I've had a little to do with brakes, the only discs I've seen that have failed structurally were ones that were abused to the point of neglect.

[Pancakes]

Yeah, in the case I mentioned above the brakes had machined thinner than they should have and the hot disc into some cold water topped it off.

[Skunk]

I've broken front front discs on my old race car. They were ventilated disc though, not cross drilled. Pulled into the pits to change the pads and the disc fell apart as the wheel was removed.

[rwh]

I guess the fancy 'petal' type disks on newer dirt bikes are for similar reasons? To get rid of the water? Would they help get rid of grit as well?

Richard

[GSVR]

You'd be hard pressed to find a modern road bike with grey iron disks. They are made of alloy steel which has little chance of cracking up. Strangely enough though some race bike disks are cast iron becuase it had alot better braking/friction properties. Moto GP bikes use carbon brakes which are completely different as they have to get hot to start working well.

I always thought the cross drilling was to provide a greater surface area and turbulence for cooling.