Topic: Drilled discs

[thelowmax]

I've read a lot of opinions on the brakes of the cb750, both front and back, drum and disc, and there seems to be some debate about the shortcomings of both systems. Though I do admit to being a bit of a speeder, I'm not a racer and drive with safety in mind, so I don't exactly push my bikes to the limits. Personally, in all my years I have never had any reason to question their effectiveness or reliability.

I have a duel front disc conversion in the works on a 77k and have all the parts from a F for a rear disc conversion that I may put on a 76k if I get the time.  Because I own several cbs, any augmentations I make are usually more for variety, aesthetics, or simply to get around paying full price (some would say I'm thrifty. I say cheap.). Also, I like to use each project as another learning guide, like when I bought a media blaster to strip and clean parts, or the powder coat system I use on small parts. Ultimately, I want to get a drill press and drilling rotors would be a good project to learn on.

So, I'm planning a build and was wondering about peoples thoughts on drilled disc brake rotors. I would be doing it for mostly aesthetic reasons because, again, I think the brakes work fine as is. And the weight lost is negligible, as far as I'm concerned. So… In you opinion, what are the pros and cons?

Here's what I'ld like to drill.

Too much?

[catsoup]

Way to close to the edge with the large holes I think. Also doing that many holes and at three/four different sizes is going to drive you (or someone else) crazy

[Steve_K]

I think you have some 224 holes in the rotor!!  My thought is  that is too many.  I would not personally  drill more than 108 and stay close to 5/16 diameter on the holes.   I am sure other people will give some opinions and you could decide from that.  That many holes look cool, but could the rotor be prone to cracking under use?

BTW, buy good quality drills and if you can get stub drills that would be better.  Slow RPMs (no more 600 RPMs for 5/16 drill) and cutting fluid will be you friend.

Steve

[70CB750]

I used cobalt bits from Home Depot - I had 2 and two discs to drill with less holes.  The bit worked well enough, but it helped to touching them up in drill doctor every so often.

The discs make some nasty sharp shavings too and it was not easy to center punch for the hole location.

[RJ CB450]

Do not drill rotors on your own!

Okie, bit of past history.  When drilled rotors started showing up, many aftermarkets tried to copy.  And those early brake companies didnt succeed because the heat cycles caused cracking.  Not as much with bike rotors as cast car ones.  In bikes, warping was a huge issue from poorly drilled..  Drilling involves more than just asthetic.  If you want drilled, best leave it to pros.

Edit: One bit I forgot.

Specific heat capacity.  Remember, brakes work by converting kinetic energy into thermal.  So take out 10% of your material, they will get 10% hotter and unless you are carbon ceramic, hot and overheated brakes dont work well.  Drilling can allow faster cooldown, but until you let off the brakes, they get hot!

[lucky]

I used cobalt bits from Home Depot - I had 2 and two discs to drill with less holes.  The bit worked well enough, but it helped to touching them up in drill doctor every so often.

The discs make some nasty sharp shavings too and it was not easy to center punch for the hole location.

 Do not center punch for the holes.
It is 400 series stainless and when you punch the metal it work hardens the metal and just makes it hard on the drill bit.
A cobalt bit is correct, but if you grind the drill bit, you will have lost the cobalt edge and only have M38 steel to drill with. The cobalt is only about .010 thousandths deep.

[70CB750]

I am afraid you are confusing Titanium nitride coating with cobalt steel alloy.  The coating (looks gold) is indeed very thin and once you resharp, it's gone.

Cobalt steel alloy is high speed steel that in addition contains cobalt for to improve hardness and higher temperatures resistance.

[thelowmax]

I think you have some 224 holes in the rotor!!  My thought is  that is too many.  I would not personally  drill more than 108 and stay close to 5/16 diameter on the holes.   I am sure other people will give some opinions and you could decide from that.  That many holes look cool, but could the rotor be prone to cracking under use?

BTW, buy good quality drills and if you can get stub drills that would be better.  Slow RPMs (no more 600 RPMs for 5/16 drill) and cutting fluid will be you friend.

Steve

 I agree. That's a lot of holes.  The largest holes are 5/16. One of the reasons I like this design is because the graduated holes leave the exact same space between the holes. I think that aids a more even heat dissipation. (though, I could be just making that up, because it sounds good). Not sure how that would effect any potential to crack.

Again, this is more of a learning process and experiment. I have a bunch of these discs so I can always bail out if it seems sketchy (the arduous process, as well as the disc.)