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Drilled Rotor Hole Size?

Shocks, Springs, Brakes, Frame, Body Work, etc

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Drilled Rotor Hole Size?

Post by BOOT » Sat Mar 09, 2019 11:54 am

I'm gonna install some drilled rotors on a car this spring and it occurred to me I could prob enlarge the holes a bit(I know small difference). Then I rem, that I also have another brand of drilled/slotted rotor and when I got them out saw they have larger drilled holes than these. I also have a 3rd brake set by another brand for another project but I don't feel like digging them out atm. So now I'm wondering what is the safest hole size?

I also noticed roughly(didn't feel like grabbing a caliper) that both chamfers were about the same OD size and of course the larger inner holes had less chamfer ID.
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Re: Drilled Rotor Hole Size?

Post by FC-Pilot » Sun Mar 10, 2019 5:12 pm

What is the application? I would say leave them. One of the beauties of slotted or drilled rotors is the lack of pad glazing we see in testing. Then again we also see about a 30% increase in pad wear. You also have the added noise you get with them. If you know what it is and anticipate it you can learn to deal with it. OEM’s that have them spend a considerable amount of time playing with different hole spacing to get the least amount of noise knowing that customers will bring their stuff in for the slightest thing

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Re: Drilled Rotor Hole Size?

Post by BOOT » Mon Mar 11, 2019 9:38 am

Noise is not a concern.

Even if I enlarged the holes past chamfer and had to rechamfer with a bit of breaking loss, is also not a concern.
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Re: Drilled Rotor Hole Size?

Post by jed » Wed Mar 13, 2019 9:51 pm

What is the purpose of larger holes?
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Re: Drilled Rotor Hole Size?

Post by Dan Timberlake » Sun Mar 17, 2019 8:19 am

In one of the late great Carroll Smith books I think he talks about detailing drilled rollers to delay cracking which can be inevitable in an application that really uses the brakes, like road racing. I think it may have been using a hardened steel ball from a ball bearing assembly and a hammer to peen each hole's chamfer at the friction surface. I think seem to recall drilled rotors were generically more problematic than slotted rotors in regards cracking.

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Re: Drilled Rotor Hole Size?

Post by BOOT » Tue Mar 19, 2019 12:25 am

weight


Interesting info, not gonna use em too hard or see a ton of use just gonna drill em a bit more like my others at least before I slap em on.
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Re: Drilled Rotor Hole Size?

Post by FC-Pilot » Tue Mar 19, 2019 2:34 pm

Admittedly I don't understand the desire to drill the holes out bigger as the weight reduction will be minimal and any performance improvement minimal at best. If you want to drill them larger and don't plan on running them hard go ahead.

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Re: Drilled Rotor Hole Size?

Post by BOOT » Wed Mar 20, 2019 2:39 pm

BOOT wrote:
Sat Mar 09, 2019 11:54 am
I'm gonna install some drilled rotors on a car this spring and it occurred to me I could prob enlarge the holes a bit(I know small difference).
I know as much as I can learn and try to keep an open mind to anything!

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Re: Drilled Rotor Hole Size?

Post by FC-Pilot » Fri Mar 22, 2019 7:03 am

BOOT wrote:
Wed Mar 20, 2019 2:39 pm
I'm gonna install some drilled rotors on a car this spring and it occurred to me I could prob enlarge the holes a bit(I know small difference).
I did not mean to be a pain in my reply, I am sorry if it came off that way. What we have seen it testing is that the holes do little to nothing positive in a general application. (I have drilled rotors on one of my vehicles because they were much cheaper than solid rotor, so I am no better than anyone else on personal stuff #-o ) We do see performance gains in high performance applications as was stated earlier, it prevents the contact surface of the pad from glazing as the holes do help to shear off that top layer of pad compound that is getting hot enough to glaze and smear. Now, the thing I like most about drilled rotors is I don't have to go and drill them first before installing thermocouples. :D That saves me time in my day (not really that much time though LOL). Now, I don't like holes as it always presents an area for cracking to start. (Again, a boring daily driver would probably never see this, but in testing we see it quite often). Now, truth time, in all the testing where we have seen visible cracking start at these holes "I" have never seen it amount to anything that would be even the slightest problem. We have even tried to make the issue worse and still were not capable of creating a safety concern. (I just like being on the safe side). Now the flip side is, having a solid rotor means there is more material to ask as a heat sink to prevent uneven rotor growth at high temps and induce DTV (disk thickness variation). Back to truth again, in the aftermarket (and even some OE parts out there) the casting variation in the rotors are a bigger cause of this than the "drilled holes". We have tested some of our (I hope you are OK with me not sharing who I work for) production rotors and due to poor quality of parts have seen as much as 75*C variation in temp on the same rotor surface, just measured at different clocking positions. Yes, that is on the same rotor with the data being collected all at once during the same fade test. I myself was blown away. After having vibration issues with my old Chevy Tahoe (which I had the cheapest rotors imaginable on) I swapped to a much higher quality rotor and have not had any issue since. It is nice when you get to learn stuff on someone else's dime (or in this case 200K). :mrgreen:

I guess my main point in sharing all this is "if you have them already then run them". If you want to go through the mess of drilling them bigger then go right ahead. Some of the things that come to my mind is are the castings thicker on one than the other and might there be differences in the cooling fins as well. I may weigh them as that might give some insight one casting to the other. I personally would be concerned if the rotor with the smaller holes already weighed less. I would not want to remove any more of the heat sink as that is your safety net. Depending on which pads you use and their ability to conduct the heat away form the friction surface and also their heat tolerance, the rotors ability to manage the heat will be important.

Again, I hope I did no offend. I wanted to share this not to detour you but to share the little bit that I have learned myself. Also, knowing that forums like this stick around I figured it might help someone down the road. Heaven knows I did not know any of this stuff 10 years ago. Have a good night (or morning now) and I hope the brake job performs well. Now I have to get back to my sick kid as that is what has me up at "stupid-O-clock" in the morning. :cry:

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Re: Drilled Rotor Hole Size?

Post by BOOT » Fri Mar 22, 2019 11:49 am

NP your fine I just want to be clear and efficient, while some post just go round and round due to people not reading the earlier material and I am not perfect in that either.

Cheaper rotors are lighter due to less material(cost) being used, why they warp easier I've always assumed, less heat sink material as you put it. My research for braking showed slots are better for even pad wear or slotted and drilled. I think anyone who expects better braking and long life is asking too much.

My double drilled rotors are just for weight reduction at the cost of pad life and some surface area/braking, they actually weigh more than cheaper rotors by a bit. It'd be nice to be able to compare the rotational inertia but I'll just assume the drilled has a bit less. I may have some unneeded mass also machined off but we'll see what the estimate is. I wouldn't suggest anyone who dailys do a lot of the things I do and the low use is why I get away with some of it.
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