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Thread: Drilled vs slotted vs solid brake disks

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    MadCat360's Avatar
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    Drilled vs slotted vs solid brake disks

    Last week I noticed that my engineer had put solid brake rotors on my kart, when we had been using drilled rotors (and the competition was as well). He said it didn't make a difference, and that they were from the same manufacturer and not to worry. I really tried to feel a difference, but I just couldn't. Not in braking force or fade or anything. But they did feel a little more numb, but only slightly, and probably wouldn't be noticed at all if we were using power assist brakes.

    I'm wondering, then, why you'd drill and/or slot rotors?
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    Captain Volvo _HighVoltage_'s Avatar
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    By drilling the rotors (discs) you create a larger overall area, which helps cooling to some extent. Same principle as the radiator - more thin ribs, better than few thick ones.
    There is a downside however. Although, you are increasing the cooling area, you are actually decreasing the friction area (the are where the pads touch the rotors).

    So in the end - you have to find the right balance between the two. The reason because you are feeling them numb right now is because they are brand new. That's always a problem with new brakes. They will reach their full stopping power in about 60-70 miles.
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    I LUV MY PRIUS!!! the Interceptor's Avatar
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    I think the difference in cooling is small. Mostly, drilled and slotted discs give you better braking capabilities when they're wet / dirty, since the surface is cleaned much faster when the dirt / water can go somewhere. Otherwise, only larger discs or pads with a better friction coefficient will provide better braking.
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    I'm not Moe bartboy9891's Avatar
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    http://www.carbibles.com/brake_bible.html

    Read the short paragraph on "rotor technology" near the top.
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    Seems like it would have more to do with fade than with stopping power. Brakes stop working well when they start to get cooked, and slots/holes seem like they'd help with that...it'll still happen, just putting it off for a bit.
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    MadCat360's Avatar
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    Quote Originally Posted by the Interceptor View Post
    Mostly, drilled and slotted discs give you better braking capabilities when they're wet / dirty, since the surface is cleaned much faster when the dirt / water can go somewhere.
    Oh no! That gives me a lot of confidence in my engineer now! It was scheduled to rain last weekend! Thankfully it didn't. :p


    Quote Originally Posted by bartboy9891 View Post
    http://www.carbibles.com/brake_bible.html

    Read the short paragraph on "rotor technology" near the top.
    Oooo that's very nice. Thanks.

    We run dual rotors so I'm not sure if drilling them would make much of a difference cooling wise. They don't have a cover of any kind and they're right on the rear axle so they get plenty of air. They didn't seem to fade or anything. They did seem hotter at the start of the race, though, going into turn 1.
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    When you brake hot gasses are formed between the pad and the rotor. Slotting and/or drilling is actually less for cooling and more to give that gas somewhere to go (and it generally looks cooler than a plain rotor ). From what I've seen, I would go slotted rather than drilled, because at least some drilled rotors have a tendency to crack under stress. But it looks like all that is mentioned in the brake bible link above too.

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    Quote Originally Posted by 73GMCSprint View Post
    From what I've seen, I would go slotted rather than drilled, because at least some drilled rotors have a tendency to crack under stress.
    Yes especially in a kart if you happen to hop the curbs you bang up the brake rotors pretty good in the rear.

    That said I've seen a lot of school karts that have some pretty big dings in them and they seem to hold up for a while. I used to hop the curbs a lot and I never had a disk explode or anything, but I've heard of it happening.
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    i am not sure about on Karts but on road cars most brake pad material (even on high performance pads) no longer gases like it used to so slotted and drilled rotors do next to nothing as far as gas evac goes. Also the slots/holes can scrape more material off the pads so they may wear faster than on a smooth rotor
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    Hedgehog Sandwich Matt2000's Avatar
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    Right I just want to clear this up. Why is it that people fell the need to use the word 'rotors'? That type of brake is called a disc brake, so therefore that big round bit is the disc.

    Calling it a rotor makes things unnecessarily complicated, especially when the two words are used in the same post to mean the same thing. Seems to be an American thing, but you knew that.

    Mini-rant over.
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    Quote Originally Posted by matt2000 View Post
    Right I just want to clear this up. Why is it that people fell the need to use the word 'rotors'? That type of brake is called a disc brake, so therefore that big round bit is the disc.

    Calling it a rotor makes things unnecessarily complicated, especially when the two words are used in the same post to mean the same thing. Seems to be an American thing, but you knew that.

    Mini-rant over.
    Americans make up their own words for everything, like "hood" and "aluminum" etc. etc.

    I don't really have a problem with it though, in fact I catch myself using some of those words on occasion. I do, however, have a problem with "shock absorber." Terrible term. It's called a DAMPER


    Anyway . . . what The_Finn says is on the money. Drilled rotors discs are old technology and not neccessary anymore. Add in the fact that they wear badly/unevenly and have a tendency to crack, I would much rather use solid ventilated discs maybe with small slots depending on the application.

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    Hedgehog Sandwich Matt2000's Avatar
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    Thought you were going to give me a roasting when I saw a reply, glad you think the same way. I do actually use 'shock absorber' though so I am partially guilty.
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    In the US, we call them rotors. Over in England, you call trucks "Lorries". They're not "lorries" they're trucks.

    A semantics debate is useless. you call your trucks lorries, we'll call our brake rotors rotors.
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    Quote Originally Posted by InfernalVortex View Post
    In the US, we call them rotors. Over in England, you call trucks "Lorries". They're not "lorries" they're trucks.

    A semantics debate is useless. you call your trucks lorries, we'll call our brake rotors rotors.
    I call it a semi. Does that make me a communist, or an adult advice columnist?
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    Quote Originally Posted by NecroJoe View Post
    I call it a semi. Does that make me a communist, or an adult advice columnist?
    Semi refers to a type of trailer; so if you called a box truck a semi, you'd just be wrong, but referring to a tractor-trailer as semi is acceptable.
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    No, a box truck is a box truck...if it's a "hinged" tractor-trailer, it's a semi.
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    I've got a hinged tractor-trailer, if you know what i mean!

    Drilling discs reduces the metal mass used to absorb heat from the pads, reducing cooling (it outweighs the increase in surface area). The only instance you would need to run drilled discs is in endurance racing, where slots would wear away.

    Slots keep the pad surface fresh, cutting off any glazing or muck that accumulates (whilst slightly increasing pad wear). If you wear the discs down considerably, the slots would disappear, and you'd lose that advantage.

    On a gokart, straight, solid discs is all you'd need, unless you went offroading. On a road car, i'd definitely go slotted. On a Lemans entrant, drilled.
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    Quote Originally Posted by The_Finn View Post
    i am not sure about on Karts but on road cars most brake pad material (even on high performance pads) no longer gases like it used to so slotted and drilled rotors do next to nothing as far as gas evac goes. Also the slots/holes can scrape more material off the pads so they may wear faster than on a smooth rotor
    My slotted rotors tear through pads faster than I ever thought possible.
    Or maybe it's my frequent trips to the mountains to play.
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    Here's something I wrote up on another forum a while back regarding brakes.

    Quote Originally Posted by Polygon View Post
    This is something I typed up at another forum.

    1. Warped Rotors:

    There is only one way a rotor can warp and this is if there is uneven torque distribution across the hub. If you are experiencing pedal pulsation and have even torque distribution then your rotors have a build up of pad material. This is usually caused by improper bead in. Have the rotors turned and re-bead your brakes.

    2. Big brake kits will help my car stop better:

    This is usually not the case for two reasons. And most of the time your braking distance will increase. Most big brake kits are only for the front rotors. You might think that your rear rotors doing almost nothing, but you'd be wrong. The problem is that the pressure is not proportioned correctly from front to back. You need to get a kit for all four corners that is proportioned or a set with an adjustable proportioning valve. The other reason is that some kits include rotors that are meant for track use. This means that you will never be able to get them to operating temperature on the street and thus they will be ineffective.

    3. Cross-drilling will help my car stop:

    Cross-drilling is a marketing ploy. Cross-drilled rotors are a complete waste of money. Cross-drilling was created about 40 years ago in racing. It was created to combat out gassing. This was a problem that occurred when the pad would heat up and release a gas. This would cause the pad to float over the rotor. The drilled rotors allowed the gas to escape to the vanes and then dissipate. Fast forward to today. Out gassing was a pad problem, not a rotor problem and no modern pad should out gas. A lot of people confuse out gassing with fade or inflating flex lines. That is one reason you don't see any race cars using them anymore.

    Now there are also problems with cross-drilled rotors.

    3A. The have a tendency to crack and more so if they're actually drilled. Good cross-drilled rotors are cast that way.

    3B. Unless you get bigger rotors you will increase your braking distance. Rotors are heat sinks. They are designed to convert mechanical energy into heat energy and dissipate it. Since you have removed material by drilling you have reduced how much heat the rotor can absorb. It will then transfer that heat to your pad, calipers, lines, and then fluid. Once the fluid boils you will get brake fade. Also, since you've removed material you have reduced the contact patch of the pad on the rotor, reducing it's effective friction surface. I understand the physics behind the whole cooling surface argument. While that is correct logic, that is hardly enough surface to make up for the other two issues.

    3C. If you want to improve braking then I suggest you get slotted rotors. They will improve initial bite but they will chew up pads faster.

    4. SS brake lines will improve my braking distance:

    Stainless Steel flex lines will not make you car stop any faster. What they do is prevent them from inflating during braking which will cause your brake pedal to feel very mushy and can make your brake hard to modulate.

    If you want a good formula for brakes for a street car then stick with some good vented blanks, SS flex lines (if your current lines are old or damaged), ATE super blue fluid, and some good pads.
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    As i stated though, drilled discs (well, either cast or properly "boeing" drilled discs) ARE used in endurance racing, where slots would wear off.

    Most LMP cars have six holes in their discs, a line of three ascending in size, on each side for balance. The largest sweeps the edges of the pad, the smallest sweeps the centre.

    For road cars i would recommend slotted discs due to the "very light braking" common in commuting. You don't want to have polished/glazed pads for that one unexpected emergency stop. Lose pad life to save your own.
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