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Drilled And Slotted Rotors

By twinky64, April 9, 2009

Hey everybody, its been awhile now. In case if you guys have forgotten, I now drive a 1998 BMW z3.

I am thinking about purchasing rotors for my z3 because the ones with the 1.9L engine are not ventilated, they are solid. Same goes for the rears. In order to improve cooling, I was thinking about purchasing some cross drilled rotors to help transfer heat. However, I am worried that the contact area between the pad and rotor will diminish.

By what factor? I have no idea. That is my question, by what factor will my braking distance worsen due to the decrease in contact area between pad and rotor? That in and of itself is probably irrelevant. The braking distance is also determined by the coefficient of friction between the tire and the pavement. The larger the rotor diameter, the better the heat capacitance and thus better cooling is primarily. Secondarily, the further out from center of rotation the more effective the clamping force is. I am not looking to increase my rotor size however because of cost.

So....if I get drilled rotors, mind you they are solid discs, it will increase cooling but decrease the structural integrity. But what about slotted rotors? Because they are channeled, there is more surface area, however, the increase in surface area is not acting as a fin (for heat dissipation). I suppose the last question is, should I say to hell with aftermarket rotors and just live with solid, non-ventilated, rotors?

Is there a difference in braking performance about how the drills or rotors are pointing backwards or forwards? Intuitively, it shouldn't matter.

Thanks guys. I'm currently a student majoring in mechanical engineering. Im already leaning toward drilled rotors (though they are more prone to cracking). Slotted rotors will help with pad cleaning and bite but I'm almost positive it will not help with cooling despite the increase in surface area exposed to the environment. I'm asking for experience.

you should be more worried about drilled ones cracking. slotted are better for street and daily driving, less chance of them cracking and breaking. unless you're doing 100-0-100 runs without cooldown period you won't feel the difference between plain, slotted, or drilled. if you're going to get beefier rotors then just get a big brake kit with larger calipers and pads as well as larger thicker rotors, that will make much more difference.

Big drawbacks from drilling, aside from the possibility of cracking and warping, is that you effectively reduced the thermal capacity of the discs (less "meat" to help act as a heat sink) as well as reduce friction contact area due to the holes. Granted there is more overall surface area which should improve cooling, but there is a crossover point between surface area and rotor mass that should not be ignored. Their original claim to fame was reducing the accumulation of friction pad particulate matter and the boundary layer of gases - but with new pad materials, this is no longer an issue.

I wouldn't worry too much about cooling unless you experienced significant brake fade recently. Solid rotors have been used in track events, coupled with the appropriate pads, and kept up with larger, slotted/drilled apps. All depends on the conditions.

If you are hard set on getting something different (not plain solid rotor) and do not want to upgrade to a BBK (Big Brake Kit) - then I would suggest a good street pad and a dimpled rotor. A dimpled rotor will have more surface area for increased cooling, but still retain a good portion of their mass - compared to a fulled drilled application. Direction of dimples doesn't make much of a difference - usually, that only results in a different sound under braking (drilled apps make a humming or whirring noise). Pads choice depends on driving situations - all street, mostly street or heavily "spirited" driving, some track use, 100% track? Some good all-purpose pads are Axxis, Carbontech, Porterfield, Hawk - each make specific compounds with different bite characteristics / rotor wear.

My type of driving is mostly commuting on freeways. Freeways in southern california are prone to halt very quickly.

So the recommendation I am receiving is picking up a set of pads right? Why? How do the pads help cool the rotor to the point of preventing warping? When I was driving my 98 corolla, my front rotors would warp often. I got pissed and bought myself a set of drilled and slotted rotors and my rotors didn't warp as a result.

The heat generated from the shear forces between rotor and pad is a conduction type of heat transfer. The heat built up in the pads would then conduct to the caliper. From the caliper, the heat would transfer by means of convection with outside air, and conduction to the brake fluid. How would selecting a "better" performing brake pad prevent warping of the rotor? The rotors on my z3 are warped up front and there is only one reason why that got too hot. I need better cooling and because they are not ventilated, a "basic" brake duct will not cut it. I will have to fab. a brake duct and channel it to both sides of the rotor since my rotors are solids.

Fishexpo, for the race cars that run solids, what did those cars do to address cooling. I observe how motorcycle brake systems are designed and by observation, they address cooling of solid floating discs with drilling. I figure, no point in getting a BBK since my calipers have enough clamping force to lock the wheels as it is.

BTW, where does MOST of the heat dissipation occur in a disc brake system: convection from caliper to air or convection from disc to air?

Most of the heat from the braking system dissipates via convention from rotor to air. Caliper generally doesn't heat up that much (compared to hte rotor), unless there was a direct thermal link between the pad and the caliper (some it is by design). On some vintage racers with solid discs all around - most were open wheel apps or had significant cooling space around the rotors. Even with repeated high speed stops, the solid rotors cooled enough to not fade during the race.

Higher performance pads generally take much higher operating temperatures than standard pads - in many cases, the pad shows a strong thermal gradient in the friction material that inhibits heat conduction to the caliper assembly. Your shims and pad backing does the rest. The pad itself will conduct a surprising amount of heat away from the rotors - many ceramic composites use carbon and copper strands to help pull heat away from the rotor and provide faster recovery.

Can't base this decision solely on motorcycle designs - as that is a completely different beast. Motorcycle brakes, being so visible, almost all are drilled, some have company logos or custom work done. Look at a track bike, nearly all are slotted or even solid - very few run drilled rotors on the track. Main rease, bikes weight considerably less than cars - the loss of weight and overall heat capacity is less of an issue on a bike. Plus, front and rear balance can be adjusted on the fly, so overall brake loading will be different for different conditions.

Warping is altogether another issue. Easy to associate warping to overheating, but in many cases, it is actually related to how the the brake friction boundary layers is deposited on the rotor surface. The terminology of "warped" brakes makes this even harder to ignore. Iron doesn't even break a sweat, even at highly elevated temperatures. Todays metallurgy and manufacturing techniques means, high quality rotors - typical street temps are under 700 degrees F, on a track - they could approach 1200 degrees F. Iron's melting temperature is nearly 2800 degrees F. To even consider deformation from heat, you need to heat up the material to around 50% of its melting temperature. Certain alloys of iron do deform at much lower temps, but most need to be over 1000 degrees, before something happens.

Truth of warping, overheated friction materials (i.e., most organic pads) tend to deposit very unevenly on the rotor surface - this is what causes the juddering or shaking in the wheels and chassis that makes up warped rotors. Same for cases where you come off several high speed stops and then anchor the brakes - the pad sitting in one spot will suck the heat from the rotor, causing it to cool unevenly and causing a significant difference in brake transfer layer. You can "warp" any rotor in that manner.

The company StopTech ran a campaign to show that there wasn't any physical warping of the metal discs - by suggesting that owners take an aggressive pad, like Hawk Blues, run them through several hard stops, but not enough to "bed" the pads - in effect, they were just polishing the rotors faces (not like turning rotors, they didn't remove significant material from the rotor). then installed pads of choice, go through a bedding procedure, brakes afterwards ran true.

I believe your case with warped brakes on the Corolla, is more likely due to larger swept area with an uneven brake transfer layer. The drilled and slotted rotors just cleaned the pads more often and had a smaller swept area that reduced the symptoms of warped brakes. If anything - I'd almost bet money that the drilled and slotted rotors ran HOTTER than the plain vented rotors. Ultimately, the choice will be up to you. I know that BMW CCA members that track a Z3 ended up going with plain vented rotors from the later Z3s (1999+). A few 1998 Z3 came with vented rotors, I believe the cutoff manufacturing date was something after 4/98 or so. With as limited material that is on the solid rotor to begin with - going with a drilled option might cause you more headaches down the road. If you are really concerned about the heat buildup - I'd go with a plain vented rotor upgrade (with the appropriate calipers) or run a two piece rotor. The two piece rotors are designed to move radially - prevents the rotor friction surfaces from "coning" out.

According to

"Brake pads contain various metals and minerals that are bound together using a resin. At high temperature, this resin turns to liquid and can bleed out onto the rotor. Glazing is dangerous because it prevents normal contact between the pad and the rotor so you lose pad bite. Glazing can also cause brake pulsation most commonly called "rotor warping". Drilled rotors prevent pad glazing as reported in SAE paper 2006-01-0691. This is one of the reasons why drilled rotors provide 12% to 37% more brake torque over stock rotors."

Also no wonder that Powerstop sells drilled/slotted aftermarket rotors. There are other studies and whitepapers that would yield opposite results - depends on what study was trying to determine.

If you are running an organic pad and running them hard - then I would agree with their assessment. Slots will help "clean" the pad surface and prevent glazing. Drilled rotors will too, but not as effectively. If you run a hotter street, composite pad, then this glazing issue becomes less of an issue to nearly a non-issue, as higher performance pads are designed to take more heat. This would also be an issue with fresh pads that have not been bedding in correctly. Once a pad has bedding in, the binding agents that hold the material together, will have been "cooked" out. Now, many pads come pre-sintered or pre-burnished to facilitate a fast bed or being able to skip the bedding process altogether.

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