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CrazyAl -> RE: Slotted and drilled Rotors (1/13/2007 5:15:38 PM)
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ORIGINAL: timothyrw
Is Wilwood giving their brakes away?
Sounds to me that Wilwood is trying to sell an "anti-establishment" product to me.
No, they're not giving brakes away. However, Wilwood's main market is actual (non street legal) race cars--Nascar, Can Am, Outlaw, and so forth. "Street" use is a small portion of their business. They don't need to hawk their products as their market is already well established. Professional race teams use highly drilled rotors on their cars for weight savings purposes, and consequently they get people who want to use their racing rotors on street cars. This has lead to problems in which a customer uses racing brakes (which are intended for a lightweight stripped-down car, and frequent replacement) on a street car which is much heavier.
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I'll admit I don't know a thing about brakes so I'm no authority. However, do a search on cross-drilled brakes and 90+% of the manufacturers say that cross-drilled/slotted increases friction and reduces heat. It makes logical sense that a surface with holes in it, spinning, will cool quicker than a solid surface...
Advertising is not exactly gospel. I see ads every day for stocks that are guaranteed to make me a millionaire overnight, and pills that will add inches to my johnson, but that doesn't mean that these things are true. Think about it. If cross-drilled rotors really were better than solid rotors, you would see much more of them on vehicles from the factory. 95% of this "cross drilling helps heat dissipation" BS comes from overseas vendors trying to sell people their snake-oil brake products. Auto makers such as Ford spend a lot of money enigneering their brake systems. They know what works and what doesn't.
As for the "logic" of the situation, that is what the marketing people are counting on to sell brakes. First off, look at one of these street-style drilled rotors like a Powerstop. Those holes are not even 1/4" in diameter and there aren't very many of them . Do you really think that an appreciable amount of air is going to move through those holes?
Let me explain in detail why the holes do not cool appreciably. When a brake disc is spinning, the hollow passages inside the rotor (the vanes and channels) act like a centrifugal pump. Air from the center of the disc (the hub area) is pulled through those channels and is blown out the "rim" of the rotor. This is how rotors cool themselves down. The interior surface area of these channels is very large, and the vanes themselves act like heat sink fins to carry heat into the air. When the disk is spinning, it literally PUMPS air through itself to cool itself down. Cheaper rotors have straight vanes in them becasue this design is easier to manufacture. Premium brake rotors have directional or curved rotors. Why? becasue just like the rotor in a proper pump, curved vanes are more efficient at moving the air through the rotor. When the rotor is spinning, air is moving in a radial direction (like spokes on a bicycle wheel). The drilled holes in a rotor are perpendicular to this direction--they are oriented axially, not radially. Consequently they are pointed the wrong direction for air to be flowing through them. If that doesn't make sense, think of it this way: Pretend you are trying to drive out of a small driveway that has a T-intersection with a very busy street. You cannot move very quickly becasue you have to wait for an opening in traffic before you can get out. Furthermore, the holes have very low surface area compared with the rest of the rotor. Think about the total effective area of a brake rotor: both sides (braking surface) plus the area of all those channels inside the rotor. Now think about the surface area of the inside of a hole. The additional surface area gain by drilling holes is a tiny fraction compared to the rest of the rotor.
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You've not provided me with any evidence (other then your word and a quote from Wilwood which I've countered with Brembo) to suggest why this isn't so.
That is not entirely true. I have suggested three books, all of which are well regarded in the professional community and have withstood the scrutiny of countless engineers and scientists.
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And, while I have no reason to believe you haven't done some research yourself, it would be totally illogical to say that Brembo and the many other manufacturers who do this for a living haven't done just "a little" research themselves...
Of course, Brembo et al have done their research. But their goals may be a lot different than yours or mine. Let's look at the claims made here. The OP post claims the reason for drilling rotors is to dissipate heat. Brembo says nothing about dissipating heat and instead says that the holes refresh the pad surface.
The OP post is simply wrong, and I have explained why and provided documentation to back it up.
Brembo has a bit of point. Holes do cause some degree of scraping of the pad surface, and that can (slightly) improve friction if used with the right kind of pads. Now then, this also happens to shorten pad life becasue the pads are being constantly scraped. That is a tradeoff, just like the highly drilled race rotors.
If you get the highly drilled rotors, you do get some benefits: decreased weight and perhaps a small increase in pad friction. But you also have to replace your parts more frequently. That may or may not be OK with you.
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Your argument regarding motorcycles makes SOME sense. However, following your logic, it doesn't stand up to existential evidence. For example, why is it my Honda 1300C has one rotor up front and the 1800C has two rotors up front? Heck, why does ANY motorcycle have two rotors up front? If a car, which you argue has a heavier load on each disc brake, can get by with only ONE rotor per tire, then why is it the higher performance motorcycles would ever require TWO per tire?
You are confusing what is needed to stop for general use with what is nice to have for performance situations. Your 1300C has one rotor becasue that is all that is needed for safe use of the motorcycle. The 1800C is an upgraded model which simply has better brakes than the 1300C. The 1800 doesn't NEED the better brakes. It just has them for better performance. This is very common in motorcycles. For example, with Ducatis there are various "grades" of each bike. The cheapest model variant is called the "Dark". Dark models typically have less expensive suspension components and they ususally only have one front brake disc. The standard models--which have exactly the same engine--have two brake discs. It's just a higher performing model. The same is true with the Mustangs. The GTs have larger brakes than the V6 models, despite being essentially the same weight.
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Put another way, if a Honda CBR1000RR needs two brake rotors per tire for effective stopping then by your logic a Corvette Z06 should need at least two and more likely 4-5!!
The CBR1000RR doesn't need two brake discs at all. It simply has them as a performance upgrade. In fact, on some racetracks when hard braking is not needed, some riders remove the second brake disc for more weight savings.
And yes, for a Z06 to have the same braking as a CBR1000RR (assuming the tires hold, of course), it would need much more rotor area.
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What I would like to
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