Damian in NJ

Drilled rotors can crack, but factory 'cast holed' rotors like my big reds and mo30 rears hold up better than 'oem style' rotors that have been machine drilled.

Dave951M

The original reasons for drilled rotors were-

1) to let gas out from the boundary layer where the pad contacts the rotor. Under racing conditions with the old pad formulations, this could act like an air hockey table, reducing friction.

2) reduce unsprung weight. The most effective weight loss you can make is unsprung, ie, wheels, rotors, hubs, tires.

That said, slotted rotors are an extension of the drilled concept, just that there is less weight loss. They do allow a bit better removal of hot gasses under heavy braking, leading to what appears to be accelerated pad wear. Cryo rotors should last longer due to better grain structure in the metal. We have had cryo treated tooling at the shop an it's worked out great in that application. Bottom line for the street, and by that, I mean non dedicated racing, you pays you're money and makes your choice. Drilled (more accurately cast holes) do have the bling factor inherited from racing days. Solids have more mass and can absorb more heat, but really, if you drove on the street like it was a race track, you'd loose your license in short order. You just don't stress the brakes the same way. SO, make your choice, it's your money.

FRporscheman

I'll throw in my 2 cents too. I'm a physics major, so I can tell you that the amount of friction only depends on two things: the materials of the two rubbing surfaces, and the force applied between them. In other words, the friction of the brakes only depends on the pads you use and how strong your calipers are. I assume all rotors are made of the same material. Those cryo rotors may have a higher "coefficient of friction", meaning they yield higher friction, but I don't know for sure. So you can get awesome pads and make your calipers squeeze your rotor with huge force, but I think too much friction will cause brake-locking.

Notice that friction doesn't depend on surface area! This is why the bottom of hiking shoes are never smooth, but they have grooves and stuff for more friction. But surface area does help with heat transfer, so if your brakes get hot (i.e. track duty) then solids are better for absorbing heat... but the drilled ones vent better...

My choice for super performance brakes is ABS with the roughest pads available, OEM rotors, and a beefed up master cylinder or brake booster to make those calipers squeeze harder. I choose OEM rotors because that setup will eat those rotors like crazy, there is no real difference in rotor performance, and OEM is the cheapest. I choose ABS because I'm a newb in sporty driving and I lock my brakes more than I'd like.

Danno

I think we need a link to the 30-page Altima brake-thread...

A couple of factors to consider that hasn't been brought up:

1. total heat generated is directly related to braking-force. So for 6 maximum-force braking efforts, you've got a certain amount of heat the system has to absorb.

2. heat per surface area is another concern. With larger brake-rotors and pad surface-area, the heat generated at the contact patch will be lower for any given clamping force. So larger pads will generate the same friction and same heat, but over a larger surface area. The contact patch will be lower in temp. which may make a bigger difference on older pads which outgas more.

3. temp-rise of rotor is based upon total BTU/Calories/KJ generated by system. Larger, heavier rotors will heat up less for the same amount of heat generated. That's why BigRed 13" brake upgrades are used on race cars. They heat up less for the same amount of braking force because that heat is spread out over a larger mass of metal in the rotors and calipers.

4. tyre compound and contact patch is ultimately what determines braking-force; at least for a single-stop. That's because the tyre has to resist the forward movement of the car when the brakes are applied. The higher the grip, the greater the braking force. This will generate greater amounts of heat in the braking system per stop... again favoring larger, heavier rotors.

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Cryogenic rotors have the same surface structure and friction co-efficient as normal rotors. They WILL NOT give you better braking. The main difference (which is still debated) is in the intergranular structure of the metal itself. Supposedly the cryo process generates a stronger rotors, thus it will last twice as long, but it will not stop faster. Here's some links to cold rotors:

http://www.cryoeng.com/images/Brake%20Rotors.htm
http://www.lmperformance.com/2135/4.html
http://www.frozenrotors.com/brakefaq.shtml
http://www.300degrees.com
http://www.stopyoucold.com/faq.html
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As for rotors casted with holes in them... it's the marketing folkds that made that up... uh ask the people who makes these things. Ask them what kind of casting technologies are used to create such a creature. Alan Coleman on this list is heavily involved in foundries and metallurgy, ask him how to cast metals with holes in them. Then ask the rotor blanks before they get machined and see if you see any holes in them....
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Ultimately in the end, you're not going to notice ANY differences in braking performance on a street car regardless of what kind of rotors you use. The only difference will be one of pad and rotor durability. Solid rotors will give you the longest pad and rotor life.

Anyone have a link to the 30-page Altima brake-thread...?

Danno

Ok, found the Altima brake-thread. It was archived as one big page on the Corner-carvers forum:
http://www.corner-carvers.com/altimathread.php.html

I still liked the original better which had pictures. But once you go through this thread, brakes will make sense.

Dave951M

You've detailed what I refered to obliquely, you're not going to notice a difference on the street. If you do, you're probably running a huge risk of losing your license.

Mighty Shilling

Yeah. Stick with stock, or go PCCB it's affordable!

inactiveuser92616

While true in theory this is not true in application. You are talking about the basic concept of friction that they teach you in physics 1. In reality there are the factors of adhesion, abrasion, and the "coefficient of friction" If these werent factors than race cars would run on skinny donut tires to reduce weight while keeping their performance. Ever notice how frikking wide race tires are? more surface area.

944J

I think the wide tires are for side to side stability since a wider tire is stronger/stiffer and wont squat as much as a thinner/skinnier tire.

I might be wrong, but there are a lot of things that SEEM to make sense but are just illusions in the world of physics. i.e. There is no such thing a centrifical force.

It would be good to get some clarification on why people use wider tires in physics terms.

inactiveuser92616

well, its more chemistry and mechanics of deformable materials at work than newtonian physics. The adhesion component is caused by the tire chemically forming bonds with the pavement, and the abrasive component has to do with the fact that on a small scale, the pavement isnt flat but has surfaces in many directions, thus you can generate "normal" forces that arent perpindicular to the plane of the pavement. Tires are an example of these concepts taken to extremes, but illustrate the point well. High performance brakes pads are based heavily upon adhesion characteristics.

Danno

Well, contact-patch on the tyres really can't be related to brake-pad contact area due to the bouyancy of tyres. The actual contact-patch with wider tyres is actually about the same, but shaped differently. The vertical-loading on the tyre is also the same becasue the weight at each corner is still the same. It's the direction cornering forces are exerted on the contact patch that makes the difference in cornering.

Example-1: Try pushing on a white picket-fence at right-angles to the fence and notice how easy it gives? Then push on it in-line (parallel) with the fence. This is an example of applying force in parallel or series. To actually quantify this effect, we'd need some expensive FEA software.

Example-2: Get a rectangular clay housebrick and lay it on the ground. Push on it sideways near the top perpendicular to the long side and notice how easy it is to flip over? Then push on it sideways from an end to the other end and see how it's not as easy to flip over? Same thing with contact patch. A long narrow contact patch allows the tyre to roll over easier and lift the inside edge and overwhelming the outside of the contact patch.. A short wide contact patch stays flatter on the ground and allows the cornering load to be evenly spread across the tyre.

Back to brakes... anyone have quantitative test data on various brake component configurations?

Imo000

FRporscheman: "Notice that friction doesn't depend on surface area! This is why the bottom of hiking shoes are never smooth, but they have grooves and stuff for more friction. But surface area does help with heat transfer, so if your brakes get hot (i.e. track duty) then solids are better for absorbing heat... but the drilled ones vent better..."

So, according to the above statement, racing slick SHOULD have as many groves and treads as possible??????? Your statement seems inaccurate.

The reason hiking shoes have groves is, to make the bottom of shoes confirm better to the adjacent surface. Free climbers have shoes that have smooth bottoms to increase surface area. Increased surface area increases traction and reduces friction.


Too much theory.

500

That the frictional force depends only upon the two materials in contact and their contact force is correct.

The reason why bigger tires, pads etc. work better is that the larger area of contact means the contact force can be spread over a larger area (lower pressure) and consequently there will be less local temperature change for a given heat build up. As the materials in contact heat up, their frictional coeffecient changes (often gets lower) which is what the problem really is. In extreme cases (like badly overworked tires) the material structure itself will change. Fat tires work on race cars because they do not heat up as much.

Of course, there are other more complex situations like a knobby tire biting into soft ground for example in which other considerations play a role to the overall traction force (as opposed to frictional force, which is a very specific type of force, sometimes confused with other forces that contribute to traction).

Yeah, that high school physics was right after all!

M758

I ran drilled rotors on my 944-spec for 3 years and lots of hard use. I went through something like 6 sets of pads. Rotors had some minor cracking and decided to replace them with solid rotors. I don't expect and change in brake performance, but I did save like $40-50. Funny since my front pads are $110 per front set rotors are cheap. Still I don't know that I saw any benefit from the drilled rotors, but they did not crack and fall apart on my either.

944Fest (aka Dan P)

For OEM rotors, use Paragon. Good prices, great service. I think most people don't bother with cryo on our rotors is because new ones are so cheap. If it costs $100 for a freeze job, you could get a second new OEM for that.