DamonB

One right here!

So you think the stomping on the brakes from 100 mph story doesn't apply either? You know you can put the car at threshold braking nearly instantaneously with ABS brakes. You feel the bigger rotor decreases your stopping distance even though you are going to trigger the ABS in milliseconds regardless of rotor size?

How brakes and rotors work

rynberg

I disagree too, because you are wrong. If the tires are at the edge of adhesion, please explain to us how having a bigger brake rotor is going to make you stop faster.

If you want evidence, you can look up in about a hundred Motor Trend mags in their Performance Trends section and compare the braking distances from 60mph-0mph between stock and modified cars. The modified car with bigger rubber and bigger brakes NEVER stop more than a few feet shorter and often the stops are LONGER. This is because, frequently, only the front brakes are upgraded and this upsets the braking distribution of the car.

DamonB

From Grassroots Motorsports; 1991


Those Poor Rotors

Let's look ot some common rotor "modification" and "performance" upgrades that you may have been exposed to. We'll try to separate the marketing from the engineering: Bigger rotors will make your friends think you are cool, bigger rotors look sexy, but bigger rotors do not stop the car. What a bigger rotor will do is lower the overall operating temperature of the brakes--which is a GREAT idea IF your temperatures are causing problems with other ports of the braking system.

Take, for exomple, a Formula 500 racer, a small 800-pound, single-seat formula car. While the brakes are certainly much smaller than those found on a 3000-pound GT1 Camaro, that does not necessarily mean that they need to be made larger. In fact, installing o GT1 brake package onto our formula car would probably do more harm than good. That's a lot of steel hanging on the wheel that needs to accelerate each time the gas pedal is pushed. So the motto of this story is bigger is better until your temperatures are under control. After that point, you are doing more harm than good, unless you really like the look. (And hey, some of us do.)

Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the '40s and 50s, not a whole lot. Rotors were first drilled because early brake pad materials gave off gasses when heated to racing temperatures, a process known as "gassing out." These gasses then formed a thin layer between the brake pad face and the rotor, acting as a lubricant and effectively lowering the coefficient of friction. The holes were implemented to give the gasses somewhere to go. It was an effective solution, but today's friction materials do not exhibit the some gassing out phenomenon as the early pads.

For this reason, the holes have carried over more as a design feature than a performance feature. Contrary to popular belief, they don't lower temperatures. (In fact, by removing weight from the rotor, they can actually cause temperatures to increase a little.) These holes create stress risers that allow the rotor to crack sooner, and make a mess of brake pads--sort of like a cheese grater rubbing against them at every stop. Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it.

The one glaring exception here is in the rare situation where the rotors are so oversized that they need to be drilled like Swiss cheese. (Look at any performance motorcycle or lighter formula car, for an example.) While the issues of stress risers and brake pad wear are still present, drilling is used to reduce the mass of the parts in spite of these concerns. Remember that nothing comes for free. If these teams switched to non-drilled rotors, they would see lower operating temperatures and longer brake pad life, at the expense of higher weight. It's all about tradeoffs.

Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the glazing often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?).


In Summary

You can take this one to the bank. Regardless of your huge rotor diameter, brake pedal ratio, magic brake pad material, or number of pistons in your calipers, your maximum deceleration is limited every time by the tire to road interface. That is the point of this whole article. Your brakes do not stop your car. Your tires stop the car.


For further reading please see the entire article, rotors are only a single part of the braking system interface. Everything about brakes

Mahjik

iTrader: (1)

Brentis

ok --- so if the clamping force of the rotor was less than that required to lock up the tires, then would you agree that the larger rotors would benefit?

On to another fun and exciting topic - has anyone ever heard that the leading edge of the brake pad contributes to up to 15% of the braking? Don't recall where I read this, but found it interesting. I think slotted and drilled rotors can achieve similar braking characteristics but by having the holes and slots apply the bite through the entire brake pad surface. This outweighs the small loss in surface area on the rotor (in regards to frictional surface area not heat dissapation) Anyone care to enlighten me further.

Another thing I picked up on once upon a time was the rumor that Porsche had cross-drilled rotors was because Ferrari had cross-drilled rotors and was merely a marketing show. Many disagreed saying that the Germans would not fall pray to the Italians hype and would only cross drill rotors if there was a functional benefit (like germans have always done).

One thing for sure... my Supra TT had the most confidence inspiring brakes of any car I've ever driven. The fact that it was a 3600lb car and could stop shorter than our 2700lb car with only 20mm of additional tire width is amazing. If what Damon, et al says is the definitive word on braking, thats some pretty valuable 20mm of rubber (approx 4/5 of an inch).

That should provide enough fodder to keep this thread going for a bit.

DamonB

Highly Depends. The only reason to have a bigger rotor is to control the thermal exchange in the system. If the rotor is bigger than you need you are carrying around unsprung weight (the worst kind!) you don't need. Less clamping force just means you don't have to squeeze the brake pedal as tightly to get the same speed reduction. Maybe for the handicapped that would be an advantage.

The trailing edge of the pad does run hotter, and if it's running hotter it's doing more of the braking. As for the actual percentage I don't know. Most true racing applications now use multiple piston calipers with the trailing pistons slightly smaller. This puts less force on the trailing edge of the pad and more on the front in an attempt to keep the temperature the same across the entire pad.

You confused me. It's already established that if you can lock the tire instantly the brakes are working at peak performance and anything that removes mass from the rotor will make the rotor overheat (and therefore heat fade) more quickly. Ever notice how dirty racecar wheels are? That's spent brake pad material. The slots are there to sweep that garbage out from between the pad and the rotor to ensure the pad stays in maximum contact with the rotor. All things being equal you'd rather not lose that mass, but the slot is preferable to allowing pad friction to degrade. The slot does not add performance, it merely helps fight the degrading of it.

I think drilled rotors look cool. They do not stop the car in a shorter distance. It's all marketing and the idea that people THINK drilled rotors are better. I know of no F1, CART, NASCAR, TransAm car etc with holes drilled through the face of its rotors.

Brake balance between front and rear plays too. Since the weight transfers to the front under braking the rears don't get to contribute a whole lot, but you want them to do as much as possible before locking. It's a fine line; maybe the Supra is setup better. Also with modern ABS equipped cars the ABS software is everything as well. In racing we pretty much have a consensus that the Z06 ABS is better than the driver and so the drivers purposely stomp it and activate the ABS; it's a great system. On my FD with its 3 channel ABS I get shorter distances by maintaining my threshold and NOT triggering the ABS. Overall it's not a huge difference but on the track we are fighting for feet and inches. On the street I would never be without ABS. Even if the total stopping distance is slightly more I like the idea that while I am drinking my Slurpee I can stomp the pedal without thinking and not go out of control. The Supra has bigger tires and perhaps better ABS software, but in the scheme of things it's always easier to stop a lighter car than a heavier one.

This isn't me that says, I have just studied and learned for myself. Nothing here I posted is original work on my part other than the stopping from 100 mph example. I don't trust people who say "well, that's just how it is". I tend to research the fundamentals and PROVE it's right. Along with the real knowledge and understanding you can solve problems on your own without relying on someone else who may be wrong. I don't get mad at being wrong at times at all, but I want to know WHY I am wrong. The answer has to be proven in its explanation.

Here's another seeming contradiction. We all know wider tires have more grip than narrower ones all things remaining equal. But physics proves that sliding friction is not dependent on the area of contact between the surfaces, only on the pressure between them. If you were to slide a book across a table on it front and then slide it across the table on its spine, you'd find it takes exactly the same amount of effort either way. On its face the book is in greater contact with the table, but the weight is distributed across a greater area and so presses down less hard. On it's spine the book has much less contact with the table, but it's weight is now distributed across a smaller area so its contact pressure is greater. The amount of pressure exterted between the book and table is the same no matter which way you slide it, therefore the friction is the same even though you'd think the face of the book would be harder to pull across due to its greater contact with the table.

If tires obeyed the same laws than for a given vehicle weight 5" wide tires would give the same grip as 10" wide tires. Tire friction is tremendously more intricate than it seems however and so tires remain very much an art as well as a science.

Brentis

shear properties or something like that.

ok - you win. Still doesn't make sense why the supra stops faster. it doesn't even have the 50/50 weight distribution that rx-7's have.

has anyone created multi-caliper brake systems per rotor (2 calipers per rotor)? Seems like with ABS the pulsations could alternate much more quickly than with just one caliper.

DamonB

At one time I believe the SAE did experiments where the calipers completely encircled both sides of the rotor and clamped it along its entire circumference on both sides. Again we already know that a "simple" four piston caliper is entirely capable of locking the tire, so why add all the weight and complexity? Unless tires were 4 feet wide the system had no real advantages. I have seen some 2 caliper brake systems in racing, but they basically turned out to be experiments that were abandoned. The limiting factor always ends up being rotor cooling, not pad area.

The ABS pules rate is strictly a function of how quickly the ABS can change the fluid pressure. You could make the argument that is would be more difficult to do this with additional calipers because the fluid volume would be greater. Seems to me pulse rate is limited by software and solenoid technology.

Actually 50-50 only counts in steady state. Since weight transfers forward under braking your 50-50 car at rest is no longer 50-50 while braking. A rearward weight bias would actually allow all four tires to contribute more evenly under braking because as the weight transfers forward the tires become more equally loaded. A rear weight bias under acceleration also gives more grip to the rear, but at the expense of front grip. That's why Top Fuel dragsters put everything on top of the rear axle; they don't have to steer. A rear weight bias on a racecar is poor because when you get on the gas to exit the corner you loose front grip. With that in mind you'd have to wait longer before accelerating the car off the apex. All that said I have no idea what a Supra's weight distribution is.

Brentis

supra = heavier up front 53%, not much but still some.

maxcooper

There have been many F1 cars with dual calipers. I was eyeing the vintage F1 cars at Long Beach this year and saw that many had two calipers per wheel on the front. They didn't have ABS, though. I think they used lots of forced air cooling and wanted more pad area inside the relatively small wheels. That is total speculation, though -- I am sure someone will chime in with better info if it was for some other specific reason.

-Max

DamonB

How "vintage"? The ones I have seen in the past were inside very small wheels. I think the old friction materials just were not up to the task as they are now.

maxcooper

Late 70s?

-Max

theloudroom

So what about crossdrilled rotors in the rain?

Crossdrilled rotors were originally used to vent gas, but it seems like they would be good for venting liquid as well. Seems like you would get a performance improvement in wet conditions. There's a benefit that seems pretty much obvious.

As for cooling, all the explanations I've every seen of this whole "cross drilled rotors aren't worth it" thing are much too simplistic. Sure our rotors act as heat sinks and the more metal they have, the more heat they can absorb, but that heat has to go somewhere eventually. Stock rotors aren't solid! They have cooling vanes/fins/whatever.
Why? Because all that heat the rotors are absoring needs to go somewhere (air) eventually. Why isn't your CPU heatsink a solid block of aluminum? It would be able to hold more heat energy.
The idea isn't just to have more mass to STORE heat, the idea is also to dissipate heat as quickly as possible.



What I would like to see is proof the crosdrilled rotors do not cool more quickly than non-drilled rotors. I'm not saying crossdrilled rotors are necessarily better (in dry conditions), but that I have yet to see conclusive evidence that they aren't.

(I'm not talking about a one-time 60-0 stop, I'm talking about autocrossing, etc.)

DamonB

What's pretty obvious to me is that water boils at 100 degrees C. Brakes operate at much higher temps. Water is not a problem, it will become steam in an instant and brake pads also make excellent squeegies Now, even if holes did provide better brake performance in the wet it doesn't matter. Why? Because if it is wet outside the tire has less grip and can't possibly put the supposed increase in performance to use anyway. Holes are a bad trade off, even in the wet. Now if my car were underwater like a submarine and there was a benefit to be had I would opt for ball shaped slots as they do not make such stress risers in the rotor.

Let's review. Large rotors only contribution to braking is mass. Holes remove mass AND surface area from the face of the rotor. Holes can't absorb heat and at the same time a drilled rotor will hold less heat than a solid faced rotor due to its less mass. As for the heat going somewhere, it dissipates into the air. That's another plus for not having holes; there is more surface area exposed to the airstream. The idea of a finned heat sink it to create more surface area to radiate heat into the air. Your finned CPU example has much more surface area due to its fins than a solid block of the same size would. The solid block has more mass but is less efficient at exchanging the thermal energy. The solid block can readily hold more heat due to it higher mass, but is does a poorer job of getting rid of the heat it collects. The cooling vanes inside the rotor use the same idea as a fin, but are actually designed to pump air from the hub area out towards the circumference of the rotor.



We know this. We are referring to the face of the discs as being solid.

Look at any NASCAR, CART, IRL, ALMS etc car where drilling the rotors is legal and count the number of cars using them. You'll get approximately zero I leave out F1 as they are not using iron rotors.