Popular falacies part 1: brakes

[graywolf624]

I have decided to create a series of threads dispelling some popular car myths of the main stream. This particular one will be on brakes. Hopefully I can contribute 1 per day for a while. Basically I figured perhaps their would be some interest and it would allow me to hone my writing skills.

So here we go.

Popular Car Myths of the mainstream
Dennis Cook

Brake Fallacies.
The Working of Brakes
There is a common fallacy out there that increasing your brake pad size will increase the stopping power of your car through greater friction. This is in fact false. The force of friction is determined by physics as the force down on the object times the coefficient of friction. The only way to change the coefficient of friction is to modify the material or design of the brake pad. The only way to modify the force down is to change the brake piston force (by size changes or number for example).
This does not mean that a larger brake pad does not help braking! The benefit of a large brake pad comes into effect when you consider thermal dissipation. The larger the pad the more this thermal temperature is spread amongst a pad the less the temperature is concentrated in one point, decreasing the likelihood that the pad itself will heat beyond operating temperature. If the pad were to go beyond operating temperature it would glaze over resulting in brake fade. Furthermore, a larger pad results in a longer service life of the pad since there is more pad material to consume. **Note: This is not to say that a huge pad is the way to go. I am simply telling you the benefits of a bigger pad. Do not. I repeat do not buy a huge pad thinking that will be the end all. However, consider a pad with a better material makeup for a large difference.
Cross-Drilled /Slotted Rotors
The second thing you can do to improve your brake performance is often to go to a larger rotor. We all know that this gives the rotor further ability to dissipate heat away from the pads. It also increases the effective radius of the force acting on the rotor. Since actual brake torque equals force of pistons* effective radius * coefficient of friction of the pad. So obviously a larger pad, a larger rotor, or both result in better brake performance by avoiding brake fade. It is also obvious that a larger rotor will give you a larger brake torque (stopping force). But what about cross drilled or slotted rotors? Well the common belief in the main stream is that somehow slotted or cross-drilled rotors allow for better performance by handling heat. This is 100 percent false. The individuals involved in such fallacies mention that air through the holes works to cool the rotor. The issue is that from physics we know that metal transfers heat better then air by a significant amount. As such the rotor begins to work as a heat sink. Now by cross drilling or slotting we are decreasing the overall amount of metal to transfer this heat to. Clearly we are decreasing performance of the rotor to dissipate heat. We are also damaging the brakes structural rigidity. The iron in a brake rotor is made of a crystalline structure. By drilling holes in said surface we cut the end grains creating a situation that breeds cracks. Furthermore, even if we were to cut the rotors correctly to avoid cutting the end grains structural rigidity is still decreased. The temperature around the holes will be slightly less then that of the entire rotor leading to temperature stress. Moreover, the decreased mass will result in lowered rigidity. Lastly, the holes of a cross-drilled rotor decrease the area of the pad that contacts the rotor. This concentrates the heat more on certain areas of the pad (similar to the idea of using a smaller pad where the pad heats up more quickly).
So what do cross drilled and slotted rotors accomplish? Well cross-drilled does not do anything for a car but perhaps give you a certain bling look. In a motorcycle or other extremely light vehicle the decrease in rotational inertia and unsprung mass might perhaps be useful (once other more efficient avenues are exhausted). However, in a street car or race car the speeds and weight of such vehicles will make the relatively miniscule decrease be outweighed by the need for more heat dissipation.
Slotted rotors meanwhile serve a few purposes. The main original purpose of a slotted rotor was to vent gases that buildup between the pads and the rotors. However, this reasoning is no longer valid. As the years have gone by pads have been designed that produce very little gas. Furthermore many pads come with groves in themselves that allow for the removal of any minor gas that is created. So does that mean that slotted rotors fall to the same fate as cross-drilled? Well, no. A slotted rotor always decreases heat dissipation, but they have their applications. A slotted rotor will clean off the brake pad as it passes the slots. As such it works really well for rally and dirt tracks. Furthermore, the slots themselves can serve to wipe off the top layer of glaze that tends to appear on your brake pads. Some racers say this last part is beneficial while others question whether the slots will fill before the deglaze affect is ever helpful. I have yet to determine the answer to this question. The answer of slotted brake usefulness seems to lie with whether the benefit of cleaning the pads outstrips the loss in heat dissipation. This question depends on your application.
Ventilated or Veined
So what do ventilated rotors accomplish? Well, the concept would be the same as the idea for cross-drilled/ slotted that have been previously perpetrated. However, there are a few distinct differences. The rotors are designed to increase surface area and to flow air in the middle of the rotors. The increased surface area to the air clearly provides for more cooling from the air at the cost of mass. So why does this method work while the others fail? The first reason is that a ventilated design flows a lot of air through a rotor. A ventilated rotor acts as a centrifugal pump sucking air into the rotors. This is why rotors with curved veins provide better braking. A slotted or cross-drilled design will flow very little air under heavy braking. As such the veins of the ventilated system are far more efficient. Moreover, air moves through the center of the rotor cooling the rotor more evenly and efficiently. Furthermore, the ventilated design does not decrease the contact patch of the pad on the rotor. Finally, The design has different structural rigidity qualities then that of a cross-drilled or slotted design.
Brake Bias
So now you know that increasing your pad size and rotor size will help to stop your brake fade. You also know that swapping the pad, increasing the rotor size, or increasing the force of the pistons on the pad can increase your stopping force at the tires. Finally, you have learned to stay away from cross-drilled and look very closely at whether to use a slotted rotor.
So does that mean it is time to go get that fancy front brake kit for your car? Well, no again. The first thing to consider is that in any braking setup the tires are the ultimate limiting piece. You cannot stop faster then your tires allow you to stop, ever. As such, if your car can lock it’s tires under braking consistently then better brakes will not improve your braking performance. (I stress the consistent part, as brake fade must also be combated.)
Furthermore, most people understand the idea of brake bias, but fail to understand its application. A typical car is setup with the front brakes being far more effective then the rear. Now the first thing we must realize is that from a dynamic stand point your car should have stronger front brakes. When you brake physics transfers more weight to the front axle that must be accounted for. However, in this dynamic state we also have brake bias. Your typical street car is slightly dynamically biased towards the front. This leads to the front tires locking up before the rear tires under heavy conditions. Such a situation is obviously not optimal for a car stopping quickly. You want the stopping bias to be roughly equal given the acceleration you are traveling at (please note that the bias depends on the acceleration of the vehicle). When you have a front bias you get a more stable stop (as opposed to a rear bias where a lock can cause spins), but you also get further brake dive and longer stopping distances. Most cars stock come with a minor front bias for the layman. So it is clearly discernable that by going with a bigger front brake kit you are actually most likely increasing your stopping distance if you do not equally modify the rear brakes (or set the clamping forces lower on the front brake to make up for the front brakes increased effective radius).

For further information please try:
Ruiz, Stephen and Smith, Carroll. “Brake Systems and Upgrade Selection”http://www.stoptech.com/whitepapers/...ons_122701.htm
McCready,Tom and Walker, James. “Brake Bias and Performance”
http://www.stoptech.com/whitepapers/...erformance.htm

Corner-Carvers.com altima brake thread discussion.
http://corner-carvers.com/forums/sho...drilled+rotors

The famous Altima brake thread.
http://www.altimas.net/forum/showthr...7&pagenumber=1

[coombsie66]

Very interesting, and all correct as far as i can see!
Thankyou for spending the time, and i hope others can learn from this!
Oh, i do think you will still get an increased braking effectiveness on a road vehicle, from changing the front brakes for larger and not altering the brake bias. Im not sure how to prove it physically, but ive ridden in a few cars with upgraded front brakes, before and after, and the difference is more than noticable.

[godspeed06]

great article.

[Garretts_turbo]

good show, good show. this should be great if you can do it everyday!!

[nchs09]

sounds great u giving us all this man! keep them coming, it was an intresting read

[DMbaseball1604]

Thanks I enjoyed this very much! Thanks for spending your time to write all this out!

[deth]

I agree with whay you say, although it does leave be with one question. Namely, why do car manufacturers still produce cars w/ cross drilled rotors is they have an adverse affect on the cars performance and perhaps safety? This is especially true in the case of hyper cars where the need for 'bling' is hardly required. some examples that ocme to mind are the CGT and the Macca F1, both of which have cross drilled brakes. I am not insinuating that i disagree with your reasoning, i was just wondering why this is the case.

[deth]

Originally Posted by graywolf624 When you have a front bias you get a more stable stop (as opposed to a rear bias where a lock can cause spins), but you also get further brake dive and longer stopping distances. Most cars stock come with a minor front bias for the layman. So it is clearly discernable that by going with a bigger front brake kit you are actually most likely increasing your stopping distance if you do not equally modify the rear brakes (or set the clamping forces lower on the front brake to make up for the front brakes increased effective radius).

in response to coombsie's reply i believe what graywolf says is true if the fronts are locked and the rears are still spinning. if the fronts are still allowed to spin the bigger brake setup should allow the car to stop more quickly since as already mentioned there is a larger torque and therefore stopping power.

[rsgti]

This is one great topic !! One can learn something new everyday !!

[BADMIHAI]

Yea, I was about the say the same thing deth said. Why do supercars still use cross-drilled rotors? I think even the carbon brakes on the Enzo are cross-drilled.

[graywolf624]

Namely, why do car manufacturers still produce cars w/ cross drilled rotors is they have an adverse affect on the cars performance and perhaps safety? This is especially true in the case of hyper cars where the need for 'bling' is hardly required.

Part of it is that carbon brakes are a different material then your typical iron rotors. As such they have different properties(I don't know much about carbon brakes because I don't have the money for a set lol).
The other part really is bling. For instance, we all know that part of the ferrari mystique is look. I mean if I recall from some readings the rear wing on several of the lambos(countach) is just for looks. The original design of cross drilled was the same as slotted rotors, to get gas out from under the pads. For some reason it now sells cars despite there uselessness.

One thing to note is there is a difference between what you buy from a vendor typically and say porsche big reds. First you will notice that the porsche brakes have smaller wider spaced holes. Then if you could look at the rotors under a microscope you could see they were molded to avoid the interuption of the crystalline structure rather then drilled later. This resists cracking a little better(albeit they crack sooner then solid rotors). The concept of course is that the larger size of the rotors(13") will make up for the loss in rotor mass and they are street rotors so they won't be subjected to the rigors of the track. As such they can continue to promote the same image from 20 years ago. You will never see a pair of cross-drilled rotors on a race car.

In closing people design neon underbody lights and type r stickers because they sell. It is the same thing with cross-drilled rotors. You make what sells.

[graywolf624]

in response to coombsie's reply i believe what graywolf says is true if the fronts are locked and the rears are still spinning. if the fronts are still allowed to spin the bigger brake setup should allow the car to stop more quickly since as already mentioned there is a larger torque and therefore stopping power.

Exactly.

[deth]

l thanx graywolf, but assuming that the brake rotors can be annealed properly in order to reduce stress concentrations, wouldnt the reduced rotating mass lead shorter stopping distances?

[graywolf624]

wouldnt the reduced rotating mass lead shorter stopping distances?

Answer this question. What do you think would help more? Saving 2 lbs on a rotor.. or being able to handle more heat with the rotor. The rotors are not where you want to be saving weight. The larger the rotor the more space to transfer the heat over. The more places to place the heat the less heat concentrated at the point of the pad. To much heat at the pad and the pads cease to work. If you want less rotating mass then you should get lighter wheels or be a drag racer and run a parachute.

[deth]

firstly, i'm really not trying to get into an argument, i'm just trying to sort the facts out.

with that out of the way, the greater the surface area the greater the rate of heat transfer correct? but dont these holes increase the SA. as you mentioned in your first statement these holes lead to an uneven heat distribution which is undesirable, but the same would occur at the ends and middle where the rotor is not in contace w/ the pad correct? and if your rotor is vented wouldn't this accentuate the problem even further?