The Big Brake Myth

[ThrillSHO]

Before you run out and buy NASCAR 6-piston calipers, F1 carbon fiber rotors and 50 feet of stainless steel braided brake lines, it would be wise to take a closer look at what brakes do (and don’t do). Write this down 100 times: “Brakes do not stop the car.” Brakes convert kinetic energy (motion) into thermal energy (heat). What stops the car? TIRES! The coefficient of friction between the tires and the road. Why does it take a longer distance to stop when the road is wet or icy? It doesn’t have anything to do with rotors, calipers, pad compounds or brake fluids. It has everything to do with the coefficient of friction between the tires and the road.
So, to decrease stopping distances you really have only 2 options: 1) Press the brake pedal harder. This theory only holds up until the tires lock up. Beyond that you just get a sore leg. 2) Increase the coefficient of friction by using stickier tires or wider tires.
So, why would anyone upgrade to larger brakes?
1. If your car is heavy, with sticky tires and small rotors, you may not be able to achieve maximum deceleration or even lock up the brakes, therefore you need bigger rotors. (this is rarely the case)
2. If you are converting too much kinetic energy into thermal energy and it is warping your rotors or causing your pads to fade. You need cooling ducts and/or larger rotors.
3. You just wanna look cool (and who doesn’t?).

Bottom line: Don’t expect shorter stopping distances thru brake upgrades. The practical reason for doing it is for thermal management. True, stopping distances will be shorter if fade is reduced, but that can be achieved with race pads who’s characteristics don’t change with temperature. For all but serious track duty, 11.7” front rotors and stock rear rotors with quality cabon metallic pads are sufficient – ie 96 brake upgrade.


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ThrillSHO Racing
93 ATX SHO
UDPs, LPM, 80mm MAF, K&N w/CAI Snorkle, 8mm Taylor wires, Koni Struts, High Rate Linear Springs, Camber plates, Adjustable rear control arms, Touque box brace, Poly bushings, 24mm front SB, 26mm rear SB, Alum SFB, Full-length SFCs, Front & Rear STBs, SS Y-pipe, Borla Cat-back, TransGo Shift Kit, Relocated battery, Five point harness, Five 2" gauges below the CD player, 225/45s on 16x7s
Very Low Very Fast

 

[1992blackmtx]

just curious but you are saying that 13" rotors and 4 piston calipers wont stop the car anyshorter then 96 breaks? is that true?

 

[ThrillSHO]

Correct. Assuming they are using the same pad compound with stable temperature characteristics (ie the pad coefficient of friction does not change with temperature and therefore will not fade). The braking distances will be the same. The only difference is the smaller brake will get hotter than the larger brake.

 

[smokesho]

by opinion is, the best bang for your buck is cross drilled rotors to resist heat and warping, high friction pads to increase friction on the rotor, and a really good set of tires. also, make sure your suspension is in check. poor suspension can make you gain several or tens of feet on stopping distances!

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white 92 MTX,
dash light mod (blue)
turbo mufflers w/ 21/2 inch tips
est 15.1
pirelli p4000 50 series
JBL 10 -pounds...
more to come....i hope
performance y-pipe

 

[Cobria]

ok if the purpse of brakes is to convert kinectic energy into thermal energy, bigger brakes would stop the car quicker. Larger rotors and larger pads would increase the surface area thus increasing the rate at which that kinetic energy could be converted to thermal, hence shorter stopping distance. Right?

 

[AutoXSHO]

Cross drilled rotors in more cases than not will actually hurt you. Rotors that are not drilled properly will crack around the holes, which eventually will lead to you to replacing the rotor.

The holes are not there for cooling, they are there for gas dissipation. If you're driving hard enough on the street that you need drilled rotors, you are endangering many lives

.

The holes, in addition, reduce the mass of the rotor which reduces its ability to dissipate heat. Again, a bad idea. Generally rotors are drilled when the rotors are so large that the decrease in mass will have little effect on the overall braking performance.

ThrillSHO sounds like he reads Grassroots Motorsports, and understands brake systems. A note on fade, though. Generally fade is a function of the brake system's overall ability to dissipate heat. The fade itself is caused by the heating and, sometimes, boiling of the brake fluid. Gases are more compressible than fluids. The fade is not as often a function of the Cf of the pads deteriorating with heat...

I must add, though, that upgrading from the one-piston floating caliper design that the SHO has is fruitful. Multiple piston designs, or fixed caliper designs will both improve brake feel. Bigger brakes help in both the thermal budget department as well as the leverage on the hub, which will help stopping distances, but not if the previous (smaller) setup could still reach the threshold of wheel lockup.

But whether you have '96 brakes or 13" brakes the limiting factor for overall stopping ability will be the tires.


John



[This message has been edited by AutoXSHO (edited 12-13-2001).]

 

[ThrillSHO]

I am an avid reader. GM had a great article on brakes several months ago from which I quoted. There is also an outstanding book "OEM and Race Braking Systems" that is great. I forget the author.

Another common myth is rotor warpage. Rotors to not warp from over heating. The most common cause of warped rotors on street cars is incorrect torquing of lugnuts. When your mechanic puts your tires back on, he cranks the lugnuts down tight with his impact wrench one at a time. As most of us know there is a recommended torque and pattern.

The most common cause on race cars (or cars driven aggresively) is not heating, but uneven cooling. The worse thing you can do is get the rotors and pads very hot, then park the car. With the car parked, 3/4 of the rotor cools faster than the portion under the hot pads and causes the rotor to warp. That's why racers do a cool down lap.

Like AutoXSHO says, cross drilling is over-hyped. If it were a magic bullet, we would see them on NASCAR and Formula1.

And I agree - There are plenty of legitimate reasons to upgrade brakes. Pedal feel, repeatability, reliability, weight reduction, ect. But don't expect shorter stopping distances from bigger rotors, more pistons and braided lines. All the magic happens where the rubber meets the road.

 

[smokesho]

cross drilling the rotors DOES help to dissipate heat!!!!!!!!!!!! more surface area= better heat dissipation. and if your gonna mess around with brakes anyways you WOULD get the properly manufactured ones that won't crack! but i think thrill shos original point was that you can't rely on bigger rotors alone to reduce stopping distance.

------------------
white 92 MTX,
dash light mod (blue)
turbo mufflers w/ 21/2 inch tips
est 15.1
pirelli p4000 50 series
JBL 10 -pounds...
more to come....i hope
performance y-pipe

 

[AutoXSHO]

Well, believe what you want. If you can produce some numbers, I'll believe you. It's a common myth that this is why people crossdrill rotors, but it is really designed for gas dissipation.

John V

 

[ThrillSHO]

Smoke - you are right. "cross drilling the rotors DOES help to dissipate heat!!!!!!!!!!!!" But, so what? How does that make the car stop sooner? If the brakes can achieve maximum deceleration (just prior to lock-up), who cares what the temperature of the rotors are? If you are running street pads whose Cf varies from .3 to .1 with temperature and at .1 the brakes can no longer achieve max deceleration, then yes, you need to lower the temperatures. But, you are probably running quality pads with your cross-drilled rotors that have a very stable Cf - ie no fade. Therefore the temperature of the rotors has no impact on braking distance.

John is right about gassing. This was a problem many years ago. The materials used for brake pads emmitted a gas under very high temperature which acted like a film or lubricant between the rotor and pad. Cross-drilling was the fix to allow the gases to escape.

 

[Jibberish18]

So let me ask you something Thrill. What does a guy do if he wants better stopping power???????????????????????

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1990 Sable 3.0
"My goal is to have the ultimate Sable in road handling performance"

 

[ThrillSHO]

Like I said in the original post you can:
1. Press the pedal harder (tongue firmly in cheek).
2. Increase the coefficient of friction between the tire and the road.

There are four ways to increase the tires grip on the road. All of these not only help braking, but also accelerating and cornering.
1. Use stickier tires.
2. Use wider tires.
3. Drive on a stickier road (ie stay in the racing groove).
4. Increase the vertical load on the tires.

Vertical load on the tire increases traction. To prove it, hold a pencil with it's eraser on the table while you slide it across the table. Now, lean on the pencil and try to push it across the table. The eraser has a better grip on the table when you are pushing down on it, right? Vertical load is also why your front brakes do 80% of the work. Most of the car's weight is transfered to the front.
How do we increase vertical load? Obviously, we don't add weight to the car. That is counter to everything we know about going fast. Aerodynamic downforce! Unfortunately, effective Aerodynamic downforce is tough to achieve at city speeds and it is not a proportional relationship. Doubling the downforce does not double the grip. There are practical limits.

 

[noSHO]

Oh great....so now you are telling me that the $5000 I just spent on the 15" cross-drilled platinum coated rotors, stainless gold brake lines, and super-stopper carbon ultra-fiber pads is all wasted money?

In all seriousness, though, I can't believe people don't know about the problems generally associated with X-drilled rotors. My rotors have been warped since I bought the car. Guy claims to have had new brakes put on right before I got the car (and I'm sure it's true). I just thought Ford put the rotors on warped like that. I wasn't aware that there ARE non-warped SHO rotors. I will have to look into this.


-Dave

 

[SHOed_up]

But Cross-drilled rotors look so cool in good 17" open rims, especially with big red calipers! You don't crossdrill for the cooling or gas relief so don't even bother addressing it. You crossdrill for the "Cool" factor and no one should say otherwise.

 

[1fastsho]

Yes, but the larger the rotor the larger the torque arm. I can't say that I have been 100% satisfied with my Baer Upgrade but I blame that mostly on the pads that I am using(Performance friction) I really want some good quality pads like Porterfield before I really make my desicion on whether or not to go with a 4 piston caliper.

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Shawn Pasley
Innovative Performance Technologies
770-725-4631
www.iptech.tv
92 SHO red/grey
3.2L(bored) Lowered compression(9:1)
stageII cams, Ported/Polished heads,Ported/polished Intake manifold(runners, endtanks, and behind the throttle body), big bore butterflies,80mm maf, LPM,equal length Long Tube Headers and custom Y pipe, Dynomax cat-back, Koni/coil-over suspension, poly-bushings everywhere,26mm FRONT sway bar, 28mm REAR sway bar,rear strut tower brace, solid subframe bushings,subframe connectors, tubular aluminum adjustable rear control arms,shortened shifter,235/45/17 Toyo Proxes T1-S's on TSW VX-1's. 15 lbs of boost here I come!

 

[90MTXSHO]

I find it VERY difficult to believe that larger brakes will not aid in slowing the car down any.

You are correct in saying that the stopping potential is at the point of friction between the tire and the ground, but that is only the highest point of friction.

When you look to your brakes, you must see that there is friction between the pads and the rotor itself. This is a given. By applying pressure to a specific sized rotor and pads, you create friction w/the rotor, thus stopping the wheel.

Let me try to explain this differently. Now, I'm just an intro physics student so far, so give me a few months and I'll probably be able to prove myself better.

When dealing with stopping a car, we want to look at rotational speed. We'll use the equation for finding final rotational speed:

Wf = Wi + at1, Wf = final rot. speed, Wi = initial rot. speed, a = acceleration (this case negative due to friction, and t1 = time for smaller brakes.

Because Wf will equal 0 for a stopped car, and Wi we'll assume is the same for both scenarios (we're starting at equal speeds), all that changes is a by x. This change is due to more friction caused by a larger brake. So...

Wf = Wi + aT1 => 0 = Wi + aT1, and Wf = Wi + (a+x)T2 => 0 = Wi + (a+x)T2. Set the two eq's equal and you get Wi + aT1 = Wi + (a+x)T2. You can remove Wi by algebra, and divide the right side by (a+x). Final result:

aT1/(a+x) = T2. The second time, due to larger friction force, causes a lower time than the first equation. This, of course, is ignoring angular momentum, etc., etc.

Considering Kinetic energy, if your radius from the center of mass of the rotor increases, you get more kinetic energy (the point is moving faster). if you have more kinetic energy converted to heat in this case, you'll also be removing energy from the rotational motion, slowing the rotor.

I dunno if my logic flows right in here, it's nearly 3am, and I'm tired. But the point I'm trying to make is, while the tires do provide a very important part to stopping the vehicle, you cannot disregard the brakes themselves. The larger the friction force, the quicker the car will come to a stop, all things being equal (tires, etc.) The tires will most likely never reach their maximum frictional tolerances against the road under normal braking, while your brakes are blowing those tolerances away when used(otherwise you would literally stop on a dime). If you increase the brake's resistance to slip by increasing friction, your wheels will come to a stop slower.

shit, did any of that even make sense? if not...let me know, i'll try to clarify when i'm awake.

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Clint
90 Elec. Red SHO
AIM/AOL: Shaft481
My Pimpin Webpage

 

[LIZARDKING]

Lots of guys who read magizines are given the impression that big brakes = an increas in performance. These articles are written by guys who either sell brakes or are sponsered by people who sell brakes. A 500-1000$ + brake upgrade is a waste of money for anyone who does not race professionaly. As alluded to here, the rotors and pads are only part of the brake system. Proper wheel alignment, properly inflated tires, proper brake adjustment, proper working of other brake system componets are just as important as the rotor and pads. If you really want to make a difference upgrade your brake fluid and dont decrease cooling to the brakes. I take a turkey baster and siphion out the fluid from my master cylinder and replace it at the start of every summer. Every brake job I replace all the fluid. Dot 4 fluid is far better than DOT 3. Dont decrease cooling by restricting airflow to the brakes with rotor covers or closed wheel types. There is another issue concerning 'unsprung weight" but I'm not sure if that has an effect on braking. Regarding the instructions on brake fluid not to save the unused portion: Any container that is not full such as a gas tank, brake fluid bottle, master cylinder: Will have surface area that condensation can form on. This condensation will adversly affect whatever system it gets into. That is why you should always keep a full tank of fuel, keep all fluids filled to the proper level, and not save any open unused petroluem products. I agree with ThrillSHO on this subject, but if your brakes are in poor condition from age or lack of maintenance and need replacement then a 96 upgrade is worth the small increase in cost.

 

[AutoXSHO]

DeaconBlue will probably disagree with me, but I think that anything more than the '96 upgrade is overkill for the street. I think a satisfactory brake system for the SHO would be:

1. '96+ SHO 11.6" rotors up front
2. Stainless brake lines all around
3. Quality pads suited to your application (i.e. Autocross/street which get up to temp fast, or race pads for the track which won't fade away)
4. Quality fluid changed often (I run Castrol LMA fluid. It's cheap and works great)
5. No brake bias - plugs in the rear proportioning valve.

With the '96 brakes, my ABS is working overtime to keep the wheels from not locking, both front and rear. This indicates that I have plenty of stopping power (tires are the limiting factor here).

I could NOT say this about my '95 factory brakes. They were horrendous.

Yes, the 13" crossdrilled rotors with aluminum rotor hats and 2-piston Cobra calipers look cool. Yes, they feel great (nice and firm with the solid lines). Yes, the car stops on a dime. Yes, they are LIGHTER than my '96 brakes. Yes, they will be more fade resistant. Do I want them? O'course.

Will they stop the car faster everthing else being equal? I doubt it.

JV

 

[ThrillSHO]

Clint - Your logic is correct.
"By applying pressure to a specific sized rotor and pads, you create friction w/the rotor, thus stopping the wheel. " "The second time, due to larger friction force, causes a lower time than the first equation. "
Yes, larger rotor, more pad surface area, high friction pads will all make the wheel stop sooner. Where you went off track is assuming the car stops when the wheel's rotational speed reaches zero. This means the tires don't skid. Unfortunately, this is not the case.

Let's look at this from a different angle. Most of you guys do the 1/4 mile thing and have figured out that the most important thing to low ETs is the launch and hooking up. Obviously, easing into the throttle doesn't yeild fast ETs, nor does smoking the tires off the line. Maximum grip occurs at 10% slip, ie when the tires are rotating 10% faster than they would if they were rolling freely. For a given tire, the fastest theoretical 1/4 mile time would occur if you could maintain 10% slip (max grip) over the entire 1/4 mile, right? Unfortunately, you would need a couple thousand hp. Most cars can only maintain that 10% slip for a short distance. Thats why you look at 60ft times. Assuming you weren't caught sleeping at the line, the 60ft time is a measure of how well the tires are hooking up. Beyond 60ft, the engine is responsible for the speed of the car and the tires are just along for the ride. So wouldn't you agree that if your engine were able to keep the tires at 10% slip for the entire run, any increase in hp would not yeild faster times? Any additional power would cause the wheels to spin and slow the car. If you wanted lower ETs you would have to use bigger tires, stickier tires or increase down force.

Fortunately, the rubber molecules in your tires are dumb, they don't know if you are braking or accelerating. Max grip during braking also occurs at 10% slip (wheel rotating 10% SLOWER than if it were rolling free). If your current brakes can maintain 10% slip for the entire braking distance, any additional braking would cause the wheels to skid and increase the braking distance. If you want to decrease the distance you would have to use bigger tires, stickier tires or increase down force.

 

[drivinhard]

Yep the stickier the tire, the more grip you can put down, the better the car stops.

But take 10 laps at Road Atlanta or Summit Point, come down that 3000 ft fronstretch and pound the binders lap after lap, and see how those stock 10" brakes do compared to some real brakes after 10 laps. It's called heat management, and 96 brakes with good fluid and proper pads are BARELY up to the challenge for heavy track use. 10" brakes are a joke on the track. If you say otherwise, you ain't driving the car hard enough.

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White 92
SR71SHO - racer '89