I really never thought this would turn into such a brain pan match. Basically the way I look at it is this...a car like mine typically is not limited by its tires under braking. If you have run hoosiers which I guess you probably have in your life givin the cars you have listed off my guess is that there are cars you can put those tires on that do not have the ABS come on no matter how hard you stand on the brakes due to the traction levels being to high at the tire point. (Imagine a perfect extreme example being a gokart with 245's all around. No way in hell your locking those babies up) This is exactly what happened to my car. Basically the tire setup that I had on the car had more force than the brakes could apply on the track. Thus ABS never kicked because of no skid and I stopped in the end with no drama. Trust me I can hit the pedal hard and both on the track and in my tests I didnt ***** foot it. Now swap out the calipers and rotors and repeat. This time the braking force generated by better calipers with larger pistons and less deflective pads, and a larger rotor which moves the braking force out further from the center of rotation giving it more torque. (The exact equation here for the torque applied is something like Torque=Friction of Pad x Center of Pad pressure.) This extra torque is translated to the tires which are now acted upon by a force greater than their traction limits and thus activates ABS. However because of the traction limits of the tire being pushed higher this time the car stops faster. My statement on this topic the whole time has been that big brake kits can and will make your car stop faster in the majority of cases. In cases that are like those defined by you where the tire is a POS and the limiting factor in your braking distance yes you will not see an increase. I argue this is not the common case and more the uncommon case in sports cars. I have driven a good deal of 348s and 355s and if you put the right tires on them and are on a smooth track/road you can mash the pedal all you want and the abs will not come on. I still stand by my own experiement as well as the fact that I have also done the same tests with my friends 350Z and his StopTech setup which we installed 2 weeks ago. Confirming a 10 foot difference in stopping distance with the exact same tires and car just changing rotors and calipers.
Brain pan match? I find discussions like this very interesting, because it shows the panopoly of different thinking on this topic. Braking comes up frequently in classroom sessions at the track, and invariably, there are students who raise this and similar points (e.g., drilled rotors, warped rotors, efficacy of ABS, trail braking, stainless brake lines, etc.). It's helpful to EVERYONE to clear up myths and conceptions, IMHO. Ok, John, to your point specifically. To summarize, if I may, you're arguing that my statements are TRUE (when applied to stock, POS tires), *except* in the case of a 348/355 with aftermarket, *stickier* tires, like Hoosiers. In that case, the amount of traction that the tire produces under braking is *MORE* than the amount of friction that the pad/rotor interface can generate. (fair summation?) With all due respect, John, this is simply wrong. Call up Stoptech and ask them. They will say that any modern braking system can generate waaaaaaaaaaaaaaaaaaaaaaaaay more pressure at the rotor than ANY tire can generate, even under ideal conditions. Think about it. Take ABS out of the equation. By your argument, you're saying that a 348/355 stock braking system is INSUFFICIENT to lock-up sticky tires, which is patently false. Real world example - I had an intermediate/advanced student in a 355 at WGI. Running Pirelli P-Zero D3 racing slicks (lots more grip than Hoosier R-compound tires). He believed that he could brake the car more efficiently than with ABS, so he disabled it. Third lap, after warming everything up, going into the bus stop chicane, he hits the brakes, IMMEDIATELY locks up, and slides right into the safety area. We get cleared to go back ontrack, brief stop in the pits to discuss what happened, but he then does the same thing, only this time in turn 9 (a much slower entry speed). Locks-up, tail comes around, amazingly, he catches it, tail wagging, avoids the armco. In each instance, on stock brakes, he managed to lock up new Pirelli racing slicks. Still don't believe me? Think about it. In performance testing, an r-compound tire will generate maybe 10% extra lateral grip (e.g., bumping it from 0.93g to 1.03g). Even if the tire can generate even more grip, percentage-wise, when braking, you would have to believe that the stock brakes can generate enough pressure to lock up "stock" tires but lack the capacity to lock up "stickier" tires that increase the amount of grip by 10%-15%? Do you really think that Ferrari (or any other manufacturer) only builds braking system with less than a 10% margin??? You also wrote: >Now swap out the calipers and rotors and repeat. This time the braking force generated by better calipers with larger pistons and less deflective pads, and a larger rotor which moves the braking force out further from the center of rotation giving it more torque. (The exact equation here for the torque applied is something like Torque=Friction of Pad x Center of Pad pressure.) This extra torque is translated to the tires which are now acted upon by a force greater than their traction limits and thus activates ABS. However because of the traction limits of the tire being pushed higher this time the car stops faster. Sorry, but this is wrong too. A given tire can only produce X amount of grip. The tire doesn't "know" or care how big the rotors are that are being used to decelerate the wheel. It simply produces the maximum amount of traction that the contact patch can produce. When that limit is exceeded (e.g., the slip angle increases to max), the tire simply locks-up, and rolling friction is now sliding friction (which is LESS). Rotor size simply goes to how much force can be transmitted to the tire. If it's in excess of what the tire can produce, it doesn't matter HOW MUCH IN EXCESS it is. To extend your fallacious logic further, if you keep increasing the size of the rotor, the shorter the braking distance you'll get, to the point that with a hugely big enough rotor, the car would stop in zero distance. Which we all know is not the case. Bottom line, as I've pointed out from the beginning, and as the Stoptech articles themselves have said, simply increasing the size of the rotors does not help improve stopping distances. Lastly, I'll mention this. Stoptech is IN THE BUSINESS OF SELLING aftermarket brake kits. They make money by having people like you and me buy new rotors and calipers and lines and other parts from them. It's in their best interest to convince us of the NEED for bigger brakes. And what do they say? To their credit, they are HONEST: "The brakes don't stop the vehicle - the tires do. The brakes slow the rotation of the wheels and tires. This means that braking distance measured on a single stop from a highway legal speed or higher is almost totally dependent upon the stopping ability of the tires in use." "In order to brake effectively, the tires must comply with and grip on the road. Your braking system is no better than your tires and suspension. The best money that you can spend is on really good tires and really good shocks." "Contrary to popular belief, the real reason sports- and racing cars use big brakes is to deal with heat. Period. There has been a bunch of stuff published which will disclaim this, but when you look at the braking system from a design standpoint, making them 'bigger' doesn't fundamentally do anything for stopping distance. It's all about the heat. " Vty, --Dennis
Brain pan match is a little term I use for an interesting conversation. So to summarize we both agree on: -Tires are the limiting factor in the equation for the most part. Tires do not care about or know about your brakes (You quote me as saying they do but I never infact said that...my point was a bigger rotor gives your more torque up until it exceeds the tires traction limits.) -Larger discs have the main function of providing more surface area for cooling. -Rotor size simply goes to how much force can be transmitted to the tire Where we disagree: -All vehicles can easily lock up without ABS or effectively have ABS come on anytime you mash the brake. I think this claim is absolutely insane as there are a lot of cars out there that have under sized brakes for the tires they run. Beyond that and even more truthful is that for optimial braking you DO NOT want ABS coming on. Honestly if you think that auto companies build cars with brake systems that are alway pegging ABS you are mistaken. They build them so that under maximum braking on a smooth road the car will be at the threshold of the ABS coming on. The ABS coming on does not stop your car faster it actually stops it slower. ABS is there as a safety procaution for rough roads, wet conditions, and ice where it is easy to lock up the tires its NOT there for optimial braking. Thus it follows from this that you are quite wrong in stating that all cars are designed to hammer on the ABS as soon as someone hammers the pedal. They are actually made to brake at the threshold of grip for the OEM tires on the car as that generates minimal braking distance. I know this as I have a friend that develops braking systems for GM (well until they lay him off ) Beyond the new car optimal braking stuff you also just have physics and wear on older cars. The force applied by the hydralic system is only of a certain maximum level. If the pad flexes, brake line flexes, caliper flexes, etc the maximum force from displacement of fluid from the master cylinder goes else where and is not transmitted to the brakes. If the torque generated by brake rotor and caliper is less than or equal to the tires maximum grip level the abs doesnt come on. - Larger Brake kits do not matter on cars. First off I still have not seen any evidence from you to back up your claims. I have two test cases where they did make a difference and you have yet to show me one that shows they did not make a difference. Yes the stoptech articles are excellent I have read them at length but to blindly say because they say a big brake kit does not help in all situations means that there is no reason to upgrade is insane. StopTech wouldnt sell you big brake kits and make claims to stopping distance improvements if they didnt make the car stop faster...look at their tests for a good variety of cars which shows that their kits make the cars stop better by either balancing the cars better under braking (thus balancing traction of the tires better for faster braking) OR providing more braking force. Actually they have a 350Z test some where I remember reading once that showed their front brake kit made a difference (Just googled it http://www.zeckhausen.com/Testing_Brakes.htm) showing that there is an improvement in their test cases as well with the stock tires on the Z. There are lots of cases where the above test holds true or actually improves that I have read over the years that backed up my testing. If honestly I didn't have actual data supporting my belief and that the change made a difference I sure as hell wouldn't argue with you as much as I am on this point. For example if this was the tornado thing that goes in the intake of cars I would stop arguing Anyway great conversation here...technical stuff always beats my general repair based conversations. PS. If your buddy in the 355 about spun under braking I would say his brake bias is way off and he locked the rear wheels. Locking the fronts wont cause you to spin at all, just push out through the turn....unless he did something strange. Sounds like the car wasnt setup properly actually as its pretty damn dangerous to have ever have the rears lock on a students car.
1. "Rotor size simply goes to how much force can be transmitted to the tire" - well, not quite, right? If one has upgraded the size of the rotor without changing anything else in the system, how does that translate into greater force? The caliper is still the same, and the brake pad is still the same size. The amount of surface area that the pad is contacting has not changed. So how? 2. Let's be careful - I didn't say that all cars ever made can brake hard enough to lock up their tires. I have said, and continue to say, that any modern car (e.g., built in the last decade or two, using disc brakes) will indeed transmit more than enough pressure to lockup its tires, even if those tires are particularly sticky. More specifically, I said that the 348/355 stock braking system is so way over-engineered that it can lock up even new racing slicks under ideal traction circumstances. You wrote, "We disagree-All vehicles can easily lock up without ABS or effectively have ABS come on anytime you mash the brake. I think this claim is absolutely insane as there are a lot of cars out there that have under sized brakes for the tires they run. Beyond that and even more truthful is that for optimial braking you DO NOT want ABS coming on. Honestly if you think that auto companies build cars with brake systems that are alway pegging ABS you are mistaken. They build them so that under maximum braking on a smooth road the car will be at the threshold of the ABS coming on. The ABS coming on does not stop your car faster it actually stops it slower. ABS is there as a safety procaution for rough roads, wet conditions, and ice where it is easy to lock up the tires its NOT there for optimial braking. Thus it follows from this that you are quite wrong in stating that all cars are designed to hammer on the ABS as soon as someone hammers the pedal. They are actually made to brake at the threshold of grip for the OEM tires on the car as that generates minimal braking distance. I know this as I have a friend that develops braking systems for GM." You're laboring under some misimpression, if I may , as to how ABS works. For those less technically oriented out there reading this (yeah, right, as if anyone but you and me are still reading this!), keep in mind that the coefficient of friction for a rolling tire is greater than that of a locked tire. In other words, you'll stop in a shorter distance if your tire is not locked up. The traditional way of preventing lock-up was to do "threshold braking", wherein the driver applies just enough brake pressure to keep the tires at the "threshold" of braking. The theoretical "ideal" for braking is around 15% of slip angle. What I mean by that a freely rolling tire has a slip angle of 0%, while a locked-up tire achieves 100% slip angle. For maximum braking, you want that tire to slip a little bit - just about 15%. HOW ABS WORKS: modern four channel ABS works by calculating incipient wheel lockup. This is NOT just by sensing when a wheel IS locked up, then releasing it, which is a common misconception. Rather, the ABS system senses the wheel rotation, compares tiny tiny tiny differences in the rate of rotation at different wheels (three channel looks at each front wheel and both back wheels as one, four channel looks at all four individually), looks at its algorithms and calculates the ideal slip angle for braking, then dances back and forth around it. ABS, when engaged, NEVER allows a wheel to lock. Rather, the wheel is question will be rotating SLOWER than the other wheels; when the difference in the rate of rotation reaches a certain point - the slip angle has increased beyond a permitted parameter - then the ABS intervenes. That intervention occurs before the slip angle reaches 100% (a locked wheel). A given ABS system might allow a slip angle to reach 20%, then decreases line pressure to THAT wheel, allowing the slip angle to fall back to 10%, then reapplies pressure until it gets to 20%. So, yes, a great driver can instead keep that wheel at 15% slip, and, yes, a theoretically perfect driver can brake the car at 100% of the tires combined braking force, at the maximum effective slip angle. Whereas a good ABS system will be slightly below that, because it's never at the max, but rather always just below and then just above it (not locking the wheel, BTW). BUT: the magic of modern ABS is that it can MODULATE EACH WHEEL INDIVIDUALLY. The driver can, at best, reduce pressure at all four wheels. You have one brake pedal, which operates all four wheels. And cars run on varying surfaces - surfaces that offer differing levels of grip TO EACH TIRE. If you have 15%, 15%, 10% and 100% slip, how do you fix that? You decrease pedal pressure, but that drops the braking power in the other three wheels as well. As one example, at New Hampshire International Speedway, turn 9, there is a dip as I enter it. I always lock up my left rear wheel. The ABS fixes that. If I didn't have ABS, I'd either have to live with that one locked wheel (and decreased braking ability), or else reduce pressure overall to get that wheel rotating again (decreased braking ability too). Don't believe me? Check out: http://www.geocities.com/nosro/abs_faq/#What%20are%20the%20advantages For those who believe that ABS makes cars slower on the racetrack, let me ask you a question. If ABS does make the car slower, then why has almost every top-tier sanctioning body banned ABS? Is it because they want cars to go faster? Nope. They make cars turn faster lap times, but reduce the "show", because they make it easy for even the best drivers in the world to brake MORE EFFECTIVELY. For example, ABS was in F1, but the FIA banned it because it made the cars too easy to drive - as well as faster. Teams didn't adopt ABS because they wanted to turn in SLOWER lap times, right? If the world's best and highest paid drivers run faster lap times with ABS installed in their cars, do you think you'd benefit from it too? The *BRAKES* aren't designed to brake at the "threshold of grip for the OEM tires", as you write - no, the brakes are designed to apply force to the rotors to decelerate the wheels and tires. The amount of force to be applied is selected by the driver's foot. The manufacturer can design brakes to apply a varying amount of pressure at the caliper in relation to the amount of pressure at the pedal, but the actual amount of force in a given situation is selected entirely by the driver. Brake systems aren't designed so that ABS kicks on immediately, or kicks on later. Those are two entirely different things. Whether ABS kicks in is SOLELY a function of HOW HARD the driver is standing on the pedal, in relation to the environment and the car's speed. You an design a system that applies enough brake torque to lock up the wheels at, say, 20 lbs of pedal pressure (in a given environment at a given speed), or you can design that system to apply enough brake torque to lock up the wheels at, say, 50 lbs. of pedal pressure. What I'm saying is that FOR EVERY MODERN CAR, the brake system WILL lock up the wheels (or trigger ABS) at some achievable pedal pressure, whether it's 20 lbs, 30 lbs. 50 lbs. or 100 lbs. 3. "proof"? Well, my "proof" is as good as your "proof" - you said that you didn't think that a 348/355 could generate enough brake pressure with stock system to lock-up hoosiers. I submitted an incident wherein enough brake pressure was generated to lock up Pirelli racing slicks, on the stock brake system, at the moment the driver (over)applied the brakes. As for your "tests" - well, alas, I don't have it here at work with me, but there is a GREAT quote from Carroll Smith (dean of race engineers) in his book about driving and car set-up, on how to test braking, and I'll quote it when I get home. But I already pointed out a potential flaw in your methodology - indeed, that's the whole point of this debate. You said that your tests showed a difference in stopping distance, because the stock brake system didn't generate enough pressure to activate the ABS, whereas the upgraded system did. My response, and the genesis of this digression, is that you simply didn't use the brake hard enough to activate the system. That's what we're debating! And I'll stick with my position. Call up Stoptech, and just ask them if they think a stock Brembo system on a 348/355 can generate enough pressure to lockup any modern street tire, any r-compound, or any racing slick. Just ask. I know what the answer is.... Oh, yeah, regarding your comment, "If your buddy in the 355 about spun under braking I would say his brake bias is way off and he locked the rear wheels. Locking the fronts wont cause you to spin at all, just push out through the turn....unless he did something strange. Sounds like the car wasnt setup properly actually as its pretty damn dangerous to have ever have the rears lock on a students car." The first time he locked up the brakes, he slid *straight* into the bus stop chicane runoff area, as I said. The second time, in turn 9, he was actually turning and braking (it's a sharp, decreasing radius left-hander); trying to trail-brake. He locked up all four wheels. The reason the car almost spun was not because of too much rear bias (resulting in locked-up rear tires only), but because he was also cornering and adding lateral forces. The fronts were slipping as well, which is why he managed to CATCH the car. If it were only the rears locked up, he almost certainly would have hit the armco (right up on the edge of the track there). Vty, --Dennis Have fun on the track!
I'm not an engineer or mechanic, but if 348/355 brakes are so bad why did they use stock rotors in the challenge series? I think heat ruins brakes. The 348 has more than enough stopping power at normal temps. I think if you upgrade anything, I would add cooling ducts and higher rated brake fliud for track use. For street i don't see the need to upgrade 348 or 355 brakes. I am trying to get Valence USA to reproduce the cooling ducts used on 348 challenge. Except crossed drilled, bigger brakes look cool especially with after market wheels.
I've just read the last 3 posts here (most relevant are two long diatribes on how brakes and ABS works) and basically, have to say that the 2nd guy is pretty much right on. My background? I'm an ABS development engineer at a major auto manufacturer. ABS will beat a drivers best effort on non-deformable surfaces every time because of the system's ability to modulate each of the 3 or 4 channels individually and with such improved precision relative to a person (or a computer) pushing on the brake pedal. If you really want to debate specific cases, you can discuss the variances in mu-slip curves between street radials and race radials or slicks...
At work so cant comment long. I am still sitting hard on my original opinion that "big brake kits" do infact improve stopping distances and no one has proven that is not the case all the talk of how ABS works is off topic (interesting but off topic). This is where the whole argument came out of and still I have not seen anything that proves your blanket statement that brake kits do not improve stopping distances. For number (1) in your long note which I havent read through yet. That is first off a quote from your post a while back AND also moving the caliper out further from the center of the wheel (ie larger rotor) does increase the torque of the brakes even if the friction generated by the pad does not change. That is just simple physics and if I remember correctly is a formula inside of one of the stoptech guys white papers. Eitherway this argument might not ever end. I personally know from experience that I am shorter in stopping, my friends 350Z is shorter in stopping with a Big Brake kit. So to me does it make a difference yes...from my scientific beliefs it makes a difference as well. Don_xvi - multichannel ABS came into being and has advanced more with the electronic traction control mechanisms correct? Plus most of the current units are a hell of a lot more intelligent and sample wheel rates at a much higher rate than older systems correct? I know my the PSM setup in most of the new Porsches is pretty damn complex and uses multichannel pickups to basicly figure out when specific wheels are locking when trail braking to make ABS interruption smoother.
Ok, I'll keep this short and sweet. John, do you or do you not think that a stock 348/355 brake system is capable of generating sufficient force to lock-up wheels shod with sticky tires? Yes or no? If "yes", a stock system **IS** capable of so doing, THEN an upgraded brake system does NOT contribute to shorter braking distances (leaving aside heat and fade issues) -- because immediately after the brake pedal is stepped on, the brakes are applied hard enough to trigger ABS and the car is stopped, with the ABS system constantly watching and controlling. In other words, it doesn't matter how much additional pressure can be brought to bear on the brakes, because the ABS is releasing line pressure anyway. If "no", then your position is (or seems to be) that the inadequate stock system cannot generate sufficient pressure initially, and only after some period of time after the brakes have been applied, will the system have generated enough force to trigger ABS (I don't understand how initially the force is insufficient, but somehow becomes sufficient after some period of time has passed). In this scenario, then braking distances are longer (with the stock system) because the upgraded system has triggered ABS immediately, while the stock system takes time to trigger ABS. I guess we can continue to disagree, but I'll stand by MY position that any 348/355 stock brakes are waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay more strong enough to lock up even racing slicks. Hey, I'm glad that you're happy with your purchase. Me, I'm happy spending my money instead of better pads, regular fluid changes, and new sets of tires. vty, --Dennis
Some experience i have with upgraded brake systems. First car with modern ABS System brakes on Front upgraded. Stopping distance on cold brakes same as stock ! But massive improvements after some heavy braking. Because the bigger brake is cooler. Also the bigger brake was more durable. Then on a car with an older abs system upgraded brakes on the front. stopping distance longer! then standard brakes because abs dosn't work good in combination with the new brake. But also there when the abs was deactive the stopping distance was better when they are hot. Last experience bigger brakes both front and rear , bad idea car spins and hit a tree. The rear brakes too much. Car went to the junkyard. Because that i would chose the brakes carefully not only bigger is better. It should fit all together. And also the improvements of a bigger brake are more in the durability and the constant brake results after many hard stops. Not to lower the overall stopping distance on cold stock brakes. If the wheels already lock on the stock brakes a 10 times bigger brake won't stop the car faster. I also saw an newer car wich had problems with much bigger brakes on an 1/4 mile event. The car (Audi rs6 or rs4 dont remember) was modified with much bigger brakes on the front and the rear after the 1/4 mile run in the braking zone the abs disengaged the brake totaly after one heavy wheellock and the car went into the border on the end of the track without any braking. But such an heavy failure i only saw that one time never experienced this myself. I only get wired regulations from the abs to long lock or to long pause between locks, .... _________________________ Samy
I also think that but they are much to small for track use. Not because of the stopping distance but because of the heat and durability. For the street i won't put an bigger break on ... maybe only for the look .. because a small brake looks bad in a big f.e. 19" wheel. _____________________ Samy
Samy, you're absolutely right; I think we're all in agreement (as has been noted since the top of this thread), upgraded brake systems can help a lot with fade issues under constant, heavy use, like in a race. The stock 348/355 brakes are perfectly fine for street use. And they're even pretty good for track use in a driver's ed-type environment, if you make sure to have fresh fluid and a good set of pads (e.g., PFC). I say this from having thousands (yes, thousands) of laps in my 348 and 355 (before I got my 355CH). I've even got some good video of me in my mostly stock street 355B chasing down and passing a bunch of Challenge cars at Lime Rock during the sport sessions at the 2004 Ferrari Racing Days event. Mostly due to outbraking 'em into Big Bend, heh-heh. Samy, good point too about anyone who upgrades their brakes to make sure to do the ENTIRE SYSTEM, and do it properly. The Stoptech articles from which I quoted are adamant about this - just willy nilly throwing money at bigger components can actually REDUCE your braking performance. vty, --Dennis
My statement to your question below is "no" - my position is that the inadequate stock system cannot generate sufficient pressure ever unless in a unstable braking zone to trigger the abs. In this scenario, then braking distances are longer (with the stock system) because the upgraded system is able to get closer to the ABS threshhold immediately thus maximizing the braking force of the tires earlier. I really dont know what to say man...I mean the proof I have is real that it makes a difference, the proof that stoptech has at http://www.zeckhausen.com/Testing_Brakes.htm proves it makes a difference. Beyond that I guess we are just free to believe what we want. I personally in my experience with multiple 348s (which who knows they may just be **** boxes compared to your cars) running slicks do not believe you can lock up the tires instantly when ever you want on a good track surface with warm tires. With the brembos you can. Bottom line thats my experience in a good deal of laps myself. Trust me I am actually happy with my purchase and would be even happier to out brake you into a turn just to prove that it works. Well except for my car weighing more than you CH car and blah blah blah...I need a challenge car someday here soon or just BillyBob my car. Out of curiousity after all the beef you have with bigger rotors and what now why would you spend money on better pads? I mean really if you can lock up your tires when ever you want with the stock pads why get new ones? I hope your answer is that you get them for friction rates over multiple laps, as well as backplate stiffness and heat BUT I just want to make sure.
I don't want to jump into an argument, but the claim that 348 brakes are not good enough for the track is wrong. '94, '95, '96 348 challenge cars by regulation were required to use stock rotors. These cars ran 18 in. slicks. I was not involved in the 348 cahllenge, but I don't here alot of talk about brake failures. In fact my '94 348 challenge had original rotors on it until 2003.
Richard...I agree with you that the stock brakes arent bad. I just think its pretty easy to make them better. I guess that is my whole argument here. Trust me I know after many MANY miles that the stock brakes can stop ya, I just think the new ones stop me better.
I don't disagree. Actually when rotors were replaced on my car the previous owner used slotted rotors and race pads. I'm sure stops better than originals.
imho , you'd get a more noticeably better braking setup by just upgrading your brake-pads with fresh brake fluids , than going big brakes or whatever. and , any modern braking system will be able to bring your tyres into lockup. (notice , I am talking about lockup , I am not talking about all the braking in between between initial pedal push and lockup). so if you're just talking about ability to lockup , then yeah , any modern brake will do it. but how the brake behaves before lockup is , probably , what everyone here is more interested in. and thats where the bigger rotors and bigger calipers , come into the picture. cheers.
I think that larger diameter rotors will decrease stopping distances. Tires do not lock instantaneously upon application of the brakes. The rotor, after it comes into contact with the pads, will slide over them some distance before it comes to a stop relative to the pad. This is when your wheels lock. Lets say that, without changing pads or rotors, we increase the mean swept diameter of the rotors from 10 to 15 inches. I don't know how far 348 rotors have to slide over 348 pads at full pedal pressure to stop, lets guess and say 200 inches. The numbers don't matter - they're just an illustration of a concept. The swept distance of the 10" rotor per revolution is pi x 10 = 31.4 inches. The swept distance of the 15" rotor per revolution is pi x 15 = 47.1 inches. If it takes 200 inches of sliding over the pad for the rotor to stop, the 10" rotor will stop in 200/31.4 = 6.37 revolutions, and the 15" rotor will stop in 200/47.1 = 4.25 revolutions. I've been talking about circumferential distance, not linear distance up to this point. If the wheel becomes locked or the ABS cuts in at 4.25 vs 6.37 revolutions, you should save 2.12 revolutions, assuming everything stays the same after the wheels lock or the ABS cuts in. If your tire's O.D. is 25", you travel pi x 25 = 78.5 linear inches per wheel revolution. Saving 2.12 revolutions means 166.4 inches or 13.9 feet shorter stopping distance. The greater rotating inertia of the bigger brake will reduce this loss, but I'm just trying to make a simple illustration, so I ignored it.
Valence, your fomulae are interesting and elegant , and the arithmetic is correct, but you started with at least two erroneous assumptions. Let's extrapolate from your conclusion and see where that takes us. 1. In your example, you used a tire with an outer diameter of 25", which would, as you say, give us pi x 25" = 78.5 linear inches per wheel revolution. If we increased the diameter to, say, 35", that would give us pi x 35" = 110 linear inches per wheel revolution, which means that the increase in rotor size from 10" to 15" (saving 2.12 revolutions) would therefore result in saving 233.2" or 19.43 feet of stopping distance! Do you see the error? You can't just apply some arithmetic and derive a real world answer. Keep uping the diameter of the tire, and you end up saving so much stopping distance, the car stops instantaneously - which you yourself said cannot happen - "Tires do not lock instantaneously upon application of the brakes." 2. Next, let's keep the tire size as a constant. By your logic, an increase in rotor size will always give us shorter stopping distances. For example, if we had a 50" rotor (yeah, I know, bigger than the tire, but pretend that the road has a groove cut in it that allows for this piece of fantasy). The swept distance of the 10" rotor per revolution is pi x 10 = 31.4 inches. The swept distance of the 30" rotor per revolution is pi x 50 = 157.1 inches. If it takes 200 inches of sliding over the pad for the rotor to stop, the 10" rotor will stop in 200/31.4 = 6.37 revolutions, and the 50" rotor will stop in 200/157.1 = 1.27 revolutions. If the wheel becomes locked or the ABS cuts in at 1.27 vs 6.37 revolutions, you should save 5.1 revolutions, assuming everything stays the same after the wheels lock or the ABS cuts in. If your tire's O.D. is 25", you travel pi x 25 = 78.5 linear inches per wheel revolution. Saving 5.1 revolutions means 400.35 inches or 33.36 feet shorter stopping distance. Again, if you keep uping the size of the rotor, you'll have saved so much stopping distance, the car stops instantaneously - which you yourself said cannot happen - "Tires do not lock instantaneously upon application of the brakes." 3. Another experiment. Jack your car up in the air. Spin the tire as fast as you want. Stand on the brake. Does the wheel lock up instaneously? Yup. Ok, now picture the car rolling on a puddle of oil on top of a frozen lake. Hit the brake. Does it still lock up instaneously? Yup. Keep making the test have more grip, until you get to, say, a cogged wheel in a track. If the car is going 100 mph, and you slam on the brake, does it lock up instantly? Probably not. In this situation, having more rotor surface area (among other things) **WILL** help slow the car down. Somewhere between these two extremes is THE REAL WORLD. Look, the bottom line is, as I keep trying to point out, the LIMIT of the modern braking system IN THE REAL WORLD is NOT at the rotor, but at the TIRE. If you built a hypothetical car, with multiple rotors and calipers at each wheel, stacked in a row, so that you effectively had an insane amount of pad/rotor contact, you still wouldn't have any effective decrease in braking distance because the modern braking system can generate enough force to cause instant lock-up at initial application. THAT'S the point of this debate. jjstecher believe oppositely that while my statement is true for most vehicles, it is not true for a 348/355. We can have an opinion as to whether my statement is true or not, but clearly, applying some math doesn't prove anything one way or the other, as elegant as it may seem. vty, --Dennis (and here I thought this debate was dead! )
THAT is the Dennis I know & admire. Great job D. Good to still read a post from you now & again. I hope your staying dry.
To respond to your numbered responses: 1) No, I do not see the error. Perhaps I was not clear, but I am assuming a constant tire diameter in both cases of 25". I did not state otherwise. The only variable in my "experiment" is rotor diameter. 2) This is a mathematical limit problem. The wheel revolutions made before the wheel locks or ABS cuts in will approach, but never reach zero, as the diameter of the brake rotor increases. Increase the rotor to 1000" diameter, and the wheel stops in .06 revolutions. Zero is never reached. Look at the x's and dashes below. The X's are the part of the braking equation that I am talking about. They can get smaller, saving braking distance. We'll never see zero x's, even with 1000" diameter massless brakes. The post-wheel lock/ABS activation is illustrated by dashes. These dashes account for a larger part of the total braking equation, and are where tire traction is the limiting factor. I am only talking about the x zone, and this is where I feel bigger brakes can make a difference. Probably less than 10% of the total picture, but a valuable 10%, and the one that most people feel in the seat of their pants most of the time. xxxxx------------------------------------------------------------ 3) No finite mass stops instantaneously, especially in what you illustrated nicely as the real world. Bigger brakes can help reduce the x's - the pre-lockup time period. Of course the bulk of the braking event's actions relate to tire performance, probably more than 90 %. As I stated above, though, if you can reduce the "x" zone time period and distance, it will likely be noticed by the driver, feel good, and those 10 feet or so of non-tire related saved stopping distances just might save the front end of your car, or that kid that just ran out into the street. Dennis, I, and I assume the rest of us, realize that the overall braking equation is governed by tire performance, but there are other factors to consider, like the one I have described. I took milliseconds and feet of the beginning of our hypothetical panic stop, and this is important and relavant. -Chris
