0 Utenti e 1 Visitatore stanno visualizzando questo topic.
Sono sicuro che i 10 pistoncini aiutino proprio a dosare la potenza per un impianto di dimesioni molto grandi, comunque ? vero che lo sbilanciamento dell'auto in freata lo da soprattutto l'assetto, però ? anche vero che irrigidire troppo la macchina (soprattutto al posteriore) porta ad avere un posteriore nervosetto... comunque con i D2 far? un p? di prove
solitamente una TA con motore anteriore ha un bilanciamento pesi 60-40.Le case costruttrici fanno in modo che i freni partano con una ripartizione 65 - 35 in modo che tu blocchi l'anteriore... ma questa ripartizione ? creata soltanto mettendo dei dischi più piccoli al posteriore, la pressione ? la stessa.poi, man mano che la macchina frena e trasferisce peso davanti la pressione viene "tagliata" al posteriore e può arrivare fino a 80 - 20!!!!quindi se hai un impianto piccolo al posteriore rispetto a davanti la centralina taglier? con trasferimenti di carico maggiori e quindi con decelerazioni maggiori (rispetto all'originale): perché solo con esse rischierai il bloccaggio al posteriore!
Fact: when upgrading your braking system, you need to upgrade all four corners. Do not upgrade the front brakes only. This will actually decrease your overall braking performance in racing.Caveat (all facts have them, right): we're talking about a street car in predominantly stock form here used for both street and track use. The principles carry through to racier cars, but the numbers change enough to yield a different outcome. We'll explore that at the end.To understand why it is important to upgrade all four corners at the same time, whether it be pads, rotor diameter, or both, we can make the simple statement that the auto manufacturer balanced the braking force of the front and rear wheels to start with for the best braking performance and stability with the given parts. This balancing takes into consideration the car's weight, weight distribution, center of gravity, wheelbase, and weight transfer during hard braking conditions. Unless you make substantial changes to the car's weight distribution or center of gravity location, whatever brake upgrades you make, you want to keep the ratio of the braking forces the same as the stock setup.To show the effect of changing front pads only, front rotors and pads only, or all four rotors & pads, we'll look at a series of calculations to show the net effect of these changes on the force applied at the road and tire interface.First, we will assume a certain clamping force generated at the caliper based on the driver's pedal pressure, pedal leverage, master cylinder, and caliper piston size. We will assume this as a constant input to see what happens throughout the rest of the braking system by changing pads and rotor diameter. Of course, the actual braking force at the wheel is variable based on the pressure the driver applies to the brake pedal.The net result of the calculations of our theoretical setup shows that we start with a stock system designed to have a 64% front and 36% rear distribution of force. By changing only the front pads to a high friction racing pad, this braking distribution changes to 71% front and 29% rear distribution. If the front rotors are upgraded in size and the racing pads are used, the distribution changes to 74% front and 26% rear. This is a dramatic change from the intended balance of the stock setup. One very noticable effect is that the car will be less stable under hard braking -- the back end will wiggle around requiring the driver to control it. With an out-of-balance setup like this, stopping your car with huge front brake upgrades would be like stopping a bicycle or motorcycle using only the front brake. It's very unstable (and nerve racking).Another noticeable effect with a setup like this, where only the front has been changed, is significant front brake and tire wear, and little rear brake and tire wear. This is often misinterpreted as "the more front brake you have, the less you neeed in the rear." The rear system wears less, and therefore appears to be needed less. In fact, the reduced wear is not because the rear brakes are needed less, it is because they are used less. We will prove this a little further on with some numbers.The NumbersSo, let's start looking at some calculations. First, let's understand this front & rear balance thing with some numbers. Let's review the calculations to show the braking forces at the wheel. First, we need an input clamping force at the rotor. This is created with a certain pedal pressure, pedal leverage, master cylinder design, and caliper piston size. We're not trying to show the actual engineering here, just the principles with enough math to back it up, so we'll start with a set of assumptions that result in a certain caliper clamping force at the front & rear calipers. The components involved to this point will remain constant through our theoretical rotor and pad upgrades, so we'll use that same clamping force for all instances.The clamping force is multiplied by the brake pad coefficient of friction. This results in a net clamping force on the moving rotor. Next, we have to convert the static force of lbs into a ft/lbs of torque (our rotor is rotating, and a rotating force is defined as torque). Next, to translate the rotor torque into force at the road surface, we have to account for the diameter of the tire. This ends up back to a simple linear force in lbs at the road / tire interface.The table below shows the results of these calculations on four scenarios: the stock setup, front pad upgrade, front rotor and pad upgrade, and all four rotor and pad upgrade. The important net result is the balance of force between the front & rear, not the actual forces (which are variable based on driver's foot pedal pressure).OK, so now we see the relationship of how changing common bolt-on upgrades in the braking system affect the balance of front and rear braking forces. What does this do at the tire / road interface where the stopping action actually occurs?Braking in racing is concerned primarily with performance during limit braking. With this type of braking, the brakes are applied with enough force to be just short of locking up the tires. One thing you must be very clear about, is that the frictional forces of the tire on the road is what slows a car down. Rotors, calipers, and pads do not stop the car. The brake rotor is used to apply a torque on the tire. That torque is translated into a linear force at the tire and road interface that resists the car's forward motion at the road surface. This is what stops the car. Therefore, when looking at brakes you must look at the impact of the forces applied at the road surface. All the other calculations in between are simply balancing the engineering for designing parts to get to that point.A tire will generate a certain coefficient of friction based on the rubber compound, the size of the contact patch, the road conditions, etc. Another major factor in a tire's stopping power is the amount of downward force on the tire. Higher downward force increases a tire's traction. Because we're dealing with limit braking (the maximum end of the braking scale), we can simplify all the factors involved into two numbers. The downward force applied on the tire (which is a combination of a vehicle weight distribution and weight transfer under braking), and the tire's coefficient of friction (which reduces all the other factors into one number). If we multiply the downforce by the coefficient of friction, we get the maximum force the tire can create before it loses traction.Let's look at some numbers and see how this actually affects braking.Let's take a 3,000 car that has a static weight distribution of 55/45. That's 1,650 lbs on the front tires (825 each), and 1,350 lbs on the rear (675 each). Now, let's say under limit braking, we have an added weight tranfser shift towards the front of 265 lbs. That is now a total downforce of 1,915 lbs up front, and 1,085 lbs in the rear. Under braking, weight distribution is now 64/36.Under our limit braking example, the front tires will have a total of 1,915 lbs of downforce. Multiply this times the coefficient of friction of a good sticky street tire of 1.3 and you have a total of 2,490 lbs of force (1,245 lbs for each tire). The rear tires contribute 1,085 lbs x 1.3 for a total of 1,410 lbs of force (705 lbs for each tire). For all four tires to be at or near their braking limit, this also means that the brake torque applied by each brake assembly must be the same as the tire forces above. So, from the arbitrary starting point in the table above, we actually need to increase pedal pressure to acheive 1,245 lbs of force at the front brakes, and 705 lbs at the rear brakes.Now, be careful to remember that this amount of force could be generated by any size rotor, and any brake pad. The pedal pressure, master cylinder, and caliper could all be changed to account for various rotors & pads to generate this braking force. At this point, it really doesn't matter what the brake design is -- they can all lock up a tire. So, be careful not to get hung up on the fact that our sample table above shows different available brake torques. These are not maximum values -- they are sample values at a given brake pedal pressure (and other factors as mentioned).If we take either the stock or the example four wheel modified brake system (both have 64/36 force ratios), and increase pedal pressure to generate 1,245 lbs of force up front, this will result in a force of 711 lbs in the rear (we used the four-wheel modifed brake system for the numbers). This is a simple ratio calculation using (1,245 / 819) * 468. That's less than 1% off -- almost perfect, and close enough for real world use. In a racing setup, this kind of small difference could be dialed out with a front / rear bias valve. Using either of these brake systems, the front and rear tires would be very close to their maximum potential at the same time under limit braking.What happens when we use the front-only modified setup with a larger rotor and race pads? Again, the pedal pressure is increased to generate 1,245 lbs of front brake force. However, the rear brake force only generates 429 lbs of force at that pedal pressure ((1,245 / 819) * 282). For the rear tires to be at their braking limit, 705 lbs are needed. With only 429 lbs available, the rear tires are nowhere near their maximum braking potential. The rear tires are contributing far less to the overall braking performance than they could be. So with big front brakes, the rear brakes are not needed less, they are used less! Under racing limit braking, the big fancy brake upgrade may actually increase stopping distances! Brakes don't stop the car -- tires do. If you're not using 100% of all four tires, stopping performance suffers.But, you swear on the street they stop fasterFront-only upgrades may feel like they stop faster on the street. In fact, brake upgrade suppliers always tout shorter stopping distances. If brakes don't stop the car, how can this be? Until you reach limit braking (where the tire is 100% in control of stopping distance), the stopping force generated by the torque created at the rotor dominates the available stopping forces. With bigger rotors, you can generate maximum braking force with less pedal pressure, less pedal travel, and therefore less time. It's this reaction time and the faster ramp up of the torque applied at the tire that creates the shorter stopping distances. When you're traveling at 60 mph, you travel 88 feet per second. If your brakes reached peak torque .1 seconds faster, then you just reached limit braking 8.8 feet sooner. So in that respect, bigger brakes can shorten braking distances. Another way brake upgrades feel better on the street is that the bigger rotor will generate greater torque for a given brake pedal pressure. Therefore, the driver can press lighter on the pedal to get equivalent stopping power. It feels like the brakes make a big improvement in stopping power -- but they really don't. You just don't have to push as hard to get the same power.So Why Upgrade at All?For the weekend road racer with a street car -- in a word, heat!Stock style brake pads are not designed for the sustained high temperatures created by repeated high force braking. Their coefficient of friction drops significantly, and the material begins to fall apart with the high temperatures involved. On a track with several hard braking zones, brand new stock pads can wear down to the metal backplate in two hours of track time. Racing pads will last 4-to 6 hours of racing time, and their co-efficient of friction will remain consistent at the high temperatures.Stock rotor sizes, likewise, may not have the ability to dissipate the heat generated by race-duty braking. The repeated, high-temperature heat cycles may destabilize the mechanical integrity of the rotor. It could warp, or generate numerous small surface cracks. Larger rotors act as larger heat sinks. Their larger mass reduces the peak temperature they reach, and they stay mechanically stable. In a nutshell, they last longer.For the part time racer, racing brake pads are almost always needed for road racing. Rotor upgrades depend on the car, and it's original design purpose. A Vette's rotors will be fine. A Honda's rotors may not. Whether you need bigger rotors depends on the track you're racing at, and how hot the rotors get. If they warp after every couple race events, then larger rotors could save you money in the long run.The Race Car DifferenceOK, so after reading this, the experienced racers among you are saying "yeah, but..."There are conditions when this "keep the balance the same as stock" conclusion does not hold true. This is when the makeup of the car has been dramatically altered from it's stock form. Primarily this involves major changes in the car's CG position, it's total weight and / or weight distribution, or in its weight transfer characteristics under braking.If you have gutted the car, outfit it with much wider than stock tires, or use very sticky race tires, the net result of the braking balance can be quite different than the stock ratios. The ratio of front and rear braking forces changes typically reducing the amount of rear brake force required.
Al momento non ho tempo/forze per leggermi tutto il papiro ... Comunque
Better Brake PerformanceFiguring out pedal ratio and master cylinder bore sizeWayne Scraba / autoMedia.comOne area hot rodders, racers and custom car (and truck) builders tend to ignore is the brake master cylinder and, in particular, the actual brake pedal ratio. After all, it doesn?t make the car one bit quicker or faster, and if the thing eventually stops, why worry? Perhaps you should.Pedal RatioThe critical component in the braking equation is the pedal ratio. In operation, the brake pedal acts as a lever to increase the force the driver applies to the master cylinder. In turn, the master cylinder forces fluid to the disc brake caliper pistons or drum brake wheel cylinders. If you examine a brake pedal, you'll see the pivot point (where the pedal swivels) and the mounting point for the master cylinder pushrod are usually different. By varying the length of the pedal, and/or the distance between the pushrod mount and the pivot, you can change how much force (from your leg) is required to energize the master cylinder. This is the "mechanical advantage" or pedal ratio. This formula will help you figure it out: Input Force x Pedal Ratio ? Brake Piston Area = PSI.Mathematical babble? The arithmetic simply equates to the amount of force exerted by your leg times the pedal ratio divided by the area of the brake piston(s). FYI, the typical adult male can exert roughly 300 pounds of force (maximum) with one leg?and that?s a bunch. Something in the order of 1/3 or 1/2 that figure is obviously more comfortable, even in a hardcore racecar.The average manual (non-power boosted) master cylinder requires somewhere between 600-1,000 PSI to be totally effective. Somehow, 100-150 pounds of leg force has to be translated into 600-1,200 PSI. The way it's accomplished is by way of pedal ratio. While changing the overall length of the pedal is possible, it's often easier and far more practical to shorten the distance between the pivot point and the master cylinder pushrod mount location. That's precisely how many racecar chassis shops modify brake pedals.Brake Line PressureBrake line pressure is a different thing than the force you apply to the pedal. Force acts in one direction and is addressed in pounds. Pressure acts in all directions against surrounding surfaces and is addressed in pounds per square inch or PSI. "Levers" (brake pedals) can be used to change the force. Inside the hydraulic system, the surface area of the piston is what is affected by pressure. Decreasing the bore size of the master cylinder increases the pressure it can build. Pistons in master cylinders are specified by bore size. But there's a hitch: The area of a circle (or bore) is Pi?R-Squared. The area of the piston surface increases or decreases as the square of the bore size or diameter. For example, the area of a common 1-1/8-inch master cylinder is approximately 0.994-inch. The area of an equally common 1.00-inch bore master cylinder is approximately 0.785-inch. Switching from the larger master cylinder to the smaller version will increase the line pressure approximately 26.5% assuming that pedal ratio hasn't changed.As the pedal force or the pedal ratio (or both) is increased, the stroke of the master cylinder is shortened (brake line pressure is unaffected). When the size of the master cylinder piston increases, the output pressure of the master cylinder decreases. A smaller master cylinder piston will exert more line pressure with the same amount of force (pedal ratio) than a master cylinder piston with a larger piston area. There's another catch: Since the brake line fluid pressure is working against the surface of the wheel cylinder (or disc brake piston), increasing the area of the cylinder will increase brake torque. Improved BrakingThe bottom line is, if the stopping power of a car needs improvement, or if there?s a need to reduce the pedal effort, several options are available: (1) Decrease the master cylinder bore size; (2) Increase the pedal ratio; (3) Increase the wheel cylinder bore size. If the pedal ratio is increased, there will be more travel at the master cylinder piston. If the master cylinder bore size is decreased, the piston has to travel further to move the same amount of fluid. Typically, a master cylinder has approximately 1-1/2-inch to 1-3/4-inch of stroke (travel). The idea here is coordinate the pedal ratio with the bore size to arrive at approximately half of the stroke (roughly 1-inch) in order to make the brakes feel comfortable, and of course, to bring the car to a grinding halt.ResourcesMark Williams Enterprises, 765 South Pierce Avenue, Louisville, CO 80027, (303) 665-6901Copyright autoMedia.com 2000-2008
raga riesumo questo topic perché ho trovato uno sfascio che mi venderebbe le pinze anteriori ma non sa neanche lui se sono per il disco da 275 o da 255 mm..mi ha mandato queste 2 foto...voi che dite?si riesce a capire per che dischi vanno bene dalle foto? e nel caso fossero per il 275mm io che ho quelle per i 255 cosa dovrei fare per mettere queste?devo comprare delle staffe apposite o no? :lol: ecco le foto:
Citazione da: "Calimerus"Il concetto ? che la potenza massima frenante ? sempre la stessa (al limite col bloccaggio) con un impianto piuttosto che un'altro, in ogni caso più di quel valore non puoi frenare.Non sono assolutamente d'accordo Cali...E come dire che i freni carboceramici non servono a niente,come dire che negli anni non sono stati fatti progressi enormi nel campo degli impianti frenanti...pensa ai freni a margherita delle moto!
Il concetto ? che la potenza massima frenante ? sempre la stessa (al limite col bloccaggio) con un impianto piuttosto che un'altro, in ogni caso più di quel valore non puoi frenare.
Certo,pi? ? alta la velocit? a cui freniamo e più si sentir? l'apporto dell'impianto maggiorato,ma non ? vero che raggiungiamo il limite della frenata possibile causa aderenza anche con gli impianti stock...anzi secondo me ? già molto difficile con quelli maggiorati che mettiamo noi...
Un impianto stock a 220Km/h in fondo al rettilineo di monza su pista asciutta e pulita non ha la potenza per far raggiungere il limite di aderenza delle gomme,e quindi proprio per quello che hai scritto tu,non hai la massima frenata possibile,cosa che invece avresti con un impianto più potente.
tante idee e confuse... :lol: quindi stando alla tua tesi, ? solo ed esclusivamente una questione di coefficenti d'attrito :grin: a questo punto, ? sufficente cambiare le pastiglie originali, con altre dal cefficente d'attrito e temperature d'esercizio differenti, per raggiungere le stesse prestazioni di un impianto completo? :?: superfici, diametri, spessori, tipologia di pinza, nr dei pompanti, materiali, ripartizione della frenata ecc.... li buttiamo tutti nel cesso? :lol:
a questo punto, ? sufficente cambiare le pastiglie originali, con altre dal cefficente d'attrito e temperature d'esercizio differenti, per raggiungere le stesse prestazioni di un impianto completo?
Autore
Topic: Lezione su impianto frenante maggiorato : (Letto 17752 volte)
