>Reply-To: Subaru-Tech@egroups.com
>To: "Jeff Fortier" <silversuby@yahoo.com>, "STi-Mlist"
><STi_Mlist@ImprezaWRX.org>, <Subaru-Tech@egroups.com>
>Subject: [Subaru-Tech] Braking question answered?
>Date: Thu, 9 Nov 2000 17:484 -0800
>
>Hello,
>
>Jeff Fortier asked me about braking. I started answering him and I thought
>I'd post my answer to the Mlist and the Tech list to see if anyone sees
>holes in my logic. Also, it's time for some tech
>
>To improve stock braking depends on what you're aiming for. It you want
>pure, unadulterated stopping power, stock brakes with Porterfield R4 race
>pads for street use or some other brand of race pad for race use. Not
>talking about the street compound pad. Works incredibly. I'd say about a
>50% improvement over stock in feel and stopping when they're cold, and 200%
>better when they're hot. My miata went from about 130 (non-ABS) feet from
>60-0 down to about 100 feet. Downside is of course, that the pads last
>longer than the rotors. How much of an effect? You might get 2-3000 miles
>on a set of rotors
>
> Stopping has several components. You push the pedal and it causes the
>master cylinder to push brake fluid to the calipers. The caliper pistons
>push the pad against the rotor. The rotor is attached to the wheels
>through
>the hub. So the slowing down occurs when the tires slow down relative to
>the road (friction between the tires and the road). So, in that chain,
>just
>like modding an engine, start from the outside and work inwards.
>
> Biggest mod to stopping distances is tires. Get better tires with
>better
>traction. A bone stock car on race tires can probably stop from any given
>speed in 20% less distance. A great compromise is RE71s, which are pretty
>much completely out of stock at tirerack because they've been replaced by
>the RE730s, which have a much better treadwear rating but are not as good
>for handling, in comparison. At $65 for a 205/50R16, you just can't beat
>these tires. Comparable to the AVS-i's and surprisingly, a good compromise
>if you can't get those two is the Kumho Ecsta whatevers. OF course, more
>expensive tires will do even more good - like Pzips, MXX3s, etc.
>
> Then, moving inwards, the next step is the rotors. Well, for the most
>part, rotor diameter determines the torque arm through which the pads get
>to
>work, so greater diameter equals greater stopping power. Slots, holes,
>etc,
>make no difference for the street (if they do, then you're driving WAY too
>hard on the street - since they mainly help with fade and you shouldn't be
>fading brakes on the street
). But otherwise, slots are good as a>compromise since they tend not to crack the rotors and they don't chew up
>brake pads, while cross drilled rotors almost always crack and they eat
>pads
>for breakfast.
>
> Calipers, well, more pistons means better force distribution on the
>pads,
>which means longer pads can be used, which means more of the pad can be
>further away from the hub, so you get to take advantage of the greater
>torque arm.
>
> Calipers and rotors basically go hand in hand unless you make custom
>mounting kits. Bigger diameter equals more torque arm. Greater piston
>surface area means more modulation (Shiv, feel free to disagree - but I'm
>not talking about ultimate braking power, just street, non ABS braking) but
>more force at the brake pedal given the same size pad. Increase the pad
>size and you get less initial bite, but you get more heat capacity
>(arguably
>immaterial) and more surface area so more friction but less force per unit
>area. Basically, there are a million tradeoffs. Since most people buy
>brake kits, you can't do much aside from compare rotor diameters.
>
> And now to brake pads. Your size and shape is dictated by your
>calipers,
>but your compound is up to you. The pad works by creating friction between
>the fixed location pads and the turning rotors. Too much friction
>generates
>lots of heat and destroys the rotors by shaving them. The various
>compounds
>can basically be separated by their coefficients of friction at various
>temperatures. OEM pads are chosen to have a decent coefficient of friction
>when cold so that your cold braking (like after driving on the freeway for
>a
>long distance, or when first driving in the morning) performance is pretty
>good. However, the pads are usually made to last a long time and not make
>much dust. When it gets too hot for the OEM pads, they start to fail. If
>they way overheat, their compound changes and becomes brittle and they
>totally suck.
>
> Racing pads on the other hand, have poor performance when cold but grip
>like hell when hot. And they can take a lot more heat before they fail.
>However, decent race pads ignore the constraints of long life and low dust.
>So like my second paragraph describes, R4 race pads suck when cold compared
>to when hot, but still, cold, have better bite than stock pads can ever
>muster. The downside is that your rotors get torn to pieces and on my
>white
>miata, you can see a slight blackish gray haze that covers the car in a
>single day's commute to work. I have to wash the car daily now (yes, I'm
>getting new pads and rotors ASAP).
>
> In between, there are the aftermarket street pads. Hawk makes several
>pads that are very good. For pure racing, their pads are optimized for a
>slightly higher temperature than the R4 race pads, so the Hawks give a
>little less initial bite, don't stop quite as nicely cold, but stop better
>when they're hot. The Hawk street pads make less dust, have better initial
>bite, but can't take as much ultimate heat.
>
> I found that all the "streetable" mintex pads feel only slightly better
>than stock pads, and can't take hard braking without failing (i.e. on the
>track they bit the big one and you should see how far a legacy outback can
>fly off course).
>
> Anyway, which pad is better than which is a separate discussion.
>
>The next step is the brake fluid, which should make no difference in feel
>so
>long as you regularly bleed the fluid. For racing, though, you'll often
>heat the pads and calipers up hot enough to boil dot-3 fluids. Then you'll
>have air bubbles in the calipers and braking will suffer. Everyone but
>grandma or grandpa should use a dot-4 fluid. Look at the boiling point.
>Motul-600 boils at 600 degrees. Crazy high temperature (and price). Some
>dot-4 normal synthetic brakefluids (not silicon based) have dry points as
>high as 500 degrees. That's great.
>
> The final step that most people upgrade is the brake booster. I don't
>know for sure, but I think the multistage boosters don't affect
>performance,
>just feel. The multistage that most people here have feels much softer
>than
>the single stage found in most of the pre-95 or maybe even pre-97 cars.
>Basically, since we have power brakes, the multistage just gives a
>non-linear power input for the brake boosting. At some point in the pedal
>travel, you get more power assist. Makes the feel and modulation ability
>drop a little.
>
>I've probably gone on way too long, in the ways of the old Joel. I've been
>kinda quiet lately, though, so I thought I'd overdose y'all.
>
>Joel
>
>
>
>
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