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NAXJA Forum User
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A few observations of my own (primarily to be considered anecdotal - I haven't run any sort of testing on these observations, either "scientific" or "real-world." This also relates to various vehicles, various drivers, and various situations - from a girlfriend I had who went through four wheels' worth of brakes every 6-8 months to myself and my wife - who can usually get 3-4 years out of a set of brakes. YMMV.)
Fluid - change at least semi-annually. Brake fluid is hygroscopic - meaning it attracts and retains atmospheric moisture. Change by sucking the fluid out of the master cylinder reservoir (don't uncover the ports in the bottom!) and wiping that section out, then refill with fresh. And by "fresh", I mean a bottle you just broke the foil seal on. Bleed at wheels in normal order (farthest to nearest,) running the vacuum on the bleeder nipple until you get clean, clear fluid through the hose. This not only will change your brake fluid, but it also helps to flush ancillary crud out of your system. And, if you note that the fluid comes out almost black - plan on changing hydraulics in short order (something else you're not likely to see with a "gravity bleed" - but you might with a "pressure bleed.") One of the two main causes of brake "fade" is steam voids forming in the fluid from accumulated water under sudden braking from speed - the water boils up. Steam is compressible, brake fluid ain't. This is why steam voids can lead to brake fade. (The other cause is "pad gassing" - more on that in a minute.)
Pads & Rotors - Opinions vary in the service world on what I'm about to say, but I picked it up from racing tech literature (Pro-Am, Street/Strip stuff) - brake friction and metal should not be replaced at the same time. Ideally, you will bed a new rotor with pads that are already bedded, and vice versa.
"Bedding?" Yes - all new pads will outgas (release trapped gasses when heated) - it's just the nature of the beast. There's nothing that can be done about it, and that's what makes break-in periods so bloody important. By properly releasing that gas with controlled heating, you preserve the structure of the friction material (yes, it is possible that pads/shoes can crack and/or shatter if you don't bed them properly.) This is why outfits like Wilwood and Baer will go so far as to sell their pads "pre-bedded" - they will "bed" them to a test rotor on an in-house brake dynamometer, and they're all set up for a new rotor when you install them (since the pads aren't technically "new" - even though they've never been on a vehicle.)
With rotors/drums, you're not dealing with outgassing. However, bedding is no less important - cf. "normalizing" of metal castings for more information (brake rotors/drums are almost invariably iron castings - typically grey iron.) In short, normalising will release internal stresses from uneven cooling through the casting, and by controlling the application and dissipation of heat you control the concentration and dissipation of those stresses, and that residual internal stress can be reduced to nil. NB: This is the same reason that "proven" (read: used - prefereably in a grocery-getter) engine component castings are preferred for high-performance buildups than brand-spankin'-new castings - residual stresses are gone, and that removes one potential mode of failure.
Rotor Surfaces - Much debate exists on the relative merits of solid/vented/drilled/slotted/milled/dimpled/whatever rotors (or a combination of any two or more of the previous.) Why are rotors sold as anything other than solid? Simple - heat. When you apply brakes to slow down or stop, you are converting kinetic energy (motion of the vehicle) to heat energy. Remember the "Law of Conservation of Energy?" "Energy can neither be created nor destroyed." Therefore, it can only be converted - and this can mean loads of heat. I don't recall the formula offhand (I'm sure I have it somewhere,) but the conversion of the KE of two tons of metal moving forward at 60 miles per hours results in one Hell of a lot of heat - whether you express it in British Thermal Units (increase the temperature of one cubic inch of water by one degree Fahrenheit) or calories (increase the temperature of one cubic centimeter of water by one degree centigrade.) I do know one thing offhand - if you are silly enough to touch a brake rotor after a hard stop from freeway speeds, you are not likely to have fingerprints afterwards.
Any surface machining of the brake rotor (drilling/milling/slotting/grooving/whatever) results in an increase in the surface area of the rotor, which allows for more heat rejection and faster cooling. This leads to a reduction in brake fade due to pad outgassing, as well as a reduction in trapped air under the pad surface (not technically "fade", but it does affect braking action.)
The catch? Any time you have a sharp edge or corner, it acts as a heat concentrator and a stress riser. Anytime you concentrate heat (for whatever reason,) you further increase thermal stress - so you've got a double-whammy if you just drill a hole. Middlin' drilled/slotted rotors will have a chamfer leading into the hole or slot - while it doubles the number of corners, it also significantly reduces the effect - and it comes out to be somewhat less than a raw drilled hole. The really good rotors will be drilled/slotted, and then be radiused going into the hole/slot, so there is no corner to concentrate heat or stress! I have yet to evaluate these, but they do make good sense from the standpoint of pure engineering. I haven't looked into them in a while, but I recall they weren't cheap.
Are these for everyone? Hell, no! If you run in sand; probably not a good idea, stick with solid/vented rotors. If you run in mud; definitely not a good idea! The holes/slots will rapidly clog up with crap you pick up from the trail.
Street/strip rigs? Sure - they're not going to see enough contamination to be a real threat. Rockers? Probably good - as long as they stay out of the gooey or shifty stuff. Stay on dry rocks, and you'll probably have a benefit to stopping action and instances where brake action will help you climb.
Brake Fluid - Stick to a quality fluid. If you've got it in mind to change DOT spec - say, DOT3 to synthetic silicone, DOT3-DOT5, or even DOT3-DOT4, then you're going to do a flush of all of the hardlines and replace all of the softlines and the hydraulic parts (rubber seals soak up fluid as well, and DOT3/DOT4/DOT5/Silicone are mutually incompatible.) You won't need to change the booster - fluid isn't supposed to be in there - but you'll have to change the master cylinder, the wheels cylinders, and anything else rubber. And when I say "flush out the hardlines," that's precisely what I mean - it may be easier to replace them outright, if you have the means to (if not, blowing a full can of brake cleaner through them might do, but I wouldn't guarantee it.) So, you see the headache you could be letting yourself in for there.
As I'd said, this is primarily anecdotal, and I may have gotten a couple of things crossed (but I doubt it.) If anyone can prove me wrong, please feel free to do so - but it won't be as easy as telling me I'm full of it. Let me know where I can look to correct my own misinformation...
Fluid - change at least semi-annually. Brake fluid is hygroscopic - meaning it attracts and retains atmospheric moisture. Change by sucking the fluid out of the master cylinder reservoir (don't uncover the ports in the bottom!) and wiping that section out, then refill with fresh. And by "fresh", I mean a bottle you just broke the foil seal on. Bleed at wheels in normal order (farthest to nearest,) running the vacuum on the bleeder nipple until you get clean, clear fluid through the hose. This not only will change your brake fluid, but it also helps to flush ancillary crud out of your system. And, if you note that the fluid comes out almost black - plan on changing hydraulics in short order (something else you're not likely to see with a "gravity bleed" - but you might with a "pressure bleed.") One of the two main causes of brake "fade" is steam voids forming in the fluid from accumulated water under sudden braking from speed - the water boils up. Steam is compressible, brake fluid ain't. This is why steam voids can lead to brake fade. (The other cause is "pad gassing" - more on that in a minute.)
Pads & Rotors - Opinions vary in the service world on what I'm about to say, but I picked it up from racing tech literature (Pro-Am, Street/Strip stuff) - brake friction and metal should not be replaced at the same time. Ideally, you will bed a new rotor with pads that are already bedded, and vice versa.
"Bedding?" Yes - all new pads will outgas (release trapped gasses when heated) - it's just the nature of the beast. There's nothing that can be done about it, and that's what makes break-in periods so bloody important. By properly releasing that gas with controlled heating, you preserve the structure of the friction material (yes, it is possible that pads/shoes can crack and/or shatter if you don't bed them properly.) This is why outfits like Wilwood and Baer will go so far as to sell their pads "pre-bedded" - they will "bed" them to a test rotor on an in-house brake dynamometer, and they're all set up for a new rotor when you install them (since the pads aren't technically "new" - even though they've never been on a vehicle.)
With rotors/drums, you're not dealing with outgassing. However, bedding is no less important - cf. "normalizing" of metal castings for more information (brake rotors/drums are almost invariably iron castings - typically grey iron.) In short, normalising will release internal stresses from uneven cooling through the casting, and by controlling the application and dissipation of heat you control the concentration and dissipation of those stresses, and that residual internal stress can be reduced to nil. NB: This is the same reason that "proven" (read: used - prefereably in a grocery-getter) engine component castings are preferred for high-performance buildups than brand-spankin'-new castings - residual stresses are gone, and that removes one potential mode of failure.
Rotor Surfaces - Much debate exists on the relative merits of solid/vented/drilled/slotted/milled/dimpled/whatever rotors (or a combination of any two or more of the previous.) Why are rotors sold as anything other than solid? Simple - heat. When you apply brakes to slow down or stop, you are converting kinetic energy (motion of the vehicle) to heat energy. Remember the "Law of Conservation of Energy?" "Energy can neither be created nor destroyed." Therefore, it can only be converted - and this can mean loads of heat. I don't recall the formula offhand (I'm sure I have it somewhere,) but the conversion of the KE of two tons of metal moving forward at 60 miles per hours results in one Hell of a lot of heat - whether you express it in British Thermal Units (increase the temperature of one cubic inch of water by one degree Fahrenheit) or calories (increase the temperature of one cubic centimeter of water by one degree centigrade.) I do know one thing offhand - if you are silly enough to touch a brake rotor after a hard stop from freeway speeds, you are not likely to have fingerprints afterwards.
Any surface machining of the brake rotor (drilling/milling/slotting/grooving/whatever) results in an increase in the surface area of the rotor, which allows for more heat rejection and faster cooling. This leads to a reduction in brake fade due to pad outgassing, as well as a reduction in trapped air under the pad surface (not technically "fade", but it does affect braking action.)
The catch? Any time you have a sharp edge or corner, it acts as a heat concentrator and a stress riser. Anytime you concentrate heat (for whatever reason,) you further increase thermal stress - so you've got a double-whammy if you just drill a hole. Middlin' drilled/slotted rotors will have a chamfer leading into the hole or slot - while it doubles the number of corners, it also significantly reduces the effect - and it comes out to be somewhat less than a raw drilled hole. The really good rotors will be drilled/slotted, and then be radiused going into the hole/slot, so there is no corner to concentrate heat or stress! I have yet to evaluate these, but they do make good sense from the standpoint of pure engineering. I haven't looked into them in a while, but I recall they weren't cheap.
Are these for everyone? Hell, no! If you run in sand; probably not a good idea, stick with solid/vented rotors. If you run in mud; definitely not a good idea! The holes/slots will rapidly clog up with crap you pick up from the trail.
Street/strip rigs? Sure - they're not going to see enough contamination to be a real threat. Rockers? Probably good - as long as they stay out of the gooey or shifty stuff. Stay on dry rocks, and you'll probably have a benefit to stopping action and instances where brake action will help you climb.
Brake Fluid - Stick to a quality fluid. If you've got it in mind to change DOT spec - say, DOT3 to synthetic silicone, DOT3-DOT5, or even DOT3-DOT4, then you're going to do a flush of all of the hardlines and replace all of the softlines and the hydraulic parts (rubber seals soak up fluid as well, and DOT3/DOT4/DOT5/Silicone are mutually incompatible.) You won't need to change the booster - fluid isn't supposed to be in there - but you'll have to change the master cylinder, the wheels cylinders, and anything else rubber. And when I say "flush out the hardlines," that's precisely what I mean - it may be easier to replace them outright, if you have the means to (if not, blowing a full can of brake cleaner through them might do, but I wouldn't guarantee it.) So, you see the headache you could be letting yourself in for there.
As I'd said, this is primarily anecdotal, and I may have gotten a couple of things crossed (but I doubt it.) If anyone can prove me wrong, please feel free to do so - but it won't be as easy as telling me I'm full of it. Let me know where I can look to correct my own misinformation...
I know they make them for the XJ, just curious. May have to go for yellows though. Running Motul 600 in it with f/r upgraded discs.
