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Thread ID: 65593	2006-01-23 21:39:00	car brake disks.....	drcspy (146)	PC World Chat

Post ID	Timestamp	Content	User
423683	2006-01-24 02:44:00	BTW: I never machine rotors on any of my own vehicles ever! If there's a little scoring on the rotor, so what? It just increases the surface area and the braking effect . Just don't allow the pads to get thinner than the steel backing plate on which they're mounted . The heat get reflected heavilly back to the rotor and it hurts the geometry of the rotor even if it's a "little old lady" type driver . They tend to drag a foot on the brake pedal while driving . Heat is the enemy . . . rotors won't distort without some excess heat . . . and forget about "soft spots" . . that's really ancient and does not apply any more . Rotors are "Spin forged" or "Spin-cast" and there's just no way a soft spot can exist in that process . (Besides, what in cast iron can have a soft spot anyway?) Customers on the other hand expect to get the rotors machined . . . like they still ask for tune-ups when there is no such thing on newer vehicles .	SurferJoe46 (51)
423684	2006-01-24 06:10:00	thank you for sharing that information and your knowledge......yes I had worn out one pad and hadn't noticed it and only noticed at very slow speeds, (when reversing), that there was a 'sound' coming from the front wheel...it turned out to haev worn out a bit and run a light score on the disk..... anyway they replaced teh pads, (both), and machined the disk rotors. It is only since then that I have noticed the 'judder' which only happens at, (in your speeds), 50Mph or over. IT's not a 'bad' judder but I thougth I should get it checked cause I thought there was no reason for it to judder after such work was done. The mechanic cant tell me WHY its juddering just to say that supposedly according to them 'when it does that after a skim and etc then theres nothign you can do cept for replacing the disk'. I seriously doubt that the disks haev bee previously skimmed and they, from memory looked ok to me thickness wize... so I'm not quite sure how this juddering is happening ? i NEVER drive with my foot on the brake as such either ! so as far as I am aware this would be the first time the rotors haev been machined. Now WHY would I get 'judder' after that ? i cant quite comprehend how the parrelel surfaces f the rotors should have any kind of warping ?	drcspy (146)
423685	2006-01-24 06:24:00	You can get judder if the discs are not running true. The discs may not be running true for various reasons. They may have been removed for machining and skimmed up on a lathe. If so they may not have been held true, so that when released they distort. They may have been skimmed with the faces out of parallel. They may have been skimmed in-situ. If the wheel bearings are a bit loose then the discs wont necessarily run true when being machined, they will move under the influence of machining forces. After all, wheel bearings can't compared with the hefty pre-loaded bearings you get in a lathe headstock. I'd get them independently checked for run out if you cant check them yourself.	Terry Porritt (14)
423686	2006-01-24 06:25:00	um......and how would I go about checkin that myself ? what proceedures ?	drcspy (146)
423687	2006-01-24 06:46:00	Have you tried going down the steepest hill in town, at full speed, and screaming the brakes full on. Repeat x five. Might just wear them in. :thumbs:	Greg (193)
423688	2006-01-24 07:06:00	um......and how would I go about checkin that myself ? what proceedures ? You need a plunger dial gauge reading to say 0.01mm per division, a magnetic base with an assortment of stout clamps and arms. Flimsy mounts are not good. Attach the magnetic base to a convenient part of the axle, making sure it doesn't wobble, and adjust the dial gauge against the disc rotor to give a reading. Rotate the disc and note the total indicated runout in a revolution. Do it both sides of the rotor. Ideally you should also measure the thickness of the rotor with a micrometer all around to check for thickness variation as this will also cause judder. Total runout should not be more than 0.05mm, preferably less. This is what the car mechanics should do after machining, whether in-situ machining, or if they have removed the discs. I wonder how many check their work? Car mechanics are not very good at working cleanly, not very good at all. It doesnt take much dirt betwen disc and hub to give excessive run out. For this reason I never trust a car mechanic to change and/or re grease wheel bearings.	Terry Porritt (14)
423689	2006-01-24 09:21:00	Car mechanics are not very good at working cleanly, not very good at all. It doesnt take much dirt betwen disc and hub to give excessive run out. For this reason I never trust a car mechanic to change and/or re grease wheel bearings.Sorry if this sounds insulting, but that's about the dumbest thing I ever heard. Who else would you trust to change/re-grease wheel bearings? I mean geez, I'm not a mechanic by any stretch of the imagination, but I've done it perfectly at least half a dozen times in my life. It's not rocket science. An experienced mechanic could probably do it with his eyes closed!	Greg (193)
423690	2006-01-24 18:33:00	Sorry if this sounds insulting, but that's about the dumbest thing I ever heard . Who else would you trust to change/re-grease wheel bearings? I mean geez, I'm not a mechanic by any stretch of the imagination, but I've done it perfectly at least half a dozen times in my life . It's not rocket science . An experienced mechanic could probably do it with his eyes closed! Actually the modern brake lathes use centerless hubs to compensate for minor . . . er . . . dirt or debris on the lathe dogs . It's pretty simple for even a chimpanzee to run one of those machines, and bad results are something that the lathe operator would have to try very hard to accomplish . The "in-situ" type of rotor machining is very accurate, and in some cases the only way to machine a rotor . The Hondas for instance have front hub bearings that are very easily damaged if they are removed from the spindles, and as such the on car machining is the best and most accurate method of surfacing the rotors . After all, the rotors will be running on that same set of bearings and using them to indicate the runout will negate the possibility of machining in a runout problem . One surprising happenstance is that when the rotors are machined, they get placed back on the hubs and the rust that is on the face that meets the back of the rotor has been disturbed and that can sometimes cause a new set of dimensions that can result in a lot of non-parallism to the rotor . Scrub both the rotor prior to machining and the hub flange face with a stiff wire brush to help re-establish factory machined surfaces again . This is another pro-argument for in-situ machining the rotors . . . nothing else gets disturbed to generate a new problem . Concerns over packing and not packing front wheel bearings is really a no-brainer . Wheel bearings have come a long way in reliability and as such, the worst thing that can be done is to pack them on regular intervals . Think of it this way: how many times in X miles do you have this done and the costs involved in labor and materials? Now take the cost of actually replacing the bearings every 100,000 miles or so . See? The labor over the service intervals verses the replacing the bearing over a much longer interval does not pencil out as cost effective . In 486,000 miles on a Pontiac Bonneville Safari (the largest station wagon offered by Pontiac) I had never packed the front bearings, and only replaced on set on one side in all that time . Dirt from contaminating the bearing during servicing them is actually more a problem than bearing failure . Modern front wheel drive vehicles are not designed to have their bearings packed as they are sealed anyway and if you are paying for a front wheel bearing pack on a front wheel drive car, I'd be asking legal questions of the shop offering that service . Another point: running down a large hill and slamming the brakes on to "surface" the rotors back into trueness is absurd! All you will accomplish is glazing the face of the pads (and shoes on conventional drum/shoe types) that will severely destroy the friction coefficiency of the materials and make the brakes fade earlier in emergency stops and might cause odd noises like "grumbling" and "chatter" or "squeals" the rest of the life of the brake material . They will from then on run hotter, require more fluid pressure to operate at normal braking effort and place undue wear and strain on the hydraulics, the fluid itself and the power brake booster . This practice will not remachine the rotors into trueness; it will exascerbate an already poor surfacing with one that is now worse and suspect as to safety and reliability . All brakes make noise, especially Volvos and Saabs . . . . . the job of the mechanic/installer is to either raise or lower the frequency of the sound out of the range of human hearing . Make them low so that only whales and submarines can hear them or raise the pitch to the range of dogs and microwave receivers . Just don't let a human hear them and all is well . The sound of a "clunk" or "thump" when using the brakes after moving in reverse is usually caused by too much clearance in the caliper/pad holders . The pads are supposed to be installed so that they preload toward the direction of rotation for forward travel . There are usually some shim-styled springs that push the pads in that direction . When the brakes are used in the retrograde (reverse) motion, a clunk can be heard and possibly felt when the pads shift position to the opposite side of the caliper . This is normal and not a flaw in the pads or the design . . . unless one has purchased some really cheap-o pads that don't have the best of backing plate tolerances built into them . This is not a dangerous condition, more like an aesthetic problem . When drum style brakes make this noise, it is usually the self-adjusters taking up some of the slack in the wear of the shoes . It too, is normal unless the self adjuster is frozen from rust and cannot compensate . Currently, it is not a good idea to add brake fluid to the resevoir as the fluid level goes down in compensation to front pad wear . Opening the master cylinder is not a good idea . Brake fluid is hygrothropic in that it loves and attracts moisture in the form of humidity from the air . Opening it up allows the air in the top of the resevoir to exchange for a new dose of moisture laden air that will find it's way to the lowest part of your fluid system, usually the calipers and cause a lot of corrosion and debris build up there . As the slowly fluid goes down, think of it as a measuring device telling you that a front brake job is coming up soon, and that's when to check the thickness of the pads more often . When the front pads are changed, the calipers will be recompressed and the fluid will return to the resevoir again . This is a very good practice in vehicles with ABS, as any dirt . . . a baby's fart in the next town can have disasterous effects on the purity of the fluid and the ruination of very small valves and solenoids in the ABS fluid control head . We are talking a lot of $$'s here, US or NZ . Rapid loss of fluid from the resevoir is a sign of another problem, as there may be a leak some place and that requires a good inspection . The fluid is in a closed system and should really never require topping up or refilling . Bleeding and flushing is another thing all together . Brake fluid does wear out from moisture and overheating . It is a good idea to change the fluid with a total flush/refill every couple of years . That black film or slime in your master cylinder is rubber debris from disintegrating piston seals and caliper seals and check valves and is not a good thing to have in the system either; just flushing of bleeding is not going to replace rubber that has sloughed off a component . It is time to consider a system overhaul soon . Be safe and be professional . . . . . .	SurferJoe46 (51)
423691	2006-01-24 18:54:00	Dirt from contaminating the bearing during servicing them is actually more a problem than bearing failure. Exactly. That is what I indicated Greg. Most bearing failures arise from: 1. Poor housing geometry 2. Bad fitting, ie hammering home. 3. Dirt contamination. There are many more reasons. Bearing failure analysis for all types of machinery was part of my job, as was bearing assembly of precision machine tools under ultra clean conditions, so it makes me squirm when I see what happens in some places. Anyone can grease and pack a bearing, but whether it is done properly is another question altogether. I nearly said 'any fool can...', but that would have been taken the wrong way :)	Terry Porritt (14)
423692	2006-01-24 19:24:00	Exactly. That is what I indicated Greg. Most bearing failures arise from: 1. Poor housing geometry 2. Bad fitting, ie hammering home. 3. Dirt contamination. There are many more reasons. Point taken.	Greg (193)
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