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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
423693	2006-01-24 19:52:00	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. 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 :) ......and don't forget over-lubrication and wrong lubricating agents. Over lubrication is a strange thing to think about; after all, some is good, more is better...right? Nope! If the quantity of oil/grease is too much then the rolling parts of the bearings skate over the non-moving parts and there is a certain amount of brinelling that happens. (See: www.sizes.com) A secondary effect of modern lubricating agents is the advent of very long polymer chains that resist shearing and uncalving. These wonder oils are not necessarily good for the old ball or roller bearing. Long molecular structures can keep metal-to-metal contact to a minimum, but there's a problem with that too. Heat is not exchanged and skating the roller/ball will just allow them to stop rotating at all, and the resulting sudden contact with the metal surface that is inevitable will cause a lot of chortling and displacement of hardened or nitrated case hardened cracks to appear. Without some very strong detergent values, the lube cannot cling to the bearing surfaces and this too causes rapid erosion of the surfaces and failure to reduce environmental contact with corrosive agents that can pickle the bearing. Hydrochloric and sulfuric acid from sulfur dioxide generated by the Vanadium in the metals themselves comes to mind as very prevalent agents in most petro-chemical based lubricants. There is always the human factor to consider too. Hands are a warehouse of agents that can kill a bearing quickly. Oxalic acid, hydrogen sulfide, free sulfur from sweat and olfactory glands are destructive in the presence of water. It's just best to keep humans out of the loop.	SurferJoe46 (51)
423694	2006-01-24 21:05:00	Right Joe so very true. Over greasing and not leaving enough space is bad as you say, it can lead to overheating. Another simple thing, if grease is taken from tubs of grease as is done in shop or factory conditions, rather than from a tube, then the surface of the grease must be smoothed flat afterwards with a paddle left in the tub for that purpose. If a depression is left, then oil from the grease will come out and collect at the bottom of the hole. The surrounding grease is then depleted of oil, that is a bad thing! Another cause of premature failure is skidding of the bearing balls or rollers due to insufficient or no pre-load, or weird lubricant as you also indicated. Metal to metal contact is indeed required in a rolling bearing. One speck of dirt, say a bit of cast iron carbide, and that can lead to local stress failure if it gets between the bearing surfaces, then that can propogate catastrophically with premature bearing failure. It is rather amazing that auto bearings after servicing dont fail more often, but then speeds and loads are very moderate. Sealed bearings are a good thing. Getting back to the juddering brakes, for whatever reason this is virtually certain to be caused by rotor run out/lack of truth/waviness etc. Therefore it must have been bad workmanship. I doubt if all auto workshops have quality in situ lathes, I suspect some back street car mechanics are much the same as when I first came here, ie they have a pre-war lathe in the corner, though some workshops I've seen are very good. Neither does NZ have a background tradition of precision engineering as you have in the US or Europe There are some things that can't be done with a farmers no 8 fencing wire attitude. ;)	Terry Porritt (14)
423695	2006-01-24 21:25:00	There are some things that can't be done with a farmers no 8 fencing wire attitude.But I doubt you would've convinced Bert Munro of that! ;)	Greg (193)
423696	2006-01-24 22:03:00	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. Though I defer to your superior knowledge, ably demonstrated already (learnt much, thanks), I wonder if you could clarify this point. I was always led to understand that rotors were often skimmed when replacing the pads if scoring was present, as to not skim would actually decrease the surface area - a smooth pad on a grooved surface is not making full contact with the rotor surface unless the rotor surface is skimmed to match the smooth pad. Or have I missed the point completely? Cheers Lizard	Lizard (2409)
423697	2006-01-24 22:43:00	. Though I defer to your superior knowledge, ably demonstrated already (learnt much, thanks), I wonder if you could clarify this point . I was always led to understand that rotors were often skimmed when replacing the pads if scoring was present, as to not skim would actually decrease the surface area - a smooth pad on a grooved surface is not making full contact with the rotor surface unless the rotor surface is skimmed to match the smooth pad . Or have I missed the point completely? Cheers Lizard There are times when a resurfacing is a good thing . If you have gotten metal-to-metal with the brake pad backing plates to the rotor surface, then yes, it's a good idea to clean up the surface to cut down the abrasive action of the roughened metal to the new lining . That's a given, but there are times when that isn't so . Minor cuts into the surface that are from a foreign body (pebble, MINOR rivet-to-rotor contact, wear irregularity of the old pads, etc) are not really that much worry . The new pads will very rapidly modify their face contact to accept the minor flaws . There are on the market here in the US a brand of brake pad (Raybestos) that has a squiggle of a burnishing compound on the face of the material . It's purpose is to remove any carbon build up and lightly score the rotor to re-establish good pad/rotor wear characteristics . This burnishing compound is supposed to be sintered bronze in a rigid paste that cleans and prepares the rotor for the new pads . I like it very much, but I bet it doesn't last long . . . it cuts the shop labor down to not have the lathe run for a while and labor is the easiest money to hide from the Internal Revenue Service (the US Tax Kneebreakers) . Making labor on turning the rotors is just monetary ($$) sense to a shop . . but not a fair shake to the owner . You are evidence of posturing by repair shops and parts manufacturers . To present a good parable, think of this: Take a sheet of paper of known (factor X) surface area, and fold it into a fan-fold . You will see that the unexpanded fan appears to have less area if it is left in the folded condition . I reality, it has stayed the same in surface area . So much for optical illusions . The effect of having any annular grooves or scores on the surface increases the swept area of the pads . . . and here's where you are right and wrong: The pads will have a minor loss of contact for a few miles, but in a very short time the pads will configure themselves to the irregularities, actually increasing the contact area . In fact some years ago, General Motors had just the simplest idea of maintaning pad alignment as a noise abatement idea on the rotors of their disk brakes when they first became production . They machined into the face of the rotor a "vee" shaped groove about 3/32nds inch deep and about the same in width to keep the pad in correct relation with the rotor at all times after the pad had configured itself to the groove . Early-on there were troubles from the old mechanics who just "knew" that a groove or gouge in a brake drum was disasterous (not really true either), and tried to machine out the groove that the factory had so technically machined into the rotors . The result was that they sold a lot of new rotors in those days, as every one that hit a repair shop had the same problem . . and they didn't know it was the design! So, unless the groove is really radical or goes into the cooling fins, it presents no real loss to braking ability, in fact increases the quality of the brakes . . UP TO A POINT! I recommend that you use judgement in viewing all things concerning the disk brakes . . they represent about 60% to 80% of your overall braking efficiency . However a few cosmetic notches in the face of the rotors are not bothersome, nor necessarily bad either . Cracks, deep grooves and caliper piston-to-rotor contact are all very dangerous and should be cared for with professionalism and good judgement .	SurferJoe46 (51)
423698	2006-01-24 22:53:00	But I doubt you would've convinced Bert Munro of that! ;) Right :) But he must have been pretty well clued up with his machining and fitting ability, especially when you look at his work . The film tended to exagerate the no 8 fencing wire for effect . I'd rather believe the article written in Classic Bike, transcribed from Munroes correspondence, and the photos it showed . Edit: Oh, and I didn't believe one bit about him carving his tires with a bread knife to make "Slick" tires . What they used to do in those days, as Iv'e seen George Brown do with his 'Nero' sprint bike in the 50s was just to burn off the rubber, front wheel against a wall .	Terry Porritt (14)
423699	2006-01-24 23:40:00	There are times when a resurfacing is a good thing . If you have gotten metal-to-metal with the brake pad backing plates to the rotor surface, then yes, it's a good idea to clean up the surface to cut down the abrasive action of the roughened metal to the new lining . That's a given, but there are times when that isn't so . Minor cuts into the surface that are from a foreign body (pebble, MINOR rivet-to-rotor contact, wear irregularity of the old pads, etc) are not really that much worry . The new pads will very rapidly modify their face contact to accept the minor flaws . There are on the market here in the US a brand of brake pad (Raybestos) that has a squiggle of a burnishing compound on the face of the material . It's purpose is to remove any carbon build up and lightly score the rotor to re-establish good pad/rotor wear characteristics . This burnishing compound is supposed to be sintered bronze in a rigid paste that cleans and prepares the rotor for the new pads . I like it very much, but I bet it doesn't last long . . . it cuts the shop labor down to not have the lathe run for a while and labor is the easiest money to hide from the Internal Revenue Service (the US Tax Kneebreakers) . Making labor on turning the rotors is just monetary ($$) sense to a shop . . but not a fair shake to the owner . You are evidence of posturing by repair shops and parts manufacturers . To present a good parable, think of this: Take a sheet of paper of known (factor X) surface area, and fold it into a fan-fold . You will see that the unexpanded fan appears to have less area if it is left in the folded condition . I reality, it has stayed the same in surface area . So much for optical illusions . The effect of having any annular grooves or scores on the surface increases the swept area of the pads . . . and here's where you are right and wrong: The pads will have a minor loss of contact for a few miles, but in a very short time the pads will configure themselves to the irregularities, actually increasing the contact area . In fact some years ago, General Motors had just the simplest idea of maintaning pad alignment as a noise abatement idea on the rotors of their disk brakes when they first became production . They machined into the face of the rotor a "vee" shaped groove about 3/32nds inch deep and about the same in width to keep the pad in correct relation with the rotor at all times after the pad had configured itself to the groove . Early-on there were troubles from the old mechanics who just "knew" that a groove or gouge in a brake drum was disasterous (not really true either), and tried to machine out the groove that the factory had so technically machined into the rotors . The result was that they sold a lot of new rotors in those days, as every one that hit a repair shop had the same problem . . and they didn't know it was the design! So, unless the groove is really radical or goes into the cooling fins, it presents no real loss to braking ability, in fact increases the quality of the brakes . . UP TO A POINT! I recommend that you use judgement in viewing all things concerning the disk brakes . . they represent about 60% to 80% of your overall braking efficiency . However a few cosmetic notches in the face of the rotors are not bothersome, nor necessarily bad either . Cracks, deep grooves and caliper piston-to-rotor contact are all very dangerous and should be cared for with professionalism and good judgement . Thanks Joe, very helpful . I guess I was operating on the assumption that the pads were of a comparable material to the rotors, and so would not relinquish their surface to the rotor's surface quite so easily . Perhaps I will refuse the offer of skimming next time, provided the rotors aren't too deeply gouged - so long as I can coax an accurate report on their condition from the garage . Cheers Lizard	Lizard (2409)
423700	2006-01-25 01:20:00	A picture is worth a thousand words. www.imagef1.net.nz This is a setup on my wifes front wheel drive 1000cc Hyundai A Mercer dial gauge 0.25 inch travel, reading to 0.0001 inches per division, mounted from a mag base which is attached to the shock absorber. Total indicated runout for this side of the rotor is not more than 0.0003 inch which is very good. The sort of precautions required include making sure the setup doesn't move when the hub is rotated. Continuous slight, very slight, tapping with a pencil on the gauge as the hub is turned to ensure that the gauge is not sticking. (This gauge is over 40 years old :) ), angle the gauge plunger very slightly so that it is 'trailing' in the direction of rotor rotation.	Terry Porritt (14)
423701	2006-01-25 05:17:00	Terry, your: "Total indicated runout for this side of the rotor is not more than 0.0003 inch which is very good." ....is 3/10,000's of an inch...and that's well beyond NASA specs..... I think .003" is more like it. Dial indicators don't usually get that close. But, yes, .003" is very good anyway.	SurferJoe46 (51)
423702	2006-01-25 05:23:00	Terry's gauge does. He said so. And he can probably still estimate another decimal place. :cool:	Graham L (2)
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