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Engineery types - brake pad/heat questions

Discussion in 'Downhill & Freeride' started by thaflyinfatman, Nov 26, 2005.

  1. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    Ok I have managed to "glaze" my brake pads by overheating them (they're stock Avid sintered jobbies). What I want to know is (and I'm not interested in bs speculation unless you have some reasoning behind it), when you heat a sintered brake pad up to whatever temperature causes glazing, is it likely to cause *all* the pad material to harden up just as (or nearly as) much or is it strictly a surface issue? Because I've been told that if you just file/sand the surface of the pads off (with 100% clean sandpaper/file obviously to avoid contamination) then bed the pads back in again, they should be fine. I have not managed to successfully do this (by successfully, I mean "get the brakes back to their original power").

    What I'm thinking is - pad heats up to some rediculous temperature (because I'm a hack and I drag the brakes too much), say ~400C right at the surface. Brake pad is only about 1.5-2mm thick even when new, so obviously it's not going to be hard for the heat to be conducted through the pad material. Is this likely to (literally) cook *all* the pad material?

    Basically, the pads (well brakes in general) were f'ing awesome when brand new. Then I glazed the pads over and now they're pretty mediocre. I tried some other pads (Koolstops), and they were nothing special, but I haven't re-tried with brand new Avid sintered pads. I am also super careful about contamination, and to the best of my knowledge have completely avoided it with these brakes. I also clean the rotor now and then (only with brake-friendly stuff though, ie water/detergent [which before the pads got glazed, worked very well] and acetone [ditto, and makes sure there's no oil residue]), so I'm convinced it's not contamination that's the case.

    Anyone got any comment on the heat thing? Is the rest of the pad material likely to be as rooted as the surface apparently is?

    I'd be especially interested to hear from anyone who has actually been involved in any kind of (real) brake testing/R&D. On that note, does anyone have an email address for Avid? The SRAM website is a fuggin POS.
     

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  2. maxyedor

    maxyedor <b>TOOL PRO</b>

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    First off you would need to get them EXTREEMLY hot to cause a total degradation of the pad. If your brakes got to the point that the faded to where the lever pulled all the way to that bar, chances are the pads are bunned completely. If this is not what happened and they just got hot you can use sand-paper to get rid of the glazing on the surface, you should see the color of the Avid pads become a gray colour when all the glazing has been romoved. Or just replace the pads, it's up to you.

    Second, check your rotor's I have seen the Avid rotors turn a deep purple color when overheated, this seriously diminishes the power because not only do the get high-hardened they get glazed with a buch of burned pad material. This reduces the friction and thus the stopping power suffers. You may try going with a bigger rotor if you can, they will not only cool more efficiantly, but they will also give you mre power, and let you drag the brakes less.

    hopefully that helped.
     
  3. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    I've never gotten them to fade or change the colour of the rotors... just hot enough (apparently) to glaze them over. I'm confident it isn't contamination because it was obvious that it was getting worse each run I did on a specific day (it was on a very heavy-braking track). And like I said, I did try sanding the pads back and yes I noticed a change in the surface finish (went from a slightly shiny finish to a dull, matte type finish) but it didn't really help much.

    Why would they need to be extremely hot to cause "total degradation" of the pad? Nearly all the surface area of the actual friction material is up against the rotor, and it's only a millimetre or so thick, like I said - so I can't see why it's unfeasible that the majority of the pad material could be a fairly similar temperature to the actual surface.
     
  4. zedro

    zedro Turbo Monkey

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    not necessarily....when you think of what the actual surface look like at the grain level, you'd have lots of points or 'asperities' (moreso with a porous material like pads have) which are what give the surface it's friction properties. The braking action occurs as these asperities drag into the rotors grain and break off, bleeding off energy (friction). I think what happens is when these broken asperities reach a certain temp (and they will achieve a higher temp in the same respect that surface water can evaporate ontop of ice) they 'glaze' (melt?) and stick into the pads and rotors porous surface which would smooth it out and change it's properties, ie. reduce friction. So i dont think it's so much the surface temp that is so high, its the material thats being torn from the surface, superheated, then re-deposited and re-bonding to the original surface with a different friction-unfriendly surface. I liken this to taking a blowtorch to a block of ice; a portion of the surface melts and on cooling it refreezes, but it's surface texture is now much smoother, the ice is clearer. Same with the pads although the chemical composions not just grain structure could be different too.

    As for your pads, who knows. Maybe theres a contaminant, maybe something that even helps/causes glazing?
     
  5. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    That's an interesting point/theory. If that is indeed the case, then sanding the top back and re-bedding the brakes should fix it, yeah?

    iirc you said you had overheating issues with your Avids too, yes? What were the causes/effects, and did you ever get around it?

    I still wonder though, if the bonding of the pad material is somehow affected by heat. How are sintered pads bonded btw (ie the powdered metal to itself... not worried about how it's bonded to the backplate)?

    It's possible there's a contaminant I guess (there's always some random stuff that touches the rotor now and then, maybe even leaves or something), but again I'm pretty sure that's not it, based mainly on the fact that most of this happened over the course of 1 day of hard-braking runs.
     
  6. dhkid

    dhkid Turbo Monkey

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    so what happenes to rotors when they get glazed? sand paper like for the pads?
     
  7. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    I dunno if rotors ever really have the same thing happen - they're much harder and are much better cooled.
     
  8. Brian HCM#1

    Brian HCM#1 MMMMMMMMM BEER!!!!!!!!!!

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    Unless you are riding in the wet & muck, you're using the wrong pad. A sinthered pad for everyday riding just sucks and they all glaze and loose performance quickly.
     
  9. johnbryanpeters

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    Another possibility is that, when you sanded or filed, you created a surface that's not coplanar with the backing plates. That would reduce braking power.

    Got some vernier calipers or a micrometer?
     
  10. spacemanspiff06

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    if you glazed the rotors over then you best off just to get a new one. you'ld have to take a file to it, and if your VERY carful you might get the surface of the rotor flat again. i don't see sand paper doing that much.
    edit: damn you johnbryanpeters! your always two steeps ahead of me!
     
  11. zedro

    zedro Turbo Monkey

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    yeah with the Hayes rotors, which also suffered a concave braking surface. When i tried the Avid rotors, the pads had already become domed, and i didnt get a ride in after i sanded the pads down. So will have to wait till spring :(
     
  12. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    I find it hard to take seriously the opinion of someone who recommends a Hope/EBC combo :p

    Nearly everyone I know uses sintered pads, and the few that use organic pads all have brakes that feel like crap. Sintered pads have way better power IMO (and that's the main thing I'm after). Maybe semi-metallics are a better choice again, but the semi-metallic Koolstops I have are pretty turdlike.

    JBP: yeah that is a possibility, however I really doubt it would be possible to sand THAT much off the pad in the few seconds of sandpapering with super fine grit wet-n-dry that I used. Filing though, yes, that could definitely be a problem.


    I have a feeling my bad braking technique (dragging too much instead of heavy stop-start braking) is probably the cause of the glazing, so I bought some new sintered Avid pads and will make an effort not to overheat these. See what happens.
     
  13. SK6

    SK6 Turbo Monkey

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    The engineering perspective is simple. When you generate that amount of heat on a surface, you have a heart transfer situation. This heat is energy, and simply does not dissipate on the surface of the pad (Glazing), but will affect a pad bellow the glazed surface.

    That being said, there are pads engineered specifically to tolerate and dissipate the heat energy, some more effectively than others, by the use of specific types of materials that will not allow heat to destroy it or alter it&#8217;s molecular structure. So the entire pad can absorb the heat generated, resulting in the less likelihood of glazing.

    That&#8217;s why years ago they used to use asbestos on brake pads because of its ability to dissipate heat. Not used anymore for the obvious reasons.
     
  14. Brian HCM#1

    Brian HCM#1 MMMMMMMMM BEER!!!!!!!!!!

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    So what do you have against EBC's? Try the reds or golds then.
     
  15. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    I have tried all the compounds EBC make. All of them were mediocre at best. All the guys I know who use them have had similar results.

    SK6 - yeah that's what I was thinking... could it be that there is enough heat to somehow harden the pad material the whole way through? Or is it even possible that the rotor does glaze over or something?

    If not, time to look elsewhere I guess.
     
  16. dhkid

    dhkid Turbo Monkey

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    thats what i want to know too.. and if there is any way exept chaging to rotos to solve the problem? :oink:
     
  17. Daver

    Daver Monkey

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    If you stayed off the brakes it wouldn't be an issue. :)
     
  18. dhkid

    dhkid Turbo Monkey

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    too late, for a whole ride i was trying that and ended up braking when it was too late/when not supposed too and had the worse arm pump i ever had!! :p :nopity:
     
  19. SK6

    SK6 Turbo Monkey

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    The best analogy I can think of in regard to the rotors is a car. Have you ever seen a rotor after it has been turned? Now, have you ever seen a rotor after it&#8217;s been on a car for a while? While the term &#8220;glazing over&#8221; has been used, it would be more of a wear. If you have ever watched a NASCAR race, especially the ones at night, the camera&#8217;s and commentators LOVE to discuss and show the brake rotors glowing red from all of the friction.

    Just from the engineering point of view, the brake pad material can transfer to the rotor face, giving it the effect of glazing over, and the typical solution is to turn the rotor. While on cars that&#8217;s a do-able option, DH bike rotors have a considerably thinner thickness then that of a car.

    The use of super fine sandpaper and a block of would, in essence would be like turning the rotor. However the point is more to remove the brake pad material off of the rotor.

    Which brings us to the, &#8220;you get what you pay for&#8221; mentality. Lower cost disk brakes, while ok for XC and trail riding, are not engineered to handle the heat generated by DH style riding. The rotors are designed a bit differently, i.e. diameter of the rotor, size of the caliper, etc&#8230;

    Certain brakes are designed DH specific for the exact same reasons as outlined above, and because of the materials used to best combat the friction to heat to material transfer, are more expensive. The rotors are engineered and designed for the higher speeds and dissipation of heat; much as the break pads are, to absorb the heat better with minimal impact on the rotor and the pads themselves.

    So, looking for a bargain can work, it&#8217;s more time consuming for the end user in the long run, and it becomes a question of what is your time worth. After experiencing something like this that becomes an issue, looking back, one ALWAYS thinks, &#8220;Why didn&#8217;t I just pay a little extra for the good stuff?&#8221;

    Hope this helps.
     
  20. atrokz

    atrokz Turbo Monkey

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    Just sand them down with something like 300 grit and try to be even. As far as the glazing goes it's due in part to what was mentioned by Zedro I think as well as metal particles embeding themselves into the pad, thus it's mostly surface. A quick clean up with the paper and a cleaning after with alcohol (the proper kind, no fragrance) will do the trick. As far as the uneven surface this may create, I wouldnt worry but dont be too heavy handed on one area. Use a mic if you like but watch it only be off by a couple thou at most which your pistons can compensate for.

    The reason I say it works is because I've done the trick (I ride at brake draging central: Quebec) and can say it works.

    cheers
    a
     
  21. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    It's probably not an issue for people like you, who sell their DH bike before the brakes even get a chance to get hot :)
     
  22. buildyourown

    buildyourown Turbo Monkey

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    If there is no discoloration on your rotors, then I doubt you got the brakes hot enough to ruin the whole pad. I've seen plenty of purple rotors that still worked fine.

    You mentioned you were using superfine wet/dry paper. Maybe that's the problem. Try something courser like 220. Make sure the paper is backed up by something flat like a table saw table or a good piece of cabinet grade plywood. I think you may not be taking enough material off.
     
  23. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    Last time I did it, I used emery paper (not sure what grit) - it changed the surface texture/finish noticeably, so again I doubt that's it.
     
  24. toodles

    toodles Turbo Monkey

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    In all seriousness, have you tried first washing your pads and rotor with water? Sometimes that's all it takes to free the compacted dust that creates the glazing effect.

    Alternatively, roll down your street or a small decline grabbing your brakes quickly and releasing them quickly until they start feeling "grabby" again. Don't drag your brakes while your doing this, just a series of quick on, then off again until your wheels start to lock up easier. Sometimes a handful of grit, sand or dirt put into your caliper helps cut the pads back in quicker.
     
  25. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    Yeah I've tried washing em with water. And detergent (rotors only, I don't let anything like that get on the pads). And acetone. And yeah I've done the quick grabbing thing quite a few times, it helps a bit but it still doesn't solve the problem, and the effect is pretty much destroyed as soon as you do any hard braking afterwards anyway.
     
  26. BSEVEER

    BSEVEER Monkey

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    Just go buy some new pads you tight ass.:blah:
     
  27. dhkid

    dhkid Turbo Monkey

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    yea, that works really well, especially with really muddy water.
     
  28. patineto

    patineto The RM Mad Scientist

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    On the motorcycle world a few methods are use to de-glaze the rotors and pads, from very simple to total sofisticated and specialized.

    well first to avoid this kind of trouble you seat the new pads and rotors the right way so the glazing never happend (well almost) the basic procedure is making several hard stops, about 40 of them from 35/40MpH to a complete stop, let the rotors and pads cool down by riding the bike and avoiding draging the brakes at all cost since heat build up is killer for pads, rotors and also brake fluid.

    now for the most comun ways to clean them.
    *sandblasting works wonders with the proper media.

    *Acid baths (sorry i don't know the exact quemical composition)

    *surface grinding,(this takes some of the material maybe to much for a bicycle rotor) but is the only way to re-surface a rotor after they achive the famous "Aplee shape" (edges are taller than the center of the rotor) but this can only be done on solid disc and not most of the "Composite" ones with Aluminum carriors since most of the Bottons stick out.

    basically for a home made grinding tool you need to avoid "Freehand" methods at all cost, since hand grinding is inacurate and can deform and afect the shape and wall thickness of the rotor, the best solution is to mimicate how a centerless surface grinder works, (same method use to finish high end ball bearings and other precision equipment) this monsters are basically made of TWo extremlly FlaT and Totally paralel surfaces that counter rotate in a random orbit fashion, this machines weigh about 3 to 5 tons, so i don't think you have space for one in your room, so the closes thing is to find Two Very flat surfaces,for example the one i use is made of two diferent materials, the bottom surface is a piece of a marvel (rock) reference table (the ultimate, 6inch thick and totally dimensionally stable) and the sand paper is not glue but held in place by water (the surface is so flat and rigid, works the same as if was glue) the upper plate I made of a 1' thick piece of steel also extremlly flat and the sand paper is glue to the surface, since is pretty heavy you don't even need to press down and you just move it around (actually is very easy on the arms) a even simpler method will be to use two stable pieces of glass (like your mothers glass dinner table or a high quality window) well hopefully you get the Two basic ideas, keep the two surface totally paralel and that the materials need to be "un-bendable" so the inacurate pressure that you bare hands can provide will not affect the overall shape of the rotor and also the wall thikiness since any deviation will generate pulsation at the lever.

    I hope this helps
     
  29. dhkid

    dhkid Turbo Monkey

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    that wouldn't work very well with two peice rotors right?
     
  30. DirtyMike

    DirtyMike Turbo Fluffer

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    What size rotors are you Running?... What Avid system??....This the Juicy's or the Mech?...I am a 250 lbs rider Running 8 inch Juicy's....No problems even when i drag them alot......Sintered pads Not glazing on me.......And yes Once there glazed There done, you see the same with Automotive pads<Master Tech> Sanding them down is just a way of geting in another ride or two before getting pads..... Each time they get glazed the Pad material will get weaker and weaker....Doesnt amtter what kind of a pad it is. Could be organic sintered Full Metallic... Ive been running Avid brakes For about four years now Started off with Mechs and Went to Juicy's the Mechs i was Going through Brake pads all teh time The Juicy's i have ridden for a Full year now and The pads are still in great shape.......Feel free to PM me if you have any other questions about it
    ________
    Hotloverzz Live
     
    #30 -   Nov 30, 2005
    Last edited: Aug 25, 2011
  31. patineto

    patineto The RM Mad Scientist

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    Nope will not work at All unless you take the bottons out and sand just the rotors.

    sadlly on the Hope's (I asume you have) is no way to do it..
     
  32. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    I'm about 220lbs, 8" Juicies, and it took a day of runs at a particularly hard-braking track to do this. Cheers for the info though, that's pretty much what I suspected.
     
  33. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    Wow. Thanks for that, I'll give it a try! Can I just ask though, what grit sandpaper do you recommend?
     
  34. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    I've already bought new pads, I just don't want to wreck these too - that's why I want to know all this stuff. And besides, if pads ARE salvageable then why throw money away on new ones? Always worth asking IMO.
     
  35. dhkid

    dhkid Turbo Monkey

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    what would new pads feel like on a glazed rotor? and are you sure that only a blueish rotor means a glazed rotor, what else is a sign of a glazed rotor? :( :confused:
     
  36. patineto

    patineto The RM Mad Scientist

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    you are more than welcome, i'm glad i can help.

    some people say, I low grit count sandpaper (50/70) will provide a agresive surface for the brakes to increase grip, personally i think that is kind of silly since the scale of contact is so minimal having "mountains and valleys" is going to decrease the contact surface and wear out the pads faster since you kind of make a "File" also sediments are more prone to get into the crevises (whatever you call the little valleys) also remenber all this things work on 5mm thick motorcycle rotors but on really thin bicycle rotors the scale does not even let you play with that.

    i will sugest you to try 180 grit with some lubricant like water, also wd40 works pretty well (I know wd is not a lubricant) but those the job fine, also sometimes if the glaze is to persistent you can use a profesional quality scotch "grinding" pad (like the ones on your kitchen made to take rust and finish materials for painting) and you can find this at many reputated metal shops and use the same glued method to the surface.

    also i forgot to mension, to achive perfect results, you need to Flip and spin the rotor from time to time so is less risk of a deviation .

    just remenebr Disc brakes, work much better in ShOrT Burts of Braking and they don't really apreciate Dragging them around since that generate a heat build up, that ussually is the reason for the glaze, somebody mension before that after the pads glaze they are done. sadlly most of the time that is the case since the pad materials get "Cook" or in other words cristalize and becomes brittle and some how like a glass.

    again I'm just glad i have something to bring to this wonderful forum.
     
  37. patineto

    patineto The RM Mad Scientist

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    actually blue have more to do with Overheated rotors than glazing,,, yes many times the two are presents togheter, but they are some how independent, actually Glazing some how will limit the amount of Overheating since the rotor becomes more slippery and less prone to be grippy generating less heat (i hope I make sense)

    usually glazing can be spot do to the shinny "Glaze" on the surface, is more like a "Laquer" finish than just a issue of color.

    also if you are really good in metalurgics you can bring a Overheat rotor back to some how decent status, this method is use a lot on vintage,, (well not so vintage since disc rotors became popular only about 30 years ago on a motorcycle) when you can not find the spare parts anymore for a restoration , the idea is to do a "Back wards" heat treament and heat up the rotor (yes on a oven) and then let the rotor cool down at room temperature, from time to time i hear this is also use on warp bicycle rotors(to make them flat again) but i will not recomend this methods for use on super thin rotors, just take it as a Historical and somehow techical background...
     
  38. BSEVEER

    BSEVEER Monkey

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    I was just trying to be a jackass. :cool: There is some very good info in this thread.
     
  39. DirtyMike

    DirtyMike Turbo Fluffer

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    Guess I should Drop the Technicals about what glazing Really is.......Glazing on Friction material<Brake pads> is the hardening of the surface Cause from Over heating and cooling and repeating.....Now a Glazed rotor is In reality a Myth.....Your Surface smoothness is teh problem with that.... Warpage....And Hotspots are your real enemy with rotors.....If your rotors are evenly Blue they will be fine.....If you have Blue Spotting or blak spotting that will Tear up your new pads.......The total Blueness is ok Just Remember to acetone/alcohol the rotos preinstall of the new pads......Sounds like you should Try and recenter your calipers and Try not to Drag them quite so much.... If that doesnt help Try the Avid Full Metallic pad.........But only if you have too.... very hard on the rotors
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    VAPORIZER WIKI
     
    #39 -   Dec 2, 2005
    Last edited: Aug 25, 2011
  40. DirtyMike

    DirtyMike Turbo Fluffer

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    <COntinuation> about the rotor glaze.... thats when your rotor Has a Mirror finish to it and has been Polished to a point Your RA number is less than 1.5<Surface Smoothness rating>thats not a worry till you replace teh pads cause with that smooth of a rotor new pads will never seat into teh rotor.... If your rotors are warped a goo way to straightne them for Mtn bike stuff is to Put it between a couple 2/4's and littereally Smack the 2.4 witha hammer........ Not like Destruciton hard But a decent amount of force and never directly on the rotor its self.....The only two times i have seen actual warped rotors so far though at teh Bike shop i p/t at was one when the guy caught a rock and tried fixing it with a Cresnet wrench.....And two Over heated rotor that was not properly Bolted to the hub <Torque issues> Any how i am rambling good luck with your brakes
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    #40 -   Dec 2, 2005
    Last edited: Aug 25, 2011