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One of the rear brake discs of my car has a long-standing 6 mm wide band of heavy corrosion at the outer diameter of the braking surface. The car has never failed an MOT but I'd like to get rid of the corrosion if possible. I envisage removing it with a small grinding wheel in a hand-held drill. Will it be a waste of time if I don't fit new pads at the same time? There's obviously a step in the existing pad and I reckon that if I remove the corrosion the corresponding part of the pad will no longer touch the disc and the corrosion will just return.
Edited by L'escargot on 11/02/2009 at 07:47
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If 'twer mine, i'd leave it alone until the salts gone and then slip a new set of discs and pads in.
As they are rears you could probably leave them until the pads are worn anyway.
At the moment you still have the whole of the pad in contact though the corroded section may not bite quite the same, if you remove that corrosion you'll not get pad contact until the pads wear down, and the way you describe will leave the disc lumpy anyway.
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AFAIK there is no reason why this can't be done, in moderation obviously.
I have often in the past ground down corroded or unworn ridges around the edges of discs. Sometimes I find the pads do not quite reach the extreme outer ring of the disc, so a ridge develops.
It might be interesting to investigate why the rust is being cleared so effectively from most of the disc, but not at the outer edge, even though the pads apparently do sweep that portion. Which came first - the corrosion or the wear in the pad?
My tip is to use the flat of the grinder blade, very carefully, and then to finish off with sandpaper on a block, rubbed in a circular motion around the disc.
Also I would not do it with an MOT imminent, because the sanding marks might draw the tester's attention to your action. A month's use will blur these marks and leave the whole disc shiny, if you fit new pads.
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Many years ago, I visited Aston Martin's at Newport Pagnell. They had a man painting the edge of discs on all new cars (the unswept part). I thought that looked like a good idea and so every time after that I did the same on any new discs I fitted.
It seemed to make not a jot of difference, they still corroded. I tried various different paints.
I don't bother now.
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Which came first - the corrosion or the wear in the pad?
The corrosion came first. When I bought the car it had only done 4000 miles in its first 16 months and all the discs were heavily corroded through lack of use. All the other discs had cleaned up satisfactorily of their own accord by the time I'd had it a few weeks.
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I found a chipping hammer was more effective on this edge rust. It seems more like scale than just rust. Mine go the same and it is due to that area being unswept by the pads.
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Mine go the same and it is due to that area being unswept by the pads.
The area I'm talking about on mine is swept by the pads.
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So lets get this right. You have a rusty ridge on the area of the disks that is swept by the pads? and the pads of got a matching grove now?
if this is the case then poncing about with an angle grinder is not the way to go. Change the pads and disks as a matched set.
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if this is the case then poncing about with an angle grinder is not the way to go. Change the pads and disks as a matched set.
I'll do that as soon as it fails an MOT ~ which it hasn't yet done. MOTs measure braking efficiency and to me that is the definitive criterion.
Edited by L'escargot on 12/02/2009 at 07:12
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. MOTs measure braking efficiency and to me that is the definitive criterion.
But MOT's don't measure the brake efficiency or balance front/rear at a given pedal pressure, the tester checks one axle at a time and plants his right hind leg hard on the pedal.
Therefore its possible to have front or rear brakes relatively poor in comparison to the other end at a given brake pedal pressure, even though they may give good readings.
A small example i've said about before.
Many years ago mk2 Golf diesel, brakes required a lot of pedal pressure for not particularly good stopping power, car very low mileage still on original pads, discs perfect.
So i slip a set of Ferodo pads in the front, what a difference, light and easy progressive feel and a huge improvement.
But the front brakes were now much better than before in relation to the rears and prone to easy locking up, so i had to put Ferodo rear shoes in to rebalance the braking, after that it was a much nicer car overall.
It was a good lesson, obviously you don't want a situation where the rears lock first, but you do need full assistance from the back to give good overall braking.
Would probably not have realised the imbalance should the car have had ABS, as locking the front wheels wouldn't have been so obvious.
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If you really want to, scrape off the rust with a piece of tool steel, or the end of a cold chisel, or something. You'll want new pads, or flat down the ones already in - if there's a ridge left in the iron, it'll take a short while for the pad to bed in again, obviously. I wouldn't grind the disk in the way you mention (however, a cup brush on a grangle winder would remove the rust well).
Me? If the brake works well enough, I wouldn't bother.
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I was surprised to see a garage 'recondition' brake discs with a grinder but another backroomer said he did the same.
My main concern would be that the discs might not be a uniform thickness afterwards, leading to reduced contact area with the pads and reduced braking efficiency.
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I used to get my discs re-skimmed after the lip had appeared. After several sets that seemed to warp, or cause wobbling/judder I eventually gave up with that practice and just buy new discs every time - the cost of most discs these days isn't too horrendous.
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just buy new discs every time - the cost of most discs these days isn't too horrendous.
I remember 20 years ago that a new disc for a Moto-Guzzi was about £110 and was amazed when I found out you could get an E-Type one for about £10 from the Jaguar Owner's Club.
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I remember being amazed at the low cost of car tyres, and how long they last, compared to motorbike tyres.
An old bike of mine had the clutch renewed using cut up wine bottle corks. As good as new!
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My main concern would be that the discs might not be a uniform thickness afterwards leading to reduced contact area with the pads and reduced braking efficiency.
They don't need to be perfectly uniform in thickness (as long as the thickness is the same going around the disc) to work properly.
Regarding the rusty bit, if the brake works well enough, I'd just ignore it.
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They don't need to be perfectly uniform in thickness (as long as the thickness is the same going around the disc) to work properly.
Err I think for maximum brake efficiency they should be.
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You don't need "maximum braking efficiency". A locked wheel is locked, however much extra friction brake:pad there is (and anyway the difference in "braking efficiency" will be very small).
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I still want all of my pad in contact with all of my disk with all of my foot pressure transmitted to the centre of my pad with no pulsing of the pedal.
Not too much to ask surely.
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I still want all of my pad in contact with all of my disk with all of my foot pressure transmitted to the centre of my pad with no pulsing of the pedal.
I don't get pulsing of the pedal ~ the corroded band is obviously the same thickness all the way round the disc.
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FT i understand the thought about not needing maximum braking efficiency all of the time but don't agree, a braking system is designed to be balanced front/rear and therefore correctly balanced braking is a must, and engineers go to great lengths to get the balance correct.
If a percentage of the rear braking is lost through non contact of the pad, but the front brakes are perfect then it must mean under a given brake pedal pressure that the front brakes will be providing more stopping power than designed in relation to the (now undersized) rears, or more exactly the rears will be providing proportionately less retardation than they were designed to.
As said above, unless the car in question is an exotic, discs are so cheap these days as to be not worth mucking about with.
Though i would in this case leave them alone until the weather breaks.
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Puzzle isn't it ? i wonder why and how it has happened, and by "long standing", do you mean before your current pads were fitted, or after, because what would concern me is how deep does the corrosion go, i am sure that you know that there is a minimum allowable thickness for a brake disc, and if this corrosion goes deep enough you may have reached that minimum thickness under it, making a weak point which - may, under stress - result in the disc cracking in that area.
If you scrape the corrosion off, you can then measure the thickness of the disc at that point, if it is within limits you could leave it, but personally i - never - scrimp on tyres or brakes, and i would be inclined to do what others have suggested, and replace the lot, and both sides too. To me it's not worth the gamble, if your engine goes bang at 70mph you stand a good chance of rolling to a stop, if the brakes go bang when you lean on them at 70mph ........
It's only money !, and you will have peace of mind.
P.S. just had a re-read of the posts, if you bought from a dealer and you havent had the vehicle long, and at 4000 miles brakes would be original eqpt, perhaps dealer could contribute something ?
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P.S. just had a re-read of the posts if you bought from a dealer and you havent had the vehicle long and at 4000 miles brakes would be original eqpt perhaps dealer could contribute something ?
I've had the car 4 years and it has now done 42,000 trouble-free miles. When I bought it at 16 months old and 4000 miles I knew about the condition of the discs (and in fact discussed it with the dealer) and I accepted that rusty discs were inevitable because of the car's history. It had probably stood for months without moving a wheel before I bought it. It was a bargain at £1000 under book price and I accepted at the time that I might need to get new discs and pads. As it happens I haven't, and it's still on the original parts. I just thought it might be a good idea (but not essential) that I got rid of the corroded band on the one face of one of the rear discs. Because it's on the outer face it's visible through the slots of the alloys. I'm only concerned (and only mildly at that!) about the looks of it. I can't get onto my garden because it's waterlogged so I've been looking at what I could tart up on my car instead!
Edited by L'escargot on 12/02/2009 at 07:36
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I would do exactly what you suggest - grind it off. I doubt if you will have to chip it off - that sort of rust usually occurs on the quarter inch wide rim. I'd chip/grind that off as well 'cos that's probably where the corrosion spread from. Then I would paint the ground area - with heat resistant paint if you've got any. I wouldn't bother to get any specially.
Eventually the main part of the pad will wear down to its step. Encourage it to do so by braking with the handbrake where possible.....e.g. approaching roundabouts on fast dry roads snick into neutral and engage a click or three of the lever. Practice arriving at traffic lights without using front brakes. Use sidelights simultaneously if being tail-gated!
I doubt if the temporary reduction in braking effect caused by the small step in the pad no longer squeezing the corroded area will be significant - even if at both sides of the disc.
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>>Encourage it to do so by braking with the handbrake where possible.....e.g. approaching roundabouts on fast dry roads snick into neutral and engage a click or three of the lever. Practice arriving at traffic lights without using front brakes.<<
Dangerous statement to make, since the vast majority of people who will read it will not know if they have a handbrake operating on the disk or additional drum! Then there will be others who try it and wonder what happened and end up facing the wrong way because the hand brake operates on the front wheels!
p
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>>end up facing the wrong way because the hand brake operates on the front wheels!
Actually, they're the only ones for whom the advice is safe! **
Locking the front wheels will not result in the vehicle swapping ends, as opposed to locking the rear wheels which is an extremely unstable situation.
** I know this may seem counter-intuitive, but, if you have a matchbox car and a smooth ramp, and jam the wheels with, say, some blue-tac between the wheels and the body, it's very easy to demonstrate that front wheel locking is stable, and rear wheel locking isn't.
At the risk of sparking a holy war, this is entirely consistent with;
- fitting the best tyres to the rear
- regulations which prevent rear wheel locking uder service brake applications
- MOT requirements for mixed radial ply and cross ply tyre types on front / rear axles
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NC Thank you for correcting that, I had second thoughts after I posted it but the phone rang and the edit function timed out.
p
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Dangerous statement to make since the vast majority of people who will read it will not know if they have a handbrake operating on the disk or additional drum! Then
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Quite so, pmh2, but I was really only replying to Mr Snail and his Focus!
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>> Dangerous statement to make since the vast majority of people who will read it will >> not know if they have a handbrake operating on the disk or additional drum! Then >> Quite so pmh2 but I was really only replying to Mr Snail and his Focus!
Very true - the rear disks on my supra had some light rusting so I was using the handbrake a bit on the approach to traffic lights etc. After a few days of this I was surprised that the disk didn't look any different and the rust was still there. When I took the wheel off to investigate further I found that the rear disks also contained a set of drum brakes...
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I think the reasonable action to take depends on why the bit of disc is corroded at all, when the main contact area is not.
If it projects beyond the pads, then i can't see that it matters. It would be quite satisfying to sand or grind it clean, but it wouldn't affect braking one way or the other.
But if the corroded area is covered by the pads, it is odd that they haven't kept it clean and shiny like the rest of the disc. It suggests that the pads aren't being applied with even pressure across their face, or that something has worn the pad at that point so that it doesn't now touch the rusty bit.
I would do the following:
1) Grind/sand/emery the rusty bit very carefully, checking its resultant thickness compared with the rest of the disc.
2) Check that the pistons retract easily, by pressing them in and pumping out a few times.
3) Fitting new pads. If you refit the old ones they won't be flat so will still miss the cleaned up rim so the rust will reappear.
The result should be a slight increase in effective disc area. I can't see any harm, and possibly some good.
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Moonshine
Snap -- I did the same - my rear discs are rusty but still passed MOT last November and I held handbrake on gently as I drove along on a clear road trying to clear rust Duh! - The disc's on my Mazda are around £150 pair. Have modern cars got rear brake compensation? more weight in car more brake pressure is allowed to rear brakes?? could explain if like me I am only one in car most trips.
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I've found this thread interesting, I missed it as I was away in Spain.
My Fiat Panda Cross has been driven less than 3,000 miles from new, over the past 4-5 months.
However, it has had 2 long periods of inactivity, one at the end of last year whilst awaiting parts for an accident repair, then 11 days in Feb at Liverpool airport. Due to the hard Winter that we have just had, the brake discs were left, coated in road salt.
The discs are all discoloured and lightly ridged but the rears have bands of corrosion as described on this thread.
Compare this with the Panda 4x4 Climbing that I just sold. With 33,000 miles on the clock. You could brush your hair in the reflection from all of the discs.
I'm not stressed about this, discs these days are relatively cheap and I know how to replace them. However, it demonstrates how road conditions and use of a car can have a huge influence on the lifespan of brake discs.
Another point, the Panda Cross, with its Diesel engine, encourages a gentler driving style with less enthusiastic braking which is making the corrosion bands harder to remove.
Ah well, the brakes work fine and MOT time is more than 2.5 years away.
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