Unfortunately I think some garages prey on drivers concerns over brakes and so almost automatically suggest replacement. It must be a great money-spinner, especially with all the current belt-tightening going on. When my wife had a Polo which was just coming out of its three year warranty the VW garage did a service and told me that the rear discs needed replacement because 'they were corroded' and suggested I had them replaced and new pads - done there and then at about £250 !
I nearly said yes, but decided to take the car home and check for myself. There was a 1-2mm band of rust around the edge of the disc, but the rest of the disc and the pads were like new. The following year, when the cambelt was replaced by a VAG independent specialist, he checked the rear brakes and declared them 'hardly worn'.
Edited by qxman {p} on 04/06/2008 at 12:09
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I've had discs pronounced as "Need replacing soon", only for them to sail through the MOT a year later.
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Of course, it worries me that a main dealer can describe a disc as corroded when all that has happened is that the outer edge of the disc which does not contact the pad is corroded. This is inevitable, no manufacturer fits a pad that contacts the disc right to its edge, or if they do they deserve all the trouble that will result.
The important areas regarding discs are 1) is the thickness of the pad contact area within wear limits, 2) does the contact area show corrosion or pitting that will impair braking efficiency, 3) is the disc runout outside specification which would cause vibration on braking.
It's so simple, why can't even a main dealer get their head around these issues.
Personally, if a disc has some corrosion on its edge and I'm fitting new pads I do file off the surface corrosion to makes things all look a bit neater.
Sometimes it worries me that the "experts" can get it so wrong regarding such a safety-sensitive area of car miantenanace.
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Doctorchris
I don't think its a case of 'experts getting it wrong'. I think it probably has to do with targets, bonuses and so forth at the dealers. I am far from an expert, but it was evident to me that the rear discs on the Polo were neither worn nor appreciably corroded. It only had 25k on it at the time and 99% of the disc was bright, smooth and there was a wear lip of no more than about 1/4mm at the edge.
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So, if the dealer is not incompetent, bad enough in itself, then they are acting fraudulently which is even worse.
Problem is, how do you prove it. One of the main reasons why I have trained myself to carry out these relatively simple jobs myself. It's a question of trust.
I have to say that, despite a lot of adverse publicity, the Fiat dealer that I use for my Panda 4x4 have always been strictly honest with me. Despite a 1yr/12,000 mile service interval they have merely said that brake parts may need to be replaced within that period and that I should look out for excess wear.
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Mind you Doc, you're obviously reasonably knowlegable about such things, wonder how they'd be with someone totally clueless, and there are many about.
Thats not meant as any accusation about your dealer, far from it, they have to protect themselves these days.
If you come over as a reasonably competent person who's not likely to let the pads wear out and carry on willy nilly, they're more likely to treat you as such.
Remember just how quickly some on here are prepared to draw the trading standards gun.
If i was a dealer mechanic and saw half worn pads, i'd be more likely to suggest replacement, and if there's grooves in the discs, i'd suggest doing them too.
Listen to the outrage on some threads over percieved wrongdoings at some garages, better to be safe in their book maybe?
Again, its hardly rocket science is it, on 90% of normal vehicles a reasonably able bodied person could easily change their own pads and discs at a fraction of the dealer costs, folllowing the instructions in a haynes, some are just bone idle anyway, you've only got to drive past the hundreds of car washes at a weekend to see them. Many of them appear to be family members of jabba the hutt, and could do with a bit of car washing excersise.
I wasn't going to tell this tale but i will after all,
My old mate Dougy and i were always tinkering with our motors, and a neighbour of Dougy's asked us to look at his Renault 11 as his brakes weren't good.
We took front wheels off, and sure as i sit here the pads had worn down to the metal months previously, and he'd carried on ignoring the feel and sound of tortured metal on metal scraping, his front discs wer approximately 1/16" or less thick, and when tapped with a hammer just shattered.
I should imagine the pro mechanics here have seen far worse than that over the years, but the vision of that has stayed with me for over 30 years, only wish i'd had a camera.
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And my speeling and typo's are getting worse. Apologies to anyone offended.
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the fact that brake discs are made of steel makes them open to corrosion, thats the nature of the beastly things , about 1 mm short of original thickness means renewal? and as for warping.. theres a thread on here somewhere that expels such rumours....
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A little confession for you gb. I had a 2CV variant once whose centrifugal clutch began to make scraping noises on takeup. As it still worked and my mind was on other things I continued to drive it until the centrifugal clutch drum wore right through and a ring of cast iron fell off. That stopped the car all right. Then all I had to do was tow it back from the embankment to Highbury with a rope behind a friend's Minor 1000, take the engine and gearbox out and put them on a crate in the front room, do the necessary and put the engine back. My wife didn't like the engine sitting there on a crate and didn't listen to my claims that it was a sort of modern sculpture in a way. The crazed American lodger who helped lift the thing into the house, a man with a murky political past who had been in jail with Wilhelm Reich (the mad ex-psychoanalyst), did his back in too.
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Interesting times, Lud. Did you by any chance partake of a Camberwell Carrot at the time?
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>>Did you by any chance partake of a Camberwell Carrot at the time?
No, Lud was far too busy growing his own firm young carrots
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They certainly were interesting, the late sixties.
I can't imagine what you mean about these parsnips though. But even if I had been eating salad, are you suggesting that any feature of that story is invented or imagined or exaggerated? Not that I really care, but it's all true as related.
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I have no doubt it's true, Lud. The Camberwell Carrot is a reference to a rather large spliff as featured in the film 'Withnail and I', well worth watching if you've never seen it. I used to know characters much like those portrayed in the film, happy days indeed.
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happy days indeed.
Yes, er, salad days... I wasn't entirely convinced by Withnail and I though. It's a representation of young-adult idiocy that doesn't begin to do justice to the real thing. And the stupid cruelty to the Jaguar got on my nerves. But I know a lot of people, mainly a bit younger than me, who admire it greatly.
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Good story Lud.
I love to hear some of the ludicrous things we've done to keep cars going.
I bet a lot of us have got the bent posture and the aches and scars due to our bodging exploits, standing on inner wings of cortina's with a rope around the neck acting as a temporary human block and tackle...should have been committed.
Talking of 2CV's i think it might have been an Ami that we looked at where one of the brake pads had dropped out and the piston was acting as a pad, well the piston resembled an American peaked cap, how it didn't pop out i shall never know.
I could make some terrible confessions about towing with Dougy all those years ago, but my halo prevents it-;)
Good times.
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>>he'd carried on ignoring the feel and sound of tortured metal on metal scraping, his front discs were approximately 1/16" or less thick, and when tapped with a hammer just shattered.
>>
A few years back, one of the managers I worked with ( a super guy) had absolutely no interest in cars except as basic transport. Exactly as you describe except after a service he came in with his disc that looked like one big shim a few thou thick-- horrifying!.
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>>doctorchris
>>
The other thing I do is to clear out the rust in the radial slots of front discs.
Finally for £9 you can get a perfectly adequate micrometer to measure the remaining thicknessso you know the situation re wear.
stores.ebay.co.uk/RDGTOOLS-ON-LINE
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Sorry but a micrometer is waste of time to try and measure brake disc thickness.
As a disc wears it has high spots and low spots. You need a measuring tool with a long neck that allows the measuring tool (type of micrometer) to mesure the disc at various points from outer edge to inner edge.
Using a conventional micrometer will just tell you the thickest part; what you need to know is the thinnest part(s)!
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A micrometer is a standard way of measuring discs & will measure most over the swept area.
If a disc has low & high spots then it'll judder like hell & will need binning!
As for disc skimming it's pretty rare these days, however some of the Porsche indies have on car disc skimmers to true discs up. The idea is that because the disc is skimmed on the car it's trued up as it's running on the hub. Porsche discs are big money & very thick so have enough meat to be skimmed. Skimming is also needed because of rust scabs, which case grief.
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a micrometer is a pointless way of measuring disc thickness.
The part it measures is the edge which is unswept by the pad and will tell you the disc is original thickness!
Uneven thickness through the disc will not automatically give you judder if the wear is in even lines in line with the circumference. ie the pad is the same shape as the disc!. If in uneven spots over the disc then yes, but a proper measure to tell you this will confim its the disc not the caliper sticking or pulsing etc!
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yb - is it possible that you're confusing a vernier caliper with a micrometer?
A micrometer will measure discs very well.
Having said that, for the low accuracy required, both are serious overkill!
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A micrometer will measure discs very well. Having said that for the low accuracy required both are serious overkill!
I tend to agree with you re overkill but trying to judge the amount of acceptable wear by looking at the lip where the disc is unswept by the pad may be easy for the pro but non experts are IMO guessing.
Cost of a cheap micrometer vs a rip of garage seems good value to me.
The higher accuracy appeals to me with regards to brakes.
The doctorchris approach can be tried, to my surprise, for as little £3.30
dm-tools.co.uk/product.php/section//sn/FAICALOUT3
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>>The higher accuracy appeals to me with regards to brakes.
The issue is that the higher accuracy is, in this application, utterly meaningless.
As yb mentions, the thickness varies from place to place across the disk, so, any individual reading to 2 decimal places is not helpful. Plus or minus 0.5mm would be the appropriate required accuracy.
Your measurement to 2 decimal places will not make your brakes any safer.
In a small modification to DrC's suggestion, it would be relatively easy to fix a scale near to the pivot of the external calipers, effectively converting the angle between the legs into a scaled distance between the points, and calibrate it with a mark to show the limit for the discs you are measuring - this would avoid the fiddle of trying to get the caliper points over the wear lip without losing the measurement.
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>>The higher accuracy appeals to me with regards to brakes. The issue is that the higher accuracy is in this application utterly meaningless.
>>
The "higher" accuracy in my case refers to the difference between my MK1 eyeball and a tool to measure the disc in some form. I consider the higher accuracy over my guess very meaningful.
I suspect the MK1 eyeball is the standard tool in the trade else real figures would be quoted to back up the "Need new discs SIR!":-(
Of course I do not need it to 2 decimal places but I would have a value to compare with the minimun safe disc thicknes set out by the car maker.
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The doctorchris approach can be tried to my surprise for as little £3.30 dm-tools.co.uk/product.php/section//sn/FAICALOUT3
Plus delivery charge of £7.95.
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>>The doctorchris approach can be tried to my surprise for as little £3.30 Plus delivery charge of £7.95.
Delivery charge - they are "local" to me but I accept your observation.
I was surprised how cheap they were. £8.50 on Ebay
>>
Still a £200+ saving according to latest feedback :-)
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This is a micrometer it can measure from the edge right in to the inside hub area unless the disc is truly massive.
en.wikipedia.org/wiki/Micrometer_(device)
I think you're thinking of a vernier which would only measure the outer lip
en.wikipedia.org/wiki/Vernier_scale
Circumfrential thickness variation is normally minimal (unless run with knackered pads)
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I just use a simple outside caliper, manoeuvre it over the lip at the edge of the disc then measure the distance across the caliper with a steel rule. accurate enough for this type of measurement.
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Some of those outside calipers have a measuring quadrant on them so you don't really have to twang them over the rusty lip of yr disc and make them all cross-billed for next time...
As you say of course, accurate enough for this kind of measurement, and you can run it all over the disc to see what the variation is.
But if there's any variation or runout to speak of you won't have bothered with measuring will you? You will know the discs are, SNAFU is it?
:o}
Edited by Lud on 04/06/2008 at 23:50
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No that's situation normal... FU then.
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We always noted them as NBG.
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But if there's any variation or runout .............
Disc brakes usually rely on the discs having a small amount of axial runout to knock the pads away from the discs when the brakes are released. In the 1960s when Dunlop made disc brakes theirs incorporated a retraction mechanism, but I think it proved to be more trouble than it was worth.
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Many years ago, I had a trip round Aston Martin's factory at Newport Pagnell. They had a man painting a band on the outside edge of the brake discs (I don't know why because their corrosion protection on the rest of the car appeared almost non-existent). I thought it seemed a good idea, and the next time I replaced some discs I did the same. I then sold the car, and haven't had to replace any discs since. I wonder if it would help to delay the corrosion.
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They had a man painting a band on the outside edge of the brake discs
Genuine Porsche discs have painted hubs to stop corrosion.
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Disc skimmings not rare these days tr7v8. It's just rare in the UK. Every workshop in the USA, Canada and Australia have one. On-car brake disc lathes are big business in 60 % of european countries, Russia, Scandanavia, Asia etc. In fact the only country that doensn't see the value in brake skimming is the UK. I suppose it's because our brake discs are different - yeah right !!!!
It's because people like Doctorchris, whilst extremely knowledgable about certain aspects of motoring - Doctorchris I really do like your posts and have used your tips in the past, just can't keep up with innovations in every aspect of the motortrade.
I could give you brand new factory braking performace, eliminate lateral run-out ensuring no pedal pulsation occurs in the future, work on solid/drilled or grooved brake discs, get rid of scoring, rust, squeaks, squeals and make your brake look brand new in 9 minutes for £20 per disc.
You've spent 10 minutes trynig to start your computer to order the parts, another 10 ordering the new parts and will waste 30 minutes fitting them. I think you need to find out some facts about brake disc skimming before coming out with a comment like that !!
When you fitted your discs did you check the runout?
It's a bit like me saying - listen to Doctor Dave from GMOT because Doctor Chris is too old and uses antiquated methods. The old style brake lathes weren't very good but the new ones are and your are biting your nose off to spite your face because you don't like change. It's like comparing a typewriter to a PC - both do the same job but ones got 50 years of innovation behind it.
My advice to everyone try getting your discs skimmed once at a garage with a good lathe - experiece it before you judge it. For those of you interested visit www.skimmydiscs.co.uk to find a brake lathe near you.
P.S garages with brake lathes know about customer service thats why they offer you three levels of brake service
1) Just pads with a bedding in period
2) Skimmed discs - new brake performance for a fraction of the price
3) New discs because your existing ones have reached minimum thickness
You have the choice of bad brakes for a period of time, good brakes at great prices or brand new brakes.
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