Brake Discs

[Sub MandrelParticipant @submandrel]

I swapped over the brake disks on my Vectra the other day. I was surprised that the material looked like aluminum alloy; a satin silver-white finish, although from the vents it was clearly sandcast and it weigh like cast iron (which I’m sure it is).

 

The finish was the same all over – as cast vents, turned rim, milled wear indicator and the very smooth disc surfaces.

 

Can anyone explain how this finish is generated – is it some sort of chemical pickle? Is the sealed plastic bag it came in essential to preserving it?

 

Neil

[Sub MandrelParticipant @submandrel]

[adyParticipant @ady]

Might be a protective coating to keep it looking bootiful.

 

I have noticed that flash rust appears overnight on many discs if you peek through the wheels.

[John OlsenParticipant @johnolsen79199]

Could it be some sort of stainless steel/rustless iron? I recall talk of stainless disks back in the seventies in the motorcycle context, they were thought to be not as good for braking. However, the advantage was supposed to be that they did not acquire a film of rust, which was said to be a potential problem with the straight cast iron ones. The theory being that you would innocently go to use your brakes after the bike had spent a night in the rain and end up on your nose. I don’t know if there was anything in these theories or not.

 

regards

John

[Ian S CParticipant @iansc]

John, I would have thought stainless steel would be a bit of a no no, because of low heat conuctivity.

Neil, could it be something like a coating of tin or similar, just to make it pretty, also if its castiron it would make handling a bit cleaner. Ian S C

[DMBParticipant @dmb]

ady,

Overnight flash rusting DOES take place on unprotected ferrous items. It used to happen a long time ago in my wooden shed workshop, cured by a number of precautions.

I am told that the London tube train rails rust overnight before first morning train.

BTW, its been snowing very fine powdery stuff for hours down here in Brighton and still only an inch or so and main roads black slush.

John

[Sub MandrelParticipant @submandrel]

When the ground is dry enough to kneel on, I’ll see if the finish is still onthe unworn bits – the brake surfaces are shiny already.

 

Quite pleased really, the car is >180,000 miles* and it needed a suspension spring (No. 3), brake pads and handbrake adjust (the disks were advisory, but the corrosion was eating inside pads). Shame I can only swing 7 1/2″ as I could have got a 1/32″ skim off each side and had a couple of year’s wear.

 

Neil

 

 

*Far cry from my previous Rover, sold cheap at less than 80K. My wife’s vectra is at about 240,000 and going strong with full compression on all cylinders!

[Gordon WParticipant @gordonw]

I don’t know what Vectra discs are either, all the ones i’ve done are only shiny on the working bit. Most standard discs used to be white  (chill cast) iron. Impossible to skim, on our sort of lathes anyway. Only set up I’ve seen, and worked, was with two tools, left and right hand, with a gap to suit, set up on top slide. This way no deflection of tools or slide, well not much.

[Sub MandrelParticipant @submandrel]

Interesting; if that’s true the old ones will have to be junked

 

I’ll put a hacksaw to an old one and see if its chilled.

 

Neil

 

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I once skimmed an old Cavaliar disc to use as a stand, that wasn’t chilled.

 

Regards Nick.

[GoCreateParticipant @gocreate]

10 years + ago I machined some brake discs for a freind, I think it was an Astra but can’t be sure. It was no problem at all, if memory serves me correct I machined up a mandrel to hold the disc by the portion that bolts to the wheel hub and machined one side,lighy cut, low speed and feed, carbide tool.  For the second side I clamped it to my skimmed face plate using studs through the disc bolt holes.

On more modern cars materials my be diffrent now, a friend of mime bought some on ebay for a vectra recently and they looked like cast iron with the brake surfaces ground, I doubt if they are genuine vectra parts though.

 

Nigel

[Bill PudneyParticipant @billpudney37759]

When the Japanese introduced disc brakes on motorcycles in the late ’60’s early ’70’s they used plain stainless steel.  We surmised at the time that this was because all the development was done in Southern California where as we all know, it never rains.  The problem was when they started being used in the UK, where as we know its always raining, they were terrifying.  At low speeds, when the water wouldn’t be flung off, there was a lag of about one wheel revolution whilst the pads wiped off the water.  This one wheel revolution would be around two meters with no braking effect.  Within a short time there was a large aftermarket trade in drilling holes and cutting grooves in the discs.  The real solution was cast iron discs.

cheers

Bill Pudney

Adelaide, Australia

[PatParticipant @pat]

Neil – I think the metal from which your Vectra disc is made will be one of the Meehanite Companies enhanced cast iron mixes.  These enhanced cast products are stronger than cast iron and have more predictable performance – factors of importance in the brake safety and in addition have improved corrosion resistance. 

 

A quick dip into the Meehanite web site will give you an insight into the advatages and some of the reasons why car makers opt for the more exotic materials over plain old cast iron. 

 

I suspect that the silver finish is simply due to shot / sand blasting of the casting prior to packaging.   The as cast finish is very good on these enhanced cast iron materials straight from the mould and the venting slots have to be accurately  cast if the disc is not to require excessive balancing after machining the friction surfaces.

 

The so called wear problem on disc brakes is due to groving caused by bits of grit being trapped between the pad and the disc.  The disc pad will polish the disc to a fine finish and due to the combination of pad and disc material deliver a high friction when pressure is applied.  In time and with heavy braking the surface can craze due to the intense heat but this sort of wear is the province of formula one not road cars.

 

If you look at the thickness of modern ventilated discs there is little thickness to support any attempts at removing groves by machining unlike the solid discs of the previous decades.  In the event of an accident there may well be insurance / legal liabilities issues if it is found that the disc has been resurfaced.  It is my understanding that this is why the pads never overlap the outer diameter of the dic so that the origional thickness can be checked and used as a refference for wear on the swept portion in contact with the pad but als that the disc has not been tampered with.  It would be interesting to hear from any accident investigators on this as I have lost ‘friends’ by not taking what they see as a light trim of their discs! 

 

Regards – Pat

[Howard JonesParticipant @howardjones35282]

Mr Pudney I have to disagree with you there.

Stainless brake disks were introduced on japanese motorcycles to remove the ugliness of rusted disks on otherwise neat motorcycles.

yes there was a problem with braking in the wet.

however it was solved by changing the friction pad materials not by reverting to cast iron disks. to my cursory knowledge japanes motorcycles still use rustless brake disks.

as they say however, our mileages may vary.

[Bill PudneyParticipant @billpudney37759]

Mr Jones, how formal!!  You wrote….

“Stainless brake disks were introduced on japanese motorcycles to remove
the ugliness of rusted disks on otherwise neat motorcycles.”

 

Isn’t that what I said??

 

So no arguments from me really, but the first 750 Honda that I inspected, closely, in March or April 1970 had a stainless steel disc, with no holes or grooves.  This was before the bike was on sale in the UK. 

The first bike that I had, with a disc brake was a 500/4 Honda in 1973, it also had a stainless disc with no drainage mechanisms; also it wouldn’t run in the rain, it even misfired in the fog!!  The undertank lightning display was a sight to behold, but that’s another story.  Maybe it was Hondas way of making sure that you couldn’t go fast enough to cause any damage in the wet, knowing that the brakes wouldn’t work.

The Japanese manufacturers have certainly  got their act together now, and whilst they may not use cast iron in their discs, it’s not the same material that they used in the early ’70’s.  Yes the pad material has changed and improved as well.

cheers

Bill Pudney

[oilcanParticipant @oilcan]

I can remember all the commotion about cast v stainless discs on motorbikes in the 70’s. Nearly everybody swore that holes in discs had an effect. Vaguely recall an article in the motorcycle press argueing about what pattern they should be. that is untill the D.o.T carried out some proper trials on them and found that the holes actualy slowed braking in the wet by holding water in them. Holes soon disappeared. As for the ugliness of rusted C.I. discs I can also recall one story of a learner rider taking his pride and joy back to the dealers complaining of poor braking. He’d oiled the discs to stop them rusting.

[Geoff TheasbyParticipant @geofftheasby]

I regularly leave my Volvo 740 parked for a week or more, even in the snow and ice, and, so far, the brake discs have never shown any sign of rust.   Braking is predictable and even, and the pads seem to be wearing well.

My past experience with ridged brake discs on other cars has been that if you let the brake pads bed in to the disc, then braking is normal.   Don’t change pads just before an MOT!

[Dinosaur EngineerParticipant @dinosaurengineer]

I too remember the Japanese stainless steel/ iron brake disc fiasco in th the early 70’s.They quickly changed to normal cast iron to resolve the problem but I think that for some time they have used some type of rust resisting iron/steel that doesn’t seem to corrode.

I noticed in a fairly recent car mechanics magazine that it was mentioned that wear indicators were machined into the central preriphery of the vented discs . Some manufacturers do this as an aid to balancing. The only wear indicators I’m aware of on vehicle brake discs are machined into the braking surface of the disc . Once the braking disc area has worn away flush with the machined areas then the disc should be replaced. If wear indicators were to be machined in the disc periphery then this would be difficult to discern as this is the area of the disc that suffers most from corrosion.

 

Most car brake discs only have a 2mm wear allowance on the overall width before they must be changed . This leaves little material for “skimming”

 

It’s unfortunate that quite a few current vehicle manufacturers seem to prioritise aesethetics over function to aid sales .My new car alloy wheel shiny “diamond finish”  coating is easily marked by auto washes . This never happened with the old silver colour paint. The use of steel fasteners in alumium alloy sub frames leads to corroded fasteners that are very difficult to remove without breakage.Most alloy wheels quickly corrode to the brake disc to make wheel removal difficult due to the inside wheel face being left as machined without any protective coating.

 

“Style over function” is the term that comes to mind.

Edited By Dinosaur Engineer on 04/12/2010 18:24:43

Edited By Dinosaur Engineer on 04/12/2010 18:32:37

[Sub MandrelParticipant @submandrel]

Couple of observations: (1)  the silver finish is also inside the vents, so it can’t be shot/sand blasting. I think its a pickle of some sort. (2) there is a milled notch on both old and new disks which the surfaces would break into when excessively worn, so i can’t see a problem with skimming if you don’t go past this wear indicator.

 

Can’t speak for motorbikes, but I remember how scary the old leather faced brakes blocks were on bicycles in the days of chromed steel rims. Modern ‘V’ brakes or disk brakes for bikes are amazing in contrast.

 

Neil

[Dinosaur EngineerParticipant @dinosaurengineer]

 

Stub Mandrel

I think the milled “notch” is done to balance the disc , not to provide a wear indicator .I’ve fitted many discs having this type of concave milling cutter notch.The ones I’ve seen would leave the disc dangerously thin if the disc was worn to this extent. Also I’ve seen these type of notches that were not centrally disposed across the width of the disc which leads me to think that they are purely for balancing. A quick check in the relevant Haynes manual will reveal the new & minimum wear limits for the disc thickness.

I would be very concerned for your welfare if you did not check this.

Edited By Dinosaur Engineer on 04/12/2010 22:32:32

[AndyBParticipant @andyb47186]

Hello chaps and season’s greetings to one and all,

 

Regarding your comments about skimming brake discs and subsequent inspection.

 

For inspection, all brake discs are given a width/thickness tolerance in the MOT (annual test) manual for every model of car.

 

Although it is obviously to ensure sufficient strength for safety, the main reason is for the travel of the caliper pistons.

 

The caliper piston has only so much travel; if the pad wears to just off the backplate and the disc is already worn, there is a danger of the piston seal leaving its seat and draining the brake fluid.

 

HGV disc brakes, being much thicker, and subject to much higher braking forces which, in turn, lead to much higher heat generation, do craze, but nowadays this tends to be only on the surface.

 

Back in the early days of HGV discs (late 80’s/early 90’s) Iveco discs on the Eurotech and Eurostar have been known to shatter…I know, it nearly happened to mine!

 

Andy

[Sub MandrelParticipant @submandrel]

Hi Dino,

 

The notch on my discs looked the same on all four disc, old and new. I though the extra holes in the hub did the balancing?

 

 Please don’t fear for my safety – I have fitted Brand new discs which will outlive teh rest of the car.

 

Neil

[Author]

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