Odd car brake issue

[Ex contributorParticipant @mgnbuk]

I had a graunching noise develope from the front OS brake on the car last week & found something I have not experienced before when I took a look yesterday.

The car is an October 2018 Hyundai i30 estate with 32000 miles showing. The grauching showed up in one specific part of my commute home – after braking to come off the motorway on a downhill sliproad , when pulling away onto the roundabout the brake would graunch. By the time I got home after 3 or 4 miles of town centre driving, the grauching was gone. The graunching also stopped when the brakes were applied.

The brakes are typical for this type of car – a single piston sliding caliper gripping a fairly substantial ventilated disc. When I removed the wheel & caliper, everything was free, the pads are fine & the disc contact faces are clean – no grooves, corrosion or signs of excessive wear. But I did notice a narrow polish mark on the rust on the edge of the disc & I presume that this is the cause of the grauching – the disc gets hot after braking down from 60-70 mph on the slip road, expands, and the rusty edge is just contacting the caliper bracket. By the time I get home the disc has cooled and shrunk back to give clearance. Application of a "toffee hammer" (1/4lb ball pein hammer) to the rust build-up shelled off around 2 mm of rust build up from the edge.

So far, so normal – the edges of discs rust. But what was a bit strange was that only half the disc was affected – the edge from the ventilation slot towards the suspension. The outer edge (ventilation slot towards the wheel) was surface rusty, but there was insufficient build up to knock off. Both discs showed the same pattern, though the NS disc build up was not enough to touch the caliper bracket.

Any ideas as to why this might have occurred ? The only thing I could think of was that the inboard side of the disc runs hotter, due to being shielded from the airflow by the stone guard, whereas the outer face is more open due to the slots in the alloy wheels. And maybe a hotter face corrodes more quickly ? I have not seen this on other ventilated discs on other cars, though.

Nigel B.

[Ex contributorParticipant @mgnbuk]

[Nick WheelerParticipant @nickwheeler]

What you've described is entirely normal for single piston sliding calipers.

[Jeff DaymanParticipant @jeffdayman43397]

Get your wheel bearings checked.

[Steve NeighbourParticipant @steveneighbour43428]

Maybe the outside of a brake disc has more exposure to the elements (rain, dirt/salt/air) because the inside is protected usually by a metal shield or guard as such is prone to corrode more ?

My other half's Renault Clio has he same issue, rust builds up on the outside outer edge and almost forms a small ridge, which like you have be removed carefully by tapping with a 'small' hammer.

Have you checked your disc thickness for wear, since the removal of asbestos from brake lining material the discs are 'sacrificial' (softer metal) and wear at a faster rate than before to help provide the same braking performance.

Steve

[noel shelleyParticipant @noelshelley55608]

Are you sure it is the brakes ? Modernish cars with coil springs have a nasty habit of them breaking and hitting other parts, often the tyre or rim. If your luck is out it will rip the front tyre open Frosty weather makes this more common. Just had it happen, strange noise, smell of burning rubber, limp home very slowly. Noel

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