Front brakes

np · Post by np » 23 Oct 2005, 10:58

I`ve just had to replace my front brakes as my pad warning light was on.On both sides,the outer pad was very worn,causing the light to come on,but the inside pads were only half worn with a good deal of life left in them.The calipers are in good condition,sliding & moving as they should with no binding etc.No matter how hard you braked,the car pulled up in a straight line.
Can anyone explain why they wore like this,or is it just one of those things?
Cheers

Stuart McB · Post by **Stuart McB** » 23 Oct 2005, 12:48

If it ain't broke, don't fix it.

steviewonder7 · Post by **steviewonder7** » 23 Oct 2005, 12:58

Hi np,
I had a very similar problem with my brakes at the front andI'm still not that satisfied with them.
Last week the steering wheel started 'juddering' under braking so I had to change my braking style to accomodate.A few days later I looked at both sides of the brakes at the front and noticed quite horrifyingly that apart from the discs having lips on both inner and outer edges (which I knew about after I purchased the car) I found the nearside pads to be in good condition and plenty of meat on them,but the offside pads were.....one down past the limit with bad scoring on it,the other fairly worn but with some time to go before need of replacing.All the time since I've owned the car it always braked in a straight line which is puzzling to me considering the type of wear the pads had on them.
Two days ago I renewed all the pads and both front discs and the car brakes very much the same(no juddering this time).The calipers were checked and cleaned at the same time of pad renewal and found to be o.k.

Dave Burns · Post by **Dave Burns** » 23 Oct 2005, 13:48

Simple, the cause is overnight rust, the outer face of the discs are more exposed to damp air, this puts a thin coating of rust on the disc and is much more abrasive than the normal smooth face of the disc.

Sometimes when the conditions that cause heavy dew are right, you can easily hear the pads rubbing the rust away on disc on the first few applications of brakes in the morning, sometimes quite noisey.

Dave

np · Post by np » 24 Oct 2005, 21:17

Thanks for that Dave.Did`nt think of that.Even after washing the car,sometimes you can hear the brakes grind when you first touch the pedal.Many thanks

Kowalski · Post by **Kowalski** » 24 Oct 2005, 22:22

Its a misnomer that the outside face of the discs are more exposed to damp air than the inside face, in fact the outside face is much more covered by the wheel than the inside which has no cover at all, in fact because of the way the wheel covers the disc, the inner face gets much more spray thrown at it as you drive along. Damp does definately make the discs rust but the way the pads wear at different rates can be explained.

The Xantia front calipers are a single piston sliding design. If the piston is a bit sticky, both pads wear prematurely, but if the slide is a little sticky the outside pad wears prematurely. The inner pad can always get itself clear by pushing the piston back (if the piston isn't stuck), the run out on the disc will be enough to make it do this. If the caliper isn't free to move but the piston is, the inner pad will get itself free but the outer pad has to move the caliper to free itself, the slide doesn't have to get absolutely solid, how stuck it gets defines how much your pad wears.

Both of my Xantias have had sticky slides, it can cause the brakes to judder with discs whose runout is within spec feel badly warped. I've lubed them a couple of times each but the problem seems to recur, I'm getting tempted to drill the caliper and fit a grease nipple but I don't think grease nipples and brakes go together, I'm sure any MOT tester that saw that sort of modification would have a fit.

Dave Burns · Post by **Dave Burns** » 26 Oct 2005, 18:43

Hehe, sticky pistons and sticky slides, even where a caliper is in tip top condition the pistons dont move that easily, the pad can't as you say free its self just like that, and for the pad to free its self it has to slide along the rusty and pitted caliper, and against the pressure of the anti rattle springs, so again not that easey to move.

But what blows that lot out of the water is the same thing happening to the rear pads, there's no sliding gubbins on them, there is however a splash shield covering the disc.

Damp air doesn't atuomaticaly get to all parts of the car during the night, only the more exposed bits are affected, and you really dont need much shelter to keep the dew off, you only have to see what happens when you park close to the side of a building during a frosty night, often times the side of the car closest to the building will be free of frost, while the other side can be covered, yet there is no actual cover, the car is in the open air.

Dave [/quote]

Kowalski · Post by **Kowalski** » 26 Oct 2005, 20:37

The rear pads on the Xantia don't wear symetrically for another reason, the dissimilar metal corrosion between caliper and suspension arm.

I've noticed that the front pads sometimes don't wear evenly and the difference can be huge, the difference side to side on the same car can be pronounced. I've also noticed that the rear pads don't wear evenly, the pattern is different but again the difference is pronounced. My cars tend to do a lot of driving and not a lot of standing around so perhaps if I had a car that stood around more and did more short journeys and didn't have the two particular design faults that the Xantia does then perhaps I'd notice an affect due to damp air.

I wonder whether the heat radiated from a warm engine or an exhaust pipe would have any effect on the dew forming on your brake discs, i.e. keeping one side slightly drier than the other. The same might apply if a building was radiating just enough heat to keep the water in the air, a bit theoretical I think.

Linegeist · Post by **Linegeist** » 26 Oct 2005, 22:51

Could the uneven pad wear side-to-side be anything to do with the fact that all braking on roundabouts is done during a right turn? If you use a lot of 'em then the outer wheels will be subjected to greater kinetic forces. no?

I only offer this because I knew a bloke who, to keep boredom at bay, habitually used one route to drive to work and a different route home, the two routes describing two halves of a complete circle - obviously, as he always arrived back where he started........

It took me a while to figure out why his nearside tyres were wearing faster than the offside counterparts.....................

Just a thought.

Clogzz · Post by **Clogzz** » 27 Oct 2005, 12:25

Kowalski wrote:The same might apply if a building was radiating just enough heat to keep the water in the air, a bit theoretical I think.

Not just theoretical, it does work like that in practice too.
I used to park a previous car along a wall, and the car stayed dry on the wall side.
Even the plants can sense the heat radiated from a wall, and grow towards the wall.
Ours certainly do.