Brake Squeal

[IanXT4]

I recently dismantled all my brakes to give them a good clean, check the pads and re-grease where appropriate. The good news is that everything seemed very straight forward and I changed the Brake fluid while I had the wheels off. If anyone is interested Ford do a special Silicon Grease for brakes, part number 1197919 (Silicon Grease to ESE-M1C171-A SPEC). It seems that a number of car manufacturers especially Ford are no longer recommending petroleum based greases for certain brake applications as it rots the rubber and plastic parts (The lower slide pins my 4.2 front brakes have hard plastic bushes).

Unfortunately, I could not get Silicon or synthetic brake grease from any of my local motor factors, so had to resort to a Ford part number :-(

Initially my brakes were silent and the grease quietened the brake pad clicking noise I had when going from reverse to forward. (caused by pad movement)

However, after a few weeks I am now getting an irritating brake squeal from front passenger side, only when reversing at slow speed and when the brakes are cold. It seems to go after a few brake applications but then re-appears when they are next cold ?

I dismantled the front brakes again this morning, checked everything and can see no obvious reason for a squeal from this wheel, all the pads have approx 7mm of pad material left.

Does anyone have any idea what the cause of this brake squeal could be ?

Many Thanks

Ian

(2004 4.2 XT4 petrol)

[anchorman]

Hi Ian

Firstly if there is 7mm of material on the pad don't worry too much about the squeal.

There are 2 reasons for the squeal;

The first relates to the pad material itself. Firstly you need to understand what pad squeal is! Put simply it is a vibration the frequency of which is detectable to the human ear. If you wet your finger and run it aound the rim of a wine glass at first nothing will happen. However, as you know, when the glass begins to dry you generate enough friction to cause the glass to resonate. If you could slow the process down enough what you would see is a succession of stick/slip occurances - each one generating its own "ping" as it breaks free from the glass. Now if you join all the pings together and speed it back up you have squeal. Friction material development is very costly and very time consuming. You can have a material that is perfectly silent on a dynamometer and yet when you fit it to a vehicle it makes and almighty din. This is because the brake itself has its own dynamic frequency (a disc makes quite a nice musical instrument) but when you add it to the rest os the suspension and steering they have their own frequencies and one can play havoc with the other. The secret of a good friction material is not necessarily high performance (seeing these adverts for high friction pads often makes me smile) but consistent performance. The problem is that friction materials are affected by all sorts of things to make the friction go up or down. Low temperature operation will fail to burn off the resin which holds all the "business" bits and pieces in place. The surface becomes polished and the true defination of "glazed". It should be remembered that pads and discs that are working in tip top condition will be shiney like mirrors!

If you work the pad normally the resin will be burned away and present the friction element of the pad to the disc normally. If you work it very hard you will still present the friction element but you will burn away the resin quickly (rapid wear) and you will make the surface porous. This in itself can raise the friction and you know what happens if the friction is high - the brake becomes prone to squeal.

So why only when cold in your case? If it is porous it will absorb moisture overnight (more the night is damp the more it will absorb) and as the moisture dries the friction will rise and.............. you've got it!!!

I have said this before but there are pads and pads. Japanese friction materials are approximately 8 times more expensive to produce than European ones as they have astonishing dimensional tolerances that nobody this side of the Pacific can mach without huge investment (trust me they are far better than they need to be). Secondly they use raw materials which are about as perfect as you can get. The problem is that because they are so expensive non of the European vehicle manufacturers accept the price which is lucky for the Europeans as they would have no friction industry!

The point I am making is that no matter what you read on the box of even the most recognised makers - Ferodo, Mintex, Pagid, Textar, Jurid etc, NONE and I mean NONE, not even the ones that profess to be their premium performance formulation are even remotely as good as the genuine bog standard Japanese pads you can buy from Kingo. For anybody that runs a Toyota made outside of Japan (Avensis, Auris, Yaris etc) - Sorry! Yours has European pads because it has to have at least 80% European content in the production of the vehicle!!!

The second reason for the squeal is more of a physical defect. If you look at this photo of the stainless steel insert which acts as in interface for the pad in the caliper;

They are capable of just working their way sideways until they just contact the disc. I doubt this is your problem as yours only does it when cold but I am just making you aware that it can cause a problem. If there is a possiblity that the pads have worn unevenly then the inner pads have a steel tang which is designed to contact the disc when worn;

.......and therfor forms an audble wear indicator;

Again in your case if you have had the pads out this is unlikely.

With regard to the clicking noise when changing direction, there should really be none if the stainless clips are all in place and the pads are of a reputable brand because the dimensions should make them a snug fit in the caliper. Now the rears are a different matter because they have a different design. It is still a reacting caliper that slides on pins to exert equal pressure on the pads when only one piston is used. However, they have a weakness. The register that locates the pad is very small. This is in principle a good idea because it reduces the risk of the pads from sticking but in practice a poor idea because the pad and caliper can wear as they reist the effects of braking forces. The result can be seen here in this photo which I have circled in orange;

The pad rattles in service even with the brake off. The only real way of curing this is new parts as I did recently and will be including in a "maintenance tip". Sneak preview!!!.......................

Good luck mate.

[shcm]

Interesting informative read

I do a lot of resonance as well, but you'll only hear it if you are an electronic super bat. Except when I'm banging my head again the lab bench of course :D

[jibbbbbb]

i must agree, great read and very usefull as are all of anchormans topics keep up the good work mate .

[IanXT4]

Thanks for the very informative reply

Ian