Strange brakes

[rallen]

Last year I changed front brake disks and pads. Disks were some American make and pads were a good European make. I also renewed the brake fluid and thus managed to put a lot of air in the system which took a while to remove and (look in the other thread) is probably still there.

Because of the thickness of the new disks and new pads, the pistons had to be pushed really far back.

After that we took the car to France, about 700 miles, almost all motorway. No problems with brakes there. We stayed at an altitude of 1200m and the motorway joins in at about 400m or less. During a descent the following days, and at the end of the descent, I noticed a terrible smell and the brakes not working well. On examination it was the brakes being extremely hot. After the brakes cooled down they were OK once more. This more or less happened on every descent (usually in the evenings after skiing). Two weeks later, on the way back home, on the motorway it started to rain heavily. I had to brake somewhat hard and noticed a great judder coming from the braking action, the car was shaking as if the disk had warped or as if the water had caused the pads to slip-bind-slip-bing creating that vibration.

On top of the above I would like to add that we have been at the same place skiing multiple times with the same car under same loads and conditions and that smell coming down the mountain and brakes going soft never ever happened. Neither did the vibration on the motorway, which was one off thank God.

It has been 6 months since and the car has been in rain and motorway and no more vibration. But I have not tried it descenting the mountain.

Would anyone know what it might be causing the heat and perhaps even that vibration?

Thanks

 

[996jimbo]

Hello again!

My guess would be that the disks were not completely clean in the first instance. Possibly if there was insufficient clearance between the new discs & pads there may have been some binding although this would generally lead to the car overheating (especially at altitude).

Probably, you had a combination of something on the surface of the disk (oil, remains of protective coating or something) combined with or leading to an extended bedding in period which caused the overheating brakes and bad reaction to water.

 

[jberks]

on the motorway it started to rain heavily. I had to brake somewhat hard and noticed a great judder coming from the braking action, the car was shaking as if the disk had warped or as if the water had caused the pads to slip-bind-slip-bing creating that vibration.
Thanks

Ok - probably daft question, but, wet, heavy braking, vibration ..- ABS cutting in as a result of aquaplaning?

 

[nicky]

I would say the discs where not propely degreased when fitted,that has now burned of and it is ok now....

 

[tom7035]

Brakes binding -expansion of the air in the system. Vibration - ABS operating.

 

[rallen]

The disks did not look covered with any kind of grease when I fitted them. The car covered approx. 800 miles before the first descent showed up the problems. I cannot believe that for 800 miles the brakes had not bedded in yet?

There is a lot of braking taking place during the descent (approx 20 mins). I would say the problems started during a period of continuous moderate braking. In the 800 miles previously there was nothing like that.

Assuming the brakes were all bedded in and proper due to the previous 800 miles, what is it that causes the brakes to get exceedingly hot during a 20 minute descent? How can air in the system cause that?

What seemed to be happening is that the pads got so hot they smelled so bad it was unbelievable. Smelled like rotting fish. Called the pads manufacturer when I came home and they said they supply big manufacturers, their pads are not second rate.

So it seems the combination of pads+disks were not man enough to take a descent, when the previous pads+disks were quite happy with same descent, same place, same weather, same loads etc.

 

[tom7035]

We all know application of the brakes generates a great deal of heat. Braking regularly for a period of 20 minutes will generate a great deal more! You've admitted there could still be a lot of air in the system. Air expands with heat, hydraulic fluid does not (at least not to the same extent) - expansion of air in the system will cause the brakes to partially apply. Proper bleeding should resolve the issue.
My theory anyway for what it's worth.
Cheers, Tom.

 

[Mid_lfe_Crisis]

Abs was the bad judder and the constant braking down the mountain caused the heat build up as said before
Test out this abs theory by driving down a country road with 2 wheels on the tarmack and 2 off hit brakes and see i u get the same fealing you did before, you dont have to be going fast about 5 to 10 mph abs should cut in and do its thing, its a verry strange fealing when abs cuts in .
when it snows i allways take my regular driving car out to a empty car park and see how it handles the eg slides and brakes and this is were i found out about the abs thing.


Ps would love to know how u get on

 

[jberks]

As far as the heat build up is concerned - the theory around slight binding due to air expansion makes sense to me. Clearly brakes will fade with heat and whilst perfectly good and adequate for most use, it may be that the original pads were still of a higher spec and so did not fade until higher temperatures. There are always differences, even when the spec sheet seems identical.
As you say, you've had no problem since, so clearly the pads you have are perfectly suitable for most use. It may even be as simple as a slightly higher air temp on that occasion so less cooling available - you may have been only fractionally below the fade point last time and an extra 1 degree in air temp put you over this time.
Presumably the new disk has the same ventilation as the original?

 

[rallen]

Will try to answer all questions.

OK the theory about the trapped air is good, I like it. There is air trapped in the calipers. WIth normal use there is no issues except that soft pedal sometimes. Coming down the mountain there is heat buidling up on the calipers. That excessive heat expands the air which causes binding of the pads which in turn results in more heat being generated even when I am not braking, positive feedback.

The judder is not ABS (I know what ABS feels like). It really felt as if the wheel was loose (don't ask how I know this), reverbarating to the steering wheel and to the car. It would happen with a minute application of the brakes and lasted about 2-3 minutes. As soon as I even touched the brakes it would start the vibration, the more I pressed the harder the vibration. Low frequency although I was doing 70mph.

I will try to bleed the front calipers next weekend and see if any air comes out.

 

[Bolide]

Brakes!

Rallen


You don't say what the car is nor how heavily it was laden during your white-knuckle ride down the mountainside... that would be useful to know

The disappearing brakes are caused by brake fade - this is when the brake fluid boils and bubbles of boiling fluid form in the caliper or brake line. The gaseous boiling brake fluid (or air, or steam, whatever nasty stuff you've got in the system) is very compressible so you effectively get no brakes. As the fluid cools the problem goes away, as you've found

I've only ever encountered this once in a Range Rover on a 1 in 4 hill in North Devon. Very exciting!

The theory about air pressure making the brakes bind is plain wrong. Air is very compressible - compared to hydraulic fluid, anyway - and, as in the fade incident, is useless for making the pads do anything at all

At the risk of sounding like a broken record (I have a habit of going on about this), why didn't you use Mercedes parts? Had you done so I guarantee you wouldn't have had this problem

One other thing: brake fluid is hygroscopic - it absorbs water. So any fluid that's been sitting unsealed, or any system that's not had the fluid changed for some time, should be replaced

If I were you I'd replace the discs & pads with Merc bits and have an expert change the fluid again for some fresh Dot 4 (or better) and bleed the system. Brake pads that smell like fish when they get hot worry me!



Nick Froome
www.w124.co.uk

 

[turnipsock]

Something else to consider...brake fade. The brake pads we use are designed for 'normal' use. It may just be some sort of brake fade you were experiencing on a long decent, were you doing a lot of heavy braking?

In my racing days, I used DS11 pads, these were designed to work at higher temps more akin to competition. However, they didn't work very well when they were cold...probably why my right leg is more developed than my left.

Is there any possibilty that the parking brake was binding, this would generate a lot problems?

Air in the system will cause some funny effects...what dot brake fluid are you using?

 

[nicky]

Will try to answer all questions.

OK the theory about the trapped air is good, I like it. There is air trapped in the calipers. WIth normal use there is no issues except that soft pedal sometimes. Coming down the mountain there is heat buidling up on the calipers. That excessive heat expands the air which causes binding of the pads which in turn results in more heat being generated even when I am not braking, positive feedback.

The judder is not ABS (I know what ABS feels like). It really felt as if the wheel was loose (don't ask how I know this), reverbarating to the steering wheel and to the car. It would happen with a minute application of the brakes and lasted about 2-3 minutes. As soon as I even touched the brakes it would start the vibration, the more I pressed the harder the vibration. Low frequency although I was doing 70mph.

I will try to bleed the front calipers next weekend and see if any air comes out.

If you where travelling down a mountain road at 70MPH no wonder you had problems with the brakes.....I have drove these roads in a Landrover and no way can you do 70MPH...

 

[jberks]

I'll accept Nick's view on the air expansion theory - Sounded good in principle but I've never heard of it before and could see arguments either way. I guess you'd need a lot of heat to cause enough air expansion to give problems.

I am curious on the fade though. I always thought that fade was caused by the disk/pad being too hot and effectively melting the pad surface. With the surface partially fluid, there is reduced friction and so reduced braking effect. Nick mentions the fluid boiling. Which is right or are they both?

 

[996jimbo]

I always thought brake fade was due to heat build up in the disk/pad reducing friction (not sure about actually melting anything), rather than boiling the fluid.

 

[nicky]

I always thought brake fade was due to heat build up in the disk/pad reducing friction (not sure about actually melting anything), rather than boiling the fluid.

Does it take a female to tell you guys what brake fade is,,it is when the brake fluid goes too warm...The brake pedal then goes to the floor,nothing to do with the pads and reducing friction......

 

[Bolide]

Brake fade

Okay, brake fade:

The disc, bell, hub & wheel are bolted together as one assembly. The pads grip the disc & friction produces heat. The assembly (see above) heats up and loses heat as fast as it can dispose of it.*

When it can't dispose of it fast enough the whole assembly heats up - as do the pads & caliper which are in intimate contact when braking - and when the whole lot reaches the boiling point of brake fluid - or the contaminants in the brake fluid - the fluid/contaminants boil & brakes disappear

This should happen at a temperature below which friction material & discs begin to "crap out" (that's a technical expression used in racing) but, if the brake pads are from the Hong Kong Fish Paste Recycling Brake Pad Manufacturing Corp, as these seem to be, the two may occur at the same time

In a nutshell, your ability to stop is as good as your brakes & tyres let it be. If you've got old brake fluid, cheap pads or bald tyres and are descending mountain passes at 70 mph then future postings on this forum may become difficult


Nick Froome
www.w124.co.uk


* This is why cars use vented discs and race cars have brake ducts for air cooling. Many cars up to the 70s had the disc inset and not in contact with the wheel flange (my BMW 2002's like this). Later cars (E30 BMW is an example) move the disc so it is next to the wheel. The wheel then acts like a heat sink to absorb & dissipate heat. Alloys wheels help here - which is why Police-spec Rover SD1s had Minilite alloys and not steel wheels after a few "interesting" moments chasing villains

 

[nicky]

Okay, brake fade:

The disc, bell, hub & wheel are bolted together as one assembly. The pads grip the disc & friction produces heat. The assembly (see above) heats up and loses heat as fast as it can dispose of it.*

When it can't dispose of it fast enough the whole assembly heats up - as do the pads & caliper which are in intimate contact when braking - and when the whole lot reaches the boiling point of brake fluid - or the contaminants in the brake fluid - the fluid/contaminants boil & brakes disappear

This should happen at a temperature below which friction material & discs begin to "crap out" (that's a technical expression used in racing) but, if the brake pads are from the Hong Kong Fish Paste Recycling Brake Pad Manufacturing Corp, as these seem to be, the two may occur at the same time

In a nutshell, your ability to stop is as good as your brakes & tyres let it be. If you've got old brake fluid, cheap pads or bald tyres and are descending mountain passes at 70 mph then future postings on this forum may become difficult


Nick Froome
www.w124.co.uk


* This is why cars use vented discs and race cars have brake ducts for air cooling. Many cars up to the 70s had the disc inset and not in contact with the wheel flange (my BMW 2002's like this). Later cars (E30 BMW is an example) move the disc so it is next to the wheel. The wheel then acts like a heat sink to absorb & dissipate heat. Alloys wheels help here - which is why Police-spec Rover SD1s had Minilite alloys and not steel wheels after a few "interesting" moments chasing villains

Well said,i was going to explain brake fade to them but thought it would have been too much for them to take in......lol..

 

[davidsl500]

Actually chaps and chapesses there are two common types of brake fade and the actual "pedal feel" determines the likely culprit.

Brake fade caused by fluid gives a mushy, longer travel pedal and pumping it can restore a bit of pressure. Wrong spec of brake fluid eg DOT3 instead of DOT4 will accentuate the problem as the fluid boils at lower temperatures - causing vapour which can be compressed hence the longer travel. Brake fluid is also Hygroscopic ie it attracts water and then the water boils causing steam which will be compressed etc etc., Hence why brake fluid should be changed every couple of years..

Friction fade gives a totally different effect. The pedal remains solid - the vehicle just doesnt stop !! This is caused by the brake friction heating up and as it heats up the coeffiecient of friction of the material reduces. This can also be exasperated by cheaper fricition materials - more importantly, the bonding agents which can vapourise at high temperature and actually form a pocket of gas between the pad and the disc - ever play air hockey? !!! Drilled and grooved disks prevent this build up as well as getting rid of heat more efficiently together with the vented disk design.

Actually, the actual coeffiecient of friction of braking material is actually very poor - there job is to get rid of heat, a block of wood has a better friction coefficient but woodent (pun intended!) last long.

 

[tom7035]

Bet it took you longer to think up the pun than the explanation David!!