Brake fade and smoke ...

[LongQ]

Mrs. LongQ mentioned something about a brake judder on her 80k mile Xsara. Took it for a quick spin and it seemed ok to me.

"You have to be slowing from quite a speed." she said. "How quite is quite?" I asked. "Quite." She said.

Well, OK, I'm game.

So up to the legal limit, a quick jab for a few more mph, check mirror, all clear, hit the brakes hard. Felt OK really. Must have got close to lock up (no ABS). Maybe a touch of shake but the steering feels the same under acceleration so I thing that is engine shake related. Back up to speed and repeat about half a mile later. Still OK ish but maybe just satring to feel a bit soft. So that's a coupld of average warm ups and 2 hard stops from around 70 to about 10-15. A couple more typical road use applications and a check or two along a dual carriageway and there is a familiar smell around (if you do track days on road pads!) and a distinct sense of brake fade after a few seconds of reasonable retardation. Getting puzzled, loads of cooling time between stops so what is going on?

Back down the dual carriage the other way and repeat the exercisefrom about, say, 80 to 20. Strong very noticable brake fade from about 2 seconds in, strong burning pads smell and clouds of smoke from each front wheel!

I rna along in normal driving mode for a couple of miles and then pulled over to have a look for anything obvious. Nothing visible but the front wheels seemed abnormally hot.

I have a suspicion that the rear brakes are not doing much for some reason. Don't know why. It get regular use and the last service was a couple of months ago iirc. But even then the heat retention seems excessive.

I will take it off somewhere and get it checked over tomorrow but if anyone has any ideas on what might produce this result I would be interested. It's a very long time since I have experienced brake fade of this order.

I am wondering if the front brakes are binding in some way but there does not seem to be any loss of performance or additional rolling resistance.

[agent006]

Bargain bucket, 40million mile life brakepads can exhibit similar symptoms.

[leorest]

LongQ said:

Mrs. LongQ mentioned something about a brake judder on her 80k mile Xsara. …. "You have to be slowing from quite a speed." she said…..

I had similar on the Mundano. Breaking from most road speeds was smooth but from 70+ (on a track of course) there was quite a violent judder from the front. This only happened with gentle breaking and the judder stopped immediately when the breaks were pressed harder. New disks, pads(with new fitting kit), and fluid sorted the problem. The old disks (only 20K-ish) were showing corrosion across 25% of the inner faces. These disks had done only motorway mileage and I only use the breaks when I have to on the motorway so I think that this problem was caused by not bedding in the pads and disks when they were new and driving only motorways with little break use.
Suffice it to say this new set have been bedded in.

Your other observations may be sorted if you end up changing disks/pads/fluid.

Hope that helps
Leo

[GreenV8S]

It's perfectly normal for the fronts to get a lot hotter than the rears. If the rear's *weren't* doing their bit you would have noticed that it consistently locked the fronts under heavy braking. To be honest what you're describing sounds perfectly normal, standard production car brakes aren't designed for that sort of abuse and I'm not surprised they started to fade. Just hope you haven't damaged them during the tests, in my experience overheating the pads knackers them, and if the fluid is overheated it goes all spongey and needs to be replaced.

[LongQ]

agent006 said:

Bargain bucket, 40million mile life brakepads can exhibit similar symptoms.

True. I'll check when we get the wheels off. However the effects were noticable much earlier then I would have expected even for cheap pads.

leorest said:

... lots of relevant stuff ...

Maybe. I have meant to check the service history bills. The car gets reasonably consistent use mostly non-motorway day by day. I didn't notice and real judder - a slight shimmy from the steering wheel if you really concentrate on it but it also exists under acceleration.

I used to get some nasty judder from my old Omega frequently but inconsistently. A couple of sets of disks only lasted 10k miles - less on one case - before picking up some sort of intermittent problem. However I think that was simply down to a porr braking system design. That said, despite it being auto, I never experienced fade like this - even when I killed the pads on a trackday. (In fact the brakes became very consistent at the end of the day - by which time one pad was about 1mm from the backplate!

[quote=GreenV8S)
It's perfectly normal for the fronts to get a lot hotter than the rears. If the rear's *weren't* doing their bit you would have noticed that it consistently locked the fronts under heavy braking. To be honest what you're describing sounds perfectly normal, standard production car brakes aren't designed for that sort of abuse and I'm not surprised they started to fade. Just hope you haven't damaged them during the tests, in my experience overheating the pads knackers them, and if the fluid is overheated it goes all spongey and needs to be replaced. [/quote]

That's exactly what I would have said EXCEPT that fade and smell appeared very early on in only the SECOND serious application of the brakes. Now that's a bit early even for a road car and I had covered about a mile at 30mph between the 2 tests and about 3 miles total since starting the journey. The third main application was a relatively normal braking activity approaching a roundabout and produced a slight smell. The fourth produced the smoke and was about a mile later after a free airflow run - so plenty of cooling available. The front wheels were very much warmer than usual - quite hot in fact - 2 miles later whan I checked them after a mile or so at 40.

So it was more like there was heat being constantly generated and was not allowing any cooling. In that case the brakes could overheat and fade after fewer applications than one might expect. Why that would be happening is what is puzzling.

No doubt I will find out later when we get the wheels off.

[LongQ]

Just checked the history.

All disks and pads replaced at 61k miles about 2.5 years ago. Front disks and pads replaced under warranty due to bad judder 6 months and 5k miles later. So the current fronts have done about 15k miles and the rears about 20k BUT the rears have been reported as 'corroded' and 'scored' on the last 2 MOT's - but not failed.

Interesting that. I assumed it was something they wrote down (along with a few other things) touting for work since only the MOT check list has ever mentioned it, not service reports.

Off to see what is what now ....

[LongQ]

Well, seems like the fronts are OK. Disks look OK and plenty left on the pads though there was a slight overlap of pad on the inner edge of the disk surface which meant a thin strip of pad had not worn down at all. Whether that could account for some of the strange symptoms - the heat build up and the smoke for example - we could not tell.

The rear disks and pads are totally shot. I suspect they have been like that for a while and not picked up at service for some reason. There may have been a one off problem a while ago with a brake balancing or compensation device (most likely) that caused binding and the excessive wear.

There's something strange about this and I suspect the new disks and pads will only treat the symptoms not the problem. So I will need to monitor what happens next.