Ceramic Brakes - Soft Pedal

[robf]

It's been raining heavily here in Kent this morning, so i got a reminder of a wet weather idiosyncrasy my car has. it is absolutely fine in the dry.

Travelling down the motorway i hit the brakes and they feel far softer. It seems the heavier the rain, the worse this gets, to the point it feels like the brakes have overheated like on a trackday. The brake pedal goes long, but then as you push further it will suddenly 'catch up' and give all of that braking force i asked for in one dollop!

I was wondering if it's a 'feature' of carbon ceramics (they're not always perfect when cold, for example), but now it's happened again i would say it is borderline dangerous. The first time it happened i had a heart stopping moment where it looked like i was going to hit the car in front of me (they pulled out suddenly).

Any ideas? I'm concious that today is a sign of what's to come weather-wise, so guess i will have to get it checked out.

[doodlebug]

Never experienced that, the ceramics have been nothing but excellent rain or shine.

[RSQ]

Never had this with mine either. Stealer visit due IMHO

[robf]

Thanks, I will do as a precaution.

I have searched for more info on t'internet and found the following on Bimmerpost with BMW M4 owners:

If the brake disc and pads become wet, all brakes undergo a drop in their friction coefficient. M carbon ceramic brakes normally have a considerably higher coefficient of friction than conventional brakes, and they feel considerably fiercer in operation. However, under the impact of moisture and road salt, for example after using a car wash, or when condensation has formed overnight, the braking effect may be equivalent to that of a conventional brake system. This may be perceived as reduced braking performance, but it can be compensated for by applying more pressure with the brake pedal; this should be continued until the pads and discs have fully dried out. Since the difference in frictional coefficient between dry and wet brakes is higher than normal with M carbon ceramic brakes, it is felt all the more clearly by the driver.

Some other people have said it is 'normal' behaviour for carbon brakes, but of course if you guys are saying you don't have the issue, surely there is something wrong.!

[RSQ]

Nonsense above. The car does a little gentle brake pressure every now and then to keep pads/discs dry IIRC (you would not feel it). When you need them therefore they work. It may be that this auto function has gone moody on yours. Then when you use them in the wet they have to dry first, maybe only 0.1s but when you are heading for another car it feels like ages.

[robf]

The odd thing is i am used to 'priming' brakes in my race cars, where you ensure the brake pad is properly located by doing a feather touch just before the braking zone. As a force of habit, i tend to do this in road cars too.

Thing is, after my initial scary moment it will be OK for the next few applications. But then, if i continue on (let's face it, you shouldn't need to brake much on the motorway), the same scary occurence will happen once again a few minutes later.

It is interesting you say it basically is meant to do this as default automatically. Clearly something amiss then...

[gorun01]

It's been raining heavily here in Kent this morning, so i got a reminder of a wet weather idiosyncrasy my car has. it is absolutely fine in the dry.

Travelling down the motorway i hit the brakes and they feel far softer. It seems the heavier the rain, the worse this gets, to the point it feels like the brakes have overheated like on a trackday. The brake pedal goes long, but then as you push further it will suddenly 'catch up' and give all of that braking force i asked for in one dollop!

Same thing happened to me once in my 1.5 years of ownership. Happened on a cold wet day on the motorway when I had not been using the brakes for a while. I pressed the pedal gently and it sucked it in from under my foot. I am pretty sure its a precaution feature when the cars thinks you are not going to get the breaking performance that you need due to external factors and it kind of helps you with some sort of brake assist. Its not very smooth but pretty reassuring that the feature exists.

[RSQ]

Found this http://www.audizine.com/forum/showthrea ... isc-wiping but didn't look hard there must be more

[MikeFish]

Rob, your ceramics are factory fitted aren't they? There is different coding for ceramic brakes which affects the disc wiping function so if factory fitted they should be set correctly unless you have been using VCDS and coded them wrongly by accident.

[mat776]

I know it's not quite what you describe, but Chris Harris noted occasional extremely sharp braking in his rs6. Speaking to Quattro gmbh they explained that they had fitted a booster to the pedal to prevent the travel getting to long. Whilst it doesn't explain the lack of braking it might explain the sharpness you noted.

About half way down the article

https://www.goodwood.com/grrc/road/the- ... is-harris/

[Zeebuilder]

I'm on steelys but I have noticed on a few occasions that when I have came off a slip road on a motor way and gently breaking that the pedal has a mind of its own and brakes much harder than I had applied my foot.

[robf]

Have my car booked in, but as it's a few weeks away they suggested phonine up VW Assistance which i did.

Their opinion is that i have a fault from removing DRC which has upset the ABS module; thereby removing the code to 'pre-wipe' the discs in wet conditions (normally the fact the windscreen wipers are going is a trigger for the car to then brush the pads past the discs every so often).

The question mark on that theory is, in part thanks to the above links, there seems to be instances where this happens on completely stock cars.

It's all a bit of a PITA to fix as i don't plan to return to stock, and i also have my OBD port hidden under the dash! I therefore think i'm going to have to pay Audi to remove the suspension fault (if they can) through the diagonistics so i can then test if the braking issue happens again.

I suppose i don't have many people to ask when it comes to how their RS6 performs with my scenario (i.e. DRC equipped cars that have aftermarket coilovers or bags fitted)!

[MikeFish]

Luke at plush has ceramics and air ride. His ceramics were retrofitted.

[MikeFish]

Also Jason has ceramics and coil overs

[drhex]

Have retrofitted ceramics on mine as well and had this behaviour until recoding as described above. Can pm you how this is done in case you have VCDS. Better to have the dealer do it in case it is original though.
Florian