Pad to Disc Clearances

[whitenemesis]

I was reading the MB info on Adaptive Brake System and this tweaked my curiosity -

"Priming is another ingenious safety measure. If the accelerator pedal is released abruptly, the Adaptive Brake System anticipates the driver’s intentions and brings the brake pads closer to the brake discs. Should the driver then apply the brakes, the response time is significantly faster, helping to reduce stopping distances"

Mercedes-Benz UK - Overview - Safety

So, why aren't the pads always this close to the discs? What is the usual pad to disc clearance?

 

[renault12ts]

I thought the pads always abutted the discs.

 

[iscaboy]

Big subject, you start getting into floating vs fixed caliper, disk and pad materials, mean disk RPM, pad area, etc etc etc

 

[flango]

Its a neat feature but the time that saves in braking could be wasted if the pad and disc material aren't right or worn. Braking is all about stopping in the shortest distance possible whilst under control. The correct choice of friction material and disc type and pattern (grooved & disc etc) + the correct breaking technique will stop the car in the shortest distance possible. MB are very good at optimising this other manufacturers like Audi & Subaru take it to another level fitting Alcon 6 pot calipers all round, some manufacturers are hopeless and anyone that has driven the new RS Focus will vouch for that, first thing any new owner does is upgrade the brakes.

 

[iscaboy]

don't forget suspension, has a huge effect on brake performance.

 

[flango]

don't forget suspension, has a huge effect on brake performance.

Indeed it does a massive effect and you are quite right braking is a complex subject based on multiple factors a lot of which people overlook as they think they don't contribute.

 

[whitenemesis]

Erm, guys, I wasn't asking about braking efficiencies, material choices or even suspension..

Just what the usual clearance between pad and disc is and why, if MB can move the pad closer to improve response time, why it can't always be 'closer'?

renault12ts - If they were always touching would they not wear out very quickly, heat up unnecessarily and we wouldn't have the water film 'problem' in wet weather..

"When driving in the wet, for example, a film of water can form on the brake discs resulting in a decrease in brake performance. But adaptive brake technology is so precise, it can remove this film of water by imperceptibly applying the brakes for an instant. This brake drying function ensures the brakes are optimised in the wet."

 

[iscaboy]

yes, bikers had this...

when the japs were building bikes in the 80's with stainless disks that did NOTHING for a second or 5 in the wet and the italians were building bikes with cast iron disks that worked brilliantly, but rusted in the sun....

experienced bikers used to "ride" the disk on jap bikes for a second or so before they needed to brake to dry the disk, so it is really nothing new.

 

[whitenemesis]

yes, bikers had this...

when the japs were building bikes in the 80's with stainless disks that did NOTHING for a second or 5 in the wet and the italians were building bikes with cast iron disks that worked brilliantly, but rusted in the sun....

experienced bikers used to "ride" the disk on jap bikes for a second or so before they needed to brake to dry the disk, so it is really nothing new.

Few things are truly 'new' but here it is automatic, transparent to the driver.

 

[megadamp]

Won't work with my wife. She is on and off the throttle all day long, like a kangeroo. Makes me feel sick.

 

[flango]

Erm, guys, I wasn't asking about braking efficiencies, material choices or even suspension..

Just what the usual clearance between pad and disc"

So why mention priming and adaptive braking in the question then?

Easy answer there is no set distance it varies from manufacturer to manufacturer depending upon the type of braking system they employ as described by Iscaboy

I thought this was going to be a thread discussing braking, types, influences etc... but obviously not!

 

[whitenemesis]

I mentioned the those because it sparked my question and I thought it would give some background to the subject and shows that MB can and do change this clearance.

Thanks for the easy answer and sorry for disappointing you.

 

[renault12ts]

Thinking about my first answer above. There is no mechanism to move the pads off the discs after the brakes have been applied. I would say there is still a light touch on the discs at all times. In the old days when drum brakes were the norm the shoes were pulled back by spring once the braking action was completed.

 

[iscaboy]

Thinking about my first answer above. There is no mechanism to move the pads off the discs after the brakes have been applied. I would say there is still a light touch on the discs at all times. In the old days when drum brakes were the norm the shoes were pulled back by spring once the braking action was completed.

Yes there is, else they would rub all the time. Think about it.

 

[renault12ts]

Yes there is, else they would rub all the time. Think about it.

What is the mechanism to move them back? Not saying you're wrong, just want to know.

 

[whitenemesis]

Could it be a return spring in the master cylinder? As the pedal comes back up the spring withdraws the piston and so 'sucks' some brake fluid back, releasing pressure on the pads?

I'd still love to know what these clearances are, purely academic interest. It's obviously something not generally considered in brake discussions

 

[renault12ts]

Could it be a return spring in the master cylinder? As the pedal comes back up the spring withdraws the piston and so 'sucks' some brake fluid back, releasing pressure on the pads?

I'd still love to know what these clearances are, purely academic interest. It's obviously something not generally considered in brake discussions

You are correct. Pressure is of course released when the pedal is released, but the pistons in the calipers are not attached to the pads, and thus the pads are not pulled back but merely relieved of pressure and still touch the discs.

I don't think you'd get a feeler gauge between pad and disc, thus no clearance. This whole thread mystifies me, because I have always assumed no clearance.

 

[whitenemesis]

You can trust MB to confuse

I'll admit at never really giving much thought until I read that description ...

 

[kth286]

the answer surely is that the brakes themselves (with this option) will not stop the car any quicker compared to brakes without this option BUT the total reaction time from the driver thinking he needs to apply the brakes to the time the brakes start being applied will be reduced. Therefore that driver will stop sooner.

 

[kth286]

the mechanism for the pistons to draw back after braking is the piston seal.

they are deformed slightly when the piston moves to brake, but as the seal is fixed in the grove, the action of the seal to regain it's composure again pulls back the piston to its position.

the fluid pressure is off at this moment.

these are very small movements.