Brake Knock-off?

[peanutaxis]

In the first few minutes of the Hungarian GP Rosberg was informed over the radio that he had some "Brake knock-off" in turn 4 which led to a long brake pedal in turn 5.

Google -> Apparently this is when the brake pads get knocked away from the discs, and so the pedal goes long because the driver has to push the pads all the way back to the discs.
But this explanation doesn't make sense. If the pads can be knocked away from the discs then there must be a gap between the piston and the brake pad. So even when brake knock-off doesn't happen the driver still has to bridge that gap which would equally make the pedal go long.


What am I missing?

[natehall]

I dont know about on a F1 car, but I assume it is at least similar to a road car.

Brake calipers have pistons in them, these are usually in contact with the brake pad at all times, just not pushing the brake pad into the disc.

Brake knock off is when the piston has retracted further than needed (in my experience no more than 1mm on a road car - probably a lot less on a F1 car) and considering when you press the brakes they normally have to move less than 1mm to grip the disc, you have double the pedal travel to get the bite on the discs.

id expect F1 clearances to be a lot less than this.

[strad]

Probably a poor job of bleeding that allowed as said for the pistons to back off more than when they started...Maybe an air bubble..the master cylinder position causes them headaches in bleeding. I'm always suprised they don't back bleed them. Or pressure fill the system.

[silente]

In my experience this could (with some old cars) happen also when driving hard on curbs: the vibrations could actually move the piston back (up to a certain point at least) needing than, on driver side, a longer pedal movement to properly brake or some very small kicks on the pedal on the straight to be sure to have again the piston in contact when properly braking at the end of the straight. That's something drivers don't like, anyway!

I guess to measure that with telemetry they should also have some sort of movement measurement on the pedal, not only brake pressure sensors.

[marcush.]

Brake pads rubbing against discs cause drag ,so you don´t want this at any cost.Usually the square shaped in crosssection caliper piston seals provide a limited piston retraction of a few tenths of a millimeter you would not be able to feel any slack in the pedal .
But then you got a car cornering at high gs ...depending on brake caliper position and hub/bearing/upright /caliper installed stiffness the loads will distort the positioning of the brake disc in relation to the caliper .Having the caliper at the bottom of the upright is maybe good for CofG but it is not good at all when cornering loads are fed into the hub/bearing assembly.
This will lead to the disc being forced a little bit from its usual line and in effect the disc making contact to the pads ...and these are knocked back in consequence .A very crude example of these happenings is a severely loose wheelbearing (taper roller design) the effect is instantly noticeable you have to pump the pedal after very short time of traveling and you get taper wear on the pads as well .
Knock back is sometimes countered by little springs behind the brake caliper pistons but you don´t ewant to do this as the drag penalty is severe -not sure how a carbon disc /pad combo reacts to permanent dragging of the pads ....I guess unpleasant...
So no negligeance on part of the mechanics it´s more a design weakness ,or better sais a tradeoff -weight vs stiffness
- in the old days they did not have ceramic bearings so the tradeoff was severe to gain stiffness ...

[zoru]

If the pads can be knocked away from the discs then there must be a gap between the piston and the brake pad.

When the brake pedal is not depressed, the brake circuit is open to the reservoir to allow fluid volume change (vapour, leak, etc.).
You can push the piston in.

[marcush.]

http://www.stoptech.com/technical-suppo ... -knockback

does explain it quite nicely

[aussiegman]

Pad knock-off can be caused by many things.

It can also be a result of movement of the disc pushing the pads away from the normal centreline of the discs rotation. I have seen this on cars where the disc is not properly set on the hub (usually improperly prepared hub surface), flex in the mounting structure of the caliper or there is other mechanical issue like wheel bearing etc.

For Rosberg as there was no mechanical failure of the braking system (though his engine did expire) it could have been poor bleeding or contaminated brake fluid as well.

Also to maintain pedal feel and prevent pad knockoff there are residual pressure valves sometime inserted in the brake lines to maintain close proximity of the pads and good pedal feel. Maybe one of these also could have failed or a pad retaining clip/spring could have failed.

Lots of possible causes when it comes to a long brake pedal...

[aussiegman]

I'm not 100% certain but I am led to believe that they may use residual pressure valves on the brake lines.

However, listening to the commentaries of the ex-F1 drivers they elude to tapping the pedal to make sure it's there for the upcoming corner.