- Brake knock off is the reason you often see in racing, where drivers are approaching
a corner after a long straight, they end up having to pump up the brake pedal before
it'll actually start working, so you'll often see when the cars are heading down the
straight towards a braking zone, you'll see the brake lights flash from the outside before the
actual braking marker, that's the driver pumping up the brakes before they get
to the braking zone.
Little pump of the brakes, not a big application, just usually a couple of
small pumps and that squeezes the pads up against the disc, not so much
that they start braking but just that they're prepared to brake so when they do want
to hit the brakes, they can go in one go and the car will start braking.
So we're looking here at a pretty typical front brake setup.
We've got the brake caliper, the disc, and the pads.
So if we look in there you can see in behind the pad, between the caliper
and the pad we've got the little pistons sitting in here.
These pistons are the parts that push out from the caliper onto the pad and
squeezes everything in together there to make the car stop.
So what's happening when you get brake knock off is the pads themselves are
getting pushed back into the caliper, that means you end up with extra
clearance in between the pad and the disc, you need to use an extra brake application
before the brakes actually start working.
The number one thing for reducing pad knock off is to reduce compliance and
any play in your hub and upright.
So that can mean at one extreme simply adjusting the wheel bearings if you've got
an adjustable type wheel bearing or it could mean increasing the stiffness of this whole
assembly so the hub and the upright, making sure that you're reducing the amount of
compliance that they've got and that can mean an entire redesign.
Here looking at this caliper on the bench again, we can look at a couple of other ways
you can look at reducing knock off.
So quite a common thing is to use anti knock off springs.
Before the caliper is assembled, you put a little spring in there behind each of
the pistons and that just keeps a little resistive force just so when they're at
rest, they're constantly wanting to move out towards the pads.
It's normal to build the calipers specifically for each circuit in professional racing so
you will have at some circuits say a relatively low average speed circcuit with
lots of bumps and curves like a street circuit, that will typically have quite heavy
knock off springs fitted whereas if you go somewhere where top speed's
really important you typically don't want any extra drag on the straights so the
driver would pay a penalty by having knock off springs which means at the end of the
long straights if there's any knock off, they will have to pump the brake pedal up
to get the pad to squeeze up against the disc so when they're actually ready to
brake, the car will brake as they're expecting but the benefit of not having those knock
off springs is they'll have less parasitic drag with the discs, will create a slight clearance
between the pads and you'll have slightly higher top speed.
Another factor in controlling knock off is the seal design.
So the seals that seal the hydraulic pressure between the piston and the
caliper also tend to have a small resistive force and the amount of drag
and the amount that the lips on those seals want to, the direction that they want
to tend to push the piston, also has an effect on knock off.
So that was a short discussion on brake knock off and some of the causes.
In summary, if you've got brake knock off problems with a long pedal, when you
approach the braking zone, particularly on bumpy circuits, what you need to do is
to reduce the compliance in the system, so reducing wheel bearing clearance,
making sure the whole thing's stiff enough and if it's still fine and you can't
do anything else to help that, you can try running anti knock off springs to
get a better quality brake pedal.
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