- Almost everyone that's modifying their car or is working in motorsports is going to have
a pretty good idea of what a brake caliper is and what its purpose is.
Today I just want to dive into some of the finer points and details around brake calipers.
The basic working principle for the braking system is that the driver presses the brake
pedal, that brake pedal transmits force to the master cylinders and the master cylinders
convert that mechanical force into a hydraulic pressure and that hydraulic pressure is what's
supplied to the calipers and is then able to squeeze the brake pads.
For a given hydraulic pressure supplied to a caliper, the clamping force you get from that
caliper is proportional to the piston sizes in the caliper.
The higher the area of the pistons inside a caliper, the more force you get for a
certain pressure.
So bigger pistons equals more force.
The important principle to understand when it comes to piston sizes in a brake caliper is
that it is the total are of all of the pistons in that caliper that defines the amount of
force you get out of it.
That could come from a single large piston in the caliper or from many smaller pistons
that add up to the same equivalent piston area.
If you're looking at changing or upgrading your brake calipers, what all of that means is
that if you need to keep the same mechanical brake bias, you need to have the same piston
area between the first and the second set of calipers.
So if you have a set of calipers that has a small number of large pistons and the new
calipers are going to have many small pistons, the total summed area of both sets of calipers
needs to be the same.
The reason we have calipers that have a number of small pistons rather than just a few large
pistons is it allows you to distribute the force of the brake pad across a larger area in a
more uniform way.
If you're going to go up in total piston area by fitting a much larger brake caliper to your
car, one thing to consider if you're keeping your master cylinders the same size while
fitting something with a larger total piston area will give you more force, at the same
time you're going to need to displace more fluid from your brake master cylinders.
That means you'll end up with a longer pedal travel as a result in order to get the
calipers to work properly.
This is an example of a pretty typical modern motorsport type brake caliper.
So this is a 4 piston caliper.
So we have two pistons in this side and two on the other side.
And the disc runs here down in the centre section with the pads between the pistons and
the brake disc.
So one of the things that's really common on modern motorsport calipers is an offset piston
size so if you look down here, if we look down here, it's pretty clear that there is
quite a large difference in the diameter of both of those pistons.
The reason we have this offset is to do with the effectiveness of the clamping on
the pads and disc at the leading edge versus the trailing edge.
What you end up with due to a gas buildup, what you end up with is the leading edge
being more effective than the trailing edge.
And when that happens that means that the leading edge of the pad tends to wear quicker
than the trailing edge of the pad so you end up with a tapered pad and that's something
we want to avoid for compliance reasons, among other things.
What I mean by leading edge is this is the part of the disc that enters the caliper and this is
the side it exits out of.
Because you've got a smaller piston on the leading edge as opposed to the trailing edge,
that means you'll end up with effectively less clamping force on the leading edge
versus the trailing edge and that's just to try and balance the effectiveness so that
you end up with a brake pad that hopefully wears in the same way at the front and
back edge.
This is the brake pad that's designed to go in this caliper.
Just sits in here like so.
And if I look down on top of it, what I mean by that taper is that this being the leading
edge, this edge of the brake pad would wear away faster than the back edge, so you end
up with a taper where this part would end up wearing quicker than this edge.
You end up with a brake pad that's skewed to the brake disc and that ends up giving you
compliance where in order for everything to get pressed against the brake pad squarely,
the whole thing has to skew over and that ends up giving you a longer pedal because
you've got pistons that are coming out on uneven angles and it's also putting a side
load on the pistons which is also a negative.
What I was talking about before with having to sum the total area of the pistons, that
means you would be individually summing the area of each of these 4 pistons and
adding them together to work out the total clamping force you're going to get
out of the caliper.
I just want to talk about a couple of points about positioning the caliper on
the car.
So fundamentally the further out, if this is the axle centreline here,
the further out the caliper is placed on the disc, the more brake torque you're going
to generate and therefore the faster you're going to retard the car.
The position of the caliper is essentially defined by the diameter of the disc.
So that's something you don't really have a lot of control over after you've already purchased
your brake disc.
Oftentimes when you're fitting aftermarket brakes to a car you're having to position
the caliper on the disc yourself.
That means making your own mounting brackets, a lot of custom work involved with that.
One of the things you really need to take care of when aligning the caliper on the disc is that
you want to make sure that the edges of the brake pads line up properly with the edges of the disc.
So what you really want to make sure is that this distance here is in the radial
location of the pad, lines up with the edge of the disc.
The maximum misalignment you can get away with here is really half a millimetre over or half
a millimetre under.
You don't want to end up with a situation where they're overhanging the discs a lot.
Means you'll end up with a lot of extra pad material overhanging the discs which is
really not idea.
When it comes to setting up your brake system, one of the really critical things is to
locate the caliper laterally on the disc properly.
So what I mean by that is in this dimension here, you want the brake disc to sit as close
as possible to the centreline of the disc.
The reason it's really critical to set the caliper up centrally on the disc is that we want our
pistons on each side to be coming out an equal amount.
You don't want the pistons on one side to be sitting a lot further out.
Each time the pistons get further and further out as the pads wear, they do tend to get
a bit of cocking over on them and that can be really destructive to both the seals and
the calipers and pistons themselves so we want to keep the amount that each one's coming out
equal side to side.
Another really critical thing when you are aligning your calipers to your discs is to
make sure that the caliper pistons are sitting exactly square to the brake disc.
So that means both, in this direction it means both in this direction, it's really critical that
the pistons and the bores are sitting parallel to the side of the disc.
If you end up with a situation where you've got the caliper not perfectly square to the disc,
you can end up with the brake pad being angled in one way or the other and all this
really leads to is giving you compliance.
That means for the brake pad to be pressed hard and square against the disc,
Something's going to have to give, that either means the brake pad is going to have to flex
or it means something in the caliper's going to have to flex or it means something in your
bracket or your upright is going to have to flex.
If there's not enoguh compliance in the system that'll just end up meaning that the brake pad
doesn't actually come into full contact with the disc which will definitely degrade your braking
performance.
One last practical thing to discuss when you're talking about mounting your calipers is the
position of the bleed nipples.
The brake nipples like you can see here on each side at the top of this caliper
need to be at the highest point hydraulically of the caliper, that means any fluid that enters
through the inlet, these nipples need to be at the highest point, any air that is in the
hydraulic part of the caliper will end up finding its way out naturally when you bleed
the brakes.
Often calipers are made in a way such that that can be mounted to either side of the car
so in this case we've got a transfer pipe going from one side of the caliper to the other
and by manufacturing it in this way it means that they can have the brake bleed nipples
on either this face or this face of the caliper meaning it can be mounted this way or this
way as long as the brake nipples are mounted on the right way on the car.
Almost all modern motorsport brake calipers are made from an aluminium alloy.
In a lot of older motorsport brake caliper designs you would see a lot of cooling fins
to help them dissipate heat, that's not something we see so much of anymore.
A lot of effort in modern brake calipers goes into dissipating the heat properly.
What you're often trying to avoid doing is transferring heat from the brake caliper
to the upright and the suspension in the hub.
That's because any excess temperature in the wheel bearings will tend to end up damaging
them or at least accelerate their wear or give you more clearance.
Generally that means that we're trying to avoid transferring as much heat as possible from
the pad to the piston to the caliper and then the caliper, because it's bolted to the upright is then
going to transfer its heat to the suspension.
It's not something we see on this particular brake caliper but one trick you'll see on a lot
of high end brake calipers is to have cooling holes drilled radially inside each of the pistons.
By helping the pistons cool, by having holes in them, it allows airflow through them which
helps to reject some of the heat that's transferred from the pad which would
otherwise be transferred from the pad to the piston to the caliper and fluid and the rest
of the suspension.
As time has gone on, monoblock calipers like this have become more and more common.
All the term monoblock means is that the entire caliper is made from a single piece
of material.
In a lot of older style motorsport calipers they were made in sections such that there
would be at least one split right down the middle.
The reason older style calipers were made in more than one piece with this split down
the middle and being bolted together is it made them a lot cheaper and quicker to
manufacture.
So making the caliper in all one piece like this makes it a lot more challenging to drill the bore
sizes for each of the pistons.
You can see, looking from the front here, there's no direct path to get through here
to make each of the piston bores.
As manufacturing techniques have got better over the years, manufacturers have found cost
effective ways of being able to precisely machine these bores while still having a monoblock caliper.
The big advantage in having a monoblock is that it increases the rigidity of the system.
So you can easily imagine if this was made in two halves with a split down the middle and
bolts going from one side through to the other to clamp it all together.
You can easily imagine that there's a lot of force in here trying to press on the pads,
that means that the caliper is naturally trying to be spread apart.
So when you've got a multi piece caliper it does always give you a lot less compliance
which all that means is less stiffness.
It actually becomes quite percetible to the driver, what the driver feels, if you've got
extra compliance in the system, whether that's from the brake pads not being properly
aligned to the disc and having to skew over as you apply the brakes, or whether it's from
having a multi piece caliper where the caliper wants to spread open when there's a lot
of force applied, the end result is you end up with a longer brake pedal for a given amount
of force.
Generally compliance in the braking system is something we're trying to avoid.
This particular caliper has what we call a bridge section bracing the two sides of the caliper
together and as you can imagine, this adds to the rigidity of the caliper quite a lot.
In endurance racing it's not uncommon to see pad changes happening during a race and to
make that easier, what you'll often see in endurance racing calipers, you'll have a removable
bridge, this means this section here will usually be held in by some sort of fastener,
usually a bolt that goes from one side to the other.
That allows the mechanics to remove that bolt, the bridge can come out, you can pull the
old pads out and slide new ones in.
In some cases to allow the mechanics to make a faster brake pad change during
a race, you can actually see sometimes, teams will decide to run without the bridge,
that means they will leave this part completely out of the car and do the race with that.
The advantage is it makes it a simpler and quicker job for the mechanics to replace
the brake pads during the race.
The downside of it is that with that bridge removed you do end up with more compliance
as when the bridge is removed, there's less stiffness in the caliper to resist this
spreading force within the caliper body.
That was a deep dive into some of the finer points of brake calipers.
i hope that helps you out when it comes time to upgrade or set up your own brake calipers
for your car.
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