Brake Physics ?

[Thrlls]

As I was walking down the street I noticed brake calipers on cars in different locations.
Some were positioned at the 3 o clock position, some at 4, 10, 9 etc. and the rear and front weren't always in the same position as each other.

This had me thinking if the front/rear were at the 9 o clock position, wouldn't this help during hard braking?

My thought is the braking force would push the front/rear down more and even out the dive having a downward effect, as opposed to having it at 3 o clock and having a lifting effect and creating more dive/lift especially if the rear is at 3 o clock. I guess any location from 12 - 6 o clock position would have some type of lifting effect.

Any thoughts on this?

I'm curious because I noticed my '84 is at 9 o clock at the front/rear.

[mpetry]

It has to do with the design of the suspension. Usually the caliper will be mounted so that the braking force is transmitted via the best load path - which is thru the control arm, swing arm, etc.

Take a look at a motorcycle's front brake, you'll see how the braking force is applied to the "back" of the fork stanchion.

There are considerations here for how braking force causes the suspension to "squat". The discussion gets complicated from there. Check out the following link

AutoZine Technical School - Suspension

[Brando]

The idea with putting the calipers behind the fork stanchion is to prevent "brake dive". Of course, it can't fully be prevented, just reduced.

[MotoSook]

Some observations:

Motorcycle front brake caliper placement - better to put it behind the fork so the braking force is transfered to the fork as a compressive force on the webbing rather than the force trying to pull the webbing off the fork if the calipers were mounted in front of the fork.

I'm not sure the dive is affected much whether the calipers are in front or behind as the force is still directed up to fork in the same plane/axis, the fork axis.

The same can be said about the caliper placements on the car with regard to dive control. Mass placement (more toward the center than to the front and/or rear) is probably more a design factor than dive control. Design of the spindle/fork probably dictates a lot of caliper placement.

[Bill Verburg]

leading vs trailing mount in car applications is solely about where they can fit the calipers w/o interfering w/ steering arms, suspension arms etc.

In a motorcycle application where the forks have much more caster and no arms to interfere w/ the placement is all about reducing the polar moment and cg of the fork. When the calipers are in the leading position they are up high(bad) and swing through a wider arc(larger polar moment) when mounted in the trailing position they are low(good) and the arc that they travel through is reduced(lower polar moment)

motorcycle anti dive has 2 components 1) the hydro-mechanical resistance due to springs & shocks 2) the geometric due to caster. Autos have that plus they can also change the geometry of the A arm axis to further aid anti dive

On a car with relatively less caster(usually not too far from zero) the arcs are very close to being equal and the relatively small % of the calipers mass wrt the rest of the suspension further reduces the effects

whether the mounts are in tension or compression is meaningless to aluminum or steel.

[PeterRT]

The whole load goes through the wheel carrier, so what the suspension sees will always be the same. Brake dive is not affected by caliper placement.

[island911]

Originally Posted by Bill Verburg

...
whether the mounts are in tension or compression is meaningless to aluminum or steel.

unless it's cast aluminum, and/or the engineer wants to minimize the size of the mounting flange. (referring to a m/c fork strut)

[MotoSook]

Originally Posted by island911

unless it's cast aluminum, and/or the engineer wants to minimize the size of the mounting flange. (referring to a m/c fork strut)

Yep..see some of the current caliper mounting designs. They're using as little material as possible. When posting my initial comment, I had visions of the ears cracking under braking.

[Bill Verburg]

Originally Posted by island911

unless it's cast aluminum, and/or the engineer wants to minimize the size of the mounting flange. (referring to a m/c fork strut)

As if that will make a significant difference one way or the other

997Cup trailing mount wheel carrier



993 & 993RS leading mount wheel carriers


the difference is wt between the 3 is much <1#

[island911]

Originally Posted by Bill Verburg

As if that will make a significant difference one way or the other
..

uhmm ... okay.

What about a chip on a shoulder? ...is it more easily knocked of, or squished off?

[Flieger]

Many F1 cars mount their calipers on the bottoms of the rotors for a lower center of mass.

[RWebb]

thx for the pics and explanation, Bill

[ShakinJoe]

nice work

[jcge]

Originally Posted by Flieger

Many F1 cars mount their calipers on the bottoms of the rotors for a lower center of mass.

Interesting - I guess road cars have historically mounted higher (3,9 or 12 oclock) to minimise the risk of debris damaging the brake line.

[Flieger]

Check out the brake photos here: What is Your Holy Grail for Components on a 911?

Here is a 956 dual-caliper setup:

[javadog]

On a car, or a motorcycle, using a brake creates a torque around the wheel center. The suspension experiences this twisting force and it is the same regardless of where the caliper is mounted. Depending upon where the caliper is mounted, various bending forces can be introcduced to a suspension part, as on a motorcycle fork. In the early- to mid-'80's certain race bikes were run with essentially the same forks turned first one way (with the caliper in the front) and then the other way (wih the caliper in the rear.) No handling changes were noted, even though the forward mounting position increased the center of mass of the fork/brake assembly and made a slight change to the moment of inertia of the assembly. The loading on the brake mount lugs did change. The lugs were beefy enough that it didn't matter but in later years, when installing larger brake rotors on a given pair of forks, the smarter racers were aware that spacing the caliper further away from the wheel center and often at a different angle, the loads on the fork lugs could be changed significantly. Loads could increase quite a bit and what was once a compression/shear load could change to a tension one.

JR

[burgermeister]

Yep - the pads generate a force tangential to the rotor, and an equal and opposite reaction occurs at the spindle center. Net result is just a moment, regardless of caliper location.

[island911]

Yep, but as Soukus sez, placement can be optimized to the primary structure.



Look at the top mount ... loaded in reverse would likely be sufficient ... for a while. Failure mode would be bad.

[my1st911]

Because caliper placement relative to the rotor is immaterial to clamping force; the considerations in caliper placement are for airflow and packaging. The front-axle packaging is typically more constrained by the steering gear, and thus calipers tend to be at the 6 and 9 o'clock positions there. Rear calipers typically face fewer placement obstacles, and then it is the parking brake cable and other packaging issues that prevail. Caliper placement has nothing to do with anti-squat or how much the front end dips in a hard stop

[RWebb]

Originally Posted by my1st911

... the considerations in caliper placement are for [1] airflow and [2] packaging. ...

I think Island is suggesting that stiffness of the moment arm may be a consideration also. Seems reasonable - w/o quantifying the forces and bending of the structure...