Why position a brake caliper in front or behind the disc?

[CritchAutomotive]

Good day people,

I am wondering if anyone can tell me the science or engineering (other than the obvious cooling and contamination prevention reasons) of positioning a brake caliper (either front or rear) behind or infront or above the brake disc?

Any help in this curious matter would be very much appreciated.

Craig Critch.

 

[evelrod]

It's pretty easy if you have ever raced a car with the calipers on top or bottom!!! As a car turns, the wheels deflect a certain amount and this causes the spindle, hub, rotor to also deflect a bit causing the brake pads to be 'bumped' back a little more than you need. The next corner when you go for the brakes all this extra space must be made up resulting in the pedal 'going to the floor'! Not the most awe inspiring event when braking for a 90 mph corner from 175 mph I can asure you (BTDT).
I place my front calipers out front as it makes pad changes quicker but, they would work equally well on the backside. The rears depends on clearance and such. My current vintage racer has drums in the rear so it's not a problem.

Rod

 

[Rob45]

Seems to me like placing the caliper at the top of the rotor, or for that matter at the bottom, would place it closer to the centerline through the upper and lower balljoints, and requiring a smaller caliper-mount, reducing inertial effects in steering the wheel from side-to-side. Can't say how great that effect would be, though.
It's certainly not uncommon to see calipers located near the top.

 

[evelrod]

Hi Rob45. What often sounds like a great idea is, in this example, NOT. On the bottom the caliper would be subject to all kinds of debris while on the top it may interfere with other suspension components. However, in both top or bottom locations the flexing of the brake rotor will bump the pads so far back as to possibly compromise the braking when the next pedal application is required. I could see how it might work as long as one NEVER turns a corner. Keep in mind it only takes a few hundreths of an inch at the pad to put the pedal on the floor. Not a good situation in any event. Calipers are often located at a slight angle above the horizontal so as to place the bleed screw at the highest location and control interferance with other suspension components. As in most all automotive suspension/brake setups, compromise is paramount.

Rod

 

[patprimmer]

I totally agree with Rod.

Stub axle and front suspension knuckle or upright flex can and this does cause the pads to be knocked back during cornering with substantial loss of braking efficiency next time they are applied. I had a car that had this problem because the calliper was probably from memory in about the 11 "O"clock 1"O"clock position. The so called fixes was to have quite a preload on the wheel bearings, but that resulted in front wheel bearing life of about 30,000 miles.

Also as Rod says, at 6"O"clock, the callipers collect retain debris, and cannot be bled. At 12"O"clock, they will interfere with the design of the suspension upright, and the calliper is in the most vulnerable position to boil the fluid.

3/9 "O"clock in front of the axle the calliper is most protected from debris, and gets most exposure to cooling air, but it shields the rotor from cooling air. At the back of the rotor increases exposure to debris, and hot air off the rotors.

I think there is nothing new here. These problems have been considered for over 50 years, and the designs are decided on for best overall compromise for the particular application.


Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gielhansen]

A reason to place it on the rear of the disc is that it will unload the wheel bearings when the brakes are applied.

In the front it will put extra load on the bearings on top of the vertical load of the car's weight.

On the other hand, the available space on the rear might be less on steered wheels because the inner wheel has a bigger steering angle.
Giel

 

[JakubMech]

You can play around with the upright internal stresses by varying the position of the brake caliper.

F1 tried the brake caliper at 6oclock purely for CG reasons, but found the caliper body being "milled" down by the wheel flexing, requiring stiffer setups.

Another point can be yaw inertia of the vehicle, having the front calipers to the back of the disc and rear calipers to the front of the disc will reduce the inertia of the vehicle, expecially using those mostor 8 pots.

Jakub

 

[notnats]

gielhansen,

Unloading the bearing under braking might not be a good idea if there is any clearance in the bearing. It may lead to shuddering of the wheel/brake assembly. The extra vertical load resulting from the caliper in front of the wheel is most likely small compared to cornering loads on the bearing and will help to stabilise the bearing.

Regards,

Jeff