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MrGalantguy

Brake Caliper postioning

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For those looking to upgrade their brakes and come across on deciding on which route to take, should take this into consideration. While planning to upgrade my brakes I came across the question does brake caliper positioning matter and the logic behind it. My dilema was buying a aftermarket BBK like Baer or Wilwood, buying the "L" bracket to fit Evo Brembos, or looking for a JDM knuckly to simply mount the Evo Brembo like it was meant from engineer design.

This question came to me while at a track event and looking at different cars setup and seeing brake setups being in the 9 or 3 o clock position and seeing a few custom setups with the calipers being in a different position besides the 9 or 3 position. I talked to a guy with over 10 years of SCCA class racing about the importance of this and made perfect sense. I requested information from him for me to post on here for explanation on the concept.

He sent me a link to the forum he is on where they have technical explanations on caliper positioning:

http://www.eng-tips.com/viewthread.cfm?qid=101874

Here are some notable quotes:

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.

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.

EFFECT OF CALIPER MOUNTING POSITION

Mark Ortiz April 2004 Chassis Newsletter

What effect on wheel loading does the positioning of the calipers in a leading or trailing location have - i.e. mounted at 3 and 9 o'clock positions? Does a trailing caliper add or subtract load on the front tires? In a rear independent suspension, does a leading caliper add or subtract wheel loading, and is it the same in a live axle situation?

The short answer is no. Caliper location has no effect whatsoever on wheel loading. Having the caliper's mass lower or higher does have a very minute effect, because it affects the CG location a tiny bit, but there is no difference between a 3 o'clock mounting position and a 9 o'clock position.

However, there is an effect on bearing loads. It might seem counterintuitive that we can change the bearing loads and not change the tire loads, but that is in fact the case. As the questioner appears to have considered, the disc tries to carry the caliper upward if the caliper is trailing, and downward if the caliper is leading. That reduces bearing loads if the caliper is trailing, and increases bearing loads if the caliper is leading. However, these forces are reacted entirely within the hub/bearing/spindle/upright/caliper/disc/hat assembly, and do not change the loads on other parts of the car.

We can think of it like this: Gravity acts downward on the car, with additions and subtractions due to inertia effects and aerodynamic effects. The road surface holds the car up. Or, we may say the road holds the tire up; the tire holds the wheel up; the wheel holds the hub up; the hub holds the bearings up; the bearings hold the spindle up; the spindle holds the upright up; the upright holds the suspension up; the suspension holds the sprung mass up. If the caliper exerts an upward force on the upright and a downward force on the disc, that just means the brake is helping the bearings and spindle hold the upright up. It doesn't change the total support force, only the load path within some of the unsprung components.

It is worth noting that in braking there are also horizontal forces acting through the wheel bearings. The car is trying to keep going forward at a constant speed. The road surface is exerting a rearward force on the car, through the tires, wheels, hubs, bearings, spindles, uprights, and suspension. We can reduce the bearing loads due to this component if we mount the caliper above center, or increase the bearing loads if we mount the caliper below center. In fact, the horizontal force may be greater than the vertical force on the tire. With racing slicks on dry pavement, the horizontal force may be 1.3 or more times as great as the vertical load on the tire. So for least bearing loads during braking, the caliper should be somewhere in the upper rear quadrant - around 1 o'clock or 11 o'clock, depending on which wheel we're looking at, and from what direction.

Now, do we actually want maximum cancellation of the bearing loads by the brakes? We might suppose so, but actually there is an argument for not having maximum cancellation. The effective radius of the brake (roughly the radius to the middle of the pad) is often less than half of the tire effective radius. This means that the force at the caliper is more than twice the rearward force at the tire contact patch, and it may also exceed the vector sum of the vertical and horizontal forces at the contact patch. Consequently, the caliper force may not only reduce the bearing loads, but reverse them. If there is any free play in the bearings, or deflection in the components, this load reversal may result in a vibration or a small variation in the steer angle of the wheel. So there is a case for building the components nice and strong, and positioning the calipers so the bearing loads will not reverse.

Of course, as a practical matter, if we are using purchased calipers we need to mount them with the bleed screws at the top, or very nearly so, just to facilitate good brake bleeding without requiring the calipers to be dismounted. This may well outweigh any theoretical considerations. If we are designing from a blank sheet of paper, we don't face this constraint, but most of us, most of the time, are designing around purchased calipers.

Another practical constraint is packaging, particularly of the steering arms and cooling ducts.

There are some ways in which we can affect wheel loads by the design of the brake system and the suspension. I am referring here to the longitudinal "anti" or "pro" effects: anti-dive or pro-dive in the front suspension, anti-lift or pro-lift at the rear. With independent suspension, it makes a difference to these effects whether the brakes are inboard or outboard. With a beam axle, it makes a difference if the calipers are mounted directly to the axle, or on birdcages or floaters that rotate on the axle and have their own linkages.

However, with all of these, we cannot significantly alter the loading on the front or rear wheel pair, nor on all four wheels. We can change the way the sprung mass moves in response to braking, and this may have small effects on CG height, with corresponding small effects on overall load transfer. But the big effects come from having geometry differences on the right and left sides of the car. These may be present even in supposedly symmetrical road racing cars, because no car stays symmetrical when it rolls. In oval track cars, we often design in, or adjust in, asymmetry even in the static condition. Such asymmetry can produce significant changes in diagonal percentage when braking, and we can use these to tune corner entry behavior.

All such effects are independent of the "clock" position of the caliper mount.

Another link on technical discussion: http://www.f1technical.net/forum/viewtopic.php?t=2444

From the searching and reading and talking to some "knowledgable people"...it seems that the 9 or 3 position is the best position for several reasons.

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