Brake Caliper Mounting

[Ballzac]

Does anyone know if there is any significant disadvantage to mounting a front brake caliper on the front of the rotor as opposed to the rear (Longitudinally)? Our caliper was mounted on the rear of the front uprights last year, but packaging is really tight this year and I want to mount it on the other side unless there are some severe dynamic reasons not to.

Thanks.

[Denny Trimble]

There are a few effects:

1) Inertial steering torque of the upright asssembly about the steering axis (probably a small effect for our cars).

2) Polar moment of inertia of the car: moving the caliper forward on the front wheels increases their distance from the CG, and polar moment is in terms of Mass * (distance from CG)^2, plus the rotational inertia of the caliper. Not a big effect really, but it adds up.

3) Reaction force on the hubs: the hubs, bearings, and spindle have to resist a force equal and opposite to the tangent braking force on the rotor. If the caliper is in front of the spindle, this force is in an upward direction, adding to the normal force from the tire. If the caliper is to the rear of the spindle, the force is downward, helping to cancel the tire normal load acting on the spindle.

This is assuming a "conventional" dead spindle, rotor outboard of the balljoints, positive scrub radius, etc.

[Nate Notta]

Ok if you decide those 1st two issues can be neglected...
how serious do you think the third is? The normal load in one tire would be ~150 statically... more than that during weight transfer. But I think it would be dwarfed by the braking load.

Do you think the design judges care?

[Tudor Miron]

Good points Denny,
Nate - sorry for this being such a long text - but you asked for it. This should be enough to make a desision http://fsae.com/groupee_common/emoticons/icon_wink.gif
Mr.Ortiz say:
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 below center, or increase the bearing loads if we mount the caliper above 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 5 o'clock or 7 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.

[Nate Notta]

Thanks for the huge post! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif Did you get that from a book?

[Tudor Miron]

At list I wasn't typing it! http://fsae.com/groupee_common/emoticons/icon_wink.gif

Thank you
Ted

[me_george]

Very good article, Tudor Miron
Do you mean Mark Ortiz by Mr. Ortiz?
One observation about this article in the sixth paragraph, he concluded " The caliper should be somewhere in the upper rear quadrant." but it is clear from the discussion that he meant lower rear quadrant, because that will lower the bearing loads since the caliper mounted below the center.. What do you guys think??

[Big D]

I agree, I think that's what he meant

[GTmule]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Big D:
I agree, I think that's what he meant <HR></BLOCKQUOTE>

It is, he put a correction this month in the Mag.

[Mikey Antonakakis]

Megabump for this good post. For teams looking "how to design brakes?" this is probably a useful read.

[billywight]

Combine some assumptions, a free body diagram, and some math.

[Owen Thomas]

Originally posted by brakeboy:
how can we analyse(calculate) forces acting on upright by brake caliper?
http://i1.kym-cdn.com/photos/images/original/000/126/314/3cd8a33a.png