A really technical question about BRAKES...

[ClubSport 911]

Now here' s one for the anoraks out there (and I count myself included). Yesterday up at Bedford autodrome, I saw something stand out so vividly in the setting sun, it has prompted me to ask this question here.

I was by trade an articled electro-mechanical power engineer, so I' ve looked at it from all sides but never reached a conclusion... here it is :-

Why is it on some Porsche' s / cars the brake caliper is in front of the Axle centre line, and with other cars, it' s behind it ?

Indeed, amongst all the cars yesterday, I saw no evidence of an " engineering rule" to guide the logic of caliper placement.

GT3 = Behind
964RS = Front.
Elise = Behind (I think)
3.2 Carrera = Front
and so on...
996 Turbo = Behind

....thoughts ?

I have considered:- Cooling, Weight distribution, axle turning moment effect, space, steering space / radius / power steering & A stub arm geometry...(in fact this is most likely)

 

[Guest]

This a is a very annoying question, mainly because I have wondered, but not asked.
Humph !

I assume simple engineering/manufacturing/cost - but I can' t justify this.

Somebody please explain !

 

[Jamie Summers]

This has bugged me for ages too - and I' m afraid I don' t have an answer either !

 

[Nick Moss]

I would think that the preferred position would be behind the axle line, allowing better cooling airflow to the disc.

 

[Alan_Armstrong]

It' s one of those questions where you think ' why didn' t I ask that' ...its bugged me for ages also...

I hope someone' s got the answer!

 

[Guest]

You should ask this over at Pelican Parts forum. There' s a guy there called Bill Verburg who knows too much about brakes, suspension and wheels.

 

[oilyrag]

Yeah me too.........at Festival of Speed I asked on the Merc stand.....they didn' t know either but " me and me mate ad a good long natter" ....only thing we came up with was better weight distribution by keeping more of the unsprung mass within the wheel base....ie less polar moment of inertia (a sort of leverage effect around the centre of rotation).
Although it could just be a fashion statement! [8|]

 

[ClubSport 911]

I think it' s all to do with geometry. Namely where it is sensible to have the steering rack / track rod ends etc...

This (of course) changes with wheelbase / car / internal layout etc etc... hence the lack of consistency

Any other thoughts ?

Steve

 

[Guest]

Namely where it is sensible to have the steering rack / track rod ends etc...

That is so obvious, it must be true.

The only problem is, why didn' t I think of it first ?

 

[oilyrag]

So...............are there any Professionals out there who could answer this question, since we all only have conjecture at this point! Interesting tho it is.
Who knows a suspension type racing mech ? He/She may be of use (that' s a very PC stance by the way)! [&:]

 

[Guest]

The literature for the 996 claims that the caliper position is behind the axle line to optimise brake cooling.
It is easier to duct the air to the front of the disc than the rear, since the rear requires the duct to bend round the stub axle. Plus the track rod ends will be in the way.

This is an easier and neater arrangement.
The issue is why it should be the other way, i.e. with the calliper in front? Perhaps the turning moment on the axle is better handled with the brake in front, but the cooling benefits negate this. The brakes are a heat sink and most road cars do not have adequate heat capacity/heat dissipation for track useage, whereas Porsche are much better. For normal road usage I doubt it makes that much difference.

Why should the 964 be OK? Lower weight means less heat has to be lost? Less optimal arrangement? Progress means that thinking has changed?

 

[hawkhead]

Areply from an engineer (not me!)

Often depends where the steering arm is, which depends on rack position, often also depends on fouling of ARB links on lock. In a clean sheet of paper, race car design the ideal place is at the bottom, or to the rear at the front of the car, and at the bottom, or to the front, at the rear of the car, to lower C og G and polar moment of inertia. In F2 they often used balanced calipers, with a pair of 2 pot calipers on the front, opposite one another. This stuck around until the late eighties, on F2 and F1. Bottom mounted calipers can be a pain though, as they usually need removing to bleed them.

 

[Guest]

Something to consider... I think that this came from a tutorial problem when I was an undergraduate mechanical engineer.

The force applied by the caliper is one that acts tangential to the disc's rotation, roughtly at the central point of the brake pads. The reaction to the force passes through the pad backplates, to the caliper, and is taken by the stub axle assembly to which the caliper is bolted.

Now, think about what happens when the caliper is directly in front of the stub axle. The force applied to the disc is directed directly upwards, so the reaction force exerted by stub axle is directly downwards - this means that any downward force being exerted by the stub axle in the absence of braking is reduced. The converse applies when the calper is directly to the rear - the reaction from the sub axle is directly upwards.

Now, consider the static situation, in which the car is at rest. Some static fraction of the weight of the car is transmitted through each sub axle, throught the wheel bearing, to the wheel and thence to the ground. The wheel bearing is exerting an upward force on the stub axle.

What happens under braking? If the caliper is in front of the axle line, the force on the wheel bearing is reduced under light braking, and probably reversed under heavy braking. If it is to the rear of the axle line, the force on the wheel bearing increases as brake force increases.

So, calipers to the front can reduce the magnitude of forces appied to the wheel bearings (and stub axle), while calipers to the rear avoid reversal of the direction of the force.

By the way, this is just a by-the-way, not an answer to the question ;-)

Alistair