Brake engineering question

[Netspeed]

If one of the ultimate goals is to reduce unsprung weight, why do manufacturers not mount the brakes inboard near the differential or some centralized braking unit up front? I can see cost as a big factor but what would some engineering explainations be?

[Superman]

Some manufacturers have done this. Of course, it depends on the axle to not fail. If the axle broke or a CV joint failed, your brakes would be gone. Brakes place a LOT more pressure on rotating parts than your engine does. Just because your CV joints can handle your engine's power does not mean they can handle braking power.

[Cory M]

This has been done in the past, on Jaguars for example.

I can think of a few reasons not to do it like serviceability, packaging, heat, stress on the halfshafts, dust. The reason manufacturers aren't doing it is probably because the additial unsprung mass doesn't make a big enough difference on the handling on the street and it is probably more expensive to manufacture and assemble.

[Netspeed]

I agree that torsionally it would become an issue but why couldn't they put it inboard of the CVs?

[BertBeagle]

My opinion on some reasons they may not use inboard brakes on modern road cars
Cost
The braking force would have to go through rotating shafting whether it is the drive axle or a brake only axle - i.e. cost
The unsprung weight saved by this on a street car would not justify the cost
Service – i. e. cost
More parts involved when the design / engineering goals may include reduced cost through reducing the number of parts
In my opinion it would boil down to cost and complexity (high and many) vs advantage (none today when the focus is on gas mileage, emissions and weight reduction).
I am not sure they have a singular goal of reducing unsprung weight. My guess - it is a byproduct of new materials.

[Cory M]

so, you think cost might be an issue?

[HawgRyder]

We mounted inboards on several racing dune buggies in the past.
One of the side effects was increased heat into the transaxle from the rear mounted calipers.
We never had an over heating problem that we could identify, but the trans got torn down on a regular basis and the fluid replaced each time.
I would investigate that problem and the other of trying to get cooling air into the centre of the car to cool both the trans and the calipers.
And...it was easy to mount the discs by the way...the flange at the inner CV joint was the same bolt pattern as the discs we used...just had to install longer bolts.
And the mounts for the calipers were just blocks bolted to the trans flanges.
Bob

[Netspeed]

Originally posted by BertBeagle
My opinion on some reasons they may not use inboard brakes on modern road cars
Cost
The braking force would have to go through rotating shafting whether it is the drive axle or a brake only axle - i.e. cost
The unsprung weight saved by this on a street car would not justify the cost
Service – i. e. cost
More parts involved when the design / engineering goals may include reduced cost through reducing the number of parts
In my opinion it would boil down to cost and complexity (high and many) vs advantage (none today when the focus is on gas mileage, emissions and weight reduction).
I am not sure they have a singular goal of reducing unsprung weight. My guess - it is a byproduct of new materials.

If cost were not a concern (i.e. F1), then there should be a reason why the F1's run the brakes outboard. I think HawgRyder is right....it must be a heat issue and that the amount of weight saved doesn't offset the additional heat or complexities (also correct BertBeagle).

[BertBeagle]

Originally posted by Netspeed
If cost were not a concern (i.e. F1), then there should be a reason why the F1's run the brakes outboard. I think HawgRyder is right....it must be a heat issue and that the amount of weight saved doesn't offset the additional heat or complexities (also correct BertBeagle).

My guess – F1 doesn’t use them because of the weight.

Fact - The brakes will produce (dissipate) more power than the engine. Thus the components to withstand the braking & acceleration would need to be more robust than those used for acceleration only.

F1 and other racing (F5000 and some prototype sportscars) have used them in the past. I know that in F5000 and Can-Am the heat migration into the transaxle was the main issue and reason why they went back outboard.

But the original question was only concerning road cars as I understood it and I believe cost is the main reason with weight factoring in also to some degree.

The only road car I can think of that has inboard brakes is a Hummer (rear). God only knows why they chose to use inboard brakes. My guess for them is an issue with the rear hubs / wheels – sort of a compromise.

[randywebb]

Audi has also done this. The 100LS had inboard mounted brakes - in fact, they were mounted close to the exhaust system and burned up the pads so fast you'd think it was using butter as a pad material.

[911pcars]

Originally posted by Netspeed
If one of the ultimate goals is to reduce unsprung weight, why do manufacturers not mount the brakes inboard near the differential or some centralized braking unit up front?

It isn't.... an ultimate goal.

Sherwood

[Porschekid962]

F1 doesnt do it because it would be a huge compromise from a performance standpoint. F1 brakes are extremely light to begin with, adding brake axles adds weight, then think of the cooling needs, they are astronomical even though carbon cools off very quickly.

At some point brakes as we know them might and probably will be deleted from cars as we know them today in favor of energy recovery systems which will be making their way into F1 as well. So much energy is lost during braking its about time that it is used for propulsion.

Anyways you will see turbocompounding and kinetic energy recovery on a street car before you seen inboard brakes is my take.

[Quicksilver]

Originally posted by BertBeagle
. . .
Fact - The brakes will produce (dissipate) more power than the engine.
. . .

This "fact" just isn't true. The only energy that brakes have to dissipate was supplied originally by the engine to begin with. So unless you are coasting down a mountain, the engine has to produce all of that kinetic energy. (On the mountain the engine probably had to push you up to begin with.)

Gasoline powered piston engines have an efficiency that is generally in the low 20% range. The wasted energy is lost as heat. This means that to get a car up to 100mph the engine is going to have to dissipate at least 4 times as much energy as the brakes will have to radiate in order to stop the car from 100mph. This heat is dissipated by radiators, oil coolers, direct heat transfer from the engine (aircooled!), radiated heat, the exhaust, etc.

The actual energy dissipated by the engine for a given quantity of speed actually goes up when you take into account aerodynamic drag, parasitic losses (alternator, oil pump, AC, etc), and losses in the drive train.

- - - -
Back to the original question...
The only reason that you would want the brakes mounted inboard on a half shaft is to reduce unsprung weight. This is an important goal but EVERY choice in automotive design is a compromise. Is the gain going to be worth the cost?

There are a number of reasons why putting the brakes in the middle of the car is a real pain. Most of them have been touched on here. Heat, cost, overall weight, mechanical complexity, etc... Almost all of this has been mentioned but lets go into some detail.

Heat - This is the big one. You are putting a major source of concentrated heat in the center of your car. You have to get the heat completely out of the car which means routing airflow into the car's center. In the engine you can dissipate your heat in remote locations on the car such as radiators, oil coolers, and the exhaust just blows heat out the back. This makes getting airflow for cooling pretty easy to keep towards the outside of a car. That makes for simple airflow management and simpler cooling design.
The heat of the brakes must be dissipated at the brakes so you need lots of air directly to the brakes. Also even though the brakes dissipate significantly less BTUs then the engine, their concentrated heat is at a much higher temperature. The radiated heat becomes an issue for every component around it (read as cook!). On a monocoque chassis getting air into the middle of the structure kind of defeats the whole premise of monocoque to begin with. You need a completely enclosed box to create the structure. They have enough trouble just creating the hole that the driver's head sticks out of!
Obviously it is easier to cool the brakes by sticking them out next to the air stream inside the wheels. This also limits the number of components that effected by the heat.

Cost - To do this you will need to add about the same complexity as you would need to add 4 wheel drive to a car while requiring a large space to actually house the brakes. You will need heat shielding and insulation. And finally you will need to add some huge brake ducts and we aren't talking about the little things we use to pipe air to the inside of the rotors. We need LOTS of air to keep the whole area cool. (Oh and all that air (read as drag) will be constantly flowing even when the brakes aren't being used.)

Weight - When we look at the weight issue on a 2 wheel drive it is obvious that you have to add additional weight to transfer the torque to the center of the car for inboard brakes. The halfshafts would have to be pretty beefy as the front brakes have to do 70% of the stopping and this will be at the maximum torque that the tires grip can maintain. This is a significant addition to the car.

Complexity - As you have seen you need to design a half shaft system that will deal with as much torque as you drive system needs except in the case of driven wheels you now have to think about torque reversals. The ultimate loads are probably higher because you usually can't accelerate at the traction limit of the tire but you can always deaccelerate at the tires traction limit.
You will have to open up an area for the brakes and for the cooling. This is a bit of work! The worst part about complexity is what happens if something goes wrong. Brakes are basically very simple. It is pretty hard to come up with a way to make them fail without a significant warning. If the brakes are on the ends of halfshafts it becomes fairly easy to come up with a way that it can fail catastrophically. Break a rear shaft while braking for a corner and you are going to discover new levels of excitement!


Because of all these reasons inboard brakes aren't really a good fit for general use cars so the quantity of development is limited. For general race cars we rely on stock chassis and stock or modified 'stock like' components so the lack of factory inboard brakes has pretty much removed that choice. For full on race cars almost all of them are designed from the lessons learned on stock vehicles and the cost of engineering a major paradigm shift such of this is exorbitant. That limits you to situations like F1 and Indy cars. The possible gains for them are minimal because carbon/carbon brakes weigh almost nothing. If you ever get a chance to actually pick up a carbon disk it will make you jaw drop. And for users of carbon/carbon brakes they would have to make all of the previously listed compromises. The easy answer is "No".


The development of ceramic brakes is probably going to make this an even easier decision to make. I wouldn't be surprised if ceramics start to show up on mainstream cars. Once they do the price will come down to the point where we will start to be able to use them on our cars.


I would still like to see someone try it on a 911. The rear should be a somewhat easier place to try it and that is really where we need the grip. It would be even more interesting if it was done on someone else's dime!

[BertBeagle]

Originally posted by Quicksilver
This "fact" just isn't true. The only energy that brakes have to dissipate was supplied originally by the engine to begin with. So unless you are coasting down a mountain, the engine has to produce all of that kinetic energy. (On the mountain the engine probably had to push you up to begin with.)

It is true - The brakes will stop the car from a given speed faster than the engine will accelerate it to that speed.

[Bill Verburg]

I understand what Quicksilver is saying, I believe that what he means though is the brakes only dissipate the energy originally supplied by the engine less the component absorbed by aero and friction.

power is the rate at which energy is transferred and so Bert is correct, 993tt/RS brakes are ~1000hp equivalents, they absorb the energy much faster than the engine can supply it.

[BertBeagle]

Originally posted by Bill Verburg
I understand what Quicksilver is saying, I believe that what he means though is the brakes only dissipate the energy originally supplied by the engine less the component absorbed by aero and friction.

power is the rate at which energy is transferred and so Bert is correct, 993tt/RS brakes are ~1000hp equivalents, they absorb the energy much faster than the engine can supply it.

I should not have used the word dissipate and I think that threw everyone off. To run inboard brakes on the rear you would need beefier drive shafts and joints. That is the point I was attempting to make - but didn't come across. The thread drifted from the original question.

[Netspeed]

Thanks for the great posts everyone! I apologize if me first post was mis-leading as regards to street cars vs. race. I just wanted fundamental engineering reasons as to the why of it and I received it. Thanks again!

[gestalt1]

i've wondered if mounting the brake even further up would have advantages. what i mean is mounting the disc before the differential and letting the differential distribute the brake force to the rear wheels. of course some type of differential that would send appropriate brake force to each of the wheels would be required otherwise one wheel would just lock up. i guess a torque sensing type that would work with brake torque might work.

[gtc]

Originally posted by Bill Verburg
power is the rate at which energy is transferred and so Bert is correct, 993tt/RS brakes are ~1000hp equivalents, they absorb the energy much faster than the engine can supply it.

I'd guess it's a component life issue.
Since the front brakes absorb ~70% of the energy, the rear axles wouldn't see any more load under braking than under acceleration in a high powered RWD car. But the reversed loading could have a significant effect on fatigue life.

Also, I don't see heat being much more of a problem. We already run air ducts to cool the brakes, so why couldn't we run them to inboard brakes?

[island911]

hmmm .. .nobody said "load path, stiffness, and natural frequency".