Brake Rotors: Why Different Sizes?

I realize that front brakes absorb a lot more energy
than rear brakes do, and that historically, this is
why front brake rotors were larger in diameter than
rear rotors.

But given the existence of brake proportioning valves,
not to mention all the electronic controls that cars
have these days, why is this still the case? Why not
save design, tooling and manufacturing costs by stan-
dardizing on one size of rotor for a given model, and
setting the proportioning valve to adjust the front-
to-rear braking bias as necessary?



Geoff

--
"When I want your opinion, I'll read it from your entrails."

(Geoff Miller) writes:

> I realize that front brakes absorb a lot more energy
> than rear brakes do, and that historically, this is
> why front brake rotors were larger in diameter than
> rear rotors.
>
> But given the existence of brake proportioning valves,
> not to mention all the electronic controls that cars
> have these days, why is this still the case? Why not
> save design, tooling and manufacturing costs by stan-
> dardizing on one size of rotor for a given model, and
> setting the proportioning valve to adjust the front-
> to-rear braking bias as necessary?

Because it's nice to have all the brakes operating in the same
temperature range. Because you can save weight and material on the
rear brakes. Because proportioning valves are actually rather crude
devices that reduce (usually) rear brake pressure above a certain
limit, sometimes sensitive to load. Because the savings from
commonality is probably less than that from smaller rear brakes.

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.

Geoff Miller wrote in rec.autos.misc

>
>
>
> I realize that front brakes absorb a lot more energy
> than rear brakes do, and that historically, this is
> why front brake rotors were larger in diameter than
> rear rotors.
>
> But given the existence of brake proportioning valves,
> not to mention all the electronic controls that cars
> have these days, why is this still the case? Why not
> save design, tooling and manufacturing costs by stan-
> dardizing on one size of rotor for a given model, and
> setting the proportioning valve to adjust the front-
> to-rear braking bias as necessary?

The front brakes do most of the braking. Most of the weight in most
cars is in the front, plus when you put on the brakes the center of
gravity will shift a bit to the front. Thus you brakes that are
physically capable of handling the heavy loads placed on them.
Big rotors mean more area for the pads to grab, and allow larger
pads. Thus providing more stopping force. Now, why aren't the rear
brakes the same size? They don't need to be, for the very reasons I
pointed out above. And since they don't need to be, why waste money
putting bigger, heavier, and more expensive parts on them?


--
Dick #1349
"Believe those who are seeking the truth; doubt those who find it."
Andre Gide, French author and critic (1869-1951).
Home Page: dickcr.iwarp.com
email:

Dick C > writes:

> The front brakes do most of the braking. Most of the weight
> in most cars is in the front, plus when you put on the brakes
> the center of gravity will shift a bit to the front. Thus you
> brakes that are physically capable of handling the heavy loads
> placed on them. Big rotors mean more area for the pads to grab,
> and allow larger pads. Thus providing more stopping force.

Yes, I understand why the front brakes are bigger; that wasn't
my question, and I phrased my post with the hope of avoiding
just such a digression. My question wasn't "Why make the front
rotors bigger than the rear ones," but rather, "Why make the rear
rotors smaller than the front ones?" Why not use "front" rotors
all the way around, in other words?


> Now, why aren't the rear brakes the same size? They don't need
> to be, for the very reasons I pointed out above. And since they
> don't need to be, why waste money putting bigger, heavier, and
> more expensive parts on them?

Because, I'd think, the additional size, weight and cost would be
insignificant, and whatever they might be, it seems at least pos-
sible that they'd be more than offset by the lower manufacturing
and logistical costs.

Now, maybe that isn't the case; and since neither of us has access
to any relevant numbers, we can only speculate. But it's certainly
a reasonable thing to wonder about, the tone of your response not-
withstanding. Industries have done weirder things than continue
to employ various methods out of habit and interia, after all.

Having read all the major U.S. car magazines lo these many years,
I note that on a given car, the front brake rotors are typically
an inch or two greater in diameter than the rear rotors, tops.
That isn't much extra weight, and it's less weight still when the
rotors are vented as opposed to solid.

And when you consider that weight reduction in most cars is only
taken so far as a design consideration (we're not talking Honda
Insight here), it seems rather unlikely that weight savings is
the reason for using smaller brake rotos in back. The weight of
that extra inch or so, multiplied by two, is vanishingly small
compared to the weight of the car as a whole.



Geoff


--
"When I want your opinion, I'll read it from your entrails."

Geoff Miller wrote in rec.autos.misc

>
>
> Dick C > writes:
>
>> The front brakes do most of the braking. Most of the weight
>> in most cars is in the front, plus when you put on the brakes
>> the center of gravity will shift a bit to the front. Thus you
>> brakes that are physically capable of handling the heavy loads
>> placed on them. Big rotors mean more area for the pads to grab,
>> and allow larger pads. Thus providing more stopping force.
>
> Yes, I understand why the front brakes are bigger; that wasn't
> my question, and I phrased my post with the hope of avoiding
> just such a digression. My question wasn't "Why make the front
> rotors bigger than the rear ones," but rather, "Why make the rear
> rotors smaller than the front ones?" Why not use "front" rotors
> all the way around, in other words?

I answered your questions below. The above was simply an explanation
for those who do not know. And yes, there are plenty of people who
fit that bill.

>
>
>> Now, why aren't the rear brakes the same size? They don't need
>> to be, for the very reasons I pointed out above. And since they
>> don't need to be, why waste money putting bigger, heavier, and
>> more expensive parts on them?
>
> Because, I'd think, the additional size, weight and cost would be
> insignificant, and whatever they might be, it seems at least pos-
> sible that they'd be more than offset by the lower manufacturing
> and logistical costs.

Cost is always significant. Accountants play a major role in the
automotive industry. If a dollar can be saved, it most likely
will be done. Read Lee Iacoca's book for an insider's perspective
on this.

>
> Now, maybe that isn't the case; and since neither of us has access
> to any relevant numbers, we can only speculate. But it's certainly
> a reasonable thing to wonder about, the tone of your response not-
> withstanding. Industries have done weirder things than continue
> to employ various methods out of habit and interia, after all.
>
> Having read all the major U.S. car magazines lo these many years,
> I note that on a given car, the front brake rotors are typically
> an inch or two greater in diameter than the rear rotors, tops.
> That isn't much extra weight, and it's less weight still when the
> rotors are vented as opposed to solid.
>
> And when you consider that weight reduction in most cars is only
> taken so far as a design consideration (we're not talking Honda
> Insight here), it seems rather unlikely that weight savings is
> the reason for using smaller brake rotos in back. The weight of
> that extra inch or so, multiplied by two, is vanishingly small
> compared to the weight of the car as a whole.

While the weight may seem small compared to the total weight of
the car, it is probably significant in that it is unsprung weight.
Ergo, weight that is connected to the wheels below the springs. This
weight plays a significant role in handling. Which plays into some
manufacturer's reasoning, while the lower cost of having the smaller
brakes, versus the higher cost of same size and much improved control
system for brakes is the major factor for most.
One thing should tell you that there is more to it than simply
the cost factor is that even the Lamborghini uses larger brakes
in the front than the rear.
http://www.sportcompactwarehouse.com...mborghini.html
And this on a car that costs over a quarter of a million dollars.
Cost is not the factor. If there were a better way of doing things
than using different sized brakes then Lamborghini almost certainly
would be doing it.


--
Dick #1349
Damn it . . . Don't you dare ask God to help me.
To her housekeeper, who had begun to pray aloud.
~~ Joan Crawford, actress, d. May 10, 1977
Home Page: dickcr.iwarp.com
email:

>And when you consider that weight reduction in most cars is only
>taken so far as a design consideration (we're not talking Honda
>Insight here), it seems rather unlikely that weight savings is
>the reason for using smaller brake rotos in back. The weight of
>that extra inch or so, multiplied by two, is vanishingly small
>compared to the weight of the car as a whole.
>
>
>
>Geoff

Geoff, although its only an inch or two, it is at the outside of the rotor
and seeing as the mass of a circle would increase as the square of the
radius, as an eg a 9 inch circle of steel plate would weigh 26% more than an
8 inch circle.
The weight is not significant relative to the weight of the total vehicle,
but the "unsprung mass" (ie the wheel, brakes and parts of the suspension
that move with the wheel) is important to the way the car handles bumps etc.
The less unsprung weight the better - there is less momentum and "resonance"
for the shocks to damp out. This is why race cars and some exotic road cars
use techniques like inboard discs (ie attached to the differential on
independent rear ends - see jaguars from 70's onwards - upside down shocks
and lightweight forged alloy suspension arms, carbon fibre brake discs,
alloy calipers etc.

I would also echo the earlier comments re having the brakes heat up more
evenly.

cheers

(Geoff Miller) writes:

<snip>

> Yes, I understand why the front brakes are bigger; that wasn't
> my question, and I phrased my post with the hope of avoiding
> just such a digression. My question wasn't "Why make the front
> rotors bigger than the rear ones," but rather, "Why make the rear
> rotors smaller than the front ones?" Why not use "front" rotors
> all the way around, in other words?
>
> > Now, why aren't the rear brakes the same size? They don't need
> > to be, for the very reasons I pointed out above. And since they
> > don't need to be, why waste money putting bigger, heavier, and
> > more expensive parts on them?
>
> Because, I'd think, the additional size, weight and cost would be
> insignificant, and whatever they might be, it seems at least pos-
> sible that they'd be more than offset by the lower manufacturing
> and logistical costs.

I don't think manufacturing costs are that different between making
200,000 of something and making 100,000 of the same thing. And they
may be offset by material savings.

And you can't make rear brakes simply a copy of the fronts, as then
you would have no parking brake. Since you already need a different
item, why not make it the right size?

<snip>

> And when you consider that weight reduction in most cars is only
> taken so far as a design consideration (we're not talking Honda
> Insight here), it seems rather unlikely that weight savings is
> the reason for using smaller brake rotos in back. The weight of
> that extra inch or so, multiplied by two, is vanishingly small
> compared to the weight of the car as a whole.

Well, there's a saying in the car industry, "If you have a thousand
engineers, each one has an improvement in mind that only costs a
buck. If you let them do it, your car just got $1,000 more expensive."

The same principle applies to weight: a single increase may seem
insignificant, but multiply it by all the parts in a car, and you've
gained way too much weight. They really *do* care what the car weighs;
after all, there are fuel economy standards. And while they might be
willing to add a few pounds for a feature that will help sell the car,
like a new nevigation computer, why do it for something that gives no
benefit?

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.

Stephen H. Westin wrote in rec.autos.misc

> (Geoff Miller) writes:
>
> <snip>
>
>> Yes, I understand why the front brakes are bigger; that wasn't
>> my question, and I phrased my post with the hope of avoiding
>> just such a digression. My question wasn't "Why make the front
>> rotors bigger than the rear ones," but rather, "Why make the rear
>> rotors smaller than the front ones?" Why not use "front" rotors
>> all the way around, in other words?
>>
>> > Now, why aren't the rear brakes the same size? They don't need
>> > to be, for the very reasons I pointed out above. And since they
>> > don't need to be, why waste money putting bigger, heavier, and
>> > more expensive parts on them?
>>
>> Because, I'd think, the additional size, weight and cost would be
>> insignificant, and whatever they might be, it seems at least pos-
>> sible that they'd be more than offset by the lower manufacturing
>> and logistical costs.
>
> I don't think manufacturing costs are that different between making
> 200,000 of something and making 100,000 of the same thing. And they
> may be offset by material savings.

The cost savings is in the material used, and the manufacturing of
the smaller parts. If they save 50 cents per car, a company that
builds a million cars would save half a million a year. Think about
that for a minute. Wouldn't you be interested in that kind of savings?

>
> And you can't make rear brakes simply a copy of the fronts, as then
> you would have no parking brake. Since you already need a different
> item, why not make it the right size?

Indeed.

>
> <snip>
>
>> And when you consider that weight reduction in most cars is only
>> taken so far as a design consideration (we're not talking Honda
>> Insight here), it seems rather unlikely that weight savings is
>> the reason for using smaller brake rotos in back. The weight of
>> that extra inch or so, multiplied by two, is vanishingly small
>> compared to the weight of the car as a whole.
>
> Well, there's a saying in the car industry, "If you have a thousand
> engineers, each one has an improvement in mind that only costs a
> buck. If you let them do it, your car just got $1,000 more expensive."

And, if you consider that it would cost quite a bit more than a buck
to replace the current smaller brakes with larger brakes and a system
to properly meter the braking force to the front and rear, the cost
would sky rocket. Plus it would be of no benefit to anyone, including
the car owner/driver, nobody would want it. Especially since it would
be prown to failure and expensive to repair. A system with high costs,
replacing what is saving the makers a few bucks, no desire from the
end user, and no benefit to the vehicle or owner, why in the hell would
any manufacture want it?

>
> The same principle applies to weight: a single increase may seem
> insignificant, but multiply it by all the parts in a car, and you've
> gained way too much weight. They really *do* care what the car weighs;
> after all, there are fuel economy standards. And while they might be
> willing to add a few pounds for a feature that will help sell the car,
> like a new nevigation computer, why do it for something that gives no
> benefit?

And costs more, and may even make the car more dangerous? Putting too
much braking in the rear can make the rear wheels lock up and slide.

--
Dick #1349
Damn it . . . Don't you dare ask God to help me.
To her housekeeper, who had begun to pray aloud.
~~ Joan Crawford, actress, d. May 10, 1977
Home Page: dickcr.iwarp.com
email:

Stephen H. Westin > writes:

> I don't think manufacturing costs are that different between
> making 200,000 of something and making 100,000 of the same
> thing. And they may be offset by material savings.

I understand that even small differences in the cost of
materials, multiplied over N thousand cars, adds up (although
the million-car figure somebody else tossed out was a bit
over the top). On the other hand, we're talking about steel,
not about some electronic gizmo or even a simple mechanical
device. If the cost of raw materials were so critical, cars
would be a lot less solid than they are.

Besides, if the cost of using "front" brake rotors on both
axles were $.50 or whatever per car, to use someone else's
example, that could easily be passed on to the consumer --
who wouldn't even notice it, because it'd be a drop in the
proverbial bucket compared to the overall cost of the car.

Consider the inside trunk release mechanisms that many cars
suddenly had following that incident of a kid getting locked
in the trunk of his parents' car a couple of years ago.
That isn't just raw materials; it's a mechanical device,
albeit a simple one, and undoubtedly more costly than mere
extra steel in two of a car's brake rotors would be. Regard-
less of whether the manufacturers absorb the cost of those
devices or it's passed along to the consumers (which is more
likely), it's clearly not a show-stopper.


> And you can't make rear brakes simply a copy of the fronts,
> as then you would have no parking brake.

You're positing *existing* brake designs. There's no reason
in the world why brakes can't be designed to incorporate a
parking brake mechanism -- which is nothing but a way to
mechanically actuate the existing brake pistons -- and simply
*not hook it up* on the wheels where it isn't needed. Stan-
dardization of that sort is trivial.


> The same principle applies to weight: a single increase
> may seem insignificant, but multiply it by all the parts
> in a car, and you've gained way too much weight. They
> really *do* care what the car weighs; after all, there
> are fuel economy standards.

If meeting fuel economy standards hinged on saving every
possible ounce, cars wouldn't be as weighed down as they
are with unnecessary gizmos and convenience features.
They'd all be designed like the Honda Insight, which omits
a remote fuel door release for the sake of weight savings.
Car seats would resemble those Aereon office chairs.


> And while they might be willing to add a few pounds for
> a feature that will help sell the car, like a new nevi-
> gation computer, why do it for something that gives no
> benefit?

It *would* give a benefit; that's my whole point. It just
wouldn't give a benefit to the consumer, like a navigation
system would. The benefit would be the simplifying and
economizing of the design and manufacturing processes, and
of the stocking of spare parts.



Geoff

--
"Necropyroxenopedocanusphilia: Sex with dead
alien puppies on fire." -- Pete Ashdown

(Geoff Miller) writes:

> Stephen H. Westin > writes:
>
> > I don't think manufacturing costs are that different between
> > making 200,000 of something and making 100,000 of the same
> > thing. And they may be offset by material savings.
>
> I understand that even small differences in the cost of
> materials, multiplied over N thousand cars, adds up (although
> the million-car figure somebody else tossed out was a bit
> over the top). On the other hand, we're talking about steel,

Cast iron, actually...

> not about some electronic gizmo or even a simple mechanical
> device. If the cost of raw materials were so critical, cars
> would be a lot less solid than they are.

And if there were no safety standards or crashworthiness ratings. And
if no one cared about squeaks, rattles, and noise.

> Besides, if the cost of using "front" brake rotors on both
> axles were $.50 or whatever per car, to use someone else's
> example, that could easily be passed on to the consumer --
> who wouldn't even notice it, because it'd be a drop in the
> proverbial bucket compared to the overall cost of the car.

Sigh. Every dime of the cost of a car is another dime. And car makers
pretty much treat it that way; they fight cost every inch of the way.
And wasn't your original proposal supposed to save on costs?

> Consider the inside trunk release mechanisms that many cars
> suddenly had following that incident of a kid getting locked
> in the trunk of his parents' car a couple of years ago.
> That isn't just raw materials; it's a mechanical device,
> albeit a simple one, and undoubtedly more costly than mere
> extra steel in two of a car's brake rotors would be. Regard-
> less of whether the manufacturers absorb the cost of those
> devices or it's passed along to the consumers (which is more
> likely), it's clearly not a show-stopper.

It is if you are staring a multimillion-dollar lawsuit in the
face. You're talking about the kind of issue that can get you
pilloried on somebody's prime-time news program, so the cost of
avoiding it is cheap by comparison.

> > And you can't make rear brakes simply a copy of the fronts,
> > as then you would have no parking brake.
>
> You're positing *existing* brake designs. There's no reason
> in the world why brakes can't be designed to incorporate a
> parking brake mechanism -- which is nothing but a way to
> mechanically actuate the existing brake pistons -- and simply
> *not hook it up* on the wheels where it isn't needed. Stan-
> dardization of that sort is trivial.

And now you've added unsprung weight to the front, as well as
additional cost. And mechanical calipers don't have a great track
record as parking brakes.

> > The same principle applies to weight: a single increase
> > may seem insignificant, but multiply it by all the parts
> > in a car, and you've gained way too much weight. They
> > really *do* care what the car weighs; after all, there
> > are fuel economy standards.
>
> If meeting fuel economy standards hinged on saving every
> possible ounce, cars wouldn't be as weighed down as they
> are with unnecessary gizmos and convenience features.
> They'd all be designed like the Honda Insight, which omits
> a remote fuel door release for the sake of weight savings.
> Car seats would resemble those Aereon office chairs.

Can't you see the difference between added weight that will sell more
cars at a higher unit profit, and added weight that nobody wants? The
car companies can.

> > And while they might be willing to add a few pounds for
> > a feature that will help sell the car, like a new nevi-
> > gation computer, why do it for something that gives no
> > benefit?
>
> It *would* give a benefit; that's my whole point. It just
> wouldn't give a benefit to the consumer, like a navigation
> system would. The benefit would be the simplifying and
> economizing of the design and manufacturing processes, and
> of the stocking of spare parts.

OK, I give up. All car manufacturers are just stupid, and they don't
know what would save them money.

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.