Brake design question - Ray? Anyone?

With a 4-wheel disc brake configuration, is the braking force (and
wear) distributed more evenly front-to-back than the use of discs in
the front and drum brakes in the rear? It may be my imagination, but I
think the rotors used on my Scion (all disc brakes) are smaller in the
front than those of my Matrix (disc/drum combination), even though the
weight of the cars is comparable, and I was wondering if the rear
brakes are intended to take more of the load.

On Tue, 30 May 2006 05:27:27 -0700, Mark wrote:
[color=blue]
> With a 4-wheel disc brake configuration, is the braking force (and
> wear) distributed more evenly front-to-back than the use of discs in
> the front and drum brakes in the rear? It may be my imagination, but I
> think the rotors used on my Scion (all disc brakes) are smaller in the
> front than those of my Matrix (disc/drum combination), even though the
> weight of the cars is comparable, and I was wondering if the rear
> brakes are intended to take more of the load.[/color]

Well, in general, most of the braking is done by the front.

In this case, I would look at the calipers. I'd be willing to bey, with a
smaller disk area that the fronts are dual piston calipers, while the
rears are single piston.

"Mark" <bogusmailmark@yahoo.com> wrote in message
news:1148992047.221682.326620@j73g2000cwa.googlegroups.com...[color=blue]
> With a 4-wheel disc brake configuration, is the braking force (and
> wear) distributed more evenly front-to-back than the use of discs in
> the front and drum brakes in the rear? It may be my imagination, but I
> think the rotors used on my Scion (all disc brakes) are smaller in the
> front than those of my Matrix (disc/drum combination), even though the
> weight of the cars is comparable, and I was wondering if the rear
> brakes are intended to take more of the load.
>[/color]

The distribution of braking force from front to rear is more dependent on
the weight distribution of the vehicle itself, suspension, vehicle speed,
and design of the master cylinder than whether the rear brakes are disc or
drum.

Brakes stop vehicles by changing kinetic energy to heat energy, and the
faster the heat is dissipated, the more efficiently it can change more
kinetic energy to heat energy. Since disc brakes are exposed to more air
than drum brakes, they can get rid of the heat more efficiently.

Factors that affect brake rotor or drum diameter are wheel diameter,
unsprung weight, vehicle weight, rotor and friction material composition,
and standard designs from rotor manufacturers.

I suspect that the rotors on the Scion may appear smaller because the wheel
has a larger diameter and so there is more space between the outside of the
rotor and the inside of the wheel rim.
--

Ray O
(correct punctuation to reply)

Thanks Ray. Would you expect rear disc brakes to wear more rapidly
than rear drum brakes, or is it impossible to make that generalization?
I would have thought that the amount of kinetic energy dissipated
front to back would be somewhat more under the control of the designer
than just set by the weight distribution and other factors you
mentioned. For instance, engaging one set of calipers slightly
"before" the other pair depending on how hard the brake pedal was
pressed. My impression has always been that drum brakes in the rear
engage only lightly unless under a panic stop, and that's why they last
a lot longer than front discs, but that maybe a mistaken impression.
On a bicycle it's desirable to mainly rely on the rear brakes to avoid
going over the handlebars. Not exactly relevant for a car, but perhaps
there are other factors that would make one preferable to the other.

"Mark" <bogusmailmark@yahoo.com> wrote in message
news:1149008234.273422.115960@j73g2000cwa.googlegroups.com...[color=blue]
> Thanks Ray. Would you expect rear disc brakes to wear more rapidly
> than rear drum brakes, or is it impossible to make that generalization?[/color]

Brake lining wear is more of a factor of lining material, brake rotor or
drum hardness, weight, speed, etc. so it is not really possible to make a
general statement that one type of brake will wear more quickly than
another.

[color=blue]
> I would have thought that the amount of kinetic energy dissipated
> front to back would be somewhat more under the control of the designer
> than just set by the weight distribution and other factors you
> mentioned. For instance, engaging one set of calipers slightly
> "before" the other pair depending on how hard the brake pedal was
> pressed.[/color]

Designers face several challenges when designing brake systems,
particuclarly with vehicles designed to carry heavier payloads. The problem
with engaging rear brakes before front brakes is avoiding premature rear
wheel lockup if the back end of the vehicle is lightly loaded.

My impression has always been that drum brakes in the rear[color=blue]
> engage only lightly unless under a panic stop, and that's why they last
> a lot longer than front discs, but that maybe a mistaken impression.
> On a bicycle it's desirable to mainly rely on the rear brakes to avoid
> going over the handlebars. Not exactly relevant for a car, but perhaps
> there are other factors that would make one preferable to the other.
>[/color]

Rear brakes last longer than front brakes because the front brakes do most
of the stopping. The heavier the vehicle, the more weight is shifted to the
front of the vehicle when stopping, and the harder the stop, more weight is
shifted to the front.
--

Ray O
(correct punctuation to reply)

Braking force on any system is distributed about 60/40 front to rear. This
explains why one goes through front brakes at a roughly 2:1 ratio over the
back brakes.





"Mark" <bogusmailmark@yahoo.com> wrote in message
news:1148992047.221682.326620@j73g2000cwa.googlegroups.com...[color=blue]
> With a 4-wheel disc brake configuration, is the braking force (and
> wear) distributed more evenly front-to-back than the use of discs in
> the front and drum brakes in the rear? It may be my imagination, but I
> think the rotors used on my Scion (all disc brakes) are smaller in the
> front than those of my Matrix (disc/drum combination), even though the
> weight of the cars is comparable, and I was wondering if the rear
> brakes are intended to take more of the load.
>[/color]

On Tue, 30 May 2006 16:39:23 -0700, "Jeff Strickland"
<crwlr@yahoo.com> wrote:
[color=blue]
>Braking force on any system is distributed about 60/40 front to rear. This
>explains why one goes through front brakes at a roughly 2:1 ratio over the
>back brakes.
>
>[/color]
It is more like 70-75%. This is true for most of the vehicles on the
road. The only exceptions are rear and mid-engines. Since these
vehicles are mostly high-performance cars (MR2, 911, NSX, 348's), they
have stiff suspensions that reduce weight transfer to the front.
These cars obviously use more of the rear brakes.

"Jeff Strickland" <crwlr@yahoo.com> wrote in message news:KuudnfzU3Iw2SuHZnZ2dneKdnZydnZ2d@ez2.net...[color=blue]
> Braking force on any system is distributed about 60/40 front to rear. This[/color]

My 86 Toyota C&C work trucks normal weight with the parts, tools and equipment I need is close to 6,000 pounds. Plus I would not
think twice about tossing 2 scoops of sand (a ton) anywhere in the bed. I doubt the sand even be noticeable when braking. That
trucks rear axle does basically all the load and braking and the front none.

My point is braking force front/rear varies greatly.

[color=blue]
> explains why one goes through front brakes at a roughly 2:1 ratio over the back brakes.
>[/color]

My Supra's front pads, rear pads, front rotors, rear rotors all wear roughly 1:1 thank god and have no weak link.

The brakes also perform the same at any speed the car can do. But if you stomp the brake /lock up all 4 at 120 mph
anyone not belted in is going to eat the dash hard

Dan

"Danny G." <dandog@pacbell.net> wrote in message
news:f7afg.36365$4L1.4788@newssvr11.news.prodigy.com...[color=blue]
>
> "Jeff Strickland" <crwlr@yahoo.com> wrote in message
> news:KuudnfzU3Iw2SuHZnZ2dneKdnZydnZ2d@ez2.net...[color=green]
>> Braking force on any system is distributed about 60/40 front to rear.
>> This[/color]
>
> My 86 Toyota C&C work trucks normal weight with the parts, tools and
> equipment I need is close to 6,000 pounds. Plus I would not think twice
> about tossing 2 scoops of sand (a ton) anywhere in the bed. I doubt the
> sand even be noticeable when braking. That trucks rear axle does basically
> all the load and braking and the front none.
>
> My point is braking force front/rear varies greatly.
>
>[color=green]
>> explains why one goes through front brakes at a roughly 2:1 ratio over
>> the back brakes.
>>[/color]
>
> My Supra's front pads, rear pads, front rotors, rear rotors all wear
> roughly 1:1 thank god and have no weak link.
>
> The brakes also perform the same at any speed the car can do. But if you
> stomp the brake /lock up all 4 at 120 mph
> anyone not belted in is going to eat the dash hard[/color]



Distributing 90+% of the load on the rear axle doesn't change things very
much. The front brakes still do the work of STOPPING. There are hardware
components that help to balance the braking duties from front to rear, but
they are "stupid" devices that don't know how you have distributed the load.

Bottom line is, it's pretty safe to say that braking is distributed 60% to
the front and 40% to the rear. There are notable exceptions -- the
mid-engine cars and highly refined sports cars, that sort of thing -- but
most passenger cars will be pretty close to the 60/40 distribution. The
reason is the kenetic energy of the car itself. As you stop, the weight is
thrown forward onto the front axle.

I do not recall that the OP listed his car among the notable exceptions.

"Jeff Strickland" <crwlr@yahoo.com> wrote in message news:FICdnQRywq1Ls-PZnZ2dnUVZ_vWdnZ2d@ez2.net...[color=blue]
>
> "Danny G." <dandog@pacbell.net> wrote in message news:f7afg.36365$4L1.4788@newssvr11.news.prodigy.com...[color=green]
>>
>> "Jeff Strickland" <crwlr@yahoo.com> wrote in message news:KuudnfzU3Iw2SuHZnZ2dneKdnZydnZ2d@ez2.net...[color=darkred]
>>> Braking force on any system is distributed about 60/40 front to rear. This[/color]
>>
>> My 86 Toyota C&C work trucks normal weight with the parts, tools and equipment I need is close to 6,000 pounds. Plus I would not
>> think twice about tossing 2 scoops of sand (a ton) anywhere in the bed. I doubt the sand even be noticeable when braking. That
>> trucks rear axle does basically all the load and braking and the front none.
>>
>> My point is braking force front/rear varies greatly.
>>
>>[color=darkred]
>>> explains why one goes through front brakes at a roughly 2:1 ratio over the back brakes.
>>>[/color]
>>
>> My Supra's front pads, rear pads, front rotors, rear rotors all wear roughly 1:1 thank god and have no weak link.
>>
>> The brakes also perform the same at any speed the car can do. But if you stomp the brake /lock up all 4 at 120 mph
>> anyone not belted in is going to eat the dash hard[/color]
>
>
>
> Distributing 90+% of the load on the rear axle doesn't change things very much. The front brakes still do the work of STOPPING.
> There are hardware components that help to balance the braking duties from front to rear, but they are "stupid" devices that don't
> know how you have distributed the load.
>
> Bottom line is, it's pretty safe to say that braking is distributed 60% to the front and 40% to the rear. There are notable
> exceptions -- the mid-engine cars and highly refined sports cars, that sort of thing -- but most passenger cars will be pretty
> close to the 60/40 distribution. The reason is the kenetic energy of the car itself. As you stop, the weight is thrown forward
> onto the front axle.
>
> I do not recall that the OP listed his car among the notable exceptions.
>[/color]


LOL I know we are off topic.
anyway...
I guess my Toyota C&C truck has huge brakes and 4 tires rear and wimpy front brakes for no reason then. 8)

Viperkiller <nothing@nothing.net> wrote:
[color=blue]
>On Tue, 30 May 2006 16:39:23 -0700, "Jeff Strickland"
><crwlr@yahoo.com> wrote:
>[color=green]
>>Braking force on any system is distributed about 60/40 front to rear. This
>>explains why one goes through front brakes at a roughly 2:1 ratio over the
>>back brakes.
>>
>>[/color]
>It is more like 70-75%. This is true for most of the vehicles on the
>road. The only exceptions are rear and mid-engines. Since these
>vehicles are mostly high-performance cars (MR2, 911, NSX, 348's), they
>have stiff suspensions that reduce weight transfer to the front.
>These cars obviously use more of the rear brakes.[/color]

I don't understand why the 'stiffness of the suspension' would
have anything to do with 'braking distribution'...isn't that
governed by the 'arm' of the lever effect? (how far above the
axles the vehicle's sprung weight is)...seems to me that the
60/40 that I see here feels about right for any average sized
automobile...no?.
--

-Gord.
(use gordon in email)

Ray O <rokigawa@tristarassociatesDOTcom> writes:
[color=blue]
> Rear brakes last longer than front brakes because the front
> brakes do most of the stopping.[/color]


On cars with 4-wheel discs, the front rotors are of larger
diameter than the rear rotors for that reason (and are often
vented as well).

Since the stopping capacity of the front brakes meet or exceed
what's needed at the rear wheels, why not use the same rotors
and calipers at all four wheels and make any necessary adjustments
with a brake proportioning valve? Expenses could be lowered by
having to procure, transport and stock fewer components.



Geoff

--
"Yep, ain't it a shame? Here we stand, with our Cokes, our
Beavis-n-Buttheads and our carrier battlegroups. We rule.
You suck." -- Ken Strayhorn

"Geoff Miller" <geoffm@u1.netgate.net> wrote in message
news:e5nlnd$cto@u1.netgate.net...[color=blue]
>
>
> Ray O <rokigawa@tristarassociatesDOTcom> writes:
>[color=green]
>> Rear brakes last longer than front brakes because the front
>> brakes do most of the stopping.[/color]
>
>
> On cars with 4-wheel discs, the front rotors are of larger
> diameter than the rear rotors for that reason (and are often
> vented as well).
>
> Since the stopping capacity of the front brakes meet or exceed
> what's needed at the rear wheels, why not use the same rotors
> and calipers at all four wheels and make any necessary adjustments
> with a brake proportioning valve? Expenses could be lowered by
> having to procure, transport and stock fewer components.
>
>
>
> Geoff
>[/color]
The reduced expenses by the factors you mentioned are probably not enough
to offset the expense of having to install larger rotors, calipers, and
pads. Also, Toyota uses a drum brake incorporated into the rear disc brake
for a parking brake.
--

Ray O
(correct punctuation to reply)

Viperkiller <nothing@nothing.net> wrote:
[color=blue]
>On Tue, 30 May 2006 16:39:23 -0700, "Jeff Strickland"
><crwlr@yahoo.com> wrote:
>[color=green]
>>Braking force on any system is distributed about 60/40 front to rear. This
>>explains why one goes through front brakes at a roughly 2:1 ratio over the
>>back brakes.
>>
>>[/color]
>It is more like 70-75%. This is true for most of the vehicles on the
>road. The only exceptions are rear and mid-engines. Since these
>vehicles are mostly high-performance cars (MR2, 911, NSX, 348's), they
>have stiff suspensions that reduce weight transfer to the front.
>These cars obviously use more of the rear brakes.[/color]

I don't understand why the 'stiffness of the suspension' would
have anything to do with 'braking distribution'...isn't that
governed by the 'arm' of the lever effect? (how far above the
axles the vehicle's sprung weight is)...seems to me that the
60/40 that I see here feels about right for any average sized
automobile...no?.
--

-Gord.
(use gordon in email)

"Gord Beaman" <gord@islandtelecom.com> wrote in message
news:qq7v72papsba9fjondtgdjgth7qhn3lm6k@4ax.com...[color=blue]
> Viperkiller <nothing@nothing.net> wrote:
>[color=green]
>>On Tue, 30 May 2006 16:39:23 -0700, "Jeff Strickland"
>><crwlr@yahoo.com> wrote:
>>[color=darkred]
>>>Braking force on any system is distributed about 60/40 front to rear.
>>>This
>>>explains why one goes through front brakes at a roughly 2:1 ratio over
>>>the
>>>back brakes.
>>>
>>>[/color]
>>It is more like 70-75%. This is true for most of the vehicles on the
>>road. The only exceptions are rear and mid-engines. Since these
>>vehicles are mostly high-performance cars (MR2, 911, NSX, 348's), they
>>have stiff suspensions that reduce weight transfer to the front.
>>These cars obviously use more of the rear brakes.[/color]
>
> I don't understand why the 'stiffness of the suspension' would
> have anything to do with 'braking distribution'...isn't that
> governed by the 'arm' of the lever effect? (how far above the
> axles the vehicle's sprung weight is)...seems to me that the
> 60/40 that I see here feels about right for any average sized
> automobile...no?.
> --
>
> -Gord.
> (use gordon in email)[/color]

Under hard braking, inertia transfers weight to the front of the vehicle.
When weight is transferred to the front of the vehicle, the springs at the
front of the vehicle are compressed and the springs at the rear of the
vehicle are stretched, compounding the effect of the weight transfer from
the inertia. If the springs are stiffer, there is less compounding of
weight transfer to the front of the vehicle.
--

Ray O
(correct punctuation to reply)