Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

I started to give him the old "inertial weight transfer to the front
while braking" reply and then found that it really wasn't making total
sense to me.

Providing you don't drive and brake like a madman neither the front or
rear tires are doing much skidding on the pavement so it's likely all
four are all making the same number of revolutions while braking. So, if
the brake pad areas and the piston diameters were all equal front and
rear I'd expect the pad wear rate to also be equal.

It's been too long since I've done a DIY brake job and I never stopped
to study the relative sizes of drums, shoes, pads and pistons back when
I used to do that stuff on all our family jalopies.

Answers please?

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

Jeff Wisnia wrote:

Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

I started to give him the old "inertial weight transfer to the front
while braking" reply and then found that it really wasn't making total
sense to me.

Providing you don't drive and brake like a madman neither the front or
rear tires are doing much skidding on the pavement so it's likely all
four are all making the same number of revolutions while braking. So, if
the brake pad areas and the piston diameters were all equal front and
rear I'd expect the pad wear rate to also be equal.

It's been too long since I've done a DIY brake job and I never stopped
to study the relative sizes of drums, shoes, pads and pistons back when
I used to do that stuff on all our family jalopies.

The braking system is designed to give the
front brakes more authority than the rear.
The increased pressure and heat causes them
to wear out faster.

And that goes back to the weight transfer
issue. The front brakes get more authority
because they can use it without breaking
traction.

"Jim Stewart" wrote in message
news
Jeff Wisnia wrote:

Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

I started to give him the old "inertial weight transfer to the front
while braking" reply and then found that it really wasn't making total
sense to me.

Providing you don't drive and brake like a madman neither the front or
rear tires are doing much skidding on the pavement so it's likely all
four are all making the same number of revolutions while braking. So, if
the brake pad areas and the piston diameters were all equal front and
rear I'd expect the pad wear rate to also be equal.

It's been too long since I've done a DIY brake job and I never stopped to
study the relative sizes of drums, shoes, pads and pistons back when I
used to do that stuff on all our family jalopies.

The braking system is designed to give the
front brakes more authority than the rear.
The increased pressure and heat causes them
to wear out faster.

And that goes back to the weight transfer
issue. The front brakes get more authority
because they can use it without breaking
traction.

I seem to remember something called a "proportioning valve", between front
and rear brakes.
--
DT

"DrollTroll" wrote in message
...

"Jim Stewart" wrote in message
news
Jeff Wisnia wrote:

Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

I started to give him the old "inertial weight transfer to the front
while braking" reply and then found that it really wasn't making total
sense to me.

Providing you don't drive and brake like a madman neither the front or
rear tires are doing much skidding on the pavement so it's likely all
four are all making the same number of revolutions while braking. So, if
the brake pad areas and the piston diameters were all equal front and
rear I'd expect the pad wear rate to also be equal.

It's been too long since I've done a DIY brake job and I never stopped
to study the relative sizes of drums, shoes, pads and pistons back when
I used to do that stuff on all our family jalopies.

The braking system is designed to give the
front brakes more authority than the rear.
The increased pressure and heat causes them
to wear out faster.

And that goes back to the weight transfer
issue. The front brakes get more authority
because they can use it without breaking
traction.

I seem to remember something called a "proportioning valve", between front
and rear brakes.

That's correct and some are adjustable. On those units it is necessary to
balance the system after changing out the master cylinder.


JC

"John R. Carroll" wrote in message

"DrollTroll" wrote in message
...

I seem to remember something called a "proportioning valve",
between front and rear brakes.

That's correct and some are adjustable. On those units it is
necessary to balance the system after changing out the master
cylinder.

But aren't the front and rear independent systems, with separate reservoirs
and all? It sounds like this valve is a single point of failure common to
both systems.


--

Reply in group, but if emailing add one more
zero, and remove the last word.

Tom Del Rosso wrote:
"John R. Carroll" wrote in message

"DrollTroll" wrote in message
...
I seem to remember something called a "proportioning valve",
between front and rear brakes.
That's correct and some are adjustable. On those units it is
necessary to balance the system after changing out the master
cylinder.

But aren't the front and rear independent systems, with separate reservoirs
and all? It sounds like this valve is a single point of failure common to
both systems.

Most cars have two independent systems,
right front/left rear and left front/right rear.

Jeff Wisnia writes:

Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

I started to give him the old "inertial weight transfer to the front
while braking" reply and then found that it really wasn't making total
sense to me.

Providing you don't drive and brake like a madman neither the front or
rear tires are doing much skidding on the pavement so it's likely all
four are all making the same number of revolutions while braking. So,
if the brake pad areas and the piston diameters were all equal front
and rear I'd expect the pad wear rate to also be equal.

With most cars its even more than just inertial transfer -- there's a
lot more weight on the fronts than the rears *before* you put the
brakes on.

If you the areas, piston diameters, and line pressures were equal
you'd lock up the rears before you were applying full force to the
fronts. The brake proportioning (however the manufacturer goes about
it) really does make the fronts exert more force than the rears.

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).

Joe Pfeiffer wrote:

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).

My guess is safety.

"Jim Stewart" wrote in message
.. .
Joe Pfeiffer wrote:

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).

My guess is safety.

It isn't safety, it's standardisation to lower costs.

JC

John R. Carroll wrote:
"Jim Stewart" wrote in message
.. .
Joe Pfeiffer wrote:

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).
My guess is safety.

It isn't safety, it's standardisation to lower costs.

Standardization of what? Every car I've ever
seen that had 4w disk brakes had bigger ones
on the front.

On Fri, 17 Oct 2008 17:21:21 -0700, the infamous Jim Stewart
scrawled the following:

John R. Carroll wrote:
"Jim Stewart" wrote in message
.. .
Joe Pfeiffer wrote:

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).
My guess is safety.

It isn't safety, it's standardisation to lower costs.

Standardization of what? Every car I've ever
seen that had 4w disk brakes had bigger ones
on the front.

80-90% of the stopping power comes from the front brakes.

--
"Politics is the art of looking for trouble, finding it whether it
exists or not, diagnosing it incorrectly, and applying the wrong
remedy." -- Ernest Benn

"Jim Stewart" wrote in message
.. .
John R. Carroll wrote:
"Jim Stewart" wrote in message
.. .
Joe Pfeiffer wrote:

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).
My guess is safety.

It isn't safety, it's standardisation to lower costs.

Standardization of what? Every car I've ever
seen that had 4w disk brakes had bigger ones
on the front.

Emergency brake systems are typically in the rear rotor housing so they'll
be different in order to make room.
To have the same area the rear rotors would necessarily require a bigger OD
and a different caliper.

JC

Joe Pfeiffer wrote:

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).


I've replaced the front pads once on my car with 160,000 miles, still waiting for rears to
give it up. Changed front pads at 121,000.

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller

"Joe Pfeiffer" wrote in message
...
Jeff Wisnia writes:

Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

I started to give him the old "inertial weight transfer to the front
while braking" reply and then found that it really wasn't making total
sense to me.

Providing you don't drive and brake like a madman neither the front or
rear tires are doing much skidding on the pavement so it's likely all
four are all making the same number of revolutions while braking. So,
if the brake pad areas and the piston diameters were all equal front
and rear I'd expect the pad wear rate to also be equal.

With most cars its even more than just inertial transfer -- there's a
lot more weight on the fronts than the rears *before* you put the
brakes on.

If you the areas, piston diameters, and line pressures were equal
you'd lock up the rears before you were applying full force to the
fronts. The brake proportioning (however the manufacturer goes about
it) really does make the fronts exert more force than the rears.

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).

Why would you want them to all wear out at the same time? That wouldn't be
safe or save money.

"ATP*" writes:

"Joe Pfeiffer" wrote in message
...

The corollary I've never been able to figure out is why they don't use
smaller brakes and more pressure in the rear, so they'd all wear out
at the same time (well, as a matter of fact most vehicles I've had
apart have had larger fronts than rears. But not by enough to make up
for the difference in how hard the fronts have to work).

Why would you want them to all wear out at the same time? That wouldn't be
safe or save money.

Well, "roughly" the same time (and you get a lot of notice from when
the wear strip first starts dragging until your brakes are unsafe).
Putting bigger rear brakes on than necessary costs weight and costs
the manufacturer money.

Jeff Wisnia writes:

I started to give him the old "inertial weight transfer to the front
while braking" reply and then found that it really wasn't making total
sense to me.

When you step on the brakes, you are thrown forward.
So is the car. There is more weight on the front wheels than on the rear.

On Oct 17, 12:07*pm, Jeff Wisnia wrote:
Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

At rest, or at moderate speeds, the front and rear tires
bear the same load (that's why you use the same, or
nearly the same, tire pressures front and rear).

When braking, the nonrotation of the car means the torque (by
the wheel/road friction) and countertorque (by imbalance of
front wheel/rear wheel load force) are equal. That
means the front wheels bear more load during the braking
of forward motion than at rest.

Since the front wheels bear more load during braking,
they can safely apply more friction force (and are sized and
proportionally engaged to do so). Higher friction force means
more wear on the front brake parts than on the rear.

Phrases like 'throws weight forward' are suggestive of
the car center-of-mass shifting with respect to the wheelbase.
That doesn't happen. Compression of the front springs
(the hood dips when you brake) is easy to see happening,
and should indicate (to folk who don't do force diagrams)
the front-tire-load situation.

On Oct 17, 7:43*pm, whit3rd wrote:
On Oct 17, 12:07*pm, Jeff Wisnia wrote:

Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

At rest, or at moderate speeds, the front and rear tires
bear the same load (that's why you use the same, or
nearly the same, tire pressures front and rear).

When braking, the nonrotation of the car means the torque (by
the wheel/road friction) and countertorque (by imbalance of
front wheel/rear wheel load force) are equal. *That
means the front wheels bear more load during the braking
of forward motion than at rest.

Since the front wheels bear more load during braking,
they can safely apply more friction force (and are sized and
proportionally engaged to do so). *Higher friction force means
more wear on the front brake parts than on the rear.

Phrases like 'throws weight forward' are suggestive of
the car center-of-mass shifting with respect to the wheelbase.
That doesn't happen. * Compression of the front springs
(the hood dips when you brake) is easy to see happening,
and should indicate (to folk who don't do force diagrams)
the front-tire-load situation.

The front wheels need bigger brakes for two, maybe three
reasons: 1. The front is often heavier. 2. The rear end tends to get a
bit light as the car rotates around the center of mass as the braking
forces are applied. 3. You DON'T want the rear end breaking loose. In
the days before ABS, rear brakes that were as strong as the front
could cause the rear wheels to lock up, and if you've ever done the
park-brake-skid thing, you'll know that once the rear wheels are
locked you might as well have a skid plate back there. There's no
particular direction the wheels will want to go, and so the car will
try to swap ends. Dangerous. Much better to make the rear brakes
weaker than to risk breakaway.

Dan