OT- Why do front brakes wear out faster than rears?

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

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).

My guess is safety.

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

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

JC

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

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

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.

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.

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

-- "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

On Fri, 17 Oct 2008 17:47:37 -0700, the infamous Jim Stewart scrawled the following:

Are you nuts, Jim? That would put a car in a spin in seconds flat if one reservoir were dry. I believe that all the cars I ever worked on up through the 90s had separate circuits for front and rear. I see no reason they'd change that. It's a real safety issue.

If you know of crossed systems, please post a link. I gotta see this.

-- "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

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.

All (modern) cars have two independent systems; a very few are like you say while the vast majority are a front system and a rear system.

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

googling for diagonal split brake turns up enough hits describing it as the other alternative to front-rear split (see