Recovered my M927 truck

In most vehicles, front brakes wear far faster than rear brakes. And it's because the weight shifts to the front upon braking. It applies even to rear-engined cars.

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Ayup, ever notice how the front end dips on stopping; the harder stop the more pronounced?

Iggy may be the exception given the way he says he drives and that many light trucks still have rear drums and disk front which can change the wear rate considerable.

I've never seen a car or a light truck that wears its rear brakes faster than the fronts. If the rear drums are really feeble, it may be that there are some out there. But I haven't seen one. And I've had a bunch (including my current 2004 focus) that have disks on front and really sad little drums on the rear.

Except in snow/salt country where rear discs can need service twice as often as front. Especially on lightly driven vehicles. The rears don't get hot enough to dry out and it's rust city. One friend who doesn't drive much is working on his rears almost annually. I remember reading that some maker had returned to rear drums because of the issue.

My various Ford Rangers go through 2 front pad changes for every 1 rear drum brake shoe change. And I dont drive hard, but I am often pulling a trailer and have about 500-800 lbs of gear on/in the truck all the time.

Gunner

Brakes not bear any vehicle weight; that is what axle bearings are for.

Brake wear is due solely to torque, which is going to identical on all four wheels until one of them locks up.

Is that true allowing for the proportioning valve front/rear pressure difference? I would think less pressure on rear = less wear?

MikeB

Ah, but ... the amount of torque that brakes can apply is proportional to the weight on the wheel. Lightly loaded wheels will lock up before heavily loaded ones, with the same torque. So, lightly loaded wheels (the back ones) are torqued less than the front - it's proportional braking.

Bob

Then let's put this another way. Because rear wheels are so lightly loaded in braking, maintaining balance (near balance; you want front brakes to lock up first) means that the brake bias runs from 70% front up to 90% front.

Rear wheels will skid first if you apply more brake friction to them than that. That's bad. So the friction you need on the rear brakes is a fraction of the friction you need on the fronts. Because either wheel diameter or brake weight limits the size of the front brakes (but not the rear; the demand for braking effort there is so much less), the brakes on the front wear faster.

The ability of the tires to produce that braking effort without skidding -- again, 70% - 90% on the front -- determines how much friction you can apply to the brakes at each end. That's a direct product of the weight shift to the front upon braking.

You can measure the weight shift by measuring the load on the suspension. It's not exactly equal ro suspension travel, because of a geometric suspension feature called "anti-dive." . But the load is still there, even if the actual travel is less than the weight shift would indicate.

The cylinder with the higher pressure will lock up first, given identical road traction and shoe contact surface area at all four (assuming that dual wheels aren't being used)

Less pressure in the rear is a very bad situation; you cannot steer an arc if the front wheels lock up, it'll just keep going straigt forward no matter which direction you point the front wheels.

Yes I know....

And thanks for clarifying.

--what you had written earlier had the potential to make lot of people even more clueless that they had been before.

Although, reading your reply again, I have to say that the reason for biasing to the front is NOT so that they will "lock up first"....in fact, if any wheels lock up it is way better for them to be the rear....

In other words, the reason brakes are biased towards the front is simply because they can be, due to the increased traction that results when the vehicle weight shifts forward.

If the rear PADS wore out (rear disk brakes) there is a very strong chance there is either a problem with the parking brake cable or actuator,(caliper) or someone has a habit of driving with the parking brake on. General rule of thumb is 2 or 3 sets of front brakes to one set of rear brakes. My ranger got new front brakes at about 166,00 (0km (8 years) - but that's highway miles, no load, and standard transmission. Still origional rears at 314,000km - and about half lining left. - well over half pads left too. They were changed because of a thump - not because they were worn out.

Yes - the drum brakes last a whole lot longer than the disks

Oh, no. Never, ever.

Front-wheel lockup occurring first causes the car to lose steering but to plow straight ahead. Rear-wheel lockup occurring first causes the rear wheel adhesion to drop to a value less than the moment of the rear end attempting to go forward (around the front wheels as a "pivot") while the front end is being braked by the front wheels.

In other words, a simple spinout, or an oscillating fishtail, and complete loss of control.

Take it from an old road racer. Seriously, you could look this up in any discussion of tire adhesion, cornering, and braking. Front wheels must lock up first. In a race car, particularly a road-racing car, you want the differential to be very small, but never for the rear to lock first. That's deadly.

Well, that's most of it. But when you're proportioning front verus rear, you proportion them so the fronts lock first. Or you should.

Shows how much you know about vehicles and brakes.

You are PARTLY right- but mostly wrong. The amount of weight on the wheels affects the amount of friction between tire and road. If you put the same amount of stopping power on the rear wheels as the front, when the weight distribution is 60/40 front /back, you will be backing into things or hitting things with your rear quarter panels on a regular basis as the rear end will be sliding around uncontrollably. That's why trucks had load sensing proportioning valves to cut the pressure to the rear wheels when they unloaded LONG before anti-lock brakes were common.

This is also why, for instance, drum brake vehicles had wider shoes on the front than the back, and MUCH larger cyls on the front than the rear - Example - 1971 Dodge Dart with drum brakes - 9 inch diameter front and rear, front shoes 2.25" wide, rear shoes 2 inches wide, front cyls 1 inch diameter, rear cyls 0.8125" diameter On a '68 or '69 Ford Falcon V8 with drum brakes, 10 inch drums, 2,25 inch wide front linings, 1.75 rear, and 1.125" front cyl and .906 rear in 68, and .875 in '69 ( due to rear brake lockup problems)

On a '68 or '69 Mustang with 390 V8, 10 inch diameter brakes, 2.5" wide front shoes, 1.5" wide rear shoes, 1.094" diameter front cyls and .813" diameter rears

This is to reduce the amount of braking torque the rear wheels can develop so they do not break the rear tires loose from the road before the front ones, due to reduced friction between the tires and the road due to reduced weight.

That's not all he missed.

Assuming the cyls are the same size - which they virtually NEVER are.

A whole lot worse to have the rears lock up first, letting the rear slide uncontrolled past the front which is still firmly planted to the road, and slowing down. If the fronts lock first, the rears drag behind, giving stability and allowing you to release the brakes enough to let the front wheels hold again before the rear wheels BECOME the front wheels.( which happens failrly quickly if the rears lock up first - particularly on a curve. Terminal oversteer.

This would be a good place for some of those vector diagrams they use to explain tire adhesion, cornering, braking, and accelerating. They make the whole thing brilliantly clear.

Or you can go take an old Corvette around a track and see how many different ways you can spin out or swap ends. It's amazing.