Front axle brake shoes wear out faster than rear axle brake shoes?

[wb9btz]

But, think about it. All brake shoes in a common hydraulic system press on the drum with the same force and all wheels turn at approximately the same speed. They will, then, wear out at the same rate. Weight transfer doesn't affect any of the aforementioned parameters.

While you are correct that the pressure exerted on the brake shoes is equal and that all wheels are turning at the same speed (at the moment the brakes are applied I might add)... Please see this:

http://www.autos.com/auto-repair/why-does-a-rear-brake-drum-last-longer-than-the-front

and:

http://www.sae.org/events/bce/tutorial-bahadur.pdf

One of the slides in the SAE presentation lists the rest of the forces (in addition to speed and brake system pressure) that affect brake pad wear -like weight and heat...

 

[JasonS]

But, think about it. All brake shoes in a common hydraulic system press on the drum with the same force and all wheels turn at approximately the same speed. They will, then, wear out at the same rate. Weight transfer doesn't affect any of the aforementioned parameters.

Weight transfer ENABLES you to use more braking force at the front without causing a skid. In these types of vehicles, the front brakes wear out faster.

While you are correct that the pressure exerted on the brake shoes is equal and that all wheels are turning at the same speed (at the moment the brakes are applied I might add)... Please see this:

http://www.autos.com/auto-repair/why-does-a-rear-brake-drum-last-longer-than-the-front

and:

http://www.sae.org/events/bce/tutorial-bahadur.pdf

One of the slides in the SAE presentation lists the rest of the forces (in addition to speed and brake system pressure) that affect brake pad wear -like weight and heat...

The braking ability is limited to tire traction. Because of weight transfer, there is more force pushing the front wheels into the pavement. This enables you to use more pressure/ force on the front brakes (compared to rear) and stop faster. A brake system with IDENTICAL brakes all around can't take advantage of weight transfer.

 

[Flyingvan911]

The van probably has larger pads in the front though?

Same size as far as I can tell.

 

[SP5]

Because the radius is squared for area, it does'nt take much diameter increase to add more force, a 1.5in WC will have almost a 19% increase in the pressure, that it applys to the shoe, for the same pedal pressure, compared to a 1.375 WC.
But nothing is free, you need to move almost 19% more fluid too, so the pedal goes down a bit more before you get shoe contact, at that point, you get your pressure increase.
Putting a set of 1.5 WCs in the front is an easy way to get a more proportional system.

 

[rustystud]

I'm doing this on my front axle. In fact I bought enough 1.5" diameter (A3) WC that I could potentially do all my wheels if I someday wanted..... (assuming my M/C and 2 airpacks had the available volume to do that safely).

I also found a 1.75" diameter bore front wheel cylinder that would fit!

Peashooter, what are the part numbers of these 1.5" wheel cylinders ? Also the 1.75" ?
Thanks.

 

[rosco]

All the shoes will not wear at the same rates. In a single WC system, such as on an M35A2, the leading shoe will tend to bite into the drum better due to drum rotation, then the rear shoe, thus wearing faster. I did the front axle a while back. The leading shoes were half worn, & the rear shoes were like new. I just reversed them, and called it good, till next time. But yes, all the shoes get the same pressure applied. I am just now doing the rear axles, and all the shoes look new. To explain the wear difference, between the front & rear axles: the front does squat more when braking (all things equal, including adjustment), and nobody is skidding, I would buy the idea that the radius changes (gets shorter), applying more force to the front axle, then the rears. Since nobody is skidding, all wheels still turn at the same number of rev's, because the circumference is assumed the same & not skidding, regardless of the length of the foot print, the rev's are still the same, front to rear.

In a system like the GMC M211, where you have two WC's per axle end, they will wear even. This assumes proper adjustment.

Consider adjustment. That front axle is much easier to adjust, then the rears.

But considering the nature of military maintenance, Its hard to come to any conclusions, unless you are there watching, start to finish.

 

[peashooter]

Peashooter, what are the part numbers of these 1.5" wheel cylinders ? Also the 1.75" ?
Thanks.

It was in G's link. The A3's front WC are NSN 2530-01-441-4866, I dont have part numbers handy. The A3 Wheel cylinders I bought were off ebay after seeing clayjames find them on there. What I did fid out was that not many dealers realize that the A3 trucks have different front wheel cylinders. Eriks does not and sells the 1-3/8" as being the correct one for all a2/a3 trucks. I know Boyce and Real Custom Trucks (Ted, member 197thhhc ) knew about the differences and I believe had or could get them (the 1.5" bore A3 size that is).

 

[SP5]

Don't hold me to this, but I believe that for the 1.5in WC, these are the part #s.
Raybestos WC 4511
Napa UP 4511
Dorman W 4511
Sorry, do not have the rebuild kit #s.

 

[peashooter]

Don't hold me to this, but I believe that for the 1.5in WC, these are the part #s.
Raybestos WC 4511
Napa UP 4511
Dorman W 4511
Sorry, do not have the rebuild kit #s.

It wasnt those because it looks like those have the slotted pistons in them. I do remember that there were lots that physically fit, but they had different pistons (some slotted, some flat, some with chamfers, generally the differences were pistons and seals, one was an exact match except the civilian versions didnt come with banjo fittings. (well the correct WC with all the right seals & pistons didnt come with banjo fittings or washers).

 

[SP5]

You'r right, it's the slotted piston version, sorry about that. There is/was a normal piston version, but I can't remember where I saw it.

 

[Wildchild467]

Just an update, I checked my front wheel cylinders and they are 1-3/8" bore. I am going to check my rear shoes sometime soon and see what those shoes look like. Updates to follow.

 

[Wildchild467]

Another update. I checked my rear brake shoes last night with my buddy ( 74M35A2 ) and they look to be in great shape, not worn looking at all. All my wheel cylinders are working as they should... I tested one while I had the drum off by having "Soldier B" lightly press the brake pedal down (careful not to blow the pistons out of the wheel cylinder) and I moved the brake shoes to the right and left at the top which move the pistons together.

As I stated before, I don't know for sure that it all had new brakes when I got the truck, but I am fairly sure they were in good shape or near new. I still don't understand how brakes that turn the same and have the same line pressure wear out faster on my truck. If I can remember to, I will check my brakes again in 20,000 miles and report back! The only thing I'm going to do soon is double check they are adjusted properly just to cover my bases.

 

[renodogs]

This is why when you stand up on the brake pedal in an all drum brake equipped 383 Road Runner or 396 Chevelle, the front rolls and the rear smokes!

Been there, done that (64 Chevy Impala SS) on 99 in Lodi, California, 1973. And right after that, I made a quick stop at the gas station to clean out the drawers and take an Elephant tranquillizer! And to chase my soul back into my body because I could have sworn I saw it run into the grape vineyards in full mutiny!

That's when I discovered those old nylon biased tires get a little 'Fred Flintstone bumpy' from the flats worn in the tires after that lovely skid. If that doesn't teach you to back off the other guy's bumper, nothing will!

 

[renodogs]

Another update. I checked my rear brake shoes last night with my buddy ( 74M35A2 ) and they look to be in great shape, not worn looking at all. All my wheel cylinders are working as they should... I tested one while I had the drum off by having "Soldier B" lightly press the brake pedal down (careful not to blow the pistons out of the wheel cylinder) and I moved the brake shoes to the right and left at the top which move the pistons together.

As I stated before, I don't know for sure that it all had new brakes when I got the truck, but I am fairly sure they were in good shape or near new. I still don't understand how brakes that turn the same and have the same line pressure wear out faster on my truck. If I can remember to, I will check my brakes again in 20,000 miles and report back! The only thing I'm going to do soon is double check they are adjusted properly just to cover my bases.

"...they are adjusted properly just to cover my bases"

Bingo.

Drum brake MUST be adjusted equally all the way around. It's a real PITA, but it's the proper way of servicing drum brakes. regardless of the type of vehicle. They should have a 'slight drag' after installation of the drum (adjust your slave cylinder plunger) all the way around (every wheel) so that they all wear proportionally. As was mentioned before, front brakes can wear faster due to vehicle 'dip' when slamming on the brakes and torque forces from turning while braking, they simply work harder at stopping you than the back brakes do. I've worked on plenty of old vehicles- changing and repairing drum brakes and they ALWAYS wear faster on the front from torque, bending and braking while in turns. It's just a matter of physics 101 and the mechanical limitation of drums.

Nonetheless, I have often seen nervous Nellie's riding the brakes (bad driving habit) in vehicles wearing out their brakes prematurely. For them, their problem will cure the ill someday when they really need those brakes and they aren't there. Ride your brakes around any mountain pass in any vehicle and you'll soon smell the liners burning; do it enough and you'll lose your brakes altogether. Do that with disc brakes and you'll not only warp the rotors, but your pad's stopping surface will glaze from the heat and now you're screwed!

Anyone notice those runaway truck ramps on I80 coming over Donner Pass? They aren't there for cat napping!

Entertainment is Saturday Night on Donner Pass, watching newbie big rig drivers whipping out the fire extinguishers to hose down brakes. You learn new foreign cuss words, new foreign dance moves, and yet, I am still amazed at the similar nature of the anguish on their faces from all cultures. OH DEAR LORD (God) (Allah) (Blessed Be) (Yahweh) (Whatever) I ALMOST DIED!!

That particular look, the one that's universal- eyes wide open, panting breath. Yeah- that one. It's killer!! HAHAHAHA

 

[gringeltaube]

"................. I've worked on plenty of old vehicles- changing and repairing drum brakes and they ALWAYS wear faster on the front ...........

OK, tell us more about your Deuce's front vs. rear brake shoes ...


G.

 

[renodogs]

OK, tell us more about your Deuce's front vs. rear brake shoes ...


G.

Oh please, I'm sure you have more to say, so why don't you pontificate your viewpoint based on your worldly experience?

 

[74M35A2]

Who says your fronts are wearing more than the rears? Maybe not the case. Only way to do the experiment correctly would be to start with 6 new sets of shoes, measure and install them, drive 40,000 miles or whatever, then remove and measure. Then, do the experiment 3 times total.

In this case, I would guess all 6 to wear the same due to the common pressure and rotational RPM. As others have mentioned, carmakers take advantage of weight transfer and use much more powerful disc up front, and lower cost drum in the rear, until recently. This is the point that is getting mixed into here, which is not the case in this situation. Now, I believe the majority of vehicles utilize rear disc also. Another removed maintenance point as they are self cleaning and self adjusting. Obviously a disc brake system is much more costly to manufacture than drum, due to higher operating pressures, much larger components, and the machining of 2 friction surfaces (rotor) vs 1 (drum). I converted my Jeep Cherokee over to rear disc (Jeep Liberty setup), mostly due to the fact that I continuously launch trailer boats/watercraft, and the drums would get wet, and stay wet, adjusters would seize, etc.... I also now enjoy a very firm brake pedal with only 1/2" of free travel before the binders are on and the pedal is a rock. I hate drum brakes, they belong on Model T's, and nothing after.......personal opinion.

Just in case anybody is confused, a proportioning valve limits or reduces the pressure to the rear brakes, so they do not lock up during panic breaking. It also contains a shuttle valve which will move and indicate that one part of the system developed a large leak. The wire connector on this valve is a detection switch going to the dash "BRAKE" light, which should also come on if the parking brake is on, or is the reservoir is low on fluid (newer vehicles). The front disc system is allowed full uninterrupted pressure directly from the master cylinder.

ABS is to maintain steering during panic stops. Though the coefficient of static friction (rolling tire) is higher than the coefficient of dynamic friction (skidding tire), the primary function of ABS is not to decrease stopping distance, as some believe. On recent vehicles, the ABS distribution block handles the proportioning of the rear brakes, and therefore a separate proportioning valve is typically not found.

Above paragraphs are totally off target, but, I'd place my money that all 6 would wear about evenly in this thread situation.


-Soldier B

 

[JasonS]

Agreed but a sample size of six is statistically insignificant.

Who says your fronts are wearing more than the rears? Maybe not the case. Only way to do the experiment correctly would be to start with 6 new sets of shoes, measure and install them, drive 40,000 miles or whatever, then remove and measure. Then, do the experiment 3 times total. -Soldier B

 

[JasonS]

I agree about the importance of properly adjusted brakes. However, you are otherwise confused. The front brakes have the same force between the drum and show and travel the same distance. Same wear.

"...they are adjusted properly just to cover my bases"

Bingo.

Drum brake MUST be adjusted equally all the way around. It's a real PITA, but it's the proper way of servicing drum brakes. regardless of the type of vehicle. They should have a 'slight drag' after installation of the drum (adjust your slave cylinder plunger) all the way around (every wheel) so that they all wear proportionally. As was mentioned before, front brakes can wear faster due to vehicle 'dip' when slamming on the brakes and torque forces from turning while braking, they simply work harder at stopping you than the back brakes do. I've worked on plenty of old vehicles- changing and repairing drum brakes and they ALWAYS wear faster on the front from torque, bending and braking while in turns. It's just a matter of physics 101 and the mechanical limitation of drums.

Nonetheless, I have often seen nervous Nellie's riding the brakes (bad driving habit) in vehicles wearing out their brakes prematurely. For them, their problem will cure the ill someday when they really need those brakes and they aren't there. Ride your brakes around any mountain pass in any vehicle and you'll soon smell the liners burning; do it enough and you'll lose your brakes altogether. Do that with disc brakes and you'll not only warp the rotors, but your pad's stopping surface will glaze from the heat and now you're screwed!

Anyone notice those runaway truck ramps on I80 coming over Donner Pass? They aren't there for cat napping!

Entertainment is Saturday Night on Donner Pass, watching newbie big rig drivers whipping out the fire extinguishers to hose down brakes. You learn new foreign cuss words, new foreign dance moves, and yet, I am still amazed at the similar nature of the anguish on their faces from all cultures. OH DEAR LORD (God) (Allah) (Blessed Be) (Yahweh) (Whatever) I ALMOST DIED!!

That particular look, the one that's universal- eyes wide open, panting breath. Yeah- that one. It's killer!! HAHAHAHA

 

[rustystud]

Who says your fronts are wearing more than the rears? Maybe not the case. Only way to do the experiment correctly would be to start with 6 new sets of shoes, measure and install them, drive 40,000 miles or whatever, then remove and measure. Then, do the experiment 3 times total.

In this case, I would guess all 6 to wear the same due to the common pressure and rotational RPM. As others have mentioned, carmakers take advantage of weight transfer and use much more powerful disc up front, and lower cost drum in the rear, until recently. This is the point that is getting mixed into here, which is not the case in this situation. Now, I believe the majority of vehicles utilize rear disc also. Another removed maintenance point as they are self cleaning and self adjusting. Obviously a disc brake system is much more costly to manufacture than drum, due to higher operating pressures, much larger components, and the machining of 2 friction surfaces (rotor) vs 1 (drum). I converted my Jeep Cherokee over to rear disc (Jeep Liberty setup), mostly due to the fact that I continuously launch trailer boats/watercraft, and the drums would get wet, and stay wet, adjusters would seize, etc.... I also now enjoy a very firm brake pedal with only 1/2" of free travel before the binders are on and the pedal is a rock. I hate drum brakes, they belong on Model T's, and nothing after.......personal opinion.

Just in case anybody is confused, a proportioning valve limits or reduces the pressure to the rear brakes, so they do not lock up during panic breaking. It also contains a shuttle valve which will move and block a circuit if that system develops a large leak. The wire connector on this valve is a detection switch going to the dash "BRAKE" light, which should also come on if the parking brake is on, or is the reservoir is low on fluid (newer vehicles). The front disc system is allowed full uninterrupted pressure directly from the master cylinder.

ABS is to maintain steering during panic stops. Though the coefficient of static friction (rolling tire) is higher than the coefficient of dynamic friction (skidding tire), the primary function of ABS is not to decrease stopping distance, as some believe. On recent vehicles, the ABS distribution block handles the proportioning of the rear brakes, and therefore a separate proportioning valve is typically not found.

Above paragraphs are totally off target, but, I'd place my money that all 6 would wear about evenly in this thread situation.


-Soldier B

Actually the proportioning valve is there in disc/drum brakes to limit the amount of pressure to the wheel cylinders since disc brakes take on average 1200psi and over to stop while drums need only 800psi. It is not just in panic mode, it's in all modes. Also there is no safety shutoff valve built into the proportioning valve. The master cylinder is split into 2 sections, this gives you the safety when one line brakes or you other-wise loss pressure. Your also wrong about "ABS" . The reason ABS cycles the brake apply is that rolling wheels are making contact with the road, thus allowing braking, while locked wheels are just skidding , not making any real contact with the road and not allowing the brakes to stop the vehicle.