Air Disc verses Drum-brakes

Not here to chat much, but I thought some of you might find this info of some value.

Last year the owner Father and Son bought 10 Freightliners with air disc brakes over my rather strong protest.
They were both sold on the high wear factor and service by the sales force. Told the average mileage before needing
pads was 400 to 500,000, {BS}. So far that isn't happening the best we will see is about 220,000 average. And so far
I have had to change out one set rotors and pads, plus three more tractors with pads only. Those ranged in mileage
from 113,000 to 134,000 miles on the first set.

Now the cost of new parts for Wabco air disc. Rotors-175.00 each, one set of pads does one axle 165.55,
wheel seal 30.00 plus gear oil. If you need a caliper also----1,300.00 per caliper.
Now the cost of new shoes and drums, Shoes-34.00 a set, spring kits 5.25, Drum 73.76

Time to do a complete pad and rotor change 1 1/2 hours per corner if your good.
Time in our shop to change out all four corners of shoes and drums, 1 hour one guy.


The technology has gone in reverse, the industry got away from inboard drums 40 years ago, with disc it
went right back where it was. Now I know someone on here will throw the safety card out about brake fade with
drums and that by having disc it gets rid of that safety issue. Fact is if you or your drivers are having brake fade with
drum brakes you and or your driver probably need to learn how to drive.

The average mileage our drivers would get with drum brakes was 350,000, some as high as 525,000.
And some of the drivers have admitted because of the slightly shorter stopping distance with disc brakes they have
found at times running up closer in traffic. Those who like them live east of the Rockies, But air disc isn't worth
spit in the mountain states.

In the photos below are some pads and rotors with 113,000 miles on them, And some new rotors.
New pads are roughly 7/8" thick, One brake shoe has enough lining to roughly make four brake pads.
And one more note-with drum brakes you will see smoke when hot-not with disc. Loose an air can on the caliper
and it will just grind the pad down to nothing in nothing flat.

Very interesting, I have no experience with air discs, never thought it was that great of an idea.

Remember the old Louisville's with the air over hydraulic disc brakes on the steer axle? What an overcomplicated joke! They work ok but just don't last. Nowadays the parts are getting hard to find for them and damn near break the bank doing a full job.

My only experience with disc brakes were the juice brakes on the C5500 GMC's at a previous shop I worked at. The service truck mechanic (who I helped out from time to time) told me we were lucky to see more than 5,000km out of rear pads during the muddy season. I'm not completely sold on disc brakes in off road applications as of yet.

I agree 100% with you Truckshop.
They have been around for years but I just don't really see them catching on. Too costly to repair.

I had a pretty good argument with an air brake instructor about the merits of air disc brakes.
His final answer was "They are coming, so get used to it"

I know in the logging industry they would not last at all, the dirt and crap from the roads would eat them up.

The only advantage I have heard from the local truckers is that they don't freeze up when parked in the winter. Still at that they may be way more costly than a hammer.

wornout wrench said:

I agree 100% with you Truckshop.
They have been around for years but I just don't really see them catching on. Too costly to repair.

I had a pretty good argument with an air brake instructor about the merits of air disc brakes.
His final answer was "They are coming, so get used to it"

I know in the logging industry they would not last at all, the dirt and crap from the roads would eat them up.

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That is a standard reply from the people on the upper rung. My reply I tell them is {who is the customer, obviously not you}. The buyer has the power if they would only use it.

old-iron-habit said:

The only advantage I have heard from the local truckers is that they don't freeze up when parked in the winter. Still at that they may be way more costly than a hammer.

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With all the electronic equipment in the cab of a truck these days plus things like air disc brakes and the main feature is disc won't freeze up. The corporate people are making truck drivers even lazier
than they already are. It's such a miserable and demeaning job to have to get off your butt to knock the frozen drums loose with a hammer, they are trying to gain weight not lose it.

What is one pad , 15 maybe 20 square inches. Compare to a shoe, almost 100 square inches. 1/5 the contact area. No thanks. Added maintenance. Don't the slides have a tendency to stick ? Or was that the earliest design. S-cams are better than wedges, no one will argue that. I don't think this was an advance. I agree with you Truckshop, I think we took a step back with air disc.

By f**k those discs and pads have been warm...……

The only way I would personally entertain dry discs is if the vehicle/machine equipped with them was also running some sort of driveline retarder (probably built into the back end of the transmission) capable of absorbing 3000ft. lbs. or (preferably) more of torque, plus a compression or an exhaust brake.

And if you have the driveline retarder and the compression/exhaust brake, you can forget ALL about brake fade whatever braking system you use, because the service brake will only be used for bringing the vehicle to a dead stop from maybe 5mph.

So do all your new trucks come from the factory with air disc only, truck shop? Or is your company ordering them that way to save weight, or is that the only thing freightliner is offering in your particular "package"?

The last 25 ordered have disc, as far as weight savings it's minimal, and yes you can order drum brakes. But like I said earlier sales force. Phil Swift here with disc brake in can!
These tractors are all equipped with the DT 12 automatic transmission, which I am not a fan of automatics in trucks. Makes a lazy driver even lazier. Fact is there is not one
driver who hasn't admitted that he uses the service brakes more with the auto trans. The new rigs are a pile of gadgetry, so many modes to operate.

My favorite come back to people who have never been on a creeper or even know what a creeper is.
A small or large fleet owner will hand the keys to a person just hired to drive or is a steady hand for ten years. The keys go to a $160.000 dollar tractor pulling a $80,000 trailer,
loaded with a $75,000 dollar commodity. The driver has the average 10th grade education. The owner of said truck also owns a 2019 Corvette-do you think the owner is going
to throw the keys to his Corvette to one of his drivers? --------------Hell no.

Truck Shop said:

The last 25 ordered have disc, as far as weight savings it's minimal, and yes you can order drum brakes. But like I said earlier sales force. Phil Swift here with disc brake in can!
These tractors are all equipped with the DT 12 automatic transmission, which I am not a fan of automatics in trucks. Makes a lazy driver even lazier. Fact is there is not one
driver who hasn't admitted that he uses the service brakes more with the auto trans. The new rigs are a pile of gadgetry, so many modes to operate.

My favorite come back to people who have never been on a creeper or even know what a creeper is.
A small or large fleet owner will hand the keys to a person just hired to drive or is a steady hand for ten years. The keys go to a $160.000 dollar tractor pulling a $80,000 trailer,
loaded with a $75,000 dollar commodity. The driver has the average 10th grade education. The owner of said truck also owns a 2019 Corvette-do you think the owner is going
to throw the keys to his Corvette to one of his drivers? --------------Hell no.

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You forgot to add the extra tire wear we discussed last week.

Theres not much built after the 80s that is good new tec. Puting a computer on a engine could have been a great leap forwared but its went the wrong way. I meet drivers some times and think he must be good behind the wheel cause he is worthless other wise.

Old Doug said:

Theres not much built after the 80s that is good new tec. Puting a computer on a engine could have been a great leap forwared but its went the wrong way.

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The computer wasn't bad... it's the emissions junk that it runs.

RZucker said:

You forgot to add the extra tire wear we discussed last week.

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Correct! The other issue I've noticed is tire wear. Same drives tries we always run have dropped in mileage about 12 to 14,000 less. We run Goodyear 399 steer tires that averaged 105 to 110,000 miles.
We will be lucky to see 90,000 miles I believe. Shorter stopping distance means the energy went directly to the road.

For every action there is a direct and opposite reaction.

Truck Shop said:

The last 25 ordered have disc, as far as weight savings it's minimal, and yes you can order drum brakes. But like I said earlier sales force. Phil Swift here with disc brake in can!
These tractors are all equipped with the DT 12 automatic transmission, which I am not a fan of automatics in trucks.

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No driveline retarder option available then I take it..?

Nige said:

No driveline retarder option available then I take it..?

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All our tractors are equipped with engine jake brakes. But because the rpm is usually on the low side when cruising over slight down grades {rolling hills} at 65 mph the jake is not very effective.
That is the down side of lower rpm fuel squeezing engines. The jake becomes less effective at highway cruising speed. On a 6% 6 mile long grade say running 45 mph in 7th at full jake rpm of
2100 rpm you should only touch the brake pedal once maybe for a corner with a gvw of 100,000 lbs. And that is set in manual mode with the automatic trans.

Almost three weeks ago I had to tow one of our rigs, truck and trailer loaded down whats called Cabbage Hill on I-84. The grade is 6% and 6 miles long total GVW including tow truck was
128,000 lbs. at 27 mph, running the jake at 2000 to 2100 I used the brakes four times with a application psi of 15 lbs for control. I had 6 axles worth of drum brakes, three on the tow truck
and three on the trailer. None on the tractor hooked to trailer being towed, never had an issue and didn't expect any.

IMHO they really need a transmission/driveline retarder in addition to the Jake. We had a fleet of Scanias equipped with Scania's own-brand driveline retarder in addition to an engine brake. The great thing about them being the fact that because the retarder functioned on the transmission output side it was totally independent of engine RPM, only on ground speed. Another plus for the retarder was the lack of noise, so you could use them anywhere. Basically you could dial in the retarder going downgrade and if you needed extra braking effort either apply more retarder, engage the engine brake, or tap the service brakes. In my experience 100% retarder application running at Max permitted GVW would just about bring the truck to a dead stop. This may have had a detrimental effect on tyre wear but I don't have any numbers. Either way we were more than happy with them.

The Volvo articulated trucks have retarders in the trans. Its incredible how quickly they pull you down to manageable speed. I don't know if that tech could be implemented onto road trucks, don't see why not. These things act like how Nige described, the heavier the load, the higher efficiency. I could totally see it eating through drive tires . It would be more betters if it would lock diff locks and power divider in before beginning the retarder. Just my 2 cents.

funwithfuel said:

The Volvo articulated trucks have retarders in the trans. Its incredible how quickly they pull you down to manageable speed. I don't know if that tech could be implemented onto road trucks, don't see why not. These things act like how Nige described, the heavier the load, the higher efficiency. I could totally see it eating through drive tires . It would be more betters if it would lock diff locks and power divider in before beginning the retarder. Just my 2 cents.

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No reason why not. The Scania's I'm referring to were on-highway units.
https://www.scania.com/group/en/a-brake-with-history/