Disc brakes or drum brakes?

One of my instructors went to a seminar on disc brakes, and said that they cost more than regular drum brakes. However, replacing just the steer tires with disc brakes shortens the stopping distance of a fully loaded tractor-trailer from the length of a football field to just three-quarters of one at 55mph. Also because of the design of disc brakes and thermal coatings they don't fade as fast as drum brakes do when used too much. Also disc brakes are inherently self adjusting, so no more crawling underneath the truck and pulling on a slack adjuster to see if there is more than one inch play in it.

 

Pros:

Superior stopping distance (200' vs 220' at 60mph & 80,000lb)
Potentially reduced liability
Very slight decrease in weight (unsprung too -- which improves ride)


Cons:

Increased initial cost
Slight increase in maintenance cost (lower labor costs offset by more expensive/less available parts). Disc brakes tend to wear faster when pulling trailers with drums (drums fade faster, making the discs do more work, in severe cases leading to warping/cracking rotors)
Slight decrease in fuel economy (disc brakes drag slightly)




In your application (local/regional fuel hauling) I would spec discs. In mine (OTR general freight), I would not.

 

Get the cost of the disc brakes on the steers and the drives. Benefits of disc is that in wet weather, since they are always slightly dragging on the rotor, the driver will have good brakes when he uses them rather than for the drums to dry out till they grab. If your company wants to limit liability issues, there may be disc brake retrofits for the other tractors and trailers. By upgrading the braking system, may give your company insurance discounts, but check with them first.

Now if disc brakes are only available for the steers, still get them. But I have seen on some tv show regarding truck building, of fire trucks i think, disc brakes can be had all the way around. It IS costlier up front (initial cost), but the longer life from them will pay off.


http://fleetowner.com/equipment/weighing-disc-brakes?page=1

http://www.kenworth.com/news/news-releases/2013/december/bendix-air-disc-brakes.aspx

http://www.peterbilt.com/about/media/2011/320/

http://www.foundationbrakes.com/media/documents/airdiscbrakes/awhitepapercaseforairdiscbrakes.pdf

 

Trailer pad around 450-500K miles.



Have had them on all the tractors since they were first availible. Around 6.5 million miles(cumulative). Had one caliper that hung up and wore one pad. At 450K miles trade they have at least 40% pad left and the rotors smooth as can be. The oldest trailers with them have over 500K miles and aren't even close to replacing.

 

Initial cost on tractors(at least FL & KW/Pete) is very little if you were ordering with aluminun hubs and centrifuse drums. Trailers are still more but coming down.

 

Disc brakes on highway trucks are clearly better.
http://trailer-bodybuilders.com/arc...t-advantages-air-disc-brakes-over-drum-brakes

 

Not too sure where someone gets that disc's drag more than drums. You set a clearance between the pads and rotor.

 

Disc brake return springs are not nearly as effective as drum brake return springs -- if they're even equipped. So after each brake application, the pad still drags slightly against the rotor.

With new, true, and properly lubricated equipmemt on a test bench, the difference is small. With the imperfect equipment/conditions found in the real world, the differences are larger (still small, but noticeable on a fleet scale)

 

Here is a very good video of several different configurations... (standard drum vs disc, wide drum vs disc, front disc\standard rear drum vs disc etc.. etc...) The standard drum vs disc is at the very beginning which I would think major of trucks would be setup with standard drum... Great video and some amazing stopping distance differences...

 

Cool video, but I hope that was staged solely for the visuals and isn't a reflection of their scientific methodology (most expensive brakes were always given the right lane).