When will FMCSA force disc brakes upon the industry?

As long as your talking about "s" cam drum brakes. Wedge brakes are another story.

 

Large truck stopping distance and braking ability has not kept up with technology. When was the last time stopping distance rules were updated? the 80's.

The only reason new technology is being considered is because the government is forcing the industries hand with 2012 stopping distance rule. The ability to stop trucks quicker and safer has been around for 15 plus years, but very few companies want to spend extra money and weight on better technology.

Mark

 

For a single stop from less than about 55 mph, disc brakes have little if any advantages over the drum brakes. Multiple stops, higher speeds give a distinct advantage to discs brakes.

Stopping distance is not just a function of good brakes. It requires suspension systems, control feel, high coefficient of friction between tire and road. You won't see trucks stop like sport cars until the suspension systems and tires get a lot better and people are willing to pay the costs. The tires could be made, the technology is common, but they wouldn't last as long. The suspension system would very expensive and complex, but again the technology exists.

Can you lock the tires on your truck with drum brakes when loaded and going 55 mph? Can you 'feather the brakes' in a panic stop holding the tires just at lock-up without the brakes fading to the point you can't lock up the wheels? If you answer yes to both, then the brakes are not the 'weak link'. Either the control system, suspension, or tires are the weak link. Keep improving the 'weak link' and eventually the brakes will be the 'weak link'.

Good ABS generally helps the control system. Active suspension systems help a lot. Any good suspension would help, but active suspension can be 'tailored' and thus handles variations in weight and weight distribution more readily.

Yes the the brakes can be 'better'. Yes it will cost money, but so do lives and accident damage. Better brakes does not always mean greater weight, though usually it does. At some point the companies, both the trucking companies and the manufacturers, will decide it more cost effective to have better brakes and shorter stopping distances. Maybe the Feds will decide for them before that time, maybe not.

Now speed management down a long hill is something else entirely different!

 

How the brakes operate versus whether or not the drums were any good ? Is that what you're saying ?
All the trucks with airbrakes that I ever drove relied on "S" cams to expand the shoes into the drums.

Now I'm not a truck mechanic so I always looked at everything from a truck drivers point of view, so without googling wedge brakes to see what they are, I will say that some late 1950's trucks that were still on the road when I started driving truck had an air over hydraulic system of operation. The trucks stopped just fine, the scary part, they had no parking brake to speak of.

The truck had a real nasty habit of dissappearing on ya while you were inside eating lunch.

 

What I was saying was that brake technology has not been stagnate. The brakes you had a 70s are not the same as what you get today. They may look similar, but there have been 40 years of improvements in materials and design. Putting a 70s braking system on a modern truck and you won't have nearly the same performance and durability.

There has been significant improvements to disc brakes over the years just like there have been to drums.

 

It should be a single stop from 65 or 75, no one goes 55 any more. We all drive faster with less following distance then in the old days. In today's real world driving a disc system will be better than a drum system.

You can apply more braking force with a disc brake system than a drum system.

Mark

 

No argument with increased speeds for the stopping distance requirement, none at all. 75 would be much more realistic and if any error it would be conservative. 55 is definitely outdated. No argument about shorter stopping distances being better and that they could/should be done.

Now a lot of people say there is not much difference between 55 or 75 for the test, but the difference is huge. About 85% more energy at 75 than 55. This alone would probably tilt the decision towards disc brakes, even if only weight were considered.

As to your statement about 'braking force', it depends on what you call 'braking force'. As engineers we refer to 'brake torque' which the force resisting the turn of the wheel. Drum brakes are the ultimate for creating this force. If you are refer to total energy absorbed (the ability to sustain a given brake torque for a period of time), then as the time is increased the disc brake gains the advantage. In short drum provide greater resistance, but fade quickly. Disc brakes provide less resistant, but can sustain it much longer. Now of course the brakes have to generate enough holding force to hold a vehicle in place (or lock a wheel when in motion). A disc brake, using common materials, that will generate enough holding force, is too big for the 55 mph stop. And a drum brake just big enough to provide the holding power would be too small for a 75 mph stop.

Personally, I like disc brakes as a rule. But I know that when asked which is better, drums or discs, it depends on many things.

 

In the early 80's in western Canada, the MOT forced the trucking companies to install front brakes on tractors that, did not have them, and if they had them and were removed, they had to retrofit them back on.
As a working mechanic I see owners, buying the cheapest brake parts, drums and shoes, and not repairing worn brake parts. If every owner repaired their equipment properly there would be no reason to have vehicle inspection programs. I was around when the inspection programs started, and vehicles were towed in for inspections, and towed out for scrap. I worked at a shop by the boarder, and we use to get units from the states, that needed to be decal-ed to do their job in Canada and when we wrote out the list, and condemned the truck, it was sent back across, and the list sent to Washington state police. The reason government has to continually upgrade the regulations is that people will not spend the money to upgrade unless they are forced to.
I have seen guys spend money on new stacks, when the drag link was worn out. and he said he did not have the money to do both.
I have seen and driven units, on road tests, with disc brakes and was impressed, they were on twin steer cement trucks and the drivers would abuse them to the braking point, and the companies removed them. The materials have improved and it will be either the government or the Insurance companies will mandate the change.
Just a thought!

 

Correct, a truck with all air disk vs a truck with "High performance" drum brakes side by side from 60mph only has 8' of distance between them. From 70mph it jumps to 92'. But you also don't have to put air disk on all axles to have an improvement, just the front axle helps a lot. If companies want to improve safety all they need to do is stop buying the cheapest shoes/drums available. I have no doubt disk brakes will some day become more common but again I'm not for anyone forcing it on the industry without proving their better in every way. Look at it this way if they cost twice as much but only offer small cut in stopping distance guys will just replace parts half as often because of cost. Then any stopping improvement is totally lost due to lack of maintenance. Not saying it's right, it's just the truth.

Suspensions do help with stopping. Spring suspensions are way more likely to lockup the brakes than air ride.


Here's a link to a video showing a side by side comparison of all the systems. Several ADB sizes and grades, standard drum, wide drum and high performance drum. Also see the difference you get just by putting them on the steering only.

 

When you compare weight drums vs disc you should compare how much drums with equal stopping power as disc weigh, then I think you will find that drums weigh more than disc do.
The last European trucks with drum brakes had 16"x8" up front and 16x10 on the drives. If you compare that to 15"x4" and 16.5"x7" that seems to be the choice today on many American trucks one understand why the drum brakes are considered light.

Many trailers still come with drums as standard equipment so the combination with a tractor with discs and a trailer with drums is no problem in Europe.