Braking Performance

Hi Bob,

First of all, do you mean 6 degree or 6%? I think 6 degrees would be a pretty gentle slope. We have a number of grades around here with signs saying 6% that are really about 20 or maybe even 30 degrees.

Next, what sort of trucks are these? 100,000 lbs is overweight for most trucks. How many axles with brakes will you have?

Thanks for your reply. The 6* reading came from a driveshaft angle gauge. I put it on the guardrail which was the same distance from the ground wherever I went. The trucks are airport fire engines. All axles will have brakes.

We are writing specs for replacement trucks. One manufacturer does not offer disc brakes. That is why I am trying to find out how the hill will effect the braking.

1. heavy truck
2. steep hill
3. poor vision
4. emergency response for a downed aircraft

I want the best brakes I can.

140,000 is the usual weight top end for western states

Get disk as most heavy vehicles are required now to have ABS in addition to air brakes - at least in the US

The weight is not that big an issue. The bridge was built with the plan of 4 trucks at once on it. The only time the trucks will use the bridge, is when an aircraft goes down on the wrong side of I95.

One manufacturer does not offer disc brakes. I want to know weather that should be a sufficient reason to reject his bid. I would like there to be as many quality bidders as possible.

All trucks must use anti-lock brakes on all axles. That is mandatory. It is the disc to drum concern. Thank you for responding.

Bob,

In my oppinion you should stick with discs.

"I want the best brakes I can"

Wouldn't that be disks, all around?

OK, taking it in the spirit intended despite my personal preference for disk (and can you do ABS with drums?)

Have they got comparable braking performance tests fully loaded?

Hang in there, someone will be along soon who can *actually* answer your question as opposed to my wild-ass opinions.

Thank you for replying. The truck can be equipped with abs brakes with drums. Most heavy weight trucks sold in US are ABS drum brakes. The ABS brakes were first mandated on big trucks back in 1977, or 78. FMVSS 121.

I also want the best. But the FAA will only pay for it if there is sufficient reason for it. I am asking for the reason from CR 4 family. Thanks again.

Bob,

There seems like only one surefire way to prove it out.

If you went out looking to buy a car, wouldn't you want to road test it. Those suppliers should be willing to let you test their vehicles before deciding. Unless there is some stupid stipulation in the bid.

I am sure that I could get two of the five manufacturers to go for that. The manufacturer of the truck without disc brakes available does not have a truck any closer to Ft. Lauderdale than Cincinnati that i know of. I do like your plan though.

"I feel that your truck would not be suitable for the conditions here. Bring your truck here, and prove me wrong."

We are scheduled for a pre bid meeting tomorrow at 10AM. I will give it a try.

Thank you for the ideas.

Or get on a plane and go there - no the real problem is they are mostly custom built...so

Who nearby has one *like* it, how do the crews like it, where on the web to fire truck drivers talk, is there a Consumer Reports for fire trucks?

It is a little more awkward than that. This brand has not been selling in the states for long. Everybody is going to be new with the truck. To balance that thought, Everyone likes their new shiny truck, because the one they just got rid of was about ten years old.

I am trying to find my own conclusions, and not blindly accept other people's opinions. I do find that the drivers of the trucks do like them. They require some adjustment to get used to them, but that is to be expected on a truck that is 10 years newer. Some components seem to raise durability questions, But the trucks are still too new do decide. The forums are all over the place, as to weather the trucks are any good. Must firefighters do not get to decide the make of truck they will work out of. But they almost all learn to like the truck they get. ( sung to the tune of "Love the one you're with")

Thank you for all of your help.

I'm familiar

Good on you for at least trying to be diligent

Should I assume you are intentionally trying not to tell us what kind of fire truck?

Wednesday at 10 AM I will be in attendance at the pre-bid conference. I expect that there will be representatives from;

• Colet SVE
• Emergency One
• KME- Walter Truck
• Oshkosh Truck
• Rosenbauer America

Anything I might say, that was detrimental to any prospective bidder could cause a protest. Any time delays might cause the monies that were planned for this project to disappear. Grant money usually has a time deadline attached to it.

After tomorrow, I should be more at liberty to identify the particulars. Thanks for putting up with my dancing.

49 CFR "Transportation" has the requirements for braking systems. What you need to determine is Vehicle Weight, Number of Axles, Number of brakes per axle, Maximum Speed and stopping distance. You've already mentioned 6* slope (6.6% grade).This will determine the braking force required to stop the vehicle. Then it's simply a mathematical model to determine if drims will supply enough force to each wheel or if you need drums.

Shadetree, and Lynlynch. Thank you for responding. The FAA, who is paying for the majority of this truck, has set a standard for the stopping distance for trucks of these weights. The truck will meet this standard. That is all that FAA demands. The particular circumstances at Ft Laud. raises the question, Will the steep hill require the superior braking performance afforded by discs? I don't have the required answer.

Will the hill increase the breaking distance by 6%? Will it increase distance by 6% for each 10 mph? What effect will the weight of the truck play in the equation?

I would like the discs. But if the drum brakes are sufficient for conditions, then I must allow the additional manufacturer to present a bid on the truck.

You might determine which system will dissipate more thermal energy away from the friction surfaces. Thermal mass of both systems will play into it,too. How much extra heat will the incline produce during the trip?

I'm not smart enough to do the math, but, when a drum heats up it expands away from the shoes, in the case of brakes. I'd rather have my disk expand into the pads.

Keep'm safe.

You brought up a good point. The hill is actually an overpass that goes over I-95. the total length is only about 1/4 mile total. That is up and down. The problem is that the incline slows you up, so the driver piles on the power. By the time he has crested the top, he has only a short distance to drop his speed before he is at a busy intersection, with limited visibility. The posted speed is 30 mph.Typically the trucks will get up to 40-45 mph by the top of the hill.

How about putting a stop light at the busy intersection that you can control remotely? That way you wouldn't have to worry about the traffic situation at the intersection. You'd still need brakes to slow down enough to turn but if you are going straight through then the brakes wouldn't be as much of an issue.

Thank you for your suggestion. Currently there is a program going on to convert some existing traffic lights to react to a transmitter that would be installed on emergency vehicles. That intersection has been identified as one that will receive the system. I am not sure how far away that program is. Programs like those tend to be slow to develop. Thank you again.

In all of this, we seem to have missed the point of "does the truck need disc brakes?"

In my mind the issue is stopping ability and that is what makes it complicated. Drum brake systems are capable of stopping a vehicle in the same distance as discs. if you go back to 1969 ( I happen to have the data because my dad bought the car new and I'm a car nut) Plymouth Roadrunners, the disc brake option would stop the car from 30 mph in 35 feet. The drums would stop the car in 34 feet, but they could only do it once before the inability to dissapate heat lengthened stopping distances. The discs didn't suffer from that problem.

Today we understand heat dissapation much better and tend to use discs where heat dissapation is required. Sports cars use four-wheel disc brakes because drivers are expected to use the brakes often and heavily. Pickup trucks use discs on the front and drums on the rear because about 70% of the braking is done with the front wheels.

There was a time when no trucker would have brakes on the front axle as they were heavy (thus reducing carrying capacity and revenue) and didn't work all that well. Front discs on trucks are lighter, simpler, easier to maintain than drums and decrease stopping distances. Drum brakes on other axles tend to be used simply because those systems are in place, effective and cheap.

From later posts it sounds like stopping distance is the issue, not repeated stops from speed. In theory you should go for the unit that has the shortest stopping distance, regardless of brake configuration. The caveat that goes with that is the shortest distance may not be the truck that is the most "driveable". It may not handle or steer or do many other things as well as its compettitors. Add to that the fact that fire departments have to plan for the unexpected and the matrix of possibilities grows.

Personally, I wouldn't buy something that I hadn't driven or had someone I trust not just drive the truck, but put it through its paces. Spec sheets don't lie, but they don't tell the whole truth either.

As suggested by others, I would get discs on at least the front axle with all wheel ABS brakes, not the rear wheel ABS only systems that some manufacturers try to use to cut costs.

Ask the manufacturers for stopping distances from the speeds you expect to be at for their unit loaded. That and a test drive will answer your question.

Thank you for all your input. All of the manufactures offer the same brake design on all axles. All disc, or all drum. All of the manufacturers MUST use ABS on all axles. The problem I face is that the only hill in the area these trucks will have to climb is at the far west end of the airport. The truck will have to be driven for at least three minuets as hard as one would ride a race horse. Full throttle until the brakes are needed. Turn the corner, and back to full throttle again. These trucks MUST accelerate from 0 to 50 mph in 25 sec for the smaller truck, and 35 seconds for the larger one. After three minutes of this, they will get to the bridge. What will be left of the brakes by the bottom of the hill? It is the repeated stopping that I am trying to improve. As a side note, I have speced transmission retarders with the trucks.

I have my response line for tomorrow well rehearsed.

Sometimes good enough just ain't good enough.

The issue is stopping distance after abusing the brakes for a couple of minutes.

Thank you again.

Bob

The other item that no one has brought up is the fact that drum brakes are more susceptible to loss of performance if they get wet which was always an inherent problem that was never able to be cured completely and also the effect of heat fade. At the weight that you are spec'd I would feel that it would be a choice of what would be the safest for the men and that would be the disc brake models. I say this based on the statement that they will go to the far end of the field slow to make the turn to go over the overpass over I95 then accelerate over the overpass and then have to be able to stop the truck in a short distance before the intersection. By using basic calculations stopping 100,000lbs. from 60 MPH would require approx. 24,000,000 ft/lbs of force which converts to about 31,00 BTU of heat generated. Disc brakes have a much larger surface area for heat dissipation than a drum brake would therefore it would make the braking efficiency much better and the down grade would actually increase these numbers proportionally to the grade of slope which 6* would be 10.51% slope.

To answer some of the questions.

The FAA requires service brakes to be able to stop the vehicle in the following distances.

35,000 lb truck---------35 feet @20 mph-------131 feet @ 40 mph

100,000 lb truck---------40 feet @ 20 mph-------160 feet @ 40 mph

The trucks all meet, or exceed these numbers.

The smaller truck will have 2 axles and four tires. Tire size is 24 R 21

The larger truck will have 4 axles and eight tires. Tire size is 24 R 21

Hi Bob,

It sounds like repeated braking and brake fade is your key issue. Disk brakes dissapate heat more efficiently than drums, because the disk is exposed to more air flow, whereas the drums retain the heat. So in short, disks are better for repeated braking operations - thats why race cars have disc brakes.

For more info on brakes check out http://www.stoptech.com/tech_info/The%20Physics%20of%20Braking%20Systems.pdf

and http://www.stoptech.com/tech_info/tech_white_papers.shtml

I don't work for stoptech, but the info is easy to follow which is why I'm suggesting this site.

The hill means that not only does the braking system have to stop the energy of the truck (kenetic energy = 1/2 the massof the truck multiplied by the speed times speed), but also deal with the potential energy developed by the hill (potential energy = mass of truck times gravity 9.81 times height of the hill).

The companies your dealing with ought to be happy to get someone to do the calculations to prove their brakes can cope.

My advice is talk to someone at your local college or high school from the mechanical engineering, math or physics department who could do the calculations for you. They'll like the challenge.

Good Luck.

Thank you. The formula you sent has a few areas I do not understand.

"potential energy = mass of truck times gravity 9.81 times height of the hill"

Mass of the truck gravity 8.91.------ Is that 8.91 figure the gravity component of the equation? The hight of the hill is 40 ft, (only an estimate right now) But there is no method to allow for the steepness of the hill. If the hill were a 12 degree decent, the breaking would have to be harder to achieve.

But your answer is the thing that I was looking for. I have contacted Stoptech directly. Thank you again.