Air Brakes or Hydraulic Brakes?

[WeBrzn2]

Stretch, the Avalanche has drum all the way around. Would be nice to have the disk up front! They are ABS and I have yet to try out that feature on the coach (tried it many times on the track with the Vette and Mini, but that does not count!).

[Stretch]

Webrzn2:

Thanx for the info...........Yea it would be a sight to see your avalanche in a 4 wheel drift around a short track...........

[Jim A]

It would be a sight, but I guess we will have to settle for the sight of the Alpine Coaches on the dragstrip at homecoming. Since homecoming is in Nashville this year, maybe we can get them to rent out the Nashville superspeedway and let us turn some laps.

[WeBrzn2]

If there is any coach that I would even consider putting into a 4 wheel drift, it would be an Alpine. The drags could be interesting, but we may find a lot of spectators falling asleep before the first 2 coaches cross the finish line in many seconds!

[JeauxRM]

My wife and I recently completed a trip to Alaska in our 25-foot Winnebago View motor home. We were, other than a class B, the smallest rig. Two others were slightly larger class Cs. Most were huge class As and huge fifth wheel rigs. We noticed that the big class As, or most of them, had air brakes. I wonder why.

I used to drive tractor-trailer rigs — 18 wheelers -- following 35 years of physics and engineering education and technical career positions. I didn’t like air brakes; but at least I understand why they are used if you have to pick up and drop trailers. Trucks could come up with hydraulic systems that work for trailers pulled by tractors; but the industry has settled on air brakes and will probably never change.

While there are advantages to air brakes for tractor-trailers, I see no advantages to having air brakes on straight trucks or motor homes. The proof that the biggest planes have hydraulic brakes. Air brakes take time to deploy because air must travel through lines and build up pressure while hydraulic pressure is transmitted almost instantly to all the wheels. With hydraulic brakes, the fluid is already in the slave cylinders. Only very slight movement of fluid is required; and since liquid is essentially non-compressible, pressure changes are transmitted at the speed of sound (which is much greater in liquids than in air) once the pressure is applied. Hydraulic brakes also apply the same pressure to each wheel while air brakes involve an adjustment to make sure all wheel brakes are applied equally, something that is not always done or done correctly.

When I drove trucks, it was an aggravation that you had to wait for a minute or more for air pressure to build up before you could release the brakes and move. The air also contains moisture which can freeze in valves and orifices preventing the application or release of brakes.

For my money, I would never have air brakes on a motor home. They are more expensive to manufacture, install and maintain, can be more trouble in cold weather and are no better than hydraulic brakes for stopping the RV. I never understood why anyone thought they would be better than hydraulic brakes for applications other than on tractor-trailers or trains. Another proof that they are not needed is that even the biggest fifth wheel RVs don’t have air brakes; they stop just fine. Most have electric brakes, which are, in my opinion, second best compared to hydraulic brakes. That makes air brakes third best.

Incidentally, there is a fourth type of brake that I use. I pull a car on a car dolly that has a surge brake built into the hitch. When the brakes are applied on the motor home, the car and car dolly push forward and press on a master cylinder. This pushes hydraulic fluid to the wheels and actuates the brakes to whatever extent is needed based on how much force is applied to the master cylinder; and that is determined by how fast you are pushing on the RVs brakes. Again, the brakes on the car dolly are hydraulic. When you take off, the pressure on the master cylinder is released and the brakes are no longer being applied. It is as if the RVs hydraulic brakes are extended to the car dolly and work as one.

This is, by the way, how tractor-trailers could be designed. The trailer hitch pin could be made to actuate a master cylinder that would apply hydraulic brakes to the trailer. The trailer brakes would actuate instantly and to the extent needed to stop the trailer simply based on inertia. That wouldn’t work to stop a trailer if the tractor-trailer rig was being stopped from backing down a hill unless some other modification to the system was made. A good engineer could solve this problem. My car and dolly don’t weigh enough that this is ever a problem.

[Kennzz05]

Originally Posted by JeauxRM

My wife and I recently completed a trip to Alaska in our 25-foot Winnebago View motor home. We were, other than a class B, the smallest rig. Two others were slightly larger class Cs. Most were huge class As and huge fifth wheel rigs. We noticed that the big class As, or most of them, had air brakes. I wonder why.



I used to drive tractor-trailer rigs — 18 wheelers -- following 35 years of physics and engineering education and technical career positions. I didn’t like air brakes; but at least I understand why they are used if you have to pick up and drop trailers. Trucks could come up with hydraulic systems that work for trailers pulled by tractors; but the industry has settled on air brakes and will probably never change.



While there are advantages to air brakes for tractor-trailers, I see no advantages to having air brakes on straight trucks or motor homes. The proof that the biggest planes have hydraulic brakes. Air brakes take time to deploy because air must travel through lines and build up pressure while hydraulic pressure is transmitted almost instantly to all the wheels. With hydraulic brakes, the fluid is already in the slave cylinders. Only very slight movement of fluid is required; and since liquid is essentially non-compressible, pressure changes are transmitted at the speed of sound (which is much greater in liquids than in air) once the pressure is applied. Hydraulic brakes also apply the same pressure to each wheel while air brakes involve an adjustment to make sure all wheel brakes are applied equally, something that is not always done or done correctly.



When I drove trucks, it was an aggravation that you had to wait for a minute or more for air pressure to build up before you could release the brakes and move. The air also contains moisture which can freeze in valves and orifices preventing the application or release of brakes.



For my money, I would never have air brakes on a motor home. They are more expensive to manufacture, install and maintain, can be more trouble in cold weather and are no better than hydraulic brakes for stopping the RV. I never understood why anyone thought they would be better than hydraulic brakes for applications other than on tractor-trailers or trains. Another proof that they are not needed is that even the biggest fifth wheel RVs don’t have air brakes; they stop just fine. Most have electric brakes, which are, in my opinion, second best compared to hydraulic brakes. That makes air brakes third best.



Incidentally, there is a fourth type of brake that I use. I pull a car on a car dolly that has a surge brake built into the hitch. When the brakes are applied on the motor home, the car and car dolly push forward and press on a master cylinder. This pushes hydraulic fluid to the wheels and actuates the brakes to whatever extent is needed based on how much force is applied to the master cylinder; and that is determined by how fast you are pushing on the RVs brakes. Again, the brakes on the car dolly are hydraulic. When you take off, the pressure on the master cylinder is released and the brakes are no longer being applied. It is as if the RVs hydraulic brakes are extended to the car dolly and work as one.



This is, by the way, how tractor-trailers could be designed. The trailer hitch pin could be made to actuate a master cylinder that would apply hydraulic brakes to the trailer. The trailer brakes would actuate instantly and to the extent needed to stop the trailer simply based on inertia. That wouldn’t work to stop a trailer if the tractor-trailer rig was being stopped from backing down a hill unless some other modification to the system was made. A good engineer could solve this problem. My car and dolly don’t weigh enough that this is ever a problem.

The answer is simple as far as trucks go loose a airline the brakes apply and prevent a runaway guess the same holds true for motorhomes
My guess when your running potentially hundreds of feet of hydraulic line the potential for leaks or damage goes up exponentially

[wolfe10]

"The answer is simple as far as trucks go loose a airline the brakes apply and prevent a runaway guess the same holds true for motorhomes"


Actually, it is even simpler.

Air lines are easy to connect/disconnect between tractor and trailer.

Hydraulics would be more complex and likely have to have air bled from them.

[slickest1]

They do have quick couplers for hydraulic lines as well. It is used in the trucking world every day. They just don't use them for brakes and contrary to the consensus here there is a sound reason for it.