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Old 03-30-2015, 07:28 PM   #15
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Nothing wrong with hydraulic brakes, just felt the air brakes is a big upgrade, personal opinion


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Old 06-14-2015, 11:05 PM   #16
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Quote:
Originally Posted by Robert L View Post
The hydraulic brakes are ineffective when towing?

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Originally Posted by halla View Post
Nothing wrong with hydraulic brakes, just felt the air brakes is a big upgrade, personal opinion

I know this thread has been silent for a few months, but I wanted to point something out--air brakes are in no way superior to hydraulic. In fact it is the other way around.

First of all, both systems use the same types of components to create the stopping friction: calipers and rotors or drums. The potential for stopping force is mainly controlled by these items. The diameter of the rotor or drum, the surface area of the brake pad or shoe, the piston diameter, and outside air flow determine the amount of stopping force and the thermal resistance of the system. Thus, stopping power is not a function of air or hydraulic fluid.

The hydraulic fluid or the air pressure is used to actuate the components mentioned above. This is where hydraulic is really better: hydraulic fluid has very little compressability. This means the brakes react instantly. Air brakes have a delay. Hydraulics also have few parts in the system and are unaffected by outside air temperature. Additionally, hydraulics lend themselves to better to technologies like ABS and stability control.

So why are air brakes so prevalent in big commercial trucks? Because you can easily couple air brakes between the tractor and the trailer. This creates a very uniformed braking system that is far superior to hydraulics on the cab and electric on the trailer.

Most of us that are towing with Super C's won't be towing a trailer with air brakes. If you have a trailer with air brakes, then you will definitely want a Super C with air brakes. If you are towing a trailer with electric or surge brakes, then you are much better off with hydraulic brakes.

Now earlier the question was raised about hydraulic brakes in the F550 vs. the air brakes of the Freightliner. If you want to know which will stop better, get the specifications for the braking hardware for each. The vehicle that generates the most brake torque retaliative to the weight of the coach and trailer will always have more stopping power. (There are calculators available online to compute brake torque.)

I do know the the Ford has a descent ABS system and the ABS system ties into the builtin electric brake controller to help prevent trailer lockup, too.

One thing I should mention is the F550 lacks an exhaust brake. This isn't a traditional brake at all. It uses exhaust system backpressure to make the engine aid in slowing the truck down. Exhaust brakes are extremely useful when coming down steep grades. Many times the need to use the air or hydraulic brakes is minimized or eliminated. This keeps heat out of the primary brake system and gives you increased braking performance.

The lack of the exhaust brake on the F550 is only true for models using the F550 Cab Chassis, though I'm not sure if anyone builds a F550 based motorhome on the truck chassis. The truck chassis version of the 6.7L uses a variable vein turbo that can be used as an exhaust brake. The Cab Chassis, such as Thor uses, doesn't have this type of turbo and thus doesn't get the exhaust brake.

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Old 06-15-2015, 06:58 AM   #17
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The F550 that I drive have a tow setting that allows the Trans assist in slowing the truck down.

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Old 06-15-2015, 07:57 AM   #18
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The F550 that I drive have a tow setting that allows the Trans assist in slowing the truck down.

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That's correct, the tow/haul mode locks the torque converter on decel, providing some engine braking. It does help, but only provides a small fraction of the braking that an exhaust brake does.

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Old 06-17-2015, 09:58 PM   #19
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That's correct, the tow/haul mode locks the torque converter on decel, providing some engine braking. It does help, but only provides a small fraction of the braking that an exhaust brake does.

-bj
Also just for clarification, an exhaust brake is not the same as engine brake. Exhaust brake basically chokes down the exhaust, increasing exhaust backpressure. Whereas the engine brake bleeds off compression pressure after the work of compressing is done.

An engine brake is much stronger than exhaust brake. An engine brake has to be built into the engine design. An exhaust brake can be added later. Not all diesel engines have an engine brake, and many do not have exhaust brake either.
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Old 06-18-2015, 01:31 PM   #20
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Also just for clarification, an exhaust brake is not the same as engine brake. Exhaust brake basically chokes down the exhaust, increasing exhaust backpressure. Whereas the engine brake bleeds off compression pressure after the work of compressing is done.

An engine brake is much stronger than exhaust brake. An engine brake has to be built into the engine design. An exhaust brake can be added later. Not all diesel engines have an engine brake, and many do not have exhaust brake either.
Thanks for clarifying. You are correct-engine braking in a diesel is actually a feature where the valve train in manipulated to cause more engine braking. These are best know as jake brakes.

A true exhaust brake is similar to a throttle body that installs in the exhaust. When activated, it closes and restricts flow, causing the engine to provide more braking. Both an exhaust brake and the jake brakes create situation where the engine resists rotating, slowing the vehicle down, but they do it in different ways with different levels of effectiveness.

Doing a little research, I've been able to figure out a bit more about how the braking assistance on the Powerstroke 6.7L works. On the latest generation 6.7 PS, the variable veins in the turbo restrict the exhaust. This provides results that are very similar to a real exhaust brake (although not quite as good).

On the first gen 6.7 PS, which is what is still in even the 2016 cab and chassis trucks that the Thor is built on, they use a single-sequential turbocharger. The exhaust brake on this model works by forcing the exhaust only through the smaller turbine path. This creates a restriction that creates the backpressure needed to slow the truck.

On both versions of the 6.7 PS, Ford uses the transmission to make it the braking assistance more effective. Basically they down shift the transmission and lock the torque converter when possible, raising the RPMs and exhaust volume. Since the exhaust volume is increased, you get more braking despite the limited restriction provided by the variable or sequential turbo.

Now to keep us guessing Ford refers to this braking assistance feature on both motors as both an exhaust brake and an engine brake. It seems their marketing department doesn't know what to call it.

To make it more confusing, on a gas motor you get engine braking too. The use of throttle bodies makes this inherent on all gas motors. It is why when you have a manual transmission gas motor car in gear and let off the gas it slows down quickly. Ford use this with a special tow/haul program on the gas motors to provide additional braking, too. My current V10 based motorhome has this. It works OK, but isn't anything like a real diesel exhaust or jake brake.


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