That sales article is just that, a sales article and should not be considered a technical discussion.
The type of actuation system boost is chosen for many different reasons depending on the particular situation. Sometimes cost, sometimes need, sometimes the complexity of integrating it into the powerplant accessories or under hood areas. It's never a conspiracy issue.
I posted this over at The Diesel Stop earlier last month in response to a question about a comment I made that I would always choose a hydroboost. So I'll confuse you guys too since I've never proofed it.
The way I write and how tired I am right now means this is going to be confusing …..
I’ve probably pushed a brake pedal 1000’s of times in a failed system mode, where either one of the two circuits of a brake hydraulic system have been failed or a booster has been shut off or vented. So I know how far and hard you have to push a pedal to the floor in order to still be able to stop under system failure. Most consumers do not and it’s not unusual to have someone who failed one of the hydraulic circuits to state they have lost all of their braking ability. They didn’t, they just had to push closer to the floor. I’ve seen it in accident cases and I’ve seen it on this forum. So from a vehicle ergonomics engineer’s position, pedal travel is of great concern.
Secondarily, I have no idea why Ford made the change for the ’12 MY, and could have been a multitude of reasons unrelated to what I’m discussing. Also the data I’m presenting is not from the ’12 SDs. We supplied friction materials to all the big three and multiple Asian and European companies. So I’m using real data as an illustration I was using as a teaching example.
Before the start of any test I would have the test drivers generate data from the vehicle so I could go back and see if the vehicle was setup right and had all the air out of the brake system. It fingerprinted the vehicle so if questions arose down the road if something other then the friction material was the reason for a poor performing test I could eliminate variables. These graphs are not industry standard, were laid out for my use, and even would give my engineers a headache sometimes. But it was my fingerprint. The data comes from having a pedal force sensor mounted in place of the rubber pad on the brake pedal, a hydraulic pressure sensor in each of the two hydraulic circuits (front and rear), and a linear transducer mounted to the brake pedal lever.
The only difference in these two vehicles is the actuation system, the booster and brake pedal leverage. The data is generated first with the vehicle running after a goose up to 3,000 rpm (highest vacuum on the downside of that goose) and then with the engine off after hitting the pedal 6 times to exhaust all vacuum/pressure in the booster or accumulator. I usually present the data a six graphs on one page, the only difference here is I split the graphs up so one page demonstrates the normal operation of both vehicles and the second under failure mode circumstances. The three graphs in a row are all the same data; just the graphs are rearranged in the presentation so it’s easier for me to understand what I am looking for. Graph curves look different depending on the presentation. As I said, other people look at it and go “Huh?”. But you’re stuck with me and I’ll bounce from one graph to another which is going to make your head spin. I’ll be using the left side graphs (pedal effort) for this discussion.
The first page shows the differences in boosted operation between the vehicles. But first let me talk about booster knees or runout. At some point in any booster operation you run out of assist, where the rest of the hydraulic pressure is attained only by direct manual force, just like if there was no booster. Depending on the design of a vacuum booster, it can be a hard or soft transition and in the case of this vacuum booster it’s a soft knee that occurs at about 60lbs PE. The hydroboosted vehicle shows a hard knee, more at 70lbs PE. Graph 1 may be better to show this
Most of our normal brake activity is done at 250-500psi, and for that the pedal efforts are pretty similar, and so is the pedal travel. But after that the hydroboost system generates more assist, or another way it takes less PE to generate hyd psi. A 13k Superduty requires about 1400-1500 hyd psi to generate a wheel skid. With a truck that has the illustrated vacuum booster setup, that is going to require the entire 150lb PE that NHTSA allows. The hydroboosted truck is only requiring 65lbs PE to do the same. And at that hydraulic pressure both require about the same pedal travel, 4.5”. A truck at 10k may only require about 900-1000 hyd psi so the PEs at that point are closer.
But lets say I was dumb enough to hitch up a 12k trailer with non-performing brakes on the back. I’m way over exceeding the design capability of the truck brakes. If the truck is relatively light, the tires may break free and just skid. But if the truck load is sufficient to keep the tires from skidding, I could really need as much brake torque as I can get. And if I’m driving in the mountains, I could get into a situation of ‘hard’ brake fade, where the heat is causing a loss of friction, but not excessive travel. For that last Hair Mary attempt at stopping, I need all of the hydraulic pressure I can get. The hydroboost system let’s me generate 500 hyd psi more at 150lbs PE then the vacuum boost system would. And at a true ‘Hail Mary’ stop, it’s been known for a driver to push 200-250lbs PE. I got the data!
The negative to that situation with the hydroboost though is pedal travel, as at 150lbs PE the travel will be 1.5” longer. And most people give up way before 6” of pedal travel. And if people are going to give up at 5” of travel thinking they are pedal to floor, you might be better off with the vacuum booster. As I said, I know how far I can go with the brake pedal and I would rather have the extra hydraulic pressure I know I can get.
Now lets look at the situation where a booster fails in operation. In reality, a rare occurrence. And the same advantages / disadvantages exist as with the systems under full operation.
If you are in a vacuum boosted vehicle with the motor off and press on the brake pedal a few times to exhaust the vacuum reserve you know you have a low travel hard pedal. The graph shows that. And with a failed vacuum booster we can only generate about 350psi. Good enough for a typical deceleration rate stop, not so good if it’s a panic stop. Pedal travel is only about 2.25” at 150lb PE (remember NHTSA limit), so there is plenty more pedal travel if someone is interested in stopping the vehicle at a higher decel rate rather then worrying that the NHTSA police are not going to hassle you for pushing too hard on the brake.
Here the hydrobooster again has an advantage in being able to generate a higher hyd psi at 150lbs PE, with the same situation of having a longer pedal travel. The reason for the higher pressure and longer travel is the pedal arms have different leverages between a hydroboost and vacuum boost system. The attachment point for the pushrod that goes out the firewall is altered. And why it’s not an easy task to just flip between the two boosters, if adding a hydroboost system in place of a vacuum boost was easy!
Again for me knowing how far I can push the brake pedal, the double the travel of the hydroboosted system is worth it to me to get another 100 hyd psi in that failure mode. The consumer may be more prone to give up with the “lost all my braking” viewpoint.
Over the last 30 years this comparison has been pretty normal with vehicles where the actuation system has been variable between the two brake assist systems. But also keep in mind this is not data from a 2012 Superduty, and all those relationships can change depending on the size, valving and pedal ratio of the actuation systems. Ford may have selected a vacuum system that is setup to be closer to the PE of a hydroboost system then I am demonstrating.
And depending on the friction level of the brake pads you put on a vehicle you can greatly change the PE required to stop depending on how the hydraulic skid points are located in relation to the booster knee. Past the knee requires a proportionally higher force. Higher friction can keep everything below the knee.