Now I have the OBS hydroboost pedal!

[FORDF250HDXLT]

You say the truck is much safer now, but I wonder if the safety you've gained is offset by the increased pedal effort? What if someone small (wife, teenager, old fart) was driving your truck? If they couldn't mash down on the pedal like you can, they won't have as much stopping power. Do you think your vacuum booster was failing and thats why the pedal was mushy? What does mushy mean in this instance? Would it go to the floor?

I would be super interested to see how you felt about the system if you were to only swap the hydro pedal in. If you only move the hydro unit down, the pedal is still going to be massively hard. This doesn't seem that safe to me?

it is extremely offset.a trade for safely vs driver comfort right now.
i wouldn't (i don't and wouldn't anyway) lend my truck out to anyone.if i had to however i would first caution them (especially a female) about the truck having a very firm brake pedal and that they need to test the brakes at slow speeds a few times before just driving down the road and that the need to use both feet on the brake pedal may be required to help them stop.
im a pretty good size guy over 6' and well over 200 lbs.i often have a very strong desire (and after work - climbing trees/arborist) i find myself using both feet on the pedal when coming to a stop just to take some pressure off my already tired legs.when fresh in the morning i usually just run 'er.it's not dangerously firm.least not for me,but id call it so for a little gal,or an old timer yes.

the truck would stop ok and felt great empty (8k lbs) but when you sat a couple ton on her back to bring it up to 12k lbs and then tow another 1 ton chipper (without brakes due to it's small size) the vac system just didn't put out enough nuts.the brake pedal would turn to fade and go way down,and the stopping distance was dangerously too long.
in fact,it was due to a close call that made me wise up and do the swap asap.no issues there since.she stops like she means it now.just takes great effort from the leg muscles.

so what your saying is,the only way to get the same mechanical advantage is to get the 3/4" pin location difference with the hydro brake pedal.it's not simply a matter of bringing the hydro unit downward in an attempt to just align the pushrod pin.the leverage point of that extra 3/4" in the pedal pivot is vital?
what if you did both?

 

[kawamatt]

so what your saying is,the only way to get the same mechanical advantage is to get the 3/4" pin location difference with the hydro brake pedal.it's not simply a matter of bringing the hydro unit downward in an attempt to just align the pushrod pin.the leverage point of that extra 3/4" in the pedal pivot is vital?
what if you did both?

That's a great question. And I can honestly say I don't know...

When we look at the situation your in, with the pedal offset and the angle on the pushrod, we can determine that the pedal pin being higher on the pedal shaft is going to give you more advantage. The question of if there will be any benefit to only moving the booster down to get the pushrod at the right angle is an interesting one.

If we are to figure out the angle of your current pushrod with the hydrobooster and vacuum pedal, we can (for lack of physical parts in hard) consider that the pushrod itself is the hypotenuse of a 90 degree triangle. The short side would measure to be the difference in the pedal pin locations, or 0.75". The other side could be measured but I'm about 3000 miles away from my truck or any other Ford F-Series, so we will just assume it is 5". This 5" would be the horizontal distance between the pedal pin and the booster hole, irrespective of the vertical offset.

Now, based on a class 2 lever, if the operator imparts of a force of 100lbs on the pedal, with a mechanical advantage of (12"/3.5")=3.43, the pedal pin will impart a force of 343 lbs on the booster pushrod in the horizontal direction (Fx). Then using a force diagram and some statics theory, we can take the sum of the moments about the end (towards the booster) of the pushrod. This tells us that with a slightly angled pushrod (5" X direction, .75" Y direction) we also see a vertical force at the booster of 51.3lb (Fy). This vertical force is opposed by the pedal shaft and bucket. If the booster is moved down to be inline with the pedal pin, therefore no pushrod angle, there then is no vertical force Fy and only the horizontal force Fx. Fx is unchanged at 343lbs for this example

So, no it doesn't appear (from my understanding of statics and the math) that you will gain back any useful (horizontal into booster) force by moving the booster down. The only benefit is that you would eliminate the vertical (off-axial) force that is currently be imparted on the booster piston (which is doing no useful work for you and possibly trying to bind the piston in the booster up).

*NOTE* I made some huge assumptions with this and the previous examples for the sake of demonstration. The pedal might be more or less than 12" and the pushrod might be more or less than 5" in length. The concept remains valid.

 

[LCAM-01XA]

This is a really good discussion. Lets see if we can put in some numbers to help understand better.

Aye, I've been meaning to run the numbers for a while now, just never got around to it. Thanks for saving me the trouble

What I am unsure about is how that extra 1" that you would have to push the pedal to achieve the same flow would feel. Is the force multiplication of the hydrobooster linear? I am thinking its going to be alot easier to get a required flow by pushing through 5" than it would 4". This is why the pedal seems easier to push with a hydroboost? I am unsure.

I doubt the hydrobooster has a linear curve, at least it sure don't feel like it - my observations are that towards the end of the pedal travel it tends to ramp up the pressure quite fast with respect to pedal travel, in other words you get lots of assist increase with very little actual pedal travel. But you gotta remember the extra 1" of pedal travel is at the pad only, the actual pin travel does not change, so any "ease" of pushing the pedal is a result of only result of the increased mechanical advantage of the hydro pedal. All this comparing just the pedals of course, keeping the master factory.

So with the super duty master and vacuum pedal, there is (2.938"/4") a 0.734 ratio of pedal movement to flow delivered when compared to stock. So a smaller pedal movement. This combo seems like a recipe for disaster. Opinions?

Reduced pedal travel itself is not the issue, it's what causes it that makes it a dangerous situation - that massive increase of piston area in the master calls for a proportional increase of the input force to create the same line pressure, and that requirement transfers all the way to the pedal. Meaning if in factory configuration you were using 100lbs of pedal force to generate 1000psi of line pressure (arbitrary numbers), now you'll need to push the pedal down with 136lbs of force to achieve the same line pressure. Flip it around and keep the 100lbs pedal force constant (because the maximum pedal force a driver is capable of is more or less a constant since it depends on their physical condition), now you're only building up 735psi of line pressure. I like looking at worst case scenarios, aka engine dead and booster reservoir depleted - in such a situation the factory system still provides semi-decent braking ability, it's far from the performance of a dedicated full manual setup but it's better than nothing. Now throw on top of it the massive reduction of line pressure cause of the large master, and things really ain't looking good at all.

So all of that doesn't take into account how the pedal is actually going to feel. I have no idea what the typical line pressures are nor what the force multiplication of the vacuum booster or hydro booster are. If we knew those things, we could actually calculate force on the pedal.

Pedal feel is a tricky subject, mostly cause it's all in the eye of the beholder. In the OEM world there are set of guidelines as to what pedal travel and forces are deemed acceptable and how they should be related, but as you can imagine that information is considered proprietary and thus is not widely available. But as far as booster capabilities go, I've seen it thrown around the web that a good vacuum booster is capable of applying 1500 lbs of force to the master pistons, the respective number for the hydrobooster being 2500lbs. At what input forces those numbers are produced I have no idea. Plus you gotta remember the hydro depends on the PS pump pressure, which varies somewhat depending on engine engine speed (not all braking occurs when engine is at idle). Therefore the whole pedal feel thing is one big clustermess that I'd rather not even get into, lol.

 

[LCAM-01XA]

So, no it doesn't appear (from my understanding of statics and the math) that you will gain back any useful (horizontal into booster) force by moving the booster down. The only benefit is that you would eliminate the vertical (off-axial) force that is currently be imparted on the booster piston (which is doing no useful work for you and possibly trying to bind the piston in the booster up).

Let's assume 300 lbs horizontal force on the pin and 30 degrees up angle of the booster pushrod. Actual pedal leverage and pushrod length do not matter as long as we know the angle. Which for sake of simplicity I set at 30 degrees.

300lbs horizontal pin force will split as 260lbs axial force on the pushrod (300*cos30) and 150lbs perpendicular to it (300*sin30). If the booster was tilted up so high it lined up with the pushrod angled down so much that would be the end of it, 260lbs input force at the booster. But that's not the case, the booster is also horizontal, so we have another 30 degree angle where pushrod enters it. Which further dissolves the 260lbs axial pushrod force into a 225lbs horizontal component (260*cos30) and a 130lbs vertical component (260*sin30) at the point where the pushrod connects to booster thru that small ball joint. So out of those initial 300lbs at the pedal pin, only 225lbs make it into the booster as an input force. Now, this is somewhat of an extreme case, as the pushrod angle is probably quite a bit less than 30 degrees, thus more of the pedal pin force will make it into the booster. But you get the idea - there are gains to he had by moving booster down so it lines up with the pedal pin.

 

[LCAM-01XA]

so what your saying is,the only way to get the same mechanical advantage is to get the 3/4" pin location difference with the hydro brake pedal.it's not simply a matter of bringing the hydro unit downward in an attempt to just align the pushrod pin.the leverage point of that extra 3/4" in the pedal pivot is vital?
what if you did both?

The leverage change is vital if you changed the master. Which you didn't. I'm using factory (for a gasser OBS at least) master size of 1-1/4" with my vacuum pedal, booster lined up with pedal so pushrod is perfectly horizontal, brakes are great in both decel rate and pedal feel/force.

Moving booster down is only needed when using it with vacuum pedal. If you moved it down AND you swapped in a hydro pedal then you'll bind the pushrod again, just in the opposite direction.

So for now bring your booster down and line it with the vacuum pedal pin. It costs you nothing but an hour or two of messing with it. See if it solves your issue (it should). If you're still unhappy with it then look for a dedicated hydro pedal, and when you find one move booster back up to line up with the new pin position.

 

[kawamatt]

Yep, your right. I thought about it for a while and couldn't decide if the pin force was the hypotenuse of the angle of the pushrod or not. Its been too long. Some research shows me that it is and the horizontal and vertical components are as you state. I'll leave my post as is because its a good lesson for me and others to understand what I did wrong.

I do have a question about your second part (beginning sentence 3) where your resolving the force at the actual booster. Your first free body diagram resolved the input force 300lbs which is the hypotenuse into the Fx (horizontal) and Fy (vertical) forces. I think it is wrong to then use the Fx (260lbs) as the hypotenuse of another triangle at the end of the pushrod at booster. Your first two sentences take the axial (I think you also confused axial and horizontal in the first sentence. The pushrod is the hypotenuse, no?) force in the pushrod (which is pedal force, 300lb) and resolve it into a horizontal force and vertical force. Because the forces have to add up to zero (equal and opposite reaction), you can't have a 260lb horizontal force component at one end of the pushrod (pedal end) and then 225lb horizontal force component at the other end of the pushrod (booster end).

What do you think?

 

[tbrumm]

Guys, you are providing a great discussion on issues that I had not really considered. Imagine all the similar iterations the Ford engineers went through on this issue. You guys also are working from a disadvatage of not having the proprietary information the Ford engineers had available to them, but with some assumptions, you are coming up with conclusions that are understandable and seem to make sense. I certainly had not really thought about the leverage involved. I knew leverage differences would result with the different pin locations, but I thought the two pin locations merely resulted from a need to fit two different booster configurations into the same model of truck. I plan to re-use my original 1-1/8" master with the hydrobooster and hydroboost pedal (and many have done so in the past with acceptable to end user results). If I don't like this combination after some real world testing on my part, I can always purchase the larger bore master and install that.

I have not brought up another little "issue" concerning the install of the OBS hydroboost pedal, and I might as well for the benefit of all that are following this thread and considering the hydroboost swap in an OBS. The brake light switch on the OBS F-Superduty is different than the switch on the vacuum booster trucks. It still mounts to the brake pedal pin in the same way, but the terminals for the wiring harness connector are perpendicular to the switch, rather that parallel. In other words, the connector on the vac booster truck plugs into the back of the switch and is "in-line" or parallel with it for lack of a better description. The switch for the hydroboost OBS F-Superduty moves the terminals to the side at the rear of the switch, so the harness plugs in perpedicular to the switch. Looking under the dash, the reason for this change is to clear a steel bracket. With the pedal pin "up" 3/4" and closer to the underside of the dash, it was necessary to move the connection to the switch so the connector and wires won't hit on the metal bracket. I don't know if this was the case on the Brick and Bull trucks as I have not really looked at them. Of course, there are probably a myriad of ways to overcome this issue as well (grind offending bracket for clearance, etc). So, if you want to avoid that, get the brake light switch when you pull the OBS F-Superduty pedal! I have checked the part number on the OBS F-Superduty brake light switch with a Ford dealer and it came back "obsolete and no longer available". I have to wonder, even if I could have ordered it, would I have received the correct part, or would I have gotten a vac booster switch (given the same part number appears to have been used on the pedal assemblies)? Fortunately, Swervyjoe pulled the switch for me too when he pulled the pedal, so I have it. Don't know what I'll do if the switch ever fails - guess I'll crosss that bridge if I ever come to it.

You must be registered for see images attach

 

[kawamatt]

Great info! That P/N opened alot of doors. I read it as F3TB-13480-A. Is that correct?

Some googling has led me to an aftermarket equivalent.

Wells F4838
Standard SLS93
Fair Sun FS-MS20

http://www.ebay.com/ctg/Standard-Motor-Products-SLS93-Brake-Light-Switch-/78970133

Nice!

 

[tbrumm]

Awesome!- hadn't gotten around to searching the aftermarket yet, so it is good to know an equivalnt switch is readliy available. The part number on the switch is F3TB-13480-AA, so the you were only missing the last "A". So anybody doing the pedal pin mod will have easy access to the correct switch to clear the underdash bracket. Thanks for looking that up - just one more little part to acquire before you start the hydroboost swap.

Edit - I may have spoken too soon. The plastic tab (the barb that retains the harness connector) in the pics of the aftermarket switch faces the wrong way or opposite what it does on the Ford switch. It is hard to see in my pic, but the barb faces "out" or "away" from the pedal pin on the Ford switch. On the aftermarket switch, the barb faces the pedal pin. Not to say that this switch would not work, but the barb would not fit into the correpsonding tab on the harness connector.

Sorry, guys - I took a pic of the barb difference, but it keeps crashing the thread when I insert the pic.

 

[LCAM-01XA]

About the BOO switch differences - just checked both the vacuum-boosted OBS and the F-Superduty OBS, and they both have identical BOO switches, the kind that has the bladed contacts facing the driver. Looks the same as a vacuum-boosted brick or bull too. The bracket you mention does make the hydro switch dang near impossible to remove, but it does not come in contact with it when pedal is released.

 

[tbrumm]

Maybe the vac switches and hydro switches were interchanged from time to time, but the switch that I got with the hydro pedal is pictured in my post above. The terminals are on the side of it, rather than on the back of it like the switch in my truck right now. Once I get the hydro pedal and bracket installed, I will see if the harness connector will still clear the underdash bracket, or if the hydro switch will be needed.

 

[LCAM-01XA]

What do you think?

What do I think? I think that it's been way too long since I've had to do FBDs and moment couples and such. As far as horizontal and vertical go, those are with respect to the firewall orientation. Axial is obviously along the axis of the pushrod, so forward and up. But the whole adding up to zero thing, yes it makes sense, and idk, I never claimed to be the brightest candle on the altar, matter of fact often it's barely lit, lol

Maybe the vac switches and hydro switches were interchanged from time to time, but the switch that I got with the hydro pedal is pictured in my post above. The terminals are on the side of it, rather than on the back of it like the switch in my truck right now. Once I get the hydro pedal and bracket installed, I will see if the harness connector will still clear the underdash bracket, or if the hydro switch will be needed.

Could be they changed designs at some point. Or it could be the hydro I'm looking at simply has the wrong switch on it... Try it and see how it fits, then act accordingly.

 

[kawamatt]

You must be registered for see images attach

Edit - I may have spoken too soon. The plastic tab (the barb that retains the harness connector) in the pics of the aftermarket switch faces the wrong way or opposite what it does on the Ford switch. It is hard to see in my pic, but the barb faces "out" or "away" from the pedal pin on the Ford switch. On the aftermarket switch, the barb faces the pedal pin. Not to say that this switch would not work, but the barb would not fit into the correpsonding tab on the harness connector.

Sorry, guys - I took a pic of the barb difference, but it keeps crashing the thread when I insert the pic.

Here is a post with both shown. You are exactly right that the locking tab is opposite on the aftermarket part. But look closer and notice that the pin polarity arraignment is also swapped. So looking at your picture the positive (+) pin is on the left and negative (G) on the right. Then the aftermarket part has the positive (+) pin on the right and negative pin (G) on the left. So, the aftermarket part will work. All's you have to do is twist the BOO plug around 180 degrees and plug it in! Make sense?

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[FORDF250HDXLT]

guys i just received my F-super 9th gen,auto trans brake pedal!
nearly 2 years after the hydro swap my right leg feels spoiled rotten!
drastically different.massive.massive.massive.thank you all very,very much! you've no idea how happy this makes me.
i always knew it wasn't right.just knew it.now she'll launch me right out onto the hood if im not careful.love it!

there isn't enough of these pedals out there to go around obviously,but you now know your pins need to be moved,either via experienced personal,or yourself if you've got the skills.be safe guys! make sure the weld is one you can trust your life with.

the numbers stamped on the 92-97 (9th gen) F-Super Duty (pre- F450) AUTO TRANS brake pedal are as follows:
F2TA-2455

holy crap this sites code is bugged.can't get all the pics,but no biggie.the best ones went.

 

[riotwarrior]

guys i just received my F-super 9th gen,auto trans brake pedal!
nearly 2 years after the hydro swap my right leg feels spoiled rotten!
drastically different.massive.massive.massive.thank you all very,very much! you've no idea how happy this makes me.
i always knew it wasn't right.just knew it.now she'll launch me right out onto the hood if im not careful.love it!

there isn't enough of these pedals out there to go around obviously,but you now know your pins need to be moved,either via experienced personal,or yourself if you've got the skills.be safe guys! make sure the weld is one you can trust your life with.

the numbers stamped on the 92-97 (9th gen) F-Super Duty (pre- F450) AUTO TRANS brake pedal are as follows:
F2TA-2455

holy crap this sites code is bugged.can't get all the pics,but no biggie.the best ones went.

Thanks for those images,

I still don't understand why people don't just upload to a photo site and link, regardless, those images make it easy for me to drill and move my pin the correct amount Thanks!

Setting the pin to 2" from pivot is simple. You can bet I'll be doing pics and a 101 on it!

Great shots, nice and clear and easy to figure out so THANKS AGAIN

Al