Issues With Big Blue!?!?!

[Gary Lewis]

WARNING - BORING GEEK STUFF AHEAD!

Guys - I'm getting lost in the physics....

I started to figure it out more completely and got lost too, so I had to go back to what my Statics TA back in college called "a fwee-ah ah-body ah-diagwam-ah" (or "free body diagram" for those of us to whom English is not our fourth language. And yes, I get that his English was MUCH better than my Chinese, so I'm really not putting him down).

What we're talking about here is how stiff the springs are. Spring stiffness is measured as the compression rate, how much force it takes to deflect the spring a certain amount. But springs are linear, so it's also how much MORE force it takes to deflect the springs a certain amount MORE.

Let's start with the F-350 suspension because it's so simple we don't need a free body diagram. Let's start with the assumptions that the front of the truck weighs 3000 lbs and we want a spring stiffness of 500 lbs/in (meaning it takes 500 lbs to compress the spring 1 inch). I just pulled that number out of the air, but it'll work for a comparison.

With 3000 lbs on the front of an F-350, that's 1500 lbs on each of the front tires, and 1500 lbs supported by each of the front springs.

Now if the springs have a rate of 500 lb/in, that would mean that if we put an additional 1000 lbs on the front of the truck we would be putting 500 lbs more on each spring and it will deflect 1" below wherever it is at ride height. And since this is a simple solid axle suspension, 1" more compression on each spring means each tire moves up (relative to the truck) the same 1".

Now lets take the F-250 front suspension and figure out what the actual spring stiffness needs to be to give an apparent stiffness of 500 lb/in. By that I mean when we put an additional 1000 lbs on the front of the truck we want each front tire to move up 1", the same as we saw in the F-350.

This is where a free body diagram becomes necessary for me. A free body diagram is a rough sketch of a "solid" piece (it doesn't have to actually be solid, or one piece, it just needs to not move with respect to itself). You show what forces are applied in what locations. Newtons law tells us that unless the piece is accelerating, the sum of all of the forces on it must be zero, and the sum of all of the moments on it must also be zero (in physics a "moment" is the torque exerted on an object by applying a force on a lever arm).

Below is the free body diagram for one tire / axle beam assembly of an F-250 front suspension. Fp is the vertical force applied to the swing arm at the pivot. Fs is the force applied to the spring perch by the spring (it's also the force on the spring). And Ft is the force applied to the tire by the ground (or the amount of weight sitting on the front tire). The measurements of 26" from pivot to spring perch and 41" from the pivot to the contact patch are approximate measurements off my '97 F-250. And they are measured parallel to the ground, not in a straight line from the pivot to the contact patch or spring perch.

Since the forces in the vertical direction need to add up to zero, the diagram tells us that Fs = Ft + Fp. Or solving for Fp (we might want to later...), Fp = Fs - Ft.

And the sum of the moments must also equal zero. Moments need to be defined around a certain point. The physics don't care what point we use, but the math will be easier if we pick the "right" point. So we'll use the pivot point. The moment the spring exerts on the body around the pivot point is Fs x 26". The moment the ground exerts on it around the pivot point is Ft x 41. So that means that Fs x 26 = Ft x 41. Or solving for Fs, Fs = Ft x 41 / 26

Let's take the same 3000 lb front end, but we'll only look at one side. The weight on the tire is half the truck weight, so still 1500 lbs, just like the F-350. But now when we do the math, the force on the spring is Ft x 41 / 26, or 1,500 lbs x 41 / 26, or 2,365 lbs! (for what it's worth, Fp = Fs - Ft = 865 lbs).

Now let's put an additional 1,000 lbs on the front of the truck to move the tire up 1", the same as we did for the F-350. Now the new Fs = 2,000 lbs x 41 / 26 = 3,154 lbs. So we've put an additional 3,154 - 2,365 = 788 lbs on the spring.

But how much has it deflected? Since the tire at a 41" radius went up 1", the spring at a 26" radius went up 26 x 1/41 = 0.63".

Spring rate is (additional) force per (additional) deflection, so 788 lbs / 0.63" = 1,243 lbs/in. Just a little stiffer than the 500 lb/in F-350 spring!

This result was so much more than I expected that I didn't believe it. I've checked my equations and my math a couple of times though, so I'm pretty sure it's right. But if anyone wants to prove me wrong I won't be at all offended.

Edit to add: A few posts later I described how the F-250 spring stiffness likely increases significantly as the TTB geometry pulls it sideways. My calculations above are for this overall spring rate. If you put an F-250 spring on a solid axle it would not get reefed on so badly, so the overall spring rate of the same spring in that case would be lower. I'm still sure that the F-250 spring is significantly stiffer than an F-350 spring, even if it were on a solid axle. But likely it isn't 2.5 times stiffer if the pulling sideways isn't taken into account.

I'd like to say that I understood every word of that. But, the truth is that I understood the principles and didn't get hung up on the words. And, while my Statics is slightly less "rusty" than Bill's, it is nonetheless about 50 years old. (Having said that, I ran statics and dynamics programs for Conoco a few years after that, so let's just say it is just very, very old.)

In any event, it looks like the F250 spring rates from this calculation are 2.5 times higher than those of an F350. (2.486 to be exact.) So, while the un-sprung weight of a D60 is higher than that of a D44HD, the fact that you get 2.5x the movement would make a huge difference in the ride as well as the articulation when on a trail.

I'm sold. And, I'm feeling like it is time to make a deal for the axle in OKC and take the trailer down and get it. Then order the stuff off ebay that David found, and shop 'round for SD springs and then order in the Sky kit.

But first I want to talk to Chris at Boomer's. Maybe tomorrow.

[FoxFord33]

..."a fwee-ah ah-body ah-diagwam-ah"...

I was in middle/high school while my dad was going to OSU for engineering. I asked him what his homework was, and he replied, "Diffahwenchah Equazioahhh." I thought he was being silly, and in a way he was. His Differential Equations prof was from an undisclosed asian country. He just about failed the class because he made the mistake of trying to listen to the prof instead of looking at the math.

[Gary Lewis]

..."a fwee-ah ah-body ah-diagwam-ah"...

I was in middle/high school while my dad was going to OSU for engineering. I asked him what his homework was, and he replied, "Diffahwenchah Equazioahhh." I thought he was being silly, and in a way he was. His Differential Equations prof was from an undisclosed asian country. He just about failed the class because he made the mistake of trying to listen to the prof instead of looking at the math.

I wonder how many of those profs there were back then? I had one at Kansas State that wore a turban. I'd forgotten all about that until this conversation, but I believe he taught Calc, not Diff E.

While I'd like to blame my poor grade in that class on his English, the truth is that I just don't care for the theory all that much. That came to light in my Physics classes. We had two teachers - one that drove a new Vette but didn't know how to work on it, and one that was a true hands-on guy. I got along great with the latter and not so much with the former. The difference in the class room was that the hands-on guy would explain how to apply the theory in every-day terms, and that clicked with me. But the other one never associated the learning to actuality.

[ArdWrknTrk]

I wonder how many of those profs there were back then? I had one at Kansas State that wore a turban. I'd forgotten all about that until this conversation, but I believe he taught Calc, not Diff E.

While I'd like to blame my poor grade in that class on his English, the truth is that I just don't care for the theory all that much. That came to light in my Physics classes. We had two teachers - one that drove a new Vette but didn't know how to work on it, and one that was a true hands-on guy. I got along great with the latter and not so much with the former. The difference in the class room was that the hands-on guy would explain how to apply the theory in every-day terms, and that clicked with me. But the other one never associated the learning to actuality.

IDK

I'm a simple man who never got to college.

Because of my differences I did well at geometry and trig, but not so well in calculus.

If I want to figure out something like this graphically I'll draw it out and factor the difference proportionally.

This (seems to me) a simple enough question that I won't get caught out and I can leave things like friction and the sideways bending moment on the spring alone.

Also, just at a glance I'd say the F-350 stack is far more progressive than the F-250's leafs.

But I'm interested, and always like to learn, so please continue.

[85lebaront2]

I wonder how many of those profs there were back then? I had one at Kansas State that wore a turban. I'd forgotten all about that until this conversation, but I believe he taught Calc, not Diff E.

While I'd like to blame my poor grade in that class on his English, the truth is that I just don't care for the theory all that much. That came to light in my Physics classes. We had two teachers - one that drove a new Vette but didn't know how to work on it, and one that was a true hands-on guy. I got along great with the latter and not so much with the former. The difference in the class room was that the hands-on guy would explain how to apply the theory in every-day terms, and that clicked with me. But the other one never associated the learning to actuality.

I had a computer programming professor at, then, Old Dominion College, now Old Dominion University. Oscar Garcia, don't know where he was from, but his favorite line was "you do that you get a teelt". He taught us Fortran to program the school's IBM 1620 which was state of the art in 1964.

I ended up washing out as ODC was spun off from William and Mary in 1963 and was still very much a Liberal Arts school at the time. The department of engineering was new for 1964 and they had hired the retired dean of engineering from NC State to establish it. Dean Lampe was working hard to eliminate a lot of the liberal arts influence and build up the technical side.

ODU now, along with Virginia Tech partneres with my former employer, Newport News Shipbuilding, for continuing education for Apprentice School graduates and higher degrees (Masters, PHD) for engineers.

What sank me was English Composition and Literature 101, 13 lengthy themes required along with lots of required reading. Engineering majors were "allowed" to carry more credit hours due to the technical classes, such as engineering graphics, calculus, chemistry, computer programming along with the electives such as PE (I elected swimming and earned my Red Cross Water Safety Instructors certificate).

[Gary Lewis]

I had a computer programming professor at, then, Old Dominion College, now Old Dominion University. Oscar Garcia, don't know where he was from, but his favorite line was "you do that you get a teelt". He taught us Fortran to program the school's IBM 1620 which was state of the art in 1964.

I ended up washing out as ODC was spun off from William and Mary in 1963 and was still very much a Liberal Arts school at the time. The department of engineering was new for 1964 and they had hired the retired dean of engineering from NC State to establish it. Dean Lampe was working hard to eliminate a lot of the liberal arts influence and build up the technical side.

ODU now, along with Virginia Tech partneres with my former employer, Newport News Shipbuilding, for continuing education for Apprentice School graduates and higher degrees (Masters, PHD) for engineers.

What sank me was English Composition and Literature 101, 13 lengthy themes required along with lots of required reading. Engineering majors were "allowed" to carry more credit hours due to the technical classes, such as engineering graphics, calculus, chemistry, computer programming along with the electives such as PE (I elected swimming and earned my Red Cross Water Safety Instructors certificate).

I think "washing out" is how I'd like to portray my demise at Kansas State - where I'd also learned Fortran on a 1620 in '65. But, I suspect that "lack of study" would be the stated cause, not English Composition, for me.

But, that doesn't mean I'm good at the "liberal arts" stuff. I had a Western Lit class where the final was to be an essay on an as-yet un-named book which we'd studied. So I wrote essays on three of the probable choices and fine-tuned them with a friend for days before the final. Sure enough, two of the books I picked were options so I chose my best essay and wrote it from memory. And I got a C. My very best work, written and re-written over several days, was barely average.

"Engineering Graphics" - My time in that class was spent at a true drawing board with scales, protractors, mechanical pencils, and lots of erasers. But the better term for the class was "mechanical drawing".

Swimming? In '65 they lined us freshmen up by the pool in our gym clothes and asked us if we could swim. If you said "yes" they pushed you into the pool to prove it. I was stunned that my best friend, standing beside me, quickly said "no". I thought everyone knew how to swim as I'd grown up doing it. Anyway, I went into Judo instead.

[Gary Lewis]

IDK

I'm a simple man who never got to college.

Because of my differences I did well at geometry and trig, but not so well in calculus.

If I want to figure out something like this graphically I'll draw it out and factor the difference proportionally.

This (seems to me) a simple enough question that I won't get caught out and I can leave things like friction and the sideways bending moment on the spring alone.

Also, just at a glance I'd say the F-350 stack is far more progressive than the F-250's leafs.

But I'm interested, and always like to learn, so please continue.

Jim - Good point. Progressive springs is an aspect of the discussion that we've not covered, and which makes it more complicated. From what I remember of the 250 springs there are two full-length leaves. Do the 350 springs, and presumably the SD springs, have different length leaves?

I'm going to guess that the 250's lack of allowable travel forced Ford to use a single-rate spring. It had to be STIFF to ensure it doesn't hit the stop all the time. But with more allowable travel the 350 springs could be softer initially and then stiffen up as you near the stop.

How much distance is there between the spring and the stop on a 350? Or, how much travel is there? How 'bout the SD's? I'm just curious how big of a difference there is.

[85lebaront2]

I think "washing out" is how I'd like to portray my demise at Kansas State - where I'd also learned Fortran on a 1620 in '65. But, I suspect that "lack of study" would be the stated cause, not English Composition, for me.

But, that doesn't mean I'm good at the "liberal arts" stuff. I had a Western Lit class where the final was to be an essay on an as-yet un-named book which we'd studied. So I wrote essays on three of the probable choices and fine-tuned them with a friend for days before the final. Sure enough, two of the books I picked were options so I chose my best essay and wrote it from memory. And I got a C. My very best work, written and re-written over several days, was barely average.

"Engineering Graphics" - My time in that class was spent at a true drawing board with scales, protractors, mechanical pencils, and lots of erasers. But the better term for the class was "mechanical drawing".

Swimming? In '65 they lined us freshmen up by the pool in our gym clothes and asked us if we could swim. If you said "yes" they pushed you into the pool to prove it. I was stunned that my best friend, standing beside me, quickly said "no". I thought everyone knew how to swim as I'd grown up doing it. Anyway, I went into Judo instead.

Gary, same thing on the engineering graphics, I still have the T-square and can still draw out a design plan. The reason I selected the WSI class, I already had Red Cross junior lifesaving (got it when I was 13), BSA Lifesaving Merit Badge, BSA Lifeguard, and BSA mile swim. The instructor was a Red Cross Chapter representative, and he was able to waive the Red Cross Senior Lifesaving as the BSA qualifications were equivalent.

[ArdWrknTrk]

Jim, how much distance is there between the spring and the stop on a 350? Or, how much travel is there? How 'bout the SD's? I'm just curious how big of a difference there is.

I'm not any expert and Shaun or Jonathan would have better answers. (so a lot of conjecture here)

I think you mean the distance between the axle housing and the snubber on the frame rail?

Well, the D60 is smaller in diameter than the TTB box and you would be measuring to axle centerline.

So half of the difference in chord.

Then you have the arch of the F-350 spring when loaded vs the negative arch of the 250 spring.

On top of this you will have the shackle in the rear pointing down vs the spring eye centered in that perch.

You can see some of that on SORD'S instruction page.

I don't know where their front perch places the front spring eye in relation to the stock Ford shackle.

I assume their page is correct in saying that you will see a 2" lift over stock 350 springs, so there is that.

IDK how much more arch SD springs have (if any) over 350 springs, or what their sack height is since they are said to be "softer"

The axle obviously is not centered on the SD spring if the same rear mount was used and the bumper needs to be cut for the front.

The SD spring is 4" longer??

But if the LoPro mounts 2" rear of stock the axle would be centered.

I just don't know these details of the SROD SD kit because I never looked into it.

 

[Nothing Special]

Jim - Good point. Progressive springs is an aspect of the discussion that we've not covered, and which makes it more complicated. From what I remember of the 250 springs there are two full-length leaves. Do the 350 springs, and presumably the SD springs, have different length leaves?

I'm going to guess that the 250's lack of allowable travel forced Ford to use a single-rate spring. It had to be STIFF to ensure it doesn't hit the stop all the time. But with more allowable travel the 350 springs could be softer initially and then stiffen up as you near the stop....

Whether the F-250 spring rate is linear or progressive isn't easy to see at first glance. By the look of it it would seem to be not nearly as progressive as the F-350 spring. But keep in mind the strange contortions the TTB geometry make it go through. As it moves it not only flexes like a normal leaf spring, but it also gets pulled sideways, a direction in which it is VERY stiff. I don't have any equations to support this (thankfully!!!), but I think the effective spring rate has to get a lot higher as it gets pulled around. (Which probably explains why the stiffness I calculated seemed too high. I was thinking about the stiffness in the normal bending direction which is likely lower than I calculated. It's the actual stiffness that I calculated, and as I just hand-waved here, that must be significantly higher than the "normal" stiffness. So using an F-250 spring on a solid axle probably isn't as completely stupid as my first calculation indicated. But it's still likely significantly stiffer than an F-350 spring, just not 2.5 times stiffer.)