FordExcursions.com Forums
Michael Walter
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• Houston, TX, USA |
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• Registered on 4/15/2004 |
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• 5 posts |
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 Posted:11/10/2004 12:23 |
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Recently, I've been planning to replace my front springs on my 2000 PSD 4x4 EX with V-Codes and my rears with F-350 springs and blocks. According to my records, all other suspension components are equal. Axle ratings of 5200 (F) and 6830 (R) are the same for EX, F250 and F350 SRW, so I assume the same Dana 50/Ford Sterling combination resides in all three. The main difference between the EX and the F350 Crew 4x4 are the springs (and rear block).
Excursion 4x4 PSD:
Front Springs - 4700 lb
Rear Springs - 5250 lb
F350 4x4 PSD (156"):
Front Springs - 4000 lb
Rear Springs - 6830 lb
My plan was to install v-codes for the front (5200 lb) and F350 springs (6830) for the back with the 4" F350 blocks. The EX would sit up a bit more like the F-350, tow heavy trailers better, and lose some of its wallow in the process.
However, while I was digging through specs for Ford's vehicles, I came across the fact that Ford's Section Modulus varies on its frames. The F350, Styleside, 156" 4x4 has a 6.0 (cu in) sectional density, where the F250 has a 5.6 (cu in) sectional density and the Excursion has a 4.6 (cu in) sectional density. this tends to vary by chassis (Example - F250 Super Cab has 6.0 sectional modulus). The yield strength of all frames is 36,000 psi, but I think that just refers to the steel itself.
I'm not a physicist, but am I wrong to interpret that the Excursion frame's yield strength is significantly less than the F350? I would appreciate anyone's thoughts or knowledge on the subject. The link below contains very detailed specs on several Ford Vehicles. Just click on the appropriate year, followed by Table of contents, then Specifications. The F-250-550 section is 75 pages. Just find the exact chassis you are looking for.
https://www.fleet.ford.com/truckbbas/topics/techspec.html
If nothing else, hopefully the link will provide some useful data to the board members, if you don't already have it.
By the way, great forum.
Thanks. |
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E. Long
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• Subscriber since 1/1/2001 |
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• Atlanta, GA, USA |
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• Registered on 1/23/2001 |
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• 2,229 posts |
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Posted:11/10/2004 23:03 |
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Michael,
Welcome to the site.
Can you define the term "Section Modulus" and "sectional density?" I'm unfamiliar with this terminology.
Also, you listed an F350 Styleside. I wasn't aware one existed. When I hear the term Styleside, I think the style of the Ford lightning, with the step on the sides of the truck bed.
-Eric
'67 Galaxie 500 - 390 FE, .030" over, FE to AOD adapter, disc brake conversion. The Daily Driver.
'00 Excursion - 7.3L PSD, LANDYOT Gen-II Radius Rods, Factory Tech Valve Body, 200K+ miles and going |
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J-Douglas
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• San Antonio, TX, USA |
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• Registered on 3/18/2004 |
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• 89 posts |
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Posted:11/10/2004 23:48 |
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Very informative! Thanks. I would be more than happy to throw in my two cents, but I have never heard the term "sectional modules", and "sectional density". Define those terms so I can understand the situation.
5.5" Fabtech, 37x12.5x17 BFG AllTerrains, 17x9 Helo Maxx 8 Chrome. 6.0 Powerstroke Diesel. |
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rozett
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• Limington, ME, USA |
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• Registered on 1/10/2003 |
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• 163 posts |
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Posted:11/11/2004 08:06 |
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Section Modulus can be thought of as a "stiffness index". It is defined as the moment of inertia of the cross-section divided by the maximum distance from the center of bending (neutral axis) to the outermost edge of the cross-section. The higher the number, the stiffer the structural element.... in this case the truck frame.
//bruce
2000 Excursion Limited V10 4x4
ART brakes, Rancho RSX shocks, Hellwig rear sway bar, SD grille, V10 badges, Sylvania Silverstars, clear corners, and Bridgestone Dueler A/T REVO 285s. |
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Michael Walter
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• Houston, TX, USA |
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• Registered on 4/15/2004 |
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• 5 posts |
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Posted:11/11/2004 10:11 |
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Thanks for the input. I apologize for not getting back sooner.
I'll try to clarify some of my statements. I am not purporting to be an expert in this field. I just happened across the information and I'm truly trying to understand what Ford's specs are telling me.
1. I should not have used the term sectional density; only section modulus. That was an error on my part.
2. I believe Ford refers to it straight side (fleet-side) pickups as Styleside. I believe their term for the step-side style is Flareside.
Bruce's definition is consistent with my interpretation, which is one thing I wanted to confirm. I was having difficulty with the way Ford was expressing section modulus in cubic inches. I was trying to visualize the actual relationship between the section modulus number and frame strength in a tangible way. For example, is the relationship linear? Is the 6.0 Section Modulus F-350 frame actually 30% stronger than the 4.6 Section Modulus Excursion frame. The same goes for the 22% relationship with some F-250 frames.
My further layman's attempt at section modulus follows:
Two factors determine the section modulus of an object, whether it be a polymer I-beam or a steel channel frame - The Moment of Inertia and the Maximum distance from the neutral axis to outermost edge of the cross section. - This is exactly how Bruce defined the relationship.
To my understanding, Moment of Inertia (MOI) is essentally a measure of an obect's resistance to rotation. In other words, how easy/difficult is it to twist the given object. For visualization, picture two boxed frame sections with the same outside dimensions. One has thick walls and the other has very thin walls. The thin-walled frame will be easier to twist. This twist occurs around a neutral axis (ie visualize an invisible center point inside the frame section).
The other factor, the maximum distance is the distance from this invisible axis to the farthest point on the frame cross section.
Again, I might get slapped down by a physicist here, but my understanding is that section modulus defines the relationship between an object's resistance to twisting (MOI) and the distance to this farthest point. In other words, all other things being equal, a larger section (wide and flat, square and large, etc) will have a lower section modulus than a smaller section. Example: a 2" x 2" boxed section will have a higher section modulus (stiffness) than a 4"x4" channel with the same MOI (twist resistance). This seems to be a very complicated concept and not given to generalization as the MOI could change also as the size changes on any particular cross section.
I hope this helps somewhat. Please feel free to add or edit anything I've written. This is just my interpretation.
What I would still be interested in knowing is why would the Excursion, at nearly 8,000 lbs, fully optioned, have a 30% more flexible frame than an F-350 Crew Cab, assuming the linear relationship is applicable? Could this be attributed to the fact that the EX is essentially one large structurally sound box sitting atop the frame, where pickups have an independent cab and bed? That's my current theory - The Excursion, by virtue of having higher overall structural rigidity than a pickup, requires a less rigid frame than a pickup to achieve nearly the same gvw rating (9200 lb in 4x4).
I'm looking to confirm or test this theory.
Thanks for the input.
Michael |
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E. Long
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• Subscriber since 1/1/2001 |
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• Atlanta, GA, USA |
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• Registered on 1/23/2001 |
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• 2,229 posts |
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Posted:11/11/2004 11:09 |
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| Quote: | | What I would still be interested in knowing is why would the Excursion, at nearly 8,000 lbs, fully optioned, have a 30% more flexible frame than an F-350 Crew Cab, assuming the linear relationship is applicable? Could this be attributed to the fact that the EX is essentially one large structurally sound box sitting atop the frame, where pickups have an independent cab and bed? That's my current theory - The Excursion, by virtue of having higher overall structural rigidity than a pickup, requires a less rigid frame than a pickup to achieve nearly the same gvw rating (9200 lb in 4x4). |
This is the same path I was heading down with regards to frame flex. Having seen many pickup trucks and Broncos on the 4x4 trails, you'd be amazed at how much more a frame on a pickup truck flexes/twists compared to a Bronco. Given this observation, I would say that this would be the primary reason why the frame is less flexible on an Excursion seeing as how it's one large body sitting on frame rails. Whatever the acceptible tolerance is, I would imagine the Ford engineers designed the 4.6 rating on the Excursion to be nearly equal to, or equal to the 6.0 rating on the 156" wheelbase F350.
The Excursion is pretty unique in terms of its body panels too...ever notice how the side panels (except for the doors) are all one piece? I've had this pointed out by a couple body shops I've gone to for repairs on the rear quarter panels (when people have hit my X in the parking lot). They were always worried that if they couldn't cut out the rear quarter and weld a new one back in perfectly, that they'd have to replace the entire side panel on the X. It's one very, very large piece:
The entire rear body panel wraps up around the top onto the roof and extends to the front windshield. This, combined with the structural design of the X's body would do quite a bit to prevent frame twisting/flexing. So yeah, maybe this makes up for the 30% difference between the F350's rating?
Interesting subject. I look forward to what others have to say. I had always assumed the frames were the same on the Excursions and SuperDuty's (other than the length, of course).
-Eric
'67 Galaxie 500 - 390 FE, .030" over, FE to AOD adapter, disc brake conversion. The Daily Driver.
'00 Excursion - 7.3L PSD, LANDYOT Gen-II Radius Rods, Factory Tech Valve Body, 200K+ miles and going |
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J-Douglas
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• San Antonio, TX, USA |
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• Registered on 3/18/2004 |
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• 89 posts |
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Posted:11/11/2004 14:55 |
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To start, there was a mix-up in responses: Michael (and the data) suggests that Excursions are 30% MORE flexible (or, similarly, 30% LESS rigid). E Long wrote that Excursions are 30% MORE rigid. What's odd, though, is that common sense disagrees with the data and I completely agree with E Long in that the Excursion would be 30% MORE rigid. Here's my two cents:
1. From everything I have gathered, the frame of the Excursion is identical to the Short Bed, Short Wheel Base (SWB), extended cab F-250. Thus refuting any differences in the frame between the two vehicles.
2. The frames of both vehicles in question are not fully enclosed (boxed). They are open channel (in the shape of a squared “C”). Any person with a lick of common sense would agree that a _-closed frame does not have near the same structural integrity as a fully boxed frame. So, why would Ford use this weaker design? COST. It saves quite a bit of money to produce millions of open rail ladder frames than a fully enclosed box frame. I mean, we're talking about millions of vehicles. So, with this assumption, we can also assume that Ford would not produce separate frames for two vehicles with identical platforms. That would cost even more money! The Excursion and the Superduty are manufactured in the same plant, thus proving that Ford's cost-cutting method of using similar parts is clear with the Superduty and Excursion. Most assumptions and knowledge about vehicle manufacturing point to the Superduty and the Excursion having the same components.
3. Were these sectional module tests performed by Ford? I say this because as a graduate student, I have found that the internet (not unlike academia) is steeped in erroneous data. Most often, however, this false data is not established to be intentionally misleading inasmuch as simply inaccurate. In these cases, the situations rarely accurately duplicate the given parameters. For example, maybe the test frame of the Excursion was a different age than the Superduty frame. Or, the climate was different at the time of the tests which allowed the steel to contract/expand. It comes down to being aware of the exact environment and circumstances under which the given test was performed. And, perhaps, this test cannot necessarily be indicative of the vehicles as a whole.
4. If the test parameters were performed by Ford, and were identical, however, one must acknowledge the inconsistent characteristics of steel. Molecularly, steel is varying in its nature, and one steel object can rarely be considered identical to another… even if they appear identical. That said, it could be assumed that even though two frames appear identical, they have the potential of being dramatically different in molecular soundness. This COULD be one small reason for inconsistencies in any ONE given stiffness test.
5. I completely agree with E Long when he says that the Excursion is stiffer due to the Excursion's nature of being one, large, “boxed” body atop a frame. The Excursion body simply does not ALLOW the frame to flex as freely. My best friend's F-250 (identical chassis to my Excursion), flexes like a rubber band. The gap between the cab and bed is always under constant flux! In contrast, my Excusion's body just does not allow that type of flex! THIS IS WHY Excursion owners have repeated complaints of failing body mount bushings! (You know, that “clunk” under the driver's feet---it's ALL over the message boards). The body mount bushings are taking the intense pressure of absorbing the flex that the frame is giving, but the body remains stiff. This flex has to be compromised somewhere… and it is all in the weak body mount bushings.
All this said, my opinion comes down to one simple precept: The frames are the same, and the data is incorrect. The frames are similar but the nature of the bodies does not allow similar flex.
5.5" Fabtech, 37x12.5x17 BFG AllTerrains, 17x9 Helo Maxx 8 Chrome. 6.0 Powerstroke Diesel. |
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Michael Walter
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• Houston, TX, USA |
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• Registered on 4/15/2004 |
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• 5 posts |
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Posted:11/11/2004 16:31 |
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Yes, there was a slight mixup in responses. No big deal. It was easy to flip the facts. We're all basically on the same page, though. I don't think it's a matter of agreement/disagreement.
The box frame was simply used as an example; not as a statement of the Excursion's frame.
My theory remains the same. Though the excursion is basically built on the Superduty chassis, its frame may be more flexible. E. Long's statements to the structural integrity of the Excursion back up my contention. The frame is allowed to be more flexible in the excursion (have a lower section modulus) than the F-Series truck because the stiff body "takes up the slack," so to speak.
I don't think this differs from the common sense opinion of J-Douglas or E. Long. If Ford realized that they could swap a flexier, cheaper to build frame under the EX and lose no rigidity due to the design of the Excursion body, it is possible they would do just that.
Testing - This data was retreived from the Ford Fleet website. This is not a "consumer relations" style specification list. It contains pages of detailed specifications. Of consequence to J-Douglas - These are not test results. They are design parameters. Design yield strength is stated, and design section modulus is stated. Furthermore, if you explore the 75 pages of specs for the various models of 2001 F-250/350/450/550, you will see that section modulus varies by in several vehicles....not as a result of the normal distribution of test results; as design changes for various models. Just read the specs in the link above if you get a chance. For example, Supercab F-250's have 6.0, but some of the Crew Cabs have 5.6.
Maybe you can make sense of it. Personally, this isn't what I set out to prove when I was digging up this info. I was out to prove that I could basically turn me EX into an "F-350+" machine by removing all elements weaker than the F-350, mostly spring sets and the rear block.
I've had the full Excursion spec for a while now. I was really just looking to CONFIRM that the EX and the F-350 had the same frame as part of my pre-modification homework. What I found wasn't what I wanted to see, but I think that with the added rigidity of the solid body, the EX is the most structurally durable machine Ford makes, with or without the F-350 frame under it.
Michael
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J-Douglas
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• San Antonio, TX, USA |
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• Registered on 3/18/2004 |
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• 89 posts |
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Posted:11/11/2004 16:57 |
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Michael,
I applaud your efforts because it got me really thinking about my vehicle. I also encourage any modifications to turn your Ex into an F-350+. That would be great! I would love to see your results.
I agree that the Excursion body "takes up the slack" for any frame weaknesses versus the F-350. However, I am curious, though... after all this dialog and speculation, do you know exactly what the weaker components are in the Ex? What components make the Ex weaker than the F-350?
5.5" Fabtech, 37x12.5x17 BFG AllTerrains, 17x9 Helo Maxx 8 Chrome. 6.0 Powerstroke Diesel. |
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E. Long
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• Subscriber since 1/1/2001 |
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• Atlanta, GA, USA |
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• Registered on 1/23/2001 |
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• 2,229 posts |
• 1
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Posted:11/12/2004 07:32 |
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| Quote: | Michael,
I applaud your efforts because it got me really thinking about my vehicle. I also encourage any modifications to turn your Ex into an F-350+. That would be great! I would love to see your results.
I agree that the Excursion body "takes up the slack" for any frame weaknesses versus the F-350. However, I am curious, though... after all this dialog and speculation, do you know exactly what the weaker components are in the Ex? What components make the Ex weaker than the F-350?
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Yes, the point I was trying to make is exactly as you stated above -- the Excursion's frame is more flexible but the body takes up the slack and thus meets or exceeds standards when compared in a side-by-side test to an F250/350 with a "stronger," less-flexible frame.
I too am curious, what should one look for and are the changes obvious on the frame that make the X frame more flexible? Sounds like we need to duck under an X and an F350 and snap some pics, but I wouldn't know which areas to focus on. A friend has a regular cab F350 with the same wheelbase as my X, so that would probably be the best "apples to apples" comparison I could do -- and I could get pics taken, but any clue as to which areas to focus on would help. There's a lot of nooks and crannies underneath these rigs. 
-Eric
'67 Galaxie 500 - 390 FE, .030" over, FE to AOD adapter, disc brake conversion. The Daily Driver.
'00 Excursion - 7.3L PSD, LANDYOT Gen-II Radius Rods, Factory Tech Valve Body, 200K+ miles and going |
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