Frame Stiffing to reduce Porpoising?

[adamis]

Got a question for you seasoned drivers with older rigs. I have a 1999 F350 Dually Quad Cab Long Bed. I've been driving it for thousands of miles and for the most part, it's been fine. However, while driving on rougher roads, I do get a porpoising effect happening. I can look in my rear view mirror and see the frame flexing at least an inch or two.

My camper is the Bigfoot 2500 and with everything loaded up is probably in the 4500lb to 5000lb range. According to the sticker on the camper the center of gravity is almost exactly over the rear wheels. Now the center of gravity does change with water in the tank as the fresh tank is in the front of the camper which puts it at the front of the bed.

The porpoising isn't a "problem" in the since that I am concerned about it causing damage. I'm well within the carrying capacity of the truck. That being said, it is an annoyance for comfort and drivability. I have already replaced the shocks on the truck and that has helped somewhat. I am also looking and seeing that I probably could move the camper another inch or two forward when I load it. I don't think this will make much of a difference though.

So, my next thought was to look at having my frame reinforced. I know there are all sorts of complications when going this route though. Ford does fully boxed frames for a reason because it gives so much more rigidity. I no there is at least one person that has done this on their 7.3 from a posting on the Ford forums. This is obviously a huge expense and extreme measure.

Has anyone else gone through this or found a way to handle excessive porpoising on older C channel frame trucks?

-------------------------Update-------------------------------
Because several people have focused in on the camper weight and that has taken this question off from what I asked, I am adding this here for anyone new reading this thread that thinks to jump in and comment about weight. Later in this discussion thread I post a response where I provide the factory weight as measured was 2733lbs and with AC and Genset just under 3000lbs. That is way lower than the 4500lb to 5000lb I stated in my original question above. I have also weighed my truck and it is 7600lbs and so the payload capacity is 3600lbs and the axle capacity of 4870lbs. I am not grossly overweight as some thought based off my original and inaccurate guesstimate in my question above.

Secondly, I also overstated the amount of "porpoising". It is not excessive in that I am concerned about the truck breaking in half. What I have observed others mainly in the overhead movement of the camper above the driver seems to be common with others.

Finally, my question in this thread is asking if someone has ever added additional bracing to the frame of the truck. I'm asking the question if this has been done before. The consensus in this thread is that it is very rare and it is very complicated to do and probably not worth the effort.

[time2roll]

I am curious if the weight rating is primarily given for a load low in the bed with less front to rear rotational mass. I mean to say a load 4000# of gravel vs 4000# gooseneck vs 4000# truck camper are very different on the dynamic frame stress even if the static downward pressure is the same.

[BigToe]

Bedlam wrote:
For frame strengthening, I was thinking along the lines of L-stock clamped with U-bolts to the rails. This would prevent the original frame from being drilled or welded. It would be the automotive version of a medical splint or brace. However, I still am not convinced that this is needed.

Not only are frame reinforcements not needed, they can cause more harm than good, by creating sudden transitions in stiffness between the reinforced and un-reinforced portions of the frame.

It is at these transitions where the frame suddenly encounters additional resistance to flexing due to the reinforcement, so the stress is then concentrated in the areas of the frame just prior to encountering the reinforcement.

The engineering of frame reinforcements to minimize transitional stress concentrations is an art and science unto itself... sufficient to challenge even the most competent repair facility, as they lack the means to mathematically model and materially test the outcomes of frame reinforcement schemes that are further hampered by the packaging constraints dictated by retrofitment to an existing frame.

Earlier I mentioned that for this same model year (1999), Ford produced L channel frame reinforcements for the Chassis Cab frames. Yet even these reinforcements are not retrofittable after the fact of production, even to identical chassis cabs built without the reinforcements.

To achieve a smoother transition between un-reinforced and reinforced sections of the frame, Ford ran the horizontal flanges of the L channel longer than the webs, and Ford tied the attachment of the flanges to the attachment of the crossmember that ties the forward rear leaf spring hangars together laterally across the frame.

The web of the reinforcement is then tapered forward, and then tapered and flanged out again on the forward end, intersecting with inverted reinforcement under the back of cab wall at a forward rake angle.

The OP talked about adding several hundred more pounds of weight in the form of a front drive axle, new transmission, and transfer case, in order to convert his truck to 4WD. But he then added the twist that he also wants the coil sprung version that Ford offered for 2005 and up, even though his frame is a 1999 that does not have the boxing behind the engine cross member that will keep the frame from cracking, as Ford found when testing the coil spring / long radius arm 4WD suspension when it was being developed in the early aughts.

Even that frame reinforcement is designed to minimize sudden stress risers. The web of the boxing is fish mouthed, and the attachment to the crossmember is tabbed. The reach back of the flanges stretches toward the existing service frame splice, spreading the strain throughout the frame, rather than attempting to shore up just the area of demonstrated frame failure from the design change in front suspension.

U Bolts not only concentrates stress/strain at attachment points, they also crush the flanges of the frame when torqued properly. Ford strongly recommends that blocks be placed between the upper and lower flanges at every U bolt.

The most logical course of action is to determine how much actual weight (by weighing the wet and loaded camper and truck on a physical scale) is being borne by the existing truck. The data derived from this physical weighing can then be entered into the mental weighing of options to solve the porpoising, as well as options to cowboy the truck into doing something more than it was originally designed too do.

[Bedlam]

For frame strengthening, I was thinking along the lines of L-stock clamped with U-bolts to the rails. This would prevent the original frame from being drilled or welded. It would be the automotive version of a medical splint or brace. However, I still am not convinced that this is needed.

[BigToe]

At issue is that it is impossible to add rigidity to the existing frame design without also adding weight.

Ford has reinforced Super Duty chassis cab frames using L reinforcements, which are split, angled along the web, and inversed at the cab to bed junction.

Let's pretend that the pickup frame is also conducive in shape to add similar frame reinforcements, and roughly estimate, using a reasonably derived example to demonstrate the added weight of material that frame stiffening reinforcement might impose.

And by the way, all of the reasonably derived estimates in my prior post were clearly identified as "EXAMPLE" assumptions, to again, demonstrate a process of estimation where the actual facts are not known.

Thanks to your clarification of the Federal Certification Label GVWR (11,200 49 state instead of 11,000 CA) and CAT Scale ticket when the truck was empty (caveat: if weight was for registering the TX truck in CA for purposes of determining fees, then most folks make every effort to make the truck as light as possible for "empty" weighings... ie, little fuel, no tools behind seat, etc), we still do not know what your actual camper weight is, beyond the dry weight of your BigFoot 25C10.6B of 2,734 lbs at manufacture.

Does that weight include the jack system? It does not appear to include the rooftop air conditioner shown in your photo, as the BigFoot label states N/A for both the air conditioner and the generator you said you added. The collective experience of truck camper owners culminates into the common understanding that there can be a signficant gap between the as manufactured weights of a camper, versus the as useable in the real world weight of the camper.

I think those of us focusing on the rig combination as typically deployed in your actual usage in the real world first, are focused on the porpoising you reported, which you stated was the principle problem you perceived and were trying to solve. One solution you proposed to solve that problem involved a question regarding the use of frame reinforcements. We are looking past that proposed solution first, in order to assess what you surmised was the root problem you were trying to solve with the frame reinforcement proposal.

But that proposal adds weight. To estimate that weight, we can use a 3" x 7" rectangular tube of 0.250" steel material thickness, which in mild A36 steel weights about 15.62 lbs per foot, and assuming we can split this tubing twice longitudinally into two L shaped reinforcements that we later split again cross-sectionally in order to invert the L at the cab transition point to retain some frame flexibilty while optimizing the placement of horizontal flange of the L to mitigate where the original frame flange is under the greatest tensile stress on the bottom flange, and the greatest compressive stress on the top flange, we might need to buy about 5 feet of this material to cut up, and hydraulically contour into a conforming shape. So that is almost 80 lbs of added material.

Because it might be best to avoid welding on the frame, we would need some hardware to mount the frame reinforcements. Whether we chose Grade 8 bolts and hardened flat washers, or Class 10.9 flange bolts and prevailing torque flange nuts, or better yet, huck bolts that are less likely to loosen over time... all that hardware will add some additional weight, say another 10 lbs. It all adds up.


As an example, from Truck Camper Magazine:
"Bigfoot 25C10.6: dry weight, 3,180 pounds + 50 gallons fresh, 417 pounds + 6 gallon water heater, 50 pounds + 2x 20-pound full propane tanks, 40 pounds + 2 batteries, 130 pounds + stuff, 500 pounds = 4,317 pounds"

Your 2000 model year 25C10.6B may start off with a lower dry weight, but the AC unit, the generator, the camper jacks, perhaps a solar panel, etc, pile the weight right back on again. In the TCMAG reported example, their actual BigFoot camper wet weight exceeded the dry weight rating on the label by 1,137 lbs, which is a 36% increase over the as manufactured dry weight on the label.

Without even considering the weights of your added generator, AC unit, and camper jack system, and instead just simply adding 36% of your dry weight to your dry weight to come up with a rough estimate of your wet weight with your pots and pans, yields a result of 3,718 lbs, which is consistent with what you are already sure of... that you are running over the 3,600 lbs capacity that you have determined from the difference between your GVWR and empty scale weight.

Now add passengers x 150 lbs each, and a full tank of diesel at an estimated 277 lbs (38 gallons x 7.3 lbs per gallon)

Along with 90 lbs of frame reinforcement, added weight that could be the final straw that might exacerbate, say, a driveshaft U joint failure from having already been pushed to a bit over maximum capacity.

You have an ideal camper, and a nearly ideal truck to haul it with. You have reported what you perceive to be a porpoising problem, and have proposed a solution. Obtaining the actual facts of the circumstances helps provide proper perspective on the problem, and may lead to a different solution than what was previously proposed.

To be solution oriented, it is strongly suggested to have your camper actually weighed as currently equipped with how you travel with it.

[valhalla360]

adamis wrote:
I don't know how this is turning into an argument about weight but since that seems to be the focal point of this thread...

..........

So you have a payload available of 3600lb (maybe less for the truck camper specific payload rating), far less than you suggested.

Your camper is 3000lb...supposedly based on the sticker but truck campers are known for being way over weight. In a best case scenario you have 600lb for gear and people. It's really easy to eat up 600lb.

As suggested before, best to get the weight fully loaded as if you are headed out camping.

[notsobigjoe]

adamis wrote:
I don't know how this is turning into an argument about weight but since that seems to be the focal point of this thread...

The first weight I provided of between 4500lbs to 5000lbs which was a guesstimate accounting for camper dry weight, water, food and gear. I was basing that estimate on what I believed the camper dry weight to be 4000lbs and everything else depending on how I loaded it another 500lbs to 1000lbs. It turns out, the actual camper weight to be 1240.06 kilograms or 2733.8lbs when it went out the factory door.



Now, that doesn't include the Onan 2500 genset added later which weighs 113lbs or the AC unit also later added which is ~130lbs give or take a few so dry weight of my camper is 2976lbs. I guess you can add in the LiFePo4 battery and round it up to an even 3000lbs.

As far as the truck is concerned, BigToe, you clearly put a lot of effort into your analysis. No, my truck is not a California truck, it actually came from Texas. It passes smog here in California and that is all I care about. As far as your breakdown of weight analysis, you are quite knowledgeable or have a lot of time, maybe both but in spite of your best efforts, you were off by quite a bit. I can't believe I am actually doing this as I had to dig it out but for the sake of putting this argument to bed...



According to the scales, my truck weight is 7600lbs. By the GVWR rating of 11200lbs on the tag of the door, that leaves me with 3600lbs of capacity, not the 2961 you estimated. The axle rating on the truck is 8250lbs for the rear. By the CAT weight scales, I only have 3380lbs on the rear. That leaves me with 4870 lbs of rear axle weight capacity.

So now that we have the truck weight and the camper weight, we now know that dry, I am 600lbs below the GVWR of the truck. Sure, add in propane, water, food, pots, pans and all the other stuff and I'm sure I'm running over that 3600lbs but it isn't by a whole lot, certainly not overweight by the 1000lb to 2000lb BigToe, GritDog and valhalla360 seem to think. With the axle capacity of 4870lbs, I am confident I am well below that by several hundred pounds or more.

valhalla360 wrote:
Excellent breakdown of the subject but I'm betting the OP just keeps insisting that it's OK because he wants confirmation, not an actual answer because it's most likely not the answer he will ike.

It might be an excellent breakdown but it was wrong. As far "insisting that it's OK because he wants confirmation" I asked for no such thing, I only asked if it had been done. I again point out that the question I started this thread was:

adamis wrote:
... So, my next thought was to look at having my frame reinforced. I know there are all sorts of complications when going this route though. Ford does fully boxed frames for a reason because it gives so much more rigidity. I no there is at least one person that has done this on their 7.3 from a posting on the Ford forums. This is obviously a huge expense and extreme measure.

Has anyone else gone through this or found a way to handle excessive porpoising on older C channel frame trucks?

There have been several people who didn't focus on the weight and answered my question about adding rigidity to the frame based on their own experiences and knowledge and for that I am thankful. Clearly this isn't something commonly done and there is limited experience. THAT is the answer to the question I asked and for all of you who did focus on the heart of my question. Thank you.

On any truck camper forum it's always about the weight and it always comes back to weight in a discussion like this one. This is one of the reasons why I left LOA for this forum. On LOA if you were a nickel overweight you were considered an outcast and treated like one. Gordon from TCM was a god over there. The weight issue is always an issue as you well know... Some times it can get out of hand.

[JRscooby]

Well now that the weight issue is settled, and we have inspected the frame for signs of movement, I have thought of a way to see how much movement you are seeing.
Using strong tape, mount a pointer at front of bed, near cab. Then attach a scale, (Grid work best, to check motion in all directions) to back of cad. Then use a mounted camera like the You-tubers use to record as you drive. Then you could slow the video, watch pointer in relation to grid.
Now it would take somebody with more gee-I'm-a-tree than me to know how much frame is flexing.

[adamis]

I don't know how this is turning into an argument about weight but since that seems to be the focal point of this thread...

The first weight I provided of between 4500lbs to 5000lbs which was a guesstimate accounting for camper dry weight, water, food and gear. I was basing that estimate on what I believed the camper dry weight to be 4000lbs and everything else depending on how I loaded it another 500lbs to 1000lbs. It turns out, the actual camper weight to be 1240.06 kilograms or 2733.8lbs when it went out the factory door.



Now, that doesn't include the Onan 2500 genset added later which weighs 113lbs or the AC unit also later added which is ~130lbs give or take a few so dry weight of my camper is 2976lbs. I guess you can add in the LiFePo4 battery and round it up to an even 3000lbs.

As far as the truck is concerned, BigToe, you clearly put a lot of effort into your analysis. No, my truck is not a California truck, it actually came from Texas. It passes smog here in California and that is all I care about. As far as your breakdown of weight analysis, you are quite knowledgeable or have a lot of time, maybe both but in spite of your best efforts, you were off by quite a bit. I can't believe I am actually doing this as I had to dig it out but for the sake of putting this argument to bed...



According to the scales, my truck weight is 7600lbs. By the GVWR rating of 11200lbs on the tag of the door, that leaves me with 3600lbs of capacity, not the 2961 you estimated. The axle rating on the truck is 8250lbs for the rear. By the CAT weight scales, I only have 3380lbs on the rear. That leaves me with 4870 lbs of rear axle weight capacity.

So now that we have the truck weight and the camper weight, we now know that dry, I am 600lbs below the GVWR of the truck. Sure, add in propane, water, food, pots, pans and all the other stuff and I'm sure I'm running over that 3600lbs but it isn't by a whole lot, certainly not overweight by the 1000lb to 2000lb BigToe, GritDog and valhalla360 seem to think. With the axle capacity of 4870lbs, I am confident I am well below that by several hundred pounds or more.

valhalla360 wrote:
Excellent breakdown of the subject but I'm betting the OP just keeps insisting that it's OK because he wants confirmation, not an actual answer because it's most likely not the answer he will ike.

It might be an excellent breakdown but it was wrong. As far "insisting that it's OK because he wants confirmation" I asked for no such thing, I only asked if it had been done. I again point out that the question I started this thread was:

adamis wrote:
... So, my next thought was to look at having my frame reinforced. I know there are all sorts of complications when going this route though. Ford does fully boxed frames for a reason because it gives so much more rigidity. I no there is at least one person that has done this on their 7.3 from a posting on the Ford forums. This is obviously a huge expense and extreme measure.

Has anyone else gone through this or found a way to handle excessive porpoising on older C channel frame trucks?

There have been several people who didn't focus on the weight and answered my question about adding rigidity to the frame based on their own experiences and knowledge and for that I am thankful. Clearly this isn't something commonly done and there is limited experience. THAT is the answer to the question I asked and for all of you who did focus on the heart of my question. Thank you.

[Grit_dog]

Pretty typical, really.
I guess we all dig our heels in from time to time on stuff. But I’ll reserve further comment other than to say that this type of behavior is seemingly habitual by some.

[valhalla360]

BigToe wrote:
Executive Summary:

.......

Excellent breakdown of the subject but I'm betting the OP just keeps insisting that it's OK because he wants confirmation, not an actual answer because it's most likely not the answer he will ike.

[BigToe]

Executive Summary:

Not enough facts were provided by Adamis, despite multiple requests from RV.net members willing to offer their experience and advise.

Available Facts:

From First Post:

adamis wrote:
I have a 1999 F350 Dually Quad Cab Long Bed. My camper is the Bigfoot 2500 and with everything loaded up is probably in the 4500lb to 5000lb range.

From Signature Photo:

Clarifies CREW Cab (not "Quad Cab," which is the Ram equivalent to 4 door Extended or Super Cab) Confirmation of Crew cab is important for looking up the correct vehicle references in the 1999 Model Year Consumer Information for Truck Camper Loading for Ford Pickup Trucks.


From Second Post:

adamis wrote:
my truck is only two wheel drive. Fine for California

From Third Post:

adamis wrote:
the reason I picked a 7.3 is the smog equipment is non existent. and I'm still legal in California. I also have close to $10k in "goodies" in this engine Larger KC Turbo, Larger Injectors, Chipped,

Contradiction:
By definition, the "larger KC Turbo, larger injectors, chipped" each individually and in combination means that the statement "I'm still legal in California" is false, and not at all a fact.


From Fourth Post:

adamis wrote:
It took me a while to find it but Max Payload for the Crew Cab Dually Long Bed 2x4 appears to be 5355 lbs and the 4x4 is 4910lbs. My camper according to the tag on it weighed 2906 and that included 50 gallons of water and 20lbs of propane. Going to a 4x4 conversion that would leave me 2000lbs for people and gear which would be way more than we would ever carry.

Questions on Maximum Payload Capacity assumptions:

From What resource did you obtain the "5,355 lbs" Maximum Payload Capacity for CREW CAB DUALLY LONG BED 4x2?

Did you account for the different Max Payload that applies to the 7.3L diesel?

Did you account for the different Max Payload that applies to California?

Did you account for the different Max Payload that applies to Model Year?

Did you account for the options that you specific truck was built with?


More Accurate Information to Consider:

It remains unknown what options your truck was built with, the weights of which reduce the maximum payload capacity of your your specific truck. Likewise, the weights of the $10K in goodies that you added is also not known.

However, given that it is well established that your truck has the 7.3L diesel, and that are in California, and therefore probable that your truck was built for California, with an 11,000 lbs GVWR... the Maximum Payload capacity for a 7.3L diesel equipped F-350 Super Duty Crew Cab 4x2 172.4" wheelbase (long bed) dual rear wheel pickup is 4410 lbs, before factory options are subtracted.

Source: 1999 Ford Source Book, Super Duty F-Series Page 86 (Dealership Hard Copy Print)


Questions on Big Foot 2500 Camper Weight determination:

How did your camper weight drop from 4,500 to 5,000 lbs down to 2,906 lbs?

Have you actually weighed your camper?

Have you actually weighed your truck?

As other rv.net members have pointed out, getting physical weights is step 1.

There is no point in guessing at what you can readily determine with certainty.




The Ford Pickup Trucks 1999 Model Year Consumer Information Truck Camper Loading guide sets forth the Minimum Requirements and Maximum Weights of truck campers on 1999 Ford Pickups.

1. The 1999 F-350 Super Duty Crew Cab DRW 172.4" WB 4x2 is only recommended for hauling truck campers if the truck is equiped with the optional CAMPER PACKAGE 532.

The CAMPER PACKAGE 532 consists of the following equipment for 7.3L diesel 4x2 applications:

A. Front Stabilizer Bar
B. Rear Stabilizer Bar
C. Auxiliary Rear Springs
D. Slide In Camper Certification

Question: Does your truck have the foregoing optional equipment A-D?


2. Factory Option Weight distribution between Front and Rear Axles for
Base 1999 F-350 4x2 Crew Cab 172.4" Wheelbase DRW 11,000 lbs GVWR

Example of Assumed Front Axle additions/subtractions:

7.3L Diesel w/4R100 Automatic: + 604 lbs.
California Emissions: + 2 lbs.
Front Stabilizer Bar: + 34 lbs.
Rear Stabilizer Bar: + 2 lbs.
Auxiliary Rear Springs: + 1 lb.
Lariat Trim: + 19 lbs.
Aluminum Wheels (-22 lbs.)
Running Boards: + 46 lbs.
Air Conditioning: + 44 lbs.
Power Windows/Locks: + 3 lbs.
40/20/40 w/Power Driver seat: + 32 lbs.
Trailer Tow Mirrors Electric: + 8 lbs.
Chrome Front Bumper: + 7 lbs.
Chrome/Painted Rear Bumper: (-17 lbs.)
LT235/85R16E A/T Tires: + 26 lbs.
LT235/85R16E A/T Spare Tire: (-11 lbs.)
AM/FM Stereo CD Player: + 8 lbs.
Trailer Tow Hitch: (-11 lbs.)

Subtotal of Example Option Weight on Front Axle: 775 lbs.

Example of Assumed Rear Axle additions/subtractions:

7.3L Diesel w/4R100 Automatic: 98 lbs.
California Emissions: 1 lbs.
Front Stabilizer Bar: (-3 lbs.)
Rear Stabilizer Bar: + 26 lbs.
Auxiliary Rear Springs: + 32 lbs.
Lariat Trim: + 20 lbs.
Aluminum Wheels: (-22 lbs.)
Running Boards: + 29 lbs.
Air Conditioning: (-3 lbs.)
Power Windows/Locks: + 3 lbs.
40/20/40 w/Power Driver seat: + 21 lbs.
Trailer Tow Mirrors Electric: + 3 lbs.
Chrome Front Bumper: (-1 lbs.)
Chrome/Painted Rear Bumper: + 80 lbs.
LT235/85R16E A/T Tires: + 51 lbs.
LT235/85R16E A/T Spare Tire: + 101 lbs.
AM/FM Stereo CD Player: + 2 lbs
Trailer Tow Hitch: + 60 lbs.

Subtotal of Example Option Weight on Rear Axle: 478 lbs.

Based on the foregoing examples of option weights, the actual cargo carrying capacity for the vehicle as equipped is in the range of 2,961 lbs., as determined on Page 43 of Ford Customer Service Publication FCS-12177-99.

The longitudinal center of gravity zone is 60.2" forward of the tailgate at the front edge of the longitudinal CG zone, to 44.7" forward from the tailgate at the aft edge of the longitudinal CG zone.

Obviously, the manner in which your specific truck is equipped may vary from the above example factory options selected for demonstration purposes.

Equally obvious is the fact that your truck does not have 5355 lbs of cargo capacity, or net carrying capacity, as you previously have cited.

It is suggested to review how the truck is engineered, before trying to re-engineer it.

The first and most basic step is to weight the truck without the camper, and then weigh the truck with the camper. With each weight, weigh the front and rear axle on separate platforms. This base line data is foundational to any future decision on how to manage camper weight on your truck within the original design limitations. The Federal Certification Label on the door jamb and the Slide In Camper Certification (if equipped) are also essential GAWR and GVWR data sources to compare your actual weights with.

[mbloof]

TxGearhead wrote:

OP: are you sure you're not just seeing the camper rock back and forth, the spring loaded Fastguns doing their job?

I always get a kick out of these sorts of comments. 🙂

So a flat hard object sitting flat on another flat hard object is "rocking"??!?!??! hahahahahahahaha

Turnbuckles have springs in them because the frame can flex and twist while the truck is going down the road and without the springs the attachment points on the camper could get torn out.



- Mark0.

[time2roll]

F450-F550 would be the method to strengthen the frame.

Otherwise post the scale weights, door sticker and a video of the frame in motion.

[Grit_dog]

OP is, or was, asserting his truck frame is flexing 2”:E Not just the camper overhang.
But then he revised his assessment to 1”. Which is also not likely. At least not as a general matter of course driving down the highway even on concrete.

[srschang]

I get concerned about our camper porpoising sometimes as well. It looks like the cabover is moving a ton when I look up and out the windshield. But in reality, it can't be moving too much or it would be hitting the sharkfin antenna. And, since ours is sitting on a rubber mat in the bed of the truck, and the center of gravity is right on the rear axle, the flex in the rubber mat probably accounts for a lot of the movement. As mbloof said, 2" of movement at the front of the cabover isn't very much movement at the front and back of the truck bed.