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Vantare Slide Cutout Structure Reinforcement
10-31-2014, 14:46 (This post was last modified: 12-05-2014 23:18 by travelite.)
Post: #1
Vantare Slide Cutout Structure Reinforcement
CC_Guy,

In the 40' Marathon thread you brought up the complexity of reinforcing the Prevost structure for aftermarket slide-out installation. A while back I did some FEA modeling of the Wanderlodge chassis in anticipation of a user installed slide. The Wanderlodge chassis has a substantial c-channel backbone which provides a great platform for slide installation. Still, design these days tends to be driven by deflection criteria as opposed to building to stress - this leads to good fit and finish and rattle free performance. The point is a Wanderlodge chassis is probably strong enough to support the slide given it's central c-channel backbone, but to keep the basement doors from falling open and the slide from leaking due to excessive flexing, a deflection based design criteria is the way to go. I don't know if the same can be said about the Prevost chassis because it's much more of a spaceframe then a central c-channel design. I imagine analysis would show that reinforcing the slide cutout is even more important in the Prevost chassis. I was initially scared of Vantare slides because they're not designed by Prevost. Seeing in person what Vantare did to reinforce the structure allayed my fears. Below is just a glimpse of the structural beams Vantare added. The Vantare beam under the salon slide is 20 inches tall with 4"x4" upper and lower chords and 2"x4" diagonal webbing. The same treatment is given to the Stateroom slide cutout. The two beams run under the slide cutouts and are joined to the Prevost chassis with 1" thick plate steel. The advantage of the H chassis is that beams of this dimension can be used while not taking away from basement storage or access. There's no doubt in my mind that Featherlite drew upon their extensive trailer building experience when making these Prevost chassis mods; the welds are excellent with good penetration:
İmage İmage İmage

david brady,
'02 Wanderlodge LXi 'Smokey' (Sold),
'04 Prevost H3 Vantare 'SpongeBob'

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12-03-2014, 21:26 (This post was last modified: 03-24-2015 19:31 by travelite.)
Post: #2
Thumbs Up RE: Vantare Slide Cutout Structure Reinforcement
Here's a load, shear, and moment diagram for a typical bus. It's a simplified model based on a uniform load distribution and unequal length overhangs:

İmage

R1 and R2 are the loads at the front and rear axles respectively; these are also the maximum shear forces. M is the max moment between the axles. From the literature:

R1 = wl(l-2c)/2b,
R2 = wl(l-2a)/2b, and
M = R1(R1/2w-a).

For this model I'm only looking at the beams added by Vantare along with Prevost's intermediate span connecting the Vantare beams. Vantare added a beam under each slide out to restore strength after it cutout the Prevost sidewalls to make room for the slides. This gives me the following measurements:

a: 24"
b: 316"
c: 134"
w: 49 lbs/in

Since the bus is symmetrical I'm focusing on a 1/2 bus model. My H3 weighs approximately 50,000 lbs, or 25,000 lbs per side. I'm conservatively assuming all this weight bears down on the wall beam composed of a 16" high Vantare section followed by a 32" high Prevost section followed by another 16" Vantare section:

İmage

And, with an applied distributed face load (in green) and axle support constraints:

İmage

The salon slide situates itself on top of the left Vantare beam while the stateroom slide is supported by the right Vantare beam. The span of this set of beams is 474 inches which after subtracting 3550 lbs of unsprung weight comes to a linear load of 49 lbs per inch.

Using the above formulas for R1, R2, and M, I get:

R1: 7571 lbs
R2: 15656 lbs
M: 33594 ft-lbs

If I resolve this moment about the extreme faces of my Vantare beam's upper and lower chords, I get a tensile force of 20156 lbs acting in compression along the top chord and a 20156 lb force acting in tension along the bottom chord.

I ran an FEA on the sidewall to check the theoretically computed values. Loaded up at 49 lbs/in and with axle constraints in place, here are the results for:

Deflection:
İmage , İmage , İmage

Tensile Force:
İmage , İmage , İmage , and

Tensile Stress:
İmage , İmage , İmage
You can see the maximum tensile stress is less than 16,000 psi, which is below the fatigue strength for low carbon steel of roughly 28,000 psi and well below the steel's ultimate tensile strength of roughly 55,000 psi.


Next I want to compare these results to the original Prevost wall beam, more to come... Smile

david brady,
'02 Wanderlodge LXi 'Smokey' (Sold),
'04 Prevost H3 Vantare 'SpongeBob'

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12-04-2014, 16:41 (This post was last modified: 12-07-2014 15:58 by davidbrady.)
Post: #3
RE: Vantare Slide Cutout Structure Reinforcement
Here's an FEA on the stock Prevost sidewall loaded up to the same 49 lbs/in and with axle constraints in place:
İmage

This model was derived from SpongeBob's Prevost documentation. Here's a view of the sidewall construction from my documents. In this view the front of the bus is to the right; whereas, in my FEA model the front is to the left:
İmage

Deflection:
İmage , İmage , İmage

Tensile Force:
İmage , İmage , İmage , and

Tensile Stress:
İmage , İmage , İmage
you can see the max stress again is less than 16,000 psi, again well below the fatigue and ultimate tensile strengths of low carbon steel.


Observations:

The max tensile stress is the same in both cases, both the Vantare beam and the Prevost beam exhibit stresses less than 16000 psi. Tensile forces on the Vantare beam chords are higher than those in the Prevost chords, maxing out at 25000 lbs versus Prevost's 13000 lbs. This is to be expected since the Prevost beam is two times as tall as the Vantare. The point to note is that the tensile stresses are the same due to the greater amount of steel in the Vantare beam, both come in less than 16000 psi. So far things are virtually unchanged by Vantare's modifications. Where things differ is in deflection. Because the partial height Vantare beam has a lower moment of inertia we'd expect it to deflect more than the Prevost beam, and this is the case. The Vantare beam's max deflection is 0.34" whereas the Prevost beam's deflection is 0.26". Remember, the loading case presented here is extreme. I'm assuming the full 50,000 lb load is supported solely by the sidewall beams. Clearly this isn't the case. Also, I'm placing all the load over 474 inches, or 40 feet; the full load of the 45 foot bus is imposed on the length made up by the two Vantare beams and the intervening Prevost beam segment. Clearly, my loading configuration is a highly conservative model; even so, the deflections are close and given the alternate load paths within the chassis in the real world I'd expect the deflection differences to be inconsequential. Still, our max stresses are close to a factor of 4 away from the steel's ultimate yield strength and well below the steels fatigue strength!

The Vantare chassis modifications are in line with restoring the 2-slide equipped chassis back to its original strength!

david brady,
'02 Wanderlodge LXi 'Smokey' (Sold),
'04 Prevost H3 Vantare 'SpongeBob'

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12-05-2014, 22:44
Post: #4
RE: Vantare Slide Cutout Structure Reinforcement
The interface between the Vantare Beam and the intervening Prevost beam segment is a 48"x16"x1" steel plate weighing in at close to 200 lbs. Here's a view of the Von Mises stresses associated with the plate:

İmage

david brady,
'02 Wanderlodge LXi 'Smokey' (Sold),
'04 Prevost H3 Vantare 'SpongeBob'

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12-14-2014, 20:08
Post: #5
RE: Vantare Slide Cutout Structure Reinforcement
David,

Your detailed analysis is way over my head, although I do understand your results. I wonder if Vantare had a mechanical engineer do all the reinforcement design. They sure seemed to have nailed the strength of their design.
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12-15-2014, 21:30 (This post was last modified: 12-15-2014 22:23 by travelite.)
Post: #6
RE: Vantare Slide Cutout Structure Reinforcement
(12-14-2014 20:08)CC_Guy Wrote:  David,

Your detailed analysis is way over my head, although I do understand your results. I wonder if Vantare had a mechanical engineer do all the reinforcement design. They sure seemed to have nailed the strength of their design.

Hi CC_Guy,

Thanks! Yes, I spoke with a couple of ex-Vantare employees who told me about the outside Engineering Consulting firm that did the design. They also did twist, bending, and vibration analysis (modal) on their chassis modifications. The end result is that the slide doesn't shake, rattle, or roll; the twin slide electric motors are very quiet and very smooth; and when the slide is cinched down against the non-pressurized D-Style bulb seal it does so evenly and without leaks. I'm very pleased with the mechanism.

david brady,
'02 Wanderlodge LXi 'Smokey' (Sold),
'04 Prevost H3 Vantare 'SpongeBob'

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