5th wheel design

[slowmover]

I just discovered I can use the Bridgestone 381 with the really low crr on the trailer.

The only BRIDGESTONE 381 I'm seeing is LRR, but of passenger car spec and a load rating of under 1,200-lbs each. The suggested weight is indicating a tire capable of in of 3,000-lbs tire (tridem) or 4,000-lbs tire (tandem) with a real world tire (and wheel) capable of greater reserve capacity than this.

Did you happen to mean the same makers 16" 3,042-lb LR-E Duravis R250?



.

[skyking]

the rough figures for axle weight is around 1800 per axle loaded. 3600 on the axles and 1200 on the pin for a max of 4800.
Those 1200 pound tires are indeed the ones I am talking about.
Perhaps I was not clear, the weight I posted was truck and trailer combined.

[slowmover]

Thanks for the clarification, you really had me thrown!

Edit: I'd still be cautious about sidewall stiffness, and would prefer that over straight LRR "benefit". The 15" LR-C YOKOHAMA RY-215's I ran on my trailer fit both criterion I felt. Even bringing them down 10-psi (but no more) they'd be quite a bit stiffer with better transient response.

[aerohead]

Originally Posted by skyking

I'm about settled down on dimensions again ( ).
This will have a frontal area of 53 sq. ft., and will follow the aft template faithfully till about the first 22 degree point. The sides will taper in a couple of small angle changes.
The rocker angle aft of the wheels needs to be a little steeper than ideal at 4.6 degrees.
The area at the back will be ~17 sq. ft.

What kind of Cd should I expect out of that, considering it will be faired smooth and tight to the tow rig and have above average fairness in general?

I spent some time with Hucho's chapter on 'Commercial vehicles'.In his 2nd Edition,page 332,Figure 8.63,'Influence of bus rear-end design on air drag',in the far right is a depiction of a boat tail and it's Cd reduction.
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*The bus has a Cd 0.341 with no tail at all.
*With the #2 configuration,the section behind the rear wheels of the bus is sloped down and in @ 5-degrees leading to Cd 0.274.
*With the #4 configuration,the boat tail is extended out to 41.7% of the 'Template' aft-body length,yielding Cd 0.218.
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The boat tail depicted in Fig. 8.63 indicates a 5-degree slope,but it is drawn with a 14.5-degree slope.
I redrew the bus @ 5-degrees and evaluated the cross-sectional area at the rear truncations using the frontal area of the 'Front Roof and Vertical Edges Rounded' bus.
*At position #2 the frontal area is reduced 18.1%/drag is reduced 19.64%
*At position #4 the frontal area is reduced 39.2%/drag is reduced 36%
*For the 1st 41.7% of aft-body,each 10% reduction in cross-section yields a 10.05% drag reduction.
*If this relationship held,then if the tail was extended back to 70%,as has been recommended with the 'Aerodynamic Streamlining Template',the the drag coefficient would fall to Cd 0.13.
*If wheel drag is removed,@ Cd 0.041,this leaves a ground-proximity Cd 0.09 for the boat-tailed bus body, which is in close agreement with the
'Template.'
--------------------------------------------------------------------------
Using this relationship as a 'loose' rule-of-thumb,then we might guestimate that if you start at 53 sq ft of fully attached frontal projected area,and then streamline the body,separation-free,down to a body truncation of only 17 sq ft at the rear,the 32% of remaining base area equates to a 67.9% reduction in area.
At 10.5%/10,the drag would be reduced 68.23%.
At a steady 55 mph (88.5 km/h) you might see a 34% mpg increase,minus any penalty for added weight/R-R if applicable.
Pretty tasty!

[skyking]

Oh man, I love it when you talk dirty like that
I have to get certain things done first, Like the truck and a bit of home remodel. I will put pencil to paper and get it out there to look at.
I am getting more truck done with the assistance of a retired finish carpenter.

[ERTW]

Your coefficient drops 68% and your frontal area increases how much? Still, Adding a trailer and coming out par is pretty amazing.

[skyking]

Originally Posted by ERTW

Your coefficient drops 68% and your frontal area increases how much? Still, Adding a trailer and coming out par is pretty amazing.

I think Phil was talking about the reduction that the template makes on a given frontal area, not in comparison to the truck without trailer.
The truck is ~34.5 ft and 0.44 cd, Cda of 15.18.
The trailer is at ~53 ft.
To get to an equal Cda, The assembly would have to get to a cd of 0.286.
The bulk of the truck's cd issues are the square back of cab and then tailgate.


I suspect that I will get a cd considerably lower than 0.286 with a little modeling help

[skyking]

I mentioned some house remodel and truck mods to do first, what 3 years ago?
I have a different truck that needs a cleanup, and I have touched every room in the house now. I am finishing up the kitchen in the next weeks so that will be done.
Now to turn back to the trailer. It has indeed changed a bit, but it is really details. My biggest hurdle will be figuring out the fabrication process so I get a smooth and strong roof structure without too much headache.
I want to use XPS foam due to high strength, but it off-gasses forever. This causes disbonds if you don't handle it. Surf board guys have vents for it, and so will I.
To get curves with a very rigid foam, I would either need to make slices and carve on it endlessly, or make kerf cuts in it.
Example of kerf cut:
I made a round peninsula end in my kitchen, using bending plywood. No need for kerf cuts there. I wainscoated it with bending oak on the circumference top and bottom, and applied a curly maple veneer. I had verticals to do on the wainscoating. I wanted a curve on that vertical like this.


It is subtle at 21.5" radius. The material is kiln dried hard maple and it really does not bend, so I kerf cut it like this:



I will test that process with 2" XPS. Single cuts along a simple radius will yield a semi-faceted radius. Not ideal but far less laborious than slicing and grinding away, but the side benefit is handling the off-gassing. If I control the resin during layup I will have channels throughout that I can drill a vent in.
This will work along the sides until I start following the compound
curves of the template. I will have to cross cut it as well, yielding a pattern of softened square elements, or diamonds if I get creative with the saw.
I may be able to soften those facets further using heat during the bending process.
I will build a male form out of wood and use stainless wire to tie it down, then glass and epoxy the outside. I will pull the wire in below the surface, and then cut it off below and leave it in the structure.
Flip it and glass the inside, and drill vents to the inside. In the gridded areas the cuts will form interconnected channels that will require few holes. The simple radius will require more.
The interior finish will be a glued on carpet or headliner that will not block the vents.

[skyking]

Phil, if you see this I had some questions.
based on hutcho's diffuser graph, he talked about a maximum dimension for the total rise of the diffuser.
The trailer's problem is overhang aft of the axle, and dragging arse in certain driveway situations.
Proposed trailer diffuser is over 8' of length and needs to rise at least 7" to help with clearance. This is where TANSTAFFL kicks in.
1) 7" or more is a lot of air.
2) It has to come from somewhere. I see it filling in from the sides, and making a nasty double vortex bucket to drag around. Correct me if I am wrong.

I can make a moving diffuser with casters on it, that can "take care of itself" in those awkward situations. Would I make aero money if I reduced that rise, that angle?

[aerohead]

Originally Posted by skyking

Phil, if you see this I had some questions.
based on hutcho's diffuser graph, he talked about a maximum dimension for the total rise of the diffuser.
The trailer's problem is overhang aft of the axle, and dragging arse in certain driveway situations.
Proposed trailer diffuser is over 8' of length and needs to rise at least 7" to help with clearance. This is where TANSTAFFL kicks in.
1) 7" or more is a lot of air.
2) It has to come from somewhere. I see it filling in from the sides, and making a nasty double vortex bucket to drag around. Correct me if I am wrong.

I can make a moving diffuser with casters on it, that can "take care of itself" in those awkward situations. Would I make aero money if I reduced that rise, that angle?

*Hucho shows a drag reduction for diffusers with up-slopes as steep as 11-degrees.The lowest drag is with 2.8-4-degrees,depending on where the slope begins underneath.

I don't have a calculator with me so it'll be tomorrow before I can check the slope.tan 2.8,or tan 4,times 8 would give you the desired rise in feet (if my brain's working).
*Square edges will induce vorticity according to Scot Funderburk's and Professor Carver's research at Texas Tech.You'd want something like this