Considering a drop axle and aeromods for my utility trailer (plus the car)

[H-Man]

Preamble: Taserface (2017 volt) gets 30 MPG with a harbor freight utility trailer hooked up to it doing 70 MPH. Currently I have a low trailer hitch and the rear wall of the trailer out so that as much of the frontal area is tucked in as possible.
Amble:
I face a few issues

1. when hauling light loads, the trailer's rear is high. The rear wall costs 6 MPG at 70 MPH.
2. when hauling tall loads (like a fridge) you are looking at 19 MPG.
3. The current axle is a non drop axle. It results in the trailer having significant amounts of clearance under it. Wasted space.
4. The rear being high means that I have to fight with appliances more to get them up and down the ramps I built.
5. The trailer is shaped like a box without a top and that probs hurts mpg
6. the hubs may be worn out or the axle bent because I get excessive wear on the inside of the tires.
7. This setup is used partially for extremely tight spaces (EG, 17' of car and 10' of trailer means that you have to worry about jackknifing the trailer)
8. Trailer has no cover.
9. the 12" rims are hard to get tires with a high speed rating for.
10. That 1.5L direct injected engine gives you about 50 HP before it starts running open loop at 3000' so steady state power must stay below this.

A drop axle with (optionally) steering would solve a lot of these issues. Might need to shift to new rims for a faster tire (The setup is stable at 95 MPH, but I have 1 set of tires that can handle north of 65 MPH). The reason to want steering is so that I can put the trailer into impossibly tight positions, but this is a low priority.


As for the cover, I'm considering using a solar panel as a cover for the trailer. I have a 300W panel in the parts pile and some LiFePO4 batteries taken from a forklift I got out of a puddle at the scrapyard. I wouldn't use this to charge the volt (well, not much, it'd be good for a few miles), the point of it would be to run tools/etc off of the pack. Air compressors, saws, fans, etc. Wouldn't be much, but I'm kinda relying on sub 25% capacity factor here. The killer application of it is that 1.2 kWh a day is enough to run my CPAP, so it's more just building the generator setup into something that's easy to move.

As for the shaped like a brick issue, I could potentially use corrugated plastic to create softer edges.


Yes, I know I could just go slower but speed limits are 75-80 MPH in my main operating areas with my trailer and that would take away a lot of the excuse to do aeromods.

[freebeard]

I support your decision.

So what do you propose for the dropped axle? My first thought is a Speedway Fad T front axle. That would enable the steering option. Steering-by-wire would be an interesting hack. Trailer plugs come in four-, six- and seven-wire configurations. Control software, an electric rack and an angle sensor at the hitch and you can have anti-fishtailing and low speed maneuvering.

Here's my low-poly approximation of aerohead's Template I made in 2013. I stayed up all night to finish it and used bottle caps for wheels.



This was designed as a foamie teardrop camper, but you could get this shape and haul tall loads with a bow and tarp cover. Four small tires for frontal area where with you setup, wheels and tires sourced from a Volt on the Speedway axle would make logistical sense. To match the Volt's width frame up a Shepherd's trailer:



If it's a 4x8 trailer, a 2-3 ft tapered extension would give good aero results. And might serve a a loading ramp.

Somewhere in the Eomodder archive is aerohead's warning about his trailer becoming airborne; consider downforce.

[Logic]

Assuming the trailer uses leaf springs, with the axel shaft bolted beneath them;
An easy lowering mod is just to bolt the shaft above the leaf springs, using the same bits, just upside down.
if the shaft's already above the leafs; some spacers.


Alternately, You might look for one of these:



Some pizza pan wheel covers and closing the wheel arch and you're good to go.
Maybe an aero teardrop 'fairing' on the axel, Made from bending thin plate.

For light loads; rounded motorbike tires would make the tire itself more aero.
The wheels on small brakeless trailers do nothing that requires contact area.

NB: This all assumes higher speed freeway driving.
If it's all low speed, stop-go city type driving: Just reduce weight as much as possible.
F=m.a...

[Vman455]

Originally Posted by H-Man

Preamble: Taserface (2017 volt) gets 30 MPG with a harbor freight utility trailer hooked up to it doing 70 MPH. Currently I have a low trailer hitch and the rear wall of the trailer out so that as much of the frontal area is tucked in as possible.

"Frontal area" is really "frontal projected area" i.e. the largest cross section of the car+trailer projected onto a plane in front of it. It isn't a physical area, and importantly, it is not a determinant of drag. It is a reference area, and reference area could be anything we want. In aircraft, reference area is typically an imaginary wing area; at other times, I've used "exposed wing area" as a reference area, or "vertical stabilizer projected area." You could use footprint area as a reference if you want, or profile area, or whatever.

What you did by removing the rear wall was reduce "wetted surface area" or Swet. Aerodynamic force is proportional to this surface area because the force is produced by the action of the stresses exerted by the air (usually decomposed into normal component--pressure--and tangential component--shear) on the entire wetted area.

Originally Posted by H-Man

Amble:
I face a few issues

1. when hauling light loads, the trailer's rear is high. The rear wall costs 6 MPG at 70 MPH.
2. when hauling tall loads (like a fridge) you are looking at 19 MPG.
3. The current axle is a non drop axle. It results in the trailer having significant amounts of clearance under it. Wasted space.
4. The rear being high means that I have to fight with appliances more to get them up and down the ramps I built.
5. The trailer is shaped like a box without a top and that probs hurts mpg
6. the hubs may be worn out or the axle bent because I get excessive wear on the inside of the tires.
7. This setup is used partially for extremely tight spaces (EG, 17' of car and 10' of trailer means that you have to worry about jackknifing the trailer)
8. Trailer has no cover.
9. the 12" rims are hard to get tires with a high speed rating for.
10. That 1.5L direct injected engine gives you about 50 HP before it starts running open loop at 3000' so steady state power must stay below this.

A drop axle with (optionally) steering would solve a lot of these issues. Might need to shift to new rims for a faster tire (The setup is stable at 95 MPH, but I have 1 set of tires that can handle north of 65 MPH). The reason to want steering is so that I can put the trailer into impossibly tight positions, but this is a low priority.


As for the cover, I'm considering using a solar panel as a cover for the trailer. I have a 300W panel in the parts pile and some LiFePO4 batteries taken from a forklift I got out of a puddle at the scrapyard. I wouldn't use this to charge the volt (well, not much, it'd be good for a few miles), the point of it would be to run tools/etc off of the pack. Air compressors, saws, fans, etc. Wouldn't be much, but I'm kinda relying on sub 25% capacity factor here. The killer application of it is that 1.2 kWh a day is enough to run my CPAP, so it's more just building the generator setup into something that's easy to move.

As for the shaped like a brick issue, I could potentially use corrugated plastic to create softer edges.


Yes, I know I could just go slower but speed limits are 75-80 MPH in my main operating areas with my trailer and that would take away a lot of the excuse to do aeromods.

Is driving slower really not an option? Because that option is free. Anything else you suggest doing will cost money. Determine your budget first.

If you're set on reducing aerodynamic drag in other ways, you will need some way to measure it. Then, to approach it:
-minimize wetted area
-reduce pressure on forward-facing surfaces
-increase pressure on rear-facing surfaces
-minimize energy loss by keeping an attached boundary layer over as much of the car+trailer combination as possible (energy is lost in shear layers/velocity gradients, and these gradients are stronger in separated flow)

Boundary layer attachment is easy to observe with tufts, and then you can try rounding corners or putting on your solar array. I would start with tuft testing; it's the easiest and will point you toward shapes that work on your particular car+trailer.

Drag changes can be extrapolated from power consumption (if the Volt has an instantaneous power output on the display somewhere), or by taking velocity measurements (calculate from dynamic pressure) upstream and downstream of the car+trailer. The smaller the difference between these, the less drag the combination produces (because less x-momentum is lost in the flow around the car+trailer).

This article explains.

Static pressure changes can be measured with surface taps/disks and the changes to drag extrapolated.

Example spoiler test.

Pressure measurement explainer.

[Gasoline Fumes]

I also have a 4x8 Chinese cargo trailer, but I don't use it enough (or very fast!) to think about its aero drag.

The wheels sticking out are probably creating a lot of drag. I would try to cover them. Airplane wheel pants!

It's probably the opposite of what you want at higher speeds, but changing to 8" wheels would be the easiest way to lower it.

A shorter tongue and/or hitch might keep the trailer better in the tow vehicle's wake.

Smooth panels on the underside of the trailer frame couldn't hurt. And rounded leading edges.

A wind deflector on the front should help with un-aerodynamic cargo. A full cover is better, but not as practical.