Trailer drag reduction from lowering

[jas67]

Hi All,

I just purchased a small flat-front box trailer, 4w x 6L x4h.
It is about a foot taller than my tow vehicle, a 2015 TDi Golf Sportwagen.

I also tow a 6x12x6 V-nose aluminum box trailer with same TV. I use a wind deflector on top of the TV for this trailer, which is almost 8 feet tall overall.

Highway MPG for the TV alone is 44-48 at 70-80MPH
Highway MPG for the TV + 4x6x4 trailer is 30-31MPG, 70-75 MPH
Highway MPG for the TV + 6x12x6 trailer with wind deflector is 20-22 MPG, 70-80 MPH
Highway MPG for the TV + 6x12x6 trailer w/o deflector is 18-19 MPG, 70-80 MPH


I'd love to do a tail for each of the trailer, and a nose-cone for the small one, but, that will be another thread.

For now, I think low hanging fruit on the smaller trailer might be to lower it. It looks like I can likely lower it 2-3 inches by adding blocks to the axel, which is above the leaf spring, without compromising suspension travel.

Has anyone here done that? How much MPG savings can I expect from lowering it 2-3 inches?

TV height is 58 inches. It is a wagon that tapers a little to the rear.
Current roof height on the trailer is 70 inches.

I tried the wind deflector with the small trailer, it may have helped MPG by 0.5 to 1 (30.5-.31.0). I'm thinking that the wind deflector I have is too severe for the shorter trailer, and that maybe a smaller one, or one with less angle might be helpful. Thoughts on this?

I wish I had access to a wind tunnel!

[freebeard]

Blocks are the old school way of lowering on leaf springs.The alternatives are re-arching the spring or reversing the eyes. Be aware of the departure angle.

For the front deflector, you could look for Hersbird's Hi-Lo as a built example, but he worked from this sketch that I can find:



This represents a pickup camper, as I was trying to resolve the cab/camper gap, but I added it to the overhang as well, as a means of adding a radius without cutting into the structure.

The [rule-of-thumb/first approximation] is 4% of overall width -- or 2" radius or 4" bubble on four feet of width. With CFD or a wind tunnel you might find that capping the pilasters with Batman ear vortex generators that create a central jet down the center of the roof.

[redpoint5]

I find taking the walls down on my trailer whenever possible massively improves MPGs.

Rarely do I need the walls of a trailer.

[freebeard]

It;s described as 'a small flat-front box trailer'.

Let's wait for a second post.

[jas67]

The previous owner had it "upgraded" to 15 inch wheels with 205-75/15 tires, which are WAY overkill, and 3 inches larger in diameter than the standard 175-80/13 tires it came with from the factory. Switching back to the 13 inch wheels/tires will lower it 1 1/2 inches. This, plus 2 inch blocks on the springs will give me a 3 1/2 inch drop. This should help, no?

I do have another small trailer with 13 inch wheels that I can borrow the wheels from, so, I can at least experiment w/o spending money.

[j-c-c]

You answered your own question. Let us know how the smaller wheels test out.
Seems to me however that your results will be rather speed dependent if we are talking about only a 3" change in elevation.

[aerohead]

Originally Posted by jas67

Hi All,

I just purchased a small flat-front box trailer, 4w x 6L x4h.
It is about a foot taller than my tow vehicle, a 2015 TDi Golf Sportwagen.

I also tow a 6x12x6 V-nose aluminum box trailer with same TV. I use a wind deflector on top of the TV for this trailer, which is almost 8 feet tall overall.

Highway MPG for the TV alone is 44-48 at 70-80MPH
Highway MPG for the TV + 4x6x4 trailer is 30-31MPG, 70-75 MPH
Highway MPG for the TV + 6x12x6 trailer with wind deflector is 20-22 MPG, 70-80 MPH
Highway MPG for the TV + 6x12x6 trailer w/o deflector is 18-19 MPG, 70-80 MPH


I'd love to do a tail for each of the trailer, and a nose-cone for the small one, but, that will be another thread.

For now, I think low hanging fruit on the smaller trailer might be to lower it. It looks like I can likely lower it 2-3 inches by adding blocks to the axel, which is above the leaf spring, without compromising suspension travel.

Has anyone here done that? How much MPG savings can I expect from lowering it 2-3 inches?

TV height is 58 inches. It is a wagon that tapers a little to the rear.
Current roof height on the trailer is 70 inches.

I tried the wind deflector with the small trailer, it may have helped MPG by 0.5 to 1 (30.5-.31.0). I'm thinking that the wind deflector I have is too severe for the shorter trailer, and that maybe a smaller one, or one with less angle might be helpful. Thoughts on this?

I wish I had access to a wind tunnel!

I don't think we could say with any certainty. There might be an advantage for the upper body airflow, while simultaneously experiencing a disadvantage in underbody flow.
From NASA, until the TV-Trailer are the same height and width, the gap is eliminated, and an aerodynamic singularity is created, only then might we be capable of applying rules to the aerodynamics.
Years ago, member freebeard shared a video of a VW Beetle '5th-wheel' RV Trailer setup which almost eliminated the gap, and an integrated trailer would be capable of completely drafting in the wake of the TV.
Matching roof height meant about a 16% drag reduction, and closing the gap meant another 16% on top of that.
'THEN' we could 'guestimate' what a trailer belly pan, wheel covers, wheel skirts, and a boat-tail would do. And 'nothing' allowed on the roof or sides of the trailer.

[freebeard]

Years ago, member freebeard shared a video of a VW Beetle '5th-wheel' RV Trailer setup which almost eliminated the gap, and an integrated trailer would be capable of completely drafting in the wake of the TV.

[citation needed] If it was the gold camper, that added a lot of frontal area. This one is no taller than the Beetle. It even has speeed holes.

1974 vw beetle trailer camper fifth wheel volkylandia

[aerohead]

As far as I'm concerned, volkylandia constitutes 'magic.'
I just love it!

[freebeard]

This is in my albums, sauce unknown, titled '36fardig'



edit: TinEye finds three other instances, the earliest links to a Korean home page of some sort.