Tow Vehical MPG...

Mickerly

I made a new nose for my 320. 15% better mileage. But the best surprise; all the bouncing non-since is gone. It pulls perfectly straight and stable. The undercut front end appears to be a bad idea aerodynamically.

irvingj

Wow, GREAT idea! So how did you determine the mpg increase? (What is your tow vehicle?)

I think you've got a great point there about the undercut front end. Wish I'd thought of that!

BTW -- really love your "whale tail" spoiler on the back... wait... Oh, guess that's your kitchen door....  

GregChris

2019 Highlander 14-16mpg if I'm driving. 12 mpg if my wife is driving. 2021 320s bd

AnOldUR

irvingj said:

 -- really love your "whale tail" spoiler on the back... wait... Oh, guess that's your kitchen door....  

I was wondering if @Mickerly had a wing (table) on both sides to get some lift while traveling at speed?

Mickerly

The 320 CS table moves outside and the kitchen hatch is open. Interesting thought though.

I've sorted the nose dynamics. I haven't started on the back yet. I think there is something to be gained. I get 25mpg from 40 to 55mph but only 17mpg at 63mph up to the tire limiting speed. Something is happening back there at approximately 60mph. I'm betting it's the hinge for the kitchen hatch. Airflow on the front of the trailer doesn't change above 40mph. Time to get out the yarn again.

Tow vehicle is a Subaru Outback Onyx XT.

LSkrabut

@ sequimite -  I used a trailer fiction sway bar when I pulled the 400. I also used a anti rattle device. There are a few different type of either devices available.

Examples:

Reese Friction Sway Control Kit - Economy - by Draw-Tite, Reese and Hidden Hitch

Item # 83660

etrailer Anti-Rattle Hitch Stabilizer for 2" Hitches - Vinyl Coated Steel

Item # e99007

GregChris

Interesting on the 40mph. Coming out of DC last fall, we went 47 miles in 90 min. I was getting 22mpg, I don't get that without towing. After the traffic thinned out mpg dropped to about 14. Incidentally the tires on 320 BD's are rated at 80mph. At least mine. I do rarely go over 65. The speed rating is on the side wall in a code, you can google how to decipher it.

subfreq111

Very interesting @Mickerly. I too have noticed the undercut front causing problems. For me, it seems to create lift and induce sway behind my fairly aerodynamic VW. At low speeds no problem, but my 220 pounds of tongue weight seems to be a lot less once at highway speeds, and I suspect the front shape of the trailer is to blame. Did you build your box out of plywood?

Mickerly

The prototype is wood and fiberglass. I'm working with a guy to make a form so we can make a mold for the body and lid. The next one will have a more curved front and a top the is water tight. I wanted to see if it worked before investing much.

Mickerly

Yes, the tires are 81mph. I prefer to travel at 65mph. I try not to go above 70mph. It used to be difficult to get above 70. Now, a moment of not paying attention and it's near 80. The fuel mileage didn't change as much as I wanted except below 55; however, it is so smooth. I don't need the anti-rattle clamp any more. I can hear the rubber in the torsion axil operate. It's a completely different trailer.

irvingj

Very interesting discussion, and although I no longer have our T@B, I still find this to be a very interesting discussion and applicable to all sorts of situations concerning trailering --and and moving a vehicle, any vehicle-- over the road at faster than walking speeds. Tounge weight, yes, a very important consideration, but there's more to it -- as demonstrated above. A teardrop is not necessarily the best shape, is it?

Take a look at the "new, aerodynamic" designs in the 30s, 40s and 50s - not only for camper trailers, but also in the rapidly expanding knowledge field of aerodynamics vis-a-vis aviation. The Chrysler "Airflow," Tatra rear-engined vehicles... They were on to something.

Your front wedge seems like a real simple solution to gain efficiency!!

kottum

The greatest problem with the Tab 320/400 design is it must push most of the air over the top, and some underneath.  Side-to-side it's basically flat, no curvature.  That's the easy way to build it, and is therefore common. Compare that with the Airstream Basecamp.

The only meaningful solution I have found is to tow at lower speeds, especially when heading into the wind.

rh5555

Mickerly said:

The 320 CS table moves outside and the kitchen hatch is open. Interesting thought though.

I've sorted the nose dynamics. I haven't started on the back yet. I think there is something to be gained. I get 25mpg from 40 to 55mph but only 17mpg at 63mph up to the tire limiting speed. Something is happening back there at approximately 60mph. I'm betting it's the hinge for the kitchen hatch. Airflow on the front of the trailer doesn't change above 40mph. Time to get out the yarn again.

Tow vehicle is a Subaru Outback Onyx XT.

Can you tell us more about how you are using yarn to optimize your aerodynamics?

Seems like you may be onto something...  Think about shipping possibilities when you get your fiberglass shield made.  You could have a profitable business there.

Mickerly

A business? In the beginning I was thinking that. After camping for 7 months with this nose, no one has had any interest until the last 2 days. I'm planning on making molds so the possibility is there to make more noses, but I would be happy to cover the cost of my own expenses.

Let me begin by saying, I've worked with engineers. I'm not an engineer. I'm a pilot. I did work for a manufacturer for a while. Measuring the air, there are two things you are looking for. What the air is doing on the surface and what the air is doing about 6 inches away. Both are important. Six inch pieces of yarn taped in certain places will only tell you what is happening at the surface. The important part is the air 6 inches to a foot away. I get to see that when driving in the rain or snow. The mist gets captured in the turbulence and becomes very visible if you are looking.

When we first got the trailer, the air from the tow vehicle hit the front of the trailer just below the window and there was a huge wake, two lanes wide on both sides, coming from the front of the trailer. Now the wake is gone from the front of the trailer and the air from the tow vehicle hits even with the top of the window.

Unfortunately, I can't see the back of the trailer without a "camera car" following. For airflow over the kitchen, I know the air separates from the trailer at about 58mph. There's a wake that appears about 10 feet behind the trailer where the air hits the street. That's also where the fuel consumption rises considerably. I think it is the kitchen hatch hinge lifting the air off the trailer. Once that is confirmed, the question becomes how to get the air back down.

Slackers

Perhaps something like a small spoiler placed forward of the hinge could be helpful.  

SLJ

I get 1.5 MPG better towing mine when I have our two canoes on top of the SUV. They affect the wind that would hit the area on the T@B above the window. The section above the tow vehicle is where the majority of the drag is. 
I don't think sloping the front boxes will do much as they're directly behind the tow vehicle and drafting it. If it made a difference tractor trailers would have a different shape on the front of the trailer. If it's that much of a problem slow down a couple miles an hour. That's where you'll see a difference in mileage.

Mickerly

You are correct. I was hoping the sloping top would push enough air up to raise the flow from the top of the tow vehicle. It got me and inch or so. I do have better flow over the top. The air used to hit and bounce. Now part goes down and part slides over the top. We're looking at a car top carrier to force the air higher.

schwartzki

I put on my bikes in the bed and the Thule air deflector on the back bar and that increased mpg 1.5mpg on the last trip before I swapped to the X5 Diesel.

AnOldUR

SLJ said:

I get 1.5 MPG better towing mine when I have our two canoes on top of the SUV.

So that could be why we got about the same amount less gas mileage on last weekends trip. We usually travel with the Jeep roof rack loaded, but since this was a one night trip we traveled lighter and with nothing on the roof. I had anticipated better mileage being a couple hundred pounds lighter, but the aerodynamics may have been an overriding factor.

Mickerly

We're driven the same route with the trailer completely empty and with the trailer completely full including water. It should be about 300lb difference. The fuel used was too close to say anything was different. The aerodynamics is everything.

Binghi

When towing our 400 BD, our VW Touareg gets roughly the same mileage whether we use a Thule rooftop carrier or not. I thought maybe the Thule would help, but not really the case for us.

kottum

That's what impressed me with our 3.0 Ecodiesel when towing.  Small changes in wind, weight, terrain or speed made little difference in fuel consumption.  The diesel low-rpm torque was such that such changes  were hardly noticeable.  Just a nice steady pull.

CharlieB

schwartzki said:

I put on my bikes in the bed and the Thule air deflector on the back bar and that increased mpg 1.5mpg on the last trip before I swapped to the X5 Diesel.

Could you post a picture of the deflector on your back bar? I’d love to see what you did there. All of this is very interesting. I stopped using a rooftop carrier on our 4Runner because I figured the wind resistance on the car would hurt our mpg. But it sounds like I should reconsider that. 

Freespirit

Improve your gas mileage with an old snowboard. A TAB modification just waiting for the creativity of the “group”. 

https://youtu.be/9bvaEYFTKS4

nbrandt

I am very intrigued with Mickerly's posts.  Looking at semi trucks and trailers for several years we have seen fiberglass bulbs on the front of trailers, pyramid shaped sails on the back as well as sheets hanging beneath the trailers between the axles - all of this is in the name of better mileage.  In the 1970's in the midst of the first "gasoline crisis" everyone was looking for better mileage out of engine and I read an article in Popular Mechanics and the author said, "Everyone wants to take a wrench to their engine to get better mileage but they should be taking a hammer to their body in improve aerodynamics - look at airplanes!"

I recently put a boondock axle on a '21 320 CS-S and am curious to see what will happen with mileage - if anything and now am having second thoughts about moving the spare to the front of the storage box.  I had been carrying it in the bed of the pickup but wanted more room there.  This too may be a negative for fuel economy.  I have also wondered if a spoiler or wind foil on the bed of a pickup might help push the air around the trailer.  I also need an airplane nose on the front of the Tundra - that is a flat faced box. 

Mr. Mickerly - if you come up with a fiberglass front for the T@B it would be worth the expense in my opinion. 

GregChris

I have a pretty much stock 2021 320s bd. Going 750 miles to Skidaway Ga. I averaged 15.1 mpg. I consider that excellent. From Skidaway  to Huntington Beach SC(200mi) I got 18.5mpg. Altitude? From Huntington Beach home(644mi) I got 17.1 mostly around or below 55mph because of weather and night driving. 2019 Toyota Highlander V6 all wheel dr.

ThatJakePerry

Over the last two years we've found our average to be around 15mpg. This is with a 2005 Volvo XC70 and a 2004 DM Tab. We've usually got a cargo box or kayaks on the roof of the wagon which definitely helps cut the wind that would normally hit the camper. 

fstop32

I'll have to say I'm in the camp with @Homebodyatheart, no need to add stress to something I want to enjoy.  Having said that I do track the mpg along with miles traveled (for trailer maintenance) and other detailed data because that's what my brain enjoys.  On average I loose 2-3 mpg with my V8 TV.  Now that I've established my "normals" I'm only concerned when they change radically which would indicate a major problem coming down the pike.
Happy Trails!

rcarlson1957

2020 Honda Ridgeline AWD pulling at 2018 T@B 320S. Avg around 15 depending on terrain and wind. Got as high as 20 for a flat stretch with wind at my back and as low as 11 uphill in the wind. In general, 8-10 lesss mpg than when I'm not pulling anything. 

kottum

rcarlson1957 and I get similar mpg with nearly identical towing combos, maybe somewhat higher average when at 60 mph.  At 55 mph better yet. I find that using a low enough gear (S position on the gear selector) improves our mileage because the transmission very seldom shifts up and down for wind or grade variations.

We do use a sway bar as a safety precaution, although this setup has little to no sway tendencies.  However for those setups where the tow vehicle is correcting for sway by applying opposing brake forces, that to me is a hit on mpg if it happens a lot.  I suspect in that case, the sway bar may benefit fuel economy.  Less corrective braking, more mpg.

We also keep our tow combo clean in the air stream.  No stuff on top of truck or trailer, use a lightweight tonneau cover, no extended mirrors (not needed with the Tab 320), no bike rack.

Add up the little things and the savings becomes real.