More power vs. less wind resistance? need better MPG.

[Rockcrawler]

I've been towing about 20-25k a yr for the 03, 04 and 05 race seasons.



03 had a cap/topper and a 30" v-nose 24' enclosed trailer

04 had the same truck and a 20' enclosed, no v-nose.

05 ditched the topper and so far running a 38' 5er race trailer with living qtrs which is a foot taller.



Mileage seems to be the same on all of them- 8-11 towing at 65-82 (7 over is typical for me). biggest variable is speed and headwinds. can get pretty sig. headwinds going west on 40 or west on 80.



On the flipside, just ran empty up to Desmoines, 410 miles each way. 19. 5 up, 20. 5 back with a tailwind, 77 mph.



I've not tried the airtabs, but never saw enough endorsement other than theory to try and or "ugly up" my trailer.

 

[RKerner]

Interesting. I found the exact opposite to be true on my '85 Toyota 4wd. I lose MPG if I take the shell off.



In the original poster's situation I would recommend a shell that tapers front/back with AirTabs on the shell and rear of trailer.



Heard/saw an interesting presentation many years ago at Cal Poly, SLO from an engineer with Peterbuilt (I think - might have been KW) regarding the aerodynamic testing they were doing on big rigs. They were able to increase MPG from 4 to 8 by adding wheel skirts/ground effects, a ball/socket type joint between the tractor & trailer and a 'teardrop' at the rear of the trailer.



Brian

I'm retesting the topper off mileage. I'll know in a month or so. My tundra 'seemed' to get better mileage with the topper on but it didn't do it consistently. So, I guess the jury is still out on that for my stuff.



The aero treatment you describe is exactly what would benefit most any vehicle -

• wheel skirts and ground effects - if you keep air from creating drag on the underside of the vehicle, you reduce overall drag. (auto OEMs spend big dough on this right now) One increased benefit is that you reduce the pressure gradient acting on the underside and just past the rear of the vehicle, which will reduce pressure drag (induced drag) . This is related to the effect Reno racers use by running as close to the ground as possible to go as fast as possible - the wingtip vortices, which create significant drag, are sort of run into the ground rather than curling around the wingtip and adversely affecting lift. Just make sure when talking to an aerodynamicist which pressure you are labeling 'ground effects'. Aero guys think 'enhanced lift' from ground effects, car guys think more downforce with ground effects.
  
• teardrop - the teardrop is fundamentally the most efficient aero shape. Its minimal frontal shape, optimum slenderness ratio (cross sectional area change versus length), and long tail to minimize the pressure gradient from the front to rear of the shape makes for minimized pressure drag and skin friction. A topper that sloped frotn to rear would help, along with about another 3 feet of extension to reduce the pressure gradient at the tailgate. This would make for a challenge in trailer towing but will theoretically fix some of the issue with drag.
  
• trailer connection - I'm guessing here but the friction in the typical fifth wheel joint is considerable and one can get incremental gains by doing less 'work' to turn the rig by using the ball and socket joint.
  

with the mention of wingtips and such, CoastyAV8r's radar will pick up the thread and help me with my aero explanations.

 

[rluft]

What are Airtabs and how do they work. Do lots of people use them? Thanks

AIRTABS are vortex generators. I understand that vortex generators are sometimes added to airplane wings to improve STOL characters. On the trailing edge of the trailer it creates a bunch of little 'tornados'. As your trailer punches through the air it leaves a hole that creates a suction on the back of the trailer. The 'tornados' allow air to break the suction on the back of the trailer and increase fuel mileage, keeps the dirt off the back too.



At least that is the theory.