Aerodynamics / Tow Hitch Spare Tire Mount

[kpaske]

I've got a Yakima Load Warrior with a full size spare. There is a wind fairing on the front of the rack, but I'm sure that this setup has to have an effect on my gas mileage. I'm considering getting a tow hitch with tire mount, and possibly having a new fairing designed. This setup would also have the added advantage of allowing me into more parking garages, as my current setup has me at about 7'. Most parking garage are about 6'8", which is just barely enough room for a lifted VX with the Yakima Load Warrior.

My questions:

How much aerodynamic benefit would I get from a) moving the full size spare to a rear tow hitch mount and b) designing a new fairing for the Yakima Load Warrior. Would it significantly improve gas mileage or acceleration?

Has anyone seen a rear tow hitch mount for a full size spare that would allow the rear door to open without removing the spare?

[Tone]

I have seen spare carriers for hitches that fold down so you can open the rear door - check with your local hitch place or online.

[kpaske]

Any thoughts from anyone on the aerodynamics issue? I figured at least Joe_Black might chime in on this one! Anyone?

[Tone]

The wind deflector on the basket will help if it is tall enough or the back your really be best and cause the least amount of resistance.

[Francesco Rizzo]

yeah, what's up Certificate to Flight Instruct.. ok.. so we're dealing with moving a large mass, the tire, through a fluid: air. Now, everything I was thought on drag pretty much always coincided with lift being generated by wings, but many principles will apply here too. There are several types of aerodynamic forces at work in regards to objects moving through air and creating "drag." The fact that the air wants to adhere to the surface of the truck is viscosity. although this friction caused by uneven surface of the tire causes some drag, it's nothing when compared to the wake created by the frontal area of the truck + tire + rack.. Basically, the truck pushes the air out of the way as if moved through it. The time and distance it takes for this air to get back together is what kills the mpg's because now the engine has to meet this increased negative force with more power from the motor.. gas
Also, the wing on our truck I would think is more of an "energy" adder than an actual down forces making the tires stick to the ground. This is important because the tire forward of the air stream colliding with the "wing" is no longer free stream, or still air, and reduces the effectiveness. The truck will always create a wake, so throwing the tire in it will create a smaller frontal area.

This is where it gets tricky though. A very big part of this picture is the speed at which truck is accelerated to. That increase will result in drag increasing at an expediential rate.. double speed get 4 times drag.. That's where you would notice the difference in fuel required, at high speeds.

to answer the question, you will notice a change in fuel, but the amount Iam unsure of. but as for the acceleration.. no.. not unless you intend driving really fast

[kpaske]

OK, so if I understand you correctly, moving the tire from the roof to a tow hitch mount would make a measurable difference, particularly for highway driving (long trips at higher speeds). This is what I suspected... Now, saying that I move the tire, how much additional benefit would I get from either removing my Load Warrior rack, or possibly designing a new wind fairing? I've always wondered whether that wind fairing, which only covers the front of the rack, but leaves a large gap underneath through which a lot of air passes, could be improved if it extended all the way to contact with the roof. Would covering this gap improve the aerodynamics or create more drag? What about if the angle was greater allowing air to flow over it better, or even if the fairing were removed to allow air to pass through the rack (as it would be empty most of the time)?

Here's a pic to help visualize my setup:

[Francesco Rizzo]

The fuel difference I believe might not be all that big of a difference, unless you take into consideration the lifetime of the vehicle and extended time at speed on the highway. I mean you will see a change, but then what is that compared to the money spent on the roof rack and then the hanger for behind the door. If you do a lot of city driving, it will porbly be more cost effective in terms of time and money to leave it as is. If you count every penny at he end of the night before you go to bed and that's all you value, switch it to the rear...

I'd say keep it as it is though, but for me the looks and utility get's the value. The cost of the parts to put the tire behind the door will most likely be much greater than the savings on fuel. I mean what are you looking at in mpg with the set-up now?

[kpaske]

I'm getting about 15 highway; but usually less because I do mostly short trips around town most of the time. I really like the look now, but I hate not being able to park in most parking garages when I'm in the city. It's not really all about the pennies (or else I'd be driving a hybrid or some other tree hugger car), but I guess I'm just trying to justify another mod. Plus I want a tow hitch anyways.

[Francesco Rizzo]

honestly, I just spent soo much money on a new exhaust and instake for the added 2 or 3 miles per gallon... so pennies add up is what I meant to say.. I get around 20 on the highway... at least before my exhaust.. it was actually 19.5 or something driving down from NY to Florida. I had JUST gotten the transfer case flushed and all the TOD fluids changed.. I think my new mpg is better but I it the gass a lot more.. which made things worse..

Keep the rack on the truck.. it's dirupting the air but not to the extent of what the tire does my making the overall stance of the truck taller.. Moving that might help a lot concidering you only get 15 mpg... that's kinda bad.. I almost think you should check some things out.. gas filter, timing and whatnot... goodluck

[kpaske]

Well, I just changed the fuel filter prior to a trip from Seattle to San Francisco and back (about 1600 miles). I use Amsoil, but admittedly don't change oil every 3000 - I change the oil filter about every 5000, and top off the oil as it burns off, but I haven't done a complete oil change in quite a while. I guess we could discuss whether this is a good idea or not, but I've been convinced from reading several articles (not all published by Amsoil, mind you) that using Amsoil and doing complete changes less often is actually better for engine longevity. At any rate, if there is something else that is causing me to get worse gas mileage than I should, I'd really like to figure out what it is.

[Daver]

My Load Warrior + spare on the roof costs me about 3mpg overall.

-Daver

[MrCrowley]

The aeronautical guru is correct, but dont forget about the mass of the tire and the prospective locations. A tire on the roof raises your center of gravity, and your handling suffers a little. Plus the aerodynamics. A tire mounted in the back will lower your COG, and improve your aerodynamics while driving down the road. But just like on the roof, the tire in the rear has negative effects also. Instead of a higher COG, you are increasing the mass behind the rear axle which will lead to more "pull around" when handling although it will also increase downward force due to gravity. Probably the same effect as a hitch mount receiver, then double it for the tire. This will also affect braking very slightly. When braking hard, less weight will tend to shift forward, which will cause the front to dive less and maintain a better stance when maneuvering and braking. This is a plus, but minor. Also the departure angle from obstacles is a factor.

All these forces are minimal, but will help you choose the right location for the right situation. 4wheel on the roof, cruise on the roof, high speed driving on the back, road trip on the back!

[colmhd]

Here's a thought get one of those aerodynamic luggage packs things and cut the front to fit across the spare and the front of the rack that will give you at least some aero dynamism.
Other than that build one.

Just a thought.
colmhd