Aero-Modding the 4Runner

[jeff88]

I am planning on building a roof rack for my 4Runner for a couple of reasons, one of which is for it to be the foundation for some eco mods. After I attach the roof rack, I will build and attach a kammback directly to the back of the RR. This way, if need be, I can take it off. I will also add in a wind fairing at the appropriate angle to help the air go around the top of the truck more smoothly. I played with the truck pic and aero template and this is what I got:



Can anybody give me any insight? Do I have it in the right position? I tried to line up the very back corner and the shape of the windshield with the template, but that leaves the whole bottom out. Is that just a result of the truck being so tall? If I lower the template, then the only way to match it would be to chop the back of the truck off! :P

I want to see how this lines up before the roof rack and then, of course, I will reevaluate once the RR is on. There is a rather large gap from the roof of the truck to the curve of the template, which I plan to fill with the RR and fairing, but is there a way to tell how tall that gap is? I'm not sure how tall I want to make the RR yet, but the template height will aide me in that decision. I want to add the spare tire to the rack, but I don't want the tire to stick up higher than the rack (I am going to make something solid and add it to the top of the rack, so the air will flow well all the way to the back, so the tire can't be taller). With the tire out of the way, I can do a better job on the planned belly pan.

Wow! Over-complicated this as always! Thanks for reading and the help!

[Xist]

Jeff, it seems like you are lining up the bottom of your vehicle with the wheel fairings, not the actual bottom of the template vehicle. Your 4Runner definitely has a high clearance, but it is tall overall, and I think that it scales decently. Also, it looks like the wheels are not completely level, so I adjusted that, and did the Template, temporarily in white, for better contrast, but it looks better in black!


Regarding the bottom of the boattail, I believe that is entirely dependent on the location of the rear wheels and\or bumper. I am not saying that what I did with this Forester was correct technique, but if you are going to have a belly pan, I do not see why you could not start curving the bottom as soon as possible:

Originally Posted by redneck

Instead of removing the existing roof rack, use it to incorporate the boat tail by building the roof line up to make it the new highest point as far forward as possible using the template. Then only a small portion behind the rear of the vehicle will then stick out. Think of it as a hump back whale.

Honestly, I keep trying to figure out how to just do the roof section. It seems counterintuitive that by increasing the frontal area, I could shorten a boat-tail.

I hope that this helps!

[Occasionally6]

Is the Kammback/boat tail the best way to reduce drag on that vehicle though? I'd be looking at reducing the drag from the fender moulds, running boards, the gap between the wheels and fenders, wheels, under the car, and an air dam (the last two would probably have a related solution) first.

[Xist]

Originally Posted by Occasionally6

Is the Kammback/boat tail the best way to reduce drag on that vehicle though? I'd be looking at reducing the drag from the fender moulds, running boards, the gap between the wheels and fenders, wheels, under the car, and an air dam (the last two would probably have a related solution) first.

I doubt that anyone does a boat tail first, it is one of the biggest modifications, both in terms of size, time investment, and looking... weird...

The largest possible modification is driving technique, but for aerodynamic improvements, I think that it would be difficult to beat a boat tail.

You listed areas of drag, but front and back wheel skirts and a belly pan might equal the improvement of a boat tail. With modifications from bumper-to-bumper, you might get attached airflow, but your vehicle will still have a giant wake.

Basjoos has all of the above modifications and plenty more.

[Sporty Modder]

Big trucks, lol. Love them but hate them at the same time. Got rid of my suburban for a Mazda pickup. Best mod for my Sub was removing the roof rack.picked up 20% mpg's.
To reduce the wake consider a rear mounted spare, with a kamm cover. Also consider a airdam, 10% of the work of a full belly and 75% of the returns.

[Xist]

Originally Posted by Sporty Modder

Big trucks, lol. Love them but hate them at the same time. Got rid of my suburban for a Mazda pickup. Best mod for my Sub was removing the roof rack.picked up 20% mpg's.
To reduce the wake consider a rear mounted spare, with a kamm cover. Also consider a airdam, 10% of the work of a full belly and 75% of the returns.

Honestly, I think that lawn edging looks surprisingly good for an air dam. I bought a roll and a box of black self-tapping screws, but I keep trying to not drive my car, while I watch for a Civic HX. When I find one of those, I would be installing the air dam immediately.

On Car MPG Efficiency Modifications Main - EcoModder, it shows

Belly pan / under tray 3.48%
3 user average

and

Front air dam 4.73%
3 user average

As always, your mileage may vary, but I am positive that we have had more than three members make belly pans, and we probably have members that have made three or more air dams. I think that we need to gather data from our members. Also, dams and pans are not necessarily mutually-exclusive. I remember reading that having both is better than one or the other.

Also, I imagine that you could make and live with enough modifications to match a boat tail easier than you could with an actual boat tail.

Prove me wrong!

[jeff88]

Meet Lester <--- Backstory; this truck isn't mine, so anything done to it has to be cool, undetectable and/or removeable. Hence the roof rack, kammback combo; cool and removeable. If somebody has an idea to make an easily removeable air dam (without using screws - no screw holes allowed! ), then I am all ears. The truck is already mostly belly panned, just not as flat as it could be. Remember this is an offroad vehicle, so things like large wheel to fender space and such are going to have to stay for now.

Also, a roof rack will be used for various events (snowboards, kayak, etc.). Even though I will be increasing frontal area, I am hoping I can improve drag to a point where I want it to be and be able to modify it as needed (I can't make major modifications to the truck itself, obviously).

FYI, I toof the cross bars off last night at about a quarter tank left, so will report back any improvements next week after a full tank of driving.

Thanks for the replies guys!

-------------------------
Xist, when I setup the template to the truck I wasn't worried about the bottom at all. I was trying to line up the template to the point where the windshield and roof meet and the point where the roof and rear hatch meet. I don't want to build a boat tail (yet), just a kammback attached to a roof rack. Do you think from the position of my template the rear end angles down too hard, too fast compared to yours, where it is much more elongated? If I add a new/modified belly pan, my plan is actually to start sloping it upwards basically immediately as much as possible, that way when it leaves the rear end, it will already be starting the "boattail". Does that make sense?

I just realized, am I supposed to line up the '0º' of the template to the end of the rear corner of the truck?

[Occasionally6]

Especially since the front bar has been bent anyway, that's a bolt on part. Either mod that (air dam) in anticipation of a replacement or buy a replacement from a junk yard and mod that.

Likewise, the running boards are bolt on, assuming you have somewhere to store them and no one needs them to get up into the vehicle.

The fender lips will be either bolt or clip on. If you can take them off (without damaging them) the mounting points will be perfect for skirts.

[jeff88]

I like that idea about the air dam, I am planning on buying a 'new' bumper from a JY anyway, so maybe I can fab something up that may or may not be removeable before I bolt it on. The nice thing about an air dam is that it gives me a lower bolt-on point for my belly pan idea.

I do have somebody who needs the running boards, so they either have to stay on or I have to get automatic ones (which I really want, but they are overly expensive - I could make them myself; another project, another much later day).

I will try out taking off the fenders on a JY 4R to see if they separate easily. I also have to check how far out the tires go from the sides of the truck. Another idea is to attach skirts, but attach the fenders over the top (that would be a looks only 'mod', a nice hybrid of looks and practicality).

[Xist]

Originally Posted by jeff88

Xist, when I setup the template to the truck I wasn't worried about the bottom at all. I was trying to line up the template to the point where the windshield and roof meet and the point where the roof and rear hatch meet. I don't want to build a boat tail (yet), just a kammback attached to a roof rack. Do you think from the position of my template the rear end angles down too hard, too fast compared to yours, where it is much more elongated? If I add a new/modified belly pan, my plan is actually to start sloping it upwards basically immediately as much as possible, that way when it leaves the rear end, it will already be starting the "boattail". Does that make sense?

I just realized, am I supposed to line up the '0º' of the template to the end of the rear corner of the truck?

You are supposed to line up 0º with the high point of the roof and for my car, that is the end. For yours, it might be also, but unless you make the roof taller, and it sounds like you plan to, then you would not start boat-tailing until the back of the vehicle.

The way we did the template on yours, you would already be tapering at 8º from your new high point, while my car would be at 0º. Transitioning from a belly pan to a boat tail should be idea. I believe this is how the bottom would be, if you built a boat-tail:

Assuming, of course, that is just the muffler tip below the bumper, and you would want that outside of the template.