Smoothing the gap between pickup cab and bed - Fuel Economy, Hypermiling, EcoModding News and Forum

fourfa · 01-14-2010, 02:19 PM

Hello, first thread here. I tried searching but haven't found anything.

Any ideas on what to use to fill a very large gap between the cab and bed/cap on my 2005 Tacoma? Needs to retain flexibility for severe offroading, and not wear away the paint inside or outside the gap in heavy dust/grit environments, etc. The truck has a shell permanently over the bed, 4" higher than the cab top.

There are some models of caps that use a rubber boot to seal the front against the back window of the cab - I've seen universal scorn for this at destroying paint. Any other thoughts?

ChazInMT · 01-14-2010, 02:51 PM

Hey Fourfa,

Here are some of my thoughts.

A 4" flat step up would seem to be an aerodynamic problem as I would think this would cause a laminar flow separation along the entire length of your cap as well as a fair amount of turbulence. Which is why I’m too sure, being the smarter than average bear that you are,

you have created this post.

I can't think of any way to put something on which is in contact with your cab that wouldn't destroy the finish. Maybe you could adhere a thin piece of metal at the contact point to act as a sacrificial wear plate.

You could fabricate a solid "AeroVisor" to attach to the top front of your cap which extends forward 6 inches or so over the back of your cab and is held 1/2 - 3/4" from the surface of the cab, it would reduce the aerodynamic upset, and not being in direct contact with you paint, would be relatively trouble free.

Either way, you are right to be concerned, the bed of your truck moves a surprising amount in relation to the cab, even during daily road use, not to mention boon dogging.

I’m certain others in this forum will chime in as well with good ideas.

Good Luck.

Charlie

fourfa · 01-14-2010, 03:01 PM

it's not a square step up, it's tapered like this:

Even so, I'd like to smooth it a bit more with a leading edge extending over the cab a little. I'm also concerned about the sides, which have a one inch gap from roofline all the way to the frame, plus additional rounding on the trailing edge of the cab and a sort of 'scoop' shape to the leading edge of the cap.

I was thinking of some clear plastic film to help protect the paint, if anyone knows any brands to search on...

bestclimb · 01-14-2010, 03:28 PM

adhesive backed wear strip on the bed and camper cover going around the edge. Then attach the rubber boot to seal the gap. The adhesive that attaches the boot and the wear strip should be pretty gentle on the paint.

LEADING EDGE TAPES from Aircraft Spruce

3M leading edge tapes

an eraser wheel for when it is time to remove the tape

ChazInMT · 01-14-2010, 03:36 PM

Thanks for the photo helping to illustrate you situation.

From my understanding, air does not need a significant taper in the front for it to be aerodynamic. A gap, as long as the surfaces are in alignment, I don't believe is significant at all either, the air simply skips over this, any airflow into or out of this area will not create an efficiency gain or loss which you would see at the pump.

I could be wrong, but I'd think any amount of money you would spend here would never be paid back to you in efficiency gains.

Again, this is just me.

I found this web site which seems to have a bunch of good info. Poke around, that list on the left seems to have a ton of info at each listing.

ClearMask Automotive Paint Protection And Headlight Protection Film - Auto Clear Bra - Scotchgard Protector - Headlight Covers

TomO · 01-14-2010, 03:59 PM

Window Air Conditioner Foam @ Home Depot
OR

Garage Door Bottom Seal @ Home Depot

And

Outdoor Mounting Tape @ Home Depot

I've personally used the tape on a freshly painted front bumper (my VX) and when I removed it the paint was un-harmed.

I would shy away from using a tape running the gap between the bed and cab as the two will move independently of each other do to frame flex. This will cause the tape to distort and possibly tear.

aerohead · 01-14-2010, 03:59 PM

Not to sound curt,but in the grand scheme of things,I don't think the gap is an issue.
Air,while not very dense,does possess inertia,and once it is directed down the roof an flanks of the truck,it's momentum will just tend to carry it,skipping over the discontinuity.
Sure there's a little penalty but maybe not worth the energy to go after.
Uni-body pickups,as VW's and Honda's don't possess remarkable mpg in spite of their complete absence of a gap.

fourfa · 01-14-2010, 04:48 PM

fair enough. Just hoping to optimize this truck for highway speeds as much as possible, lots of long miles getting to wherever I'm offroading/camping/backpacking/climbing etc. Unfortunately the 32" knobby tires, full-size spares, chassis lift, and lots of gear and tools in the back are not negotiable. I'm pretty clear on how to proceed with a belly cover and grill smoothing, but don't want to leave other low-hanging fruit unplucked.

An experiment with a car-top box yielded a 6mpg loss (FAIL). I have it set up now so I can store it in the bed until I get to the trailhead, then move it on top to make sleeping room. The bigger tires yielded about a 1mpg loss (corrected for true distance). If aerodynamics could get that back, I'd be happy. 21mpg at 65 is the best hwy I ever got fully stock (4.0L V6), which already seems much better than the usual off-roading crowd gets.

aerohead · 01-14-2010, 05:06 PM

Quote:

Originally Posted by fourfa

fair enough. Just hoping to optimize this truck for highway speeds as much as possible, lots of long miles getting to wherever I'm offroading/camping/backpacking/climbing etc. Unfortunately the 32" knobby tires, full-size spares, chassis lift, and lots of gear and tools in the back are not negotiable. I'm pretty clear on how to proceed with a belly cover and grill smoothing, but don't want to leave other low-hanging fruit unplucked.

An experiment with a car-top box yielded a 6mpg loss (FAIL). I have it set up now so I can store it in the bed until I get to the trailhead, then move it on top to make sleeping room. The bigger tires yielded about a 1mpg loss (corrected for true distance). If aerodynamics could get that back, I'd be happy. 21mpg at 65 is the best hwy I ever got fully stock (4.0L V6), which already seems much better than the usual off-roading crowd gets.

The taller cap has aggravated your frontal area,increasing the wake area canceling out some of the gains a cap can give.
If you could exploit something like JEEP's swing-away spare tire holder,you might be able to fashion a short boat tail which doubled as storage area,which allowed easy access to the rear hatch.
You might see 3-4 mpg with this single mod.

fourfa · 01-14-2010, 05:26 PM

the tall cap didn't seem to increase or decrease MPG. However I'm a tall guy, and I can sit up in the back comfortably - function trumps all else with this truck (I basically live in there like an RV all summer/climbing season).

There is a hitch receiver tiregate for around $600. Maybe I could add some storage for gear and gas cans and eliminate the need for the cartop box. Can you describe more what you were envisioning as a swing-away short boat tail? I'm all ears.

It would need a very tight fit to the receiver though. There are full bumper replacements which would be sturdier, but more like $1000. 4 mpg savings could be $4-500 a year for me so it's worth looking at...

also, being a double cab truck with 6 foot bed, it's already unreasonably long for maneuvering. Any length extension would have to not detract from departure angle, and allow other cars to park their front bumper underneath (street parking in San Francisco).