Project Aero Car Trailer

[skyking]

Hello from a fellow trailer tower.
IF (note I use the big IF) you had a clean slate, you could build a functional popup style trailer that lowers right down over the Miata, but pops up when you get to the track. I know, probably out of the time and $$$ budget, but this is the place to dream, right?
Can you enclose the trailer tires with skirts?

[Fat Charlie]

Originally Posted by mateospeed

...and the fact that the bed is currently open, and offers the potential to soak everything in the bed when it rains.

I think there's the most effective aero mod. You need a cap. Slap a cap on the truck and you've faired over the worst of the problem. A nose fairing and side skirts can follow, but a cap will stop you from doubling your frontal area unnecessarily. Plus, you'll get more storage and sleeping space.

[mateospeed]

So, I have to post once more before posting a link or image. Here's post #5.

-Matt

[mateospeed]

Got it. Imageshack saves the day, once I coverted it to a JPG. They're different sizes, I'm not sure why, but click the first one to see it full size. It is 2 sheets, not the same image.





Enjoy my amazing drawing skills. Yes, I know, I should've been an artist. I'm flattered that you think so!

[mateospeed]

Originally Posted by Fat Charlie

I think there's the most effective aero mod. You need a cap. Slap a cap on the truck and you've faired over the worst of the problem. A nose fairing and side skirts can follow, but a cap will stop you from doubling your frontal area unnecessarily. Plus, you'll get more storage and sleeping space.

I think I'm going to jump on this bandwagon. In the current setup, I'm pushing the truck through the air, with its Cd of, let's say 0.5, then an additional portion of a box, at Cd (pretty dang close to) 1.0. Now, the whole trailer face isn't whetted, but about 48 sq ft of it is directly impacting the air, judging by the bug splatter.

Then, if I add a bed cap, I'll probably see no significant drag increase as far as the truck is concerned. However, I'll have decreased my frontal area on the trailer by 10 sq ft. (3x5 cap, but the airflow will get sucked back into the gap b/w truck and trailer). I'm just not sure that's enough to make a significant difference. At about $400 for a used "extra headroom" bed cap, it's difficult to justify. I also don't have a good basis yet for estimating the aerodynamic gains to see how long it'd take to make a return on the investment.

Now, if I add a wing/ 3 wings to it, I could very effectively drop my frontal area of the trailer to very little, and the portion of the trailer that does see some air is mostly rounded. I'll take pictures the next time I go to the storage unit (which is soon, because I left my jack stands in there!).

Does anyone have any experience with gains made by adding a bed cap alone? And how much difference would a standard (cab) height cap vs. a slanted or extra-headroom cap make? Hmmmmm..


Thank you all for taking the time to scratch your heads with me. It's nice too see!
-Matt

[mateospeed]

Originally Posted by skyking

Hello from a fellow trailer tower.
IF (note I use the big IF) you had a clean slate, you could build a functional popup style trailer that lowers right down over the Miata, but pops up when you get to the track. I know, probably out of the time and $$$ budget, but this is the place to dream, right?
Can you enclose the trailer tires with skirts?

That would be pretty cool! Yeah, it's probably just a dream, but it would be pretty neat. Yesterday, I saw a Honda Element cruising down the beach road with a pop-up roof, like the VW campers have/had. It made me think 2 things: 1) Cool! and 2) If they can do it, i could do it for the trailer!

Maybe one day, when I'm a big-shot manager somewhere. Or maybe not, since I don't really want to be in management.

Yes, I could enclose the tires with skirts, but the tires, wheels, and brakes are already getting warm. I'm hesitant to consider that option, since higher tire temps equate to quicker wear rates. That would quickly negate the savings in aerodynamics.


On another note, the website for NoseCone has the attached image demonstrating Cd's for several vehicles, including faired and un-faired trucks. Does that appear to match other research y'all have seen?

-Matt

[KamperBob]

Originally Posted by mateospeed

I've heard a ton about Mr. Hucho, and hadn't even considered finding his book at a library. I simply wrote it off as too expensive to purchase, and didn't think it further.

---

Again, I forgot to mention that the 8.5x8x24 trailer is pretty much necessary for interior space. I wish it wasn't so heavy, but I got it for a steal, so it's what I've got.

---

When you say 'major surgery', are you referring to add-on aero mods to the exterior of the truck, or transplanting engine/ transmissions into the truck. If you're talking about the powertrain, I'm going to agree. I've run a few examples, and it's WAY cheaper to trade. I'll never find an ideal vehicle, but it'll be cheaper overall.

Matt, thanks for the added context. We can't possibly help much if we're not on the same page. And we are about "major" surgery to the truck.

Are you currently using a ScanGauge (et al)? I know you said you tightened the nut behind the wheel. I did too but the ScanGauge showed room for improvement I missed prior. Keeping the transmission torque converter clutch locked, for example, makes a big difference in fuel economy; even though ATF temperature is my first priority when towing. Real time metrics to predict when overdrive was okay versus preemptively locking drive helped immensely. Quantifying the costs of cruise control versus gaming the hills was beneficial too. All good.

Originally Posted by mateospeed

Then, if I add a bed cap, I'll probably see no significant drag increase as far as the truck is concerned. However, I'll have decreased my frontal area on the trailer by 10 sq ft. (3x5 cap, but the airflow will get sucked back into the gap b/w truck and trailer). (snip)

Now, if I add a wing/ 3 wings to it, I could very effectively drop my frontal area of the trailer to very little, and the portion of the trailer that does see some air is mostly rounded. I'll take pictures the next time I go to the storage unit (which is soon, because I left my jack stands in there!).

Does anyone have any experience with gains made by adding a bed cap alone? And how much difference would a standard (cab) height cap vs. a slanted or extra-headroom cap make? Hmmmmm..

I smell a rookie mistake about terminology. Frontal area is a feature of end-on silhouette. Nothing added (fairings, etc) reduce it. Instead they reduce Cd. Drag area is the product of the two which can be decreased accordingly. Not trying to nit pick but this terminology it's pretty important to get clear and make sure you're having the same conversation and not risk talking past others.

I recently borrowed Hucho's compendium from my public library. They ordered it from a university. It took a couple weeks and reference desk staff had to use advanced search tools beyond patrons at kiosks. But it was worth the wait. Section 4.4.13 Car with Trailer will especially interest you regarding leading edge effects. Section 5.6.2 Road Tests has good work on cross winds. Several other sections offers piecemeal gems. I'm planning to buy a copy. It's that good.

[mateospeed]

Originally Posted by KamperBob

Are you currently using a ScanGauge (et al)? I know you said you tightened the nut behind the wheel. I did too but the ScanGauge showed room for improvement I missed prior. Keeping the transmission torque converter clutch locked, for example, makes a big difference in fuel economy; even though ATF temperature is my first priority when towing. Real time metrics to predict when overdrive was okay versus preemptively locking drive helped immensely. Quantifying the costs of cruise control versus gaming the hills was beneficial too. All good.

I don't have a scangauge, or anything like it. I'm hesitant to buy one, since I'm planning to ditch this Tow Vehicle and go to a diesel manual when possible. I'm assuming the scangauge will not transfer to a diesel, but this might not be a good assumption. I've been doing my best to judge the load on the truck, whether or not to unlock the coverter ascending hills, etc. Transmission temp never seems to change much, perhaps because it has a giant cooler, and is a 3/4 ton to start with. I very rarely use cruise control, btw. It almost always throttles up, drops into 3rd and unlocks the converter when starting uphill, whereas I would rather not, unless it's a very long, steep hill.

Originally Posted by KamperBob

I smell a rookie mistake about terminology. Frontal area is a feature of end-on silhouette. Nothing added (fairings, etc) reduce it. Instead they reduce Cd. Drag area is the product of the two which can be decreased accordingly. Not trying to nit pick but this terminology it's pretty important to get clear and make sure you're having the same conversation and not risk talking past others.

Perhaps I'm not being clear on the way I'm approaching the problem. I'm considering 2 different thought paths: one of seperate truck and trailer bodies, and one with a connected truck and trailer.

If considering seperate truck and trailer, its seems a viable thought path to consider the trailer to have a Cd of near 1.0, but only be partially wetted, if that makes sense. If it this is simply an incorrect way of thinking, then that's acceptable, and I'll drop it. When adding a bed cap, you would effectively reduce the "effective" frontal area of the trailer.

Now, all that said, I've come to a realization: none of that makes sense. All vehicles have some gaps in them; even the truck has a gap b/w cab and bed. The seperate vehicles and effective frontal area approach does not follow accepted aerodynamic principles. I will indeed drop that thought path. Thanks for helping me get there!

SO, what I'm looking at are different ways to reduce the Cd of the entire rig, by smoothing the transition from one frontal area "block" to another. That makes more sense. Now, where's the smiley with a lightbulb?!

Originally Posted by KamperBob

I recently borrowed Hucho's compendium from my public library. They ordered it from a university. It took a couple weeks and reference desk staff had to use advanced search tools beyond patrons at kiosks. But it was worth the wait. Section 4.4.13 Car with Trailer will especially interest you regarding leading edge effects. Section 5.6.2 Road Tests has good work on cross winds. Several other sections offers piecemeal gems. I'm planning to buy a copy. It's that good.

My library is pathetic. "we don't have it." Is about all I'm getting. No response to "can you get it," "is a transfer possible," or any talk like that. They looked at me like I was completely out of line for suggesting such a thing. Perhaps I approached it from the wrong angle... time to send in the hot wife:-)

-Matt

[euromodder]

Originally Posted by mateospeed

Yeah, match the streamlining on the front of the trailer with the rear of the tow vehicle .

Reduce the wake of the trailer a fair bit with a boattail.

I wouldn't flare out the sides much beyond the rounded corners of the trailer.

Add wheel skirts.

Make everything foldable, especially the top spoiler, so you don't have the extra drag of the increased frontal area when not towing the trailer.
If you make it so that the top spoiler and sides can angle down when not towing, you'll decrease the wake area and increase the MPG of the single truck as well.


Close the gap between truck and trailer with a vertical baffle plate on the trailer front - one on the centerline can go right up to the pivot point on the hitch, without interfering with turning and backing up.
More, smaller plates can also help to reduce the cross-flow.

If the set-up allows for it when turning, extend the vertical sides further aft to better close the gap between the truck and the trailer.
(only illustrated in plan view)

[KamperBob]

Hucho's compendium is helpful. He references work by Kunstner showing interactions between tug and trailer. Compromises are inherent. The optimal teared tug bobtailing takes a bigger hit when towing. A more SUV/wagon tug interacts better (FE wise) with trailer. His wind tunnel pix clarify what problem front transition effects solve: bow wave splash from too-blunt leading edge. Quantification by Peschke and Mankau is consistent and further shows that sloped trailers are better at keeping hook load down to control sway with speed (safety first) but a deflector proved better for FE. Waters showed that when it comes to front rounding a small percent does a lot of good but a lot does very little more (diminishing returns). That's all in Chapter 4. Hucho rocks!

Other research shows more drag coming from the rear of truncated boxes (think big eddy currents and vacuum effect) than the front. So funneling down rear flow offers more bang for the buck. Some thread on this site referenced a great summary of this by Kevin Cooper from CNRC in a 1976 article for Cycle magazine. Airstream and Don-Bur both apply that knowledge (plus some other trailers though I don't know any from Elkhart, IN).