Truck aero help please

[Telco]

Originally Posted by freebeard

I don't get the string falling short of the bed part.

I read a bunch of different studies on truck aerodynamics, and all the ones I read had an angle for the rear cab spoiler that appears to follow the line you would have if you ran a string from the top of the cab to 6 inches short of the bed. The air attach lines showed the air off the cab hitting the last few inches of the bed. That's all it was.

[freebeard]

An interesting result. Do you have any way of directly measuring the downforce? I looked at the thread for Julian Edgar's throttle stop testing technique, but the info is in two Youtube videos that have been deleted. I did find a description at ecomodder.com/forum/showthread.php/throttle-stop-testing-diesel-38964.html#post639782

At around 75MPH the truck starts struggling a bit to maintain speed (clapped out V6) when it normally doesn't start to struggle until around 90MPH... It was also tested up to 95MPH

Help me understand; 'start to struggle' 15 MPH sooner, but you took it 20 MPH faster.

[Telco]

Originally Posted by freebeard

An interesting result. Do you have any way of directly measuring the downforce? I looked at the thread for Julian Edgar's throttle stop testing technique, but the info is in two Youtube videos that have been deleted. I did find a description at ecomodder.com/forum/showthread.php/throttle-stop-testing-diesel-38964.html#post639782



Help me understand; 'start to struggle' 15 MPH sooner, but you took it 20 MPH faster.

Nah, no way to measure other than the seat-o-meter, and for my purposes a seat-o-meter is sufficient. Same with the speed. But, since at speed I stop getting the harsh ride those load adjusting shocks give when unloaded, say maybe 500lbs at highway speeds? I have no idea what a thin piece of cedar floor planking will handle. Dimensions are 40 inches by about 4 inches by half an inch thick, supported every 20 inches. Whatever it handled, it was not enough to damage it, but enough to feel it.

Not sure what there is to not understand about struggling though. If you need an analogy, treat it like carrying a heavy load that is well within its weight limit vs unloaded. A truck will carry a rated load without difficulty at lower speed, but when you get to higher speeds the weight starts affecting performance. The performance effect for weight is felt across all speeds, but the effect increases exponentially with speed - the faster you try to go, the more the weight hurts performance. A spoiler's whole purpose is to add downforce to a vehicle when moving, and it sees downforce as weight. What's more, the faster you go, the more the downforce, or weight increases. So, I start noticing a dropoff on performance at high speed, at a lower speed with the spoiler than without it.

My weight listings are total guesses based off me knowing how my truck acts when carrying loads vs unloaded. When not being used as a test mule it's my farm truck and I work it hard.

[freebeard]

Thanks.

I can start feeling the effects at around 30MPH, and it is very noticeable over 50MPH. At around 75MPH the truck starts struggling a bit to maintain speed (clapped out V6) when it normally doesn't start to struggle until around 90MPH

Whatever the metric (call it the 'struggle point') fell 15 MPH. Probably aero drag rather than 'weight'. Was 95 the top end?

[Logic]

Originally Posted by Telco

OK, experiment 1 is in progress. I decided to do the underspoiler first because it was the easiest to figure out mounting. I used steel strapping and assorted hardware to build a non-permanent mount. The side straps are attached to the factory bumper mounts and will allow me to adjust the attack angle. The actual spoiler is a piece of cedar flooring, 40 inches long by, I dunno, 4 inches wide. And man does it look like a mess. This thing is not pretty even with me trying to disguise it with some flat black paint. Since there's nothing underneath to direct air, I had to hang it low enough to see some air from under the axle. The truck I'm testing this for won't have that problem as it's IRS and lowered.

And, I can report that it is a resounding success despite being miles from optimized. I can start feeling the effects at around 30MPH, and it is very noticeable over 50MPH. At around 75MPH the truck starts struggling a bit to maintain speed (clapped out V6) when it normally doesn't start to struggle until around 90MPH. The most telling part - the truck has new helper springs on the rear shocks and when unloaded rides like a kidney beater. When I put enough of a load in the bed to collapse the springs off their full extension, the truck rides like a Cadillac. I start getting that Caddy ride at around 50MPH now. It was also tested up to 95MPH, and did not deflect, warp or come apart. When cornering, the truck felt quite a bit more planted. I adjusted the angle after today's test, taking it from about 40 degrees or so (unmeasured) to maybe 30 degrees. I figure a properly designed and built one will be effective at even lower speeds, without affecting top speed. And, I have some design ideas that will allow a limited about of speed based angle adjustment.

I did not drive it enough to test for mileage, and am not sure what the effect will be. Normally I see 17.5 to 18MPG driving the way I do. On the one hand, a spoiler is wind resistance. On the other hand, the wind it is resisting is already screwed up from passing under the truck. Further, it is deflecting air into the area directly behind the tailgate. Once I get the angle set to the most it can be without affecting my normal cruising speed I'll run it for mileage to see what it does. This will take a couple of months due to my driving habits - I work from home so really don't drive much. If mileage does not decrease, I will call this a success due to the noticeably better handling.

Did you round the leading edge of the piece of wood any?
From the pics all I can tell is that there was no sharpening of the trailing edge.

Your departure angles will suffer a lot from a 'wing' that low down.
Have you considered moving it up a bit to compare the 'seat of the pants' results?

[Telco]

Originally Posted by freebeard

Thanks.


Whatever the metric (call it the 'struggle point') fell 15 MPH. Probably aero drag rather than 'weight'. Was 95 the top end?

95 was as fast as I went with it, although the truck will (eventually) do 110. I wanted to make sure it wouldn't fly apart in front of someone else. I half expected it to at least bend the steel mounts a bit but they remained as rigid as they were when installed.

A wing/spoiler generates downforce which is measured as weight on the car. Drag feels like weight, so it's a useful way to think about it.
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Originally Posted by Logic

Did you round the leading edge of the piece of wood any?
From the pics all I can tell is that there was no sharpening of the trailing edge.

Your departure angles will suffer a lot from a 'wing' that low down.
Have you considered moving it up a bit to compare the 'seat of the pants' results?

The wood I used is a piece of cedar planking that is designed to interlock with the piece next to it, so it was somewhat rounded on its own but not from what I did. The trailing edge is sharpened in that it is the female side of the interlock. I took one of the sides off so it has a trailing edge of sorts. Well, I took a side off because the wood had a split in it and I didn't want it getting bigger. The trailing edge is kind of hard to see because I also sprayed the whole thing down with matte black paint, but it's there.

I'm not concerned with departure angles because it's probably close to a foot higher than the ground where it is. Dodge made these trucks kind of tall from the factory. That being said, since this test has shown (to me at least) that this is effective, the next version will be quite a bit cleaner underneath. For the truck I'm testbedding for, the departure angle will be slightly more important as it will be lowered quite a bit with the lowest part of the truck 5 inches from the ground.
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Update: I adjusted the angle by raising the front by one hole on the metal supports. This flattened the angle by enough that it doesn't feel nearly as effective now. I'm not really seeing a benefit until highway speeds, and the struggling to maintain speed is now higher than 80MPH where I normally live on the highway. However, the truck doesn't feel quite as stuck to the road as it did with the sharper angle and then it's only at highway speeds. The problem here is the board is about 3.5 inches long and the adjustment holes are 1 inch apart so, eyeballing it, moving it one hole adjusts the angle of attack by 15 degrees. I've run half a tank through on this, and will run through about 3 tanks before I start seriously working on the next version.

Also, feel free to ask questions but don't expect quick answers. My internet time is limited in the summer. I probably won't even start on Mk2 until winter hits.

[freebeard]

The wood I used is a piece of cedar planking that is designed to interlock with the piece next to it, so it was somewhat rounded on its own but not from what I did. The trailing edge is sharpened in that it is the female side of the interlock.

AKA tongue and groove. It sounds like the mounts are adequate. Cedar would respond well to a hand plane, to make an airfoil cross section.

[Logic]

Originally Posted by Telco

95 was as fast as I went with it, although the truck will (eventually) do 110. I wanted to make sure it wouldn't fly apart in front of someone else. I half expected it to at least bend the steel mounts a bit but they remained as rigid as they were when installed.

A wing/spoiler generates downforce which is measured as weight on the car. Drag feels like weight, so it's a useful way to think about it.
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The wood I used is a piece of cedar planking that is designed to interlock with the piece next to it, so it was somewhat rounded on its own but not from what I did. The trailing edge is sharpened in that it is the female side of the interlock. I took one of the sides off so it has a trailing edge of sorts. Well, I took a side off because the wood had a split in it and I didn't want it getting bigger. The trailing edge is kind of hard to see because I also sprayed the whole thing down with matte black paint, but it's there.

I'm not concerned with departure angles because it's probably close to a foot higher than the ground where it is. Dodge made these trucks kind of tall from the factory. That being said, since this test has shown (to me at least) that this is effective, the next version will be quite a bit cleaner underneath. For the truck I'm testbedding for, the departure angle will be slightly more important as it will be lowered quite a bit with the lowest part of the truck 5 inches from the ground.
================================================== ==============

Update: I adjusted the angle by raising the front by one hole on the metal supports. This flattened the angle by enough that it doesn't feel nearly as effective now. I'm not really seeing a benefit until highway speeds, and the struggling to maintain speed is now higher than 80MPH where I normally live on the highway. However, the truck doesn't feel quite as stuck to the road as it did with the sharper angle and then it's only at highway speeds. The problem here is the board is about 3.5 inches long and the adjustment holes are 1 inch apart so, eyeballing it, moving it one hole adjusts the angle of attack by 15 degrees. I've run half a tank through on this, and will run through about 3 tanks before I start seriously working on the next version.

Also, feel free to ask questions but don't expect quick answers. My internet time is limited in the summer. I probably won't even start on Mk2 until winter hits.

The drag and lift or downforce will change DRAMATICALLY with shape.
To the point where your 'seat of the pants-o-meter' will probably notice!
NB the Cd numbers alone here differing by a factor of ~25!



Then there's lift and the lift/drag ratio.
As things stand the 'wing' is likely 'flying stalled' which is WAY more drag for very little lift.

ie: Proper shaping of the plank to a wing shape should have a dramatically different effect to the point where it changes your perceptions/decisions on angle and when the vehicle is slowed and feels loaded by it etc.

Also obviously; if the 'wing' was closer to the full width of the rear you would very likely get the more comfortable, loaded feeling for way less angle (drag) and not get that slowed down feeling (and worse economy) until a noticeably higher speed.

ie:
I fear that your 'seat of the pants-o-meter' is being misinformed and getting it wrong thx to a very bad shape and then 'wing' length..?