sepp's second aerocap

[sepp]

Originally Posted by Big Dave

Excellent work, Sepp.

I think you are very close to the optimal shape. Yeah, a boat-tail or teardrp might have a lower Cd, the vehicle gets too long to be generally practical and a pickup is all about general practicality.

I like the curved shape. From the simulator ,the curved shape and the straight cap seem to have about the same drag characteristics, but the curved shape looks soooooo much more graceful.

yup, the simulation showed how much more effective the theardrop shape was. i think something "could" be made, like a solid backpack that clips onto the rear to complete the virtual teardrop shape. and that would only be used when going on vacation or when doing trips that don't require city traffic or finding parking spots

it did seem to me from the simulation that mainly with the straight cap and also somewhat with my previous cap model, that the airflow tended to detach before reaching the end, and that the airflow coming from underneath was sucked upward back on the cap. wich all pretty much didn't happen with this model, but i think the outward curling rear edge of the cap helps preventing the rear turbulences from disturbing the airflow over the cap. i think an inward curling edge, like on bondo's cap looks nicer visually. but i think it encourages the rear turbulences to climb on the cap.

anyway there is already a notable difference on the road, i can coast down longer and i keep my speed much longer. on a downward slope where i used to lose about 10-15km/h when coasting down, im now only loosing 3-4 km/h wich caused some surprises as i need to adjust my driving again because i keep to much speed and need to waste energy through braking because i keep coming on to fast when i need to take a turn or so.

[Thalass]

Awesome! It looks great, and works well. I'm going to have to mimic this design myself in the future.

[COcyclist]

Sepp,

'Glad to hear of your better coasting with the curved cap. On a slightly off-topic note: newer fuel injected cars completely cut-off the fuel supply while engine braking in gear. Depending on how new your truck is you may get better fuel economy leaving the truck in gear down a hill if you have to stop or slow down at the bottom. (You can test you vehicle by driving down a gentle hill in gear with the window down and see if you can hear the engine quit firing)

[sepp]

i suppose mine does cut off the fuel supply when using the engine brake. i never coast down on strong hills where the truck would start accelerating. there i must use the engine brake anyway, otherwise i would be driving all the way down with my foot on the brakes.
but i do coast down in every other situation because the engine brake is quite strong on a diesel truck so if i release the accelerator it brakes way to fast on hills that have anything less than 5% downward slope

[sepp]

little update, got my first tank done with the cap on. i averaged 8,1 l/100 km (29 mpg), wich is the verry best i have ever done. my previous record without a cap was 8,5l.(27.5 mpg) but its safer to say i was closer to 26 mpg on average.
it's not a spectacular increase in FE as i was hoping to reach the 7 liter limit, but i guess it can be explained that the cap's efficiency is multiplied greatly with speed. and i never exceed 90 km/h on highway (56 mph) so i must already be driving too efficiently for the cap to make a huge difference.

anyway i e-mailed nissan belgium yesterday to make sure my pickup is ODB2 compatible (it has the adapter, but its not certain to be compatible as its a 2000 model) so i can finally get a scangauge. i think that i need it to increase my mpg further, because i think i may be driving too softly and in too high gears wich could pull down my FE i heard.

[FastPlastic]

Originally Posted by sepp

little update, got my first tank done with the cap on. i averaged 8,1 l/100 km (29 mpg), wich is the verry best i have ever done. my previous record without a cap was 8,5l.(27.5 mpg) but its safer to say i was closer to 26 mpg on average.
it's not a spectacular increase in FE as i was hoping to reach the 7 liter limit, but i guess it can be explained that the cap's efficiency is multiplied greatly with speed. and i never exceed 90 km/h on highway (56 mph) so i must already be driving too efficiently for the cap to make a huge difference.

29mpg's for the first tank is pretty darn good I know when I started the aero mods on my Jeep, it took me a while to re-learn how soon I needed to take my foot off the gas to glide to a stop sign or what not. So I wouldn't be surprised if you can push it past 30mpg's.

[sepp]

yup 29 is good when thinking that i started with 23 mpg initially. suddenly the magical 30 mpg barrier doesn't seem as unreachable anymore, and there is so much more to improve. i want to delete my mudflaps and make horizontal flaps along the body to catch the mud while not catching air. and then i really want a belly pan, but somehow it scares me. the underside is so damn complex and irregular compared to a car, plus the suspensions can move alot more and i must keep highest ground clearance. the good side is that it has a true chassis that forms a good and solid base to build upon, but it's gonna be a mindbreaker anyway.

[winkosmosis]

Do you have a transfer case skid plate? That would be a good base for bolting stuff to.

[Big Dave]

Your were getting 23 and now you are getting 26-29? That's a 13-26% improvement and you aren't happy?

Greedy rascal!

Of course when it comes to MPG, greed is good.

Your truck sounds like mine - a lot going on under it.

[sepp]

Originally Posted by winkosmosis

Do you have a transfer case skid plate? That would be a good base for bolting stuff to.

yes there is one with a few nice bolt holes in it, but it doesn't reach out up to the front bumber. there is like a big 2 feet gap.
but actually between the front and rear tires the underside is surprisingly flat and easy to cover. the problem is that gigantic rear differential that is one foot lower than anything else, and thanks to the leaf springs it can do some acrobatic up and down and twisting movements.
the front isn't gonna be easy either, my brain starts overheating as soon as i try to figure how on earth im gonna cover such a complex thing.