Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)

[slowmover]

Originally Posted by orbywan

OK, that makes sense. 3.42 sounds low to me but I'm sure you know your stuff. I have a question for you if I may. Where do I go here in Tucson to get my Class C weighed? Do they have weigh stations at some truck stops?

Nationally repeatable results on weight = CAT Scale Search

$10 approximately for first weigh, then $1 for each re-weigh that day. In setting up trailer hitch rigging we may run across the scale several times.

I think the site linked above has instructions on how to use their equipment. PM me with any questions if they don't. It is essentially a matter of having the different axles on each of the three pads.

An "empty" weight (adjusted) is with driver, full fuel, full fresh water and propane plus items always aboard. A "full" weight is with all passengers, gear, supplies, etc. That is the one where I'd do individual wheel position weights to look for FF-RR or side-side discrepancies to be able to dial in tire pressure: instead of an axle average, use the position with the highest weight (with respect to vehicle manufacturer instructions / pressure range).

Capriracer has instructions on his site about pressure rise measurements to dial things in. Load versus pressure is the mantra for trucks of any size.

.

[orbywan]

Originally Posted by slowmover

Nationally repeatable results on weight = CAT Scale Search

$10 approximately for first weigh, then $1 for each re-weigh that day. In setting up trailer hitch rigging we may run across the scale several times.

I think the site linked above has instructions on how to use their equipment. PM me with any questions if they don't. It is essentially a matter of having the different axles on each of the three pads.

An "empty" weight (adjusted) is with driver, full fuel, full fresh water and propane plus items always aboard. A "full" weight is with all passengers, gear, supplies, etc. That is the one where I'd do individual wheel position weights to look for FF-RR or side-side discrepancies to be able to dial in tire pressure: instead of an axle average, use the position with the highest weight (with respect to vehicle manufacturer instructions / pressure range).

Capriracer has instructions on his site about pressure rise measurements to dial things in. Load versus pressure is the mantra for trucks of any size.

.

Thanks slowmover, I'll find out what this thing weighs.

[skyking]

Here is what I could find on your E4OD transmission:
What are the gear ratios of the E4OD Transmission?

1st = 2.71
2nd = 1.538
3rd = 1
4th = .712

Take a gander at those numbers and the OD ratio in the US gear and gear vendors OD auxiliary transmissions. In some cases you will run in 3rd in the ford and OD on the other.

[orbywan]

Originally Posted by skyking

Here is what I could find on your E4OD transmission:
What are the gear ratios of the E4OD Transmission?

1st = 2.71
2nd = 1.538
3rd = 1
4th = .712

Take a gander at those numbers and the OD ratio in the US gear and gear vendors OD auxiliary transmissions. In some cases you will run in 3rd in the ford and OD on the other.

Duh, it's been so long since I ran an additional overdrive I forgot about running in anything other than OD on the E40D. Slow or what? Did it tell you the final drive ratio when running in OD of the E40D? Thanks for the info.

[rickjames8]

Hello,

Not sure if its better netiquette to reply to an old thread or start a new one, but as my question is about this original mod, I thought it would be best to reply here. If it's better to start a new thread, let me know and I'll do that next time.

As far as your class C boat tail is concerned, do you think it would function the same if it just had the sides, and no back? This way you could have access to that space, and even place a cargo box or spare tire in there. Rear lights would simply be mounted to the edges. Any rear window would still let in light.

Also, I've noticed that coroplast seems to be the material of choice for all boat tails. I have always enjoyed working with fiberglass. What is the main reason people do not use fiberglass? It has strength when it hardens, so you could save weight with a lighter frame while still having the structure rigid. I think in the end, the weight might be equal. Thats just a guess.

Lastly, how long do you think a boat tail on a RV would need to be to be very effective? I'm looking at 21-foot RVs primarily because I didn't like driving a longer one when I owned a big class A. I'd like to not add any more than 18 inches to the back, but would that get noticable results or be worth the effort?

Thanks,
-Rick

[BamZipPow]

Originally Posted by rickjames8

Also, I've noticed that coroplast seems to be the material of choice for all boat tails. I have always enjoyed working with fiberglass. What is the main reason people do not use fiberglass? It has strength when it hardens, so you could save weight with a lighter frame while still having the structure rigid. I think in the end, the weight might be equal. Thats just a guess.

I think it comes down to skill sets. Working with Coroplast doesn't take a lot of skills or waiting fer something to cure or setup. It's readily available and is considered low cost. It's a good starting point fer most beginners to work from.

[rickjames8]

Originally Posted by BamZipPow

I think it comes down to skill sets. Working with Coroplast doesn't take a lot of skills or waiting fer something to cure or setup. It's readily available and is considered low cost. It's a good starting point fer most beginners to work from.

I guess it does come down to skill sets. For me, I'm more intimidated by welding a steel frame. Having never welded, it seems like a much more precise skill....

[wyatt]

Originally Posted by rickjames8

As far as your class C boat tail is concerned, do you think it would function the same if it just had the sides, and no back? This way you could have access to that space, and even place a cargo box or spare tire in there. Rear lights would simply be mounted to the edges. Any rear window would still let in light.

I have seen where the optimal shape has a cavity at the back end that is ~12% the depth of the opening's minimal width. I think for our purposes, either is acceptable, but the closed back end keeps dust out, and the OP is planning on expanding his living space (if he hasn't already).

Also, I've noticed that coroplast seems to be the material of choice for all boat tails. I have always enjoyed working with fiberglass. What is the main reason people do not use fiberglass? It has strength when it hardens, so you could save weight with a lighter frame while still having the structure rigid. I think in the end, the weight might be equal. Thats just a guess.

I worked some with coroplast (search for Suzuki Swift Boat Tail). It's fairly easy to work with, and if the shape isn't what you hoped for you can easily shim it to change the shape. It also has a nice smooth surface with no real effort. I built a 4' boat tail and took it for it's maiden run in an afternoon with coroplast. Mine looked ok. Another user has built a 4' out of fiberglass, and it took him something like a year of all his spare time. His looks OEM gorgeous! I guess it just depends.

Lastly, how long do you think a boat tail on a RV would need to be to be very effective? I'm looking at 21-foot RVs primarily because I didn't like driving a longer one when I owned a big class A. I'd like to not add any more than 18 inches to the back, but would that get noticable results or be worth the effort?

I think it would be worth the effort, definitely. Actually, at 18" long, you might be better off leaving it hollow. Welding isn't so bad, and some screws hold coroplast firmly in place, but if you want to use fiberglass and make a real beauty, I will certainly watch .

[kafer65]

One thing, turbo is load sensing irrespective of rpm. If you make more aero improvement s or slow down to reduce aero load the boost will subside even if your at max rpm. Changing to as smaller gear may put you where you have an increased load on the engine and increased boost even when your rpm is reduced. Great job, buy the way. I have an old dodgeslant6 pickup with 3.55 gear and I only get 13mpg empty. I'm so ready to put an airdam and aero bed cover together now.

[orbywan]

Originally Posted by rickjames8

Hello,

Not sure if its better netiquette to reply to an old thread or start a new one, but as my question is about this original mod, I thought it would be best to reply here. If it's better to start a new thread, let me know and I'll do that next time.

Either way is OK RJ. If you make your own thread let me know, I'll subscribe to it, I'm interested to see what you'll do.

As far as your class C boat tail is concerned, do you think it would function the same if it just had the sides, and no back? This way you could have access to that space, and even place a cargo box or spare tire in there. Rear lights would simply be mounted to the edges. Any rear window would still let in light.

Just sides and no back is way more desirable than no sides at all. Mine is closed in the back, with a door, and is great for adding storage space. I have an extended hitch back there also which makes for super support for the back floor. I'm still adapting and building back there but I carry tons of stuff in the boat tail that I wouldn't be able to carry ordinarily.

Also, I've noticed that coroplast seems to be the material of choice for all boat tails. I have always enjoyed working with fiberglass. What is the main reason people do not use fiberglass? It has strength when it hardens, so you could save weight with a lighter frame while still having the structure rigid. I think in the end, the weight might be equal. Thats just a guess.

I think the reason you see so many coroplast mods is it is one of the best materials of choice for prototypes. It is easy to cut, shim, remount, take off and change the frame, or whatever. To me it is a temporary medium (except for the bellypans maybe) to work out the shape I want before I go to fiberglass or sheet metal or foam and fiberglass.

Lastly, how long do you think a boat tail on a RV would need to be to be very effective? I'm looking at 21-foot RVs primarily because I didn't like driving a longer one when I owned a big class A. I'd like to not add any more than 18 inches to the back, but would that get noticable results or be worth the effort?

Thanks,
-Rick

I don't know what the accurate answer is to the 18 inch question. Hopefully several of the experts here can answer that for you. If all else fails PM aerohead, he'll know for sure. My rig is 25 feet without the boat tail, and the tail is a little over 4 feet long. So far the added length has not caused me any problems and I take it some crazy places, everywhere from RV shows to off road locations like Cherry Creek in the Sierra Ancha's in Southern Arizona.

This afternoon I'll dig up a link to the study that NASA did on a 69 Econoline van at Dryden AF base back in the late 60's, I think you would enjoy reading that. A very interesting real vehicle (and wind tunnel) project on the particulars of changing the aero characteristics of a 'brick' shaped vehicle. Good stuff.