Factoring drag into design

[PaulC]

http://en.wikipedia.org/wiki/Bernoulli's_principle
If you want to learn about the principles of drag, and the effect it has, I suggest you all have a quiet read at the link shown.
Cheers
Paul

Paul, you trying to learn us something?



Randy

Yeah Randy but I'm also trying to make people realize that it's more a weight thing to worry about and to try and keep it within the confines of the tow vehicle. Once you exceed those limits you're trying to tow through a mini tornado all the time. Any aerodynamics go out the window when your trying to do that. A square fronted van will give almost the same mpg regardless.
Cheers
Paul

[angib]

Harvey (nobody), a couple of thoughts for you about the Scamp:

- The one place where the Scamp is well out of your Tacoma's wind shadow is underneath - it looks like the space under the Scamp is maybe less than half the space under the Tacoma. So half the air that went under the Tacoma now has to go over or around the sides of the Scamp - it's an air dam effect.

- On a related matter, the photo of the Scamp makes it look like the torsion axle may be on its last legs. They rode low from the factory back in the 1970s/80s but that looks even lower. Time to do a jump test - get someone inside to jump up and down and see how much the suspension moves - if there's little movement, the axle is a goner. I'm sure you've been over on FiberglassRV.com where there's lots more information about this, as 20-30 years appears to be the lifespan of most heavily-loaded torsion axles.

[happy_camper]

Happy camper I dare you to build a 3 foot wide teardrop. infact I double dog dare you to build one.

I don't own a dog, much less two dogs, so can't take you up on your kind offer. What if I was unable to build a three foot wide teardrop; how would I pay you, I'm dog-less. I own and use two different camping trailers now so building a third is redundant, and as I said, I don't own dogs.

Another question how big a rig have you drivien in your life time and how big a trailer have you pulled.

Larger than the subject of this builder's thread, but I don't see the point of the question really.

how many diffrent types of trailers have you pulled?

More than the subject of this builder's thread, but I don't see the point of the question really.

Happy camper have you ever owned a rv other than your teardrop?

Yes, but I don't see the point of the question really.

have you ever owned a REAL rv not a home made teardrop?

Yes, but I don't see the point of the question really.

have you ever drivin anything bigger than your half sized toyata truck and have you ever pulled anything behind it over 16 feet long in lenght?

Yes, but I don't see the point of the question really.

do you have any experance to shoot down a persons post to back up what your to claim.

Far from shooting down anyone's post, this is a "forum". By definition, a forum is a place to exchange ideas. If there was only one idea, there would be no need for a forum for others to exchange their ideas. Simple really. As for "backing up" what I claim, I guess I could post my gas receipts for the additional fuel used to tow, but I think almost everyone else here doesn't want to see those, so I'll pass.

Its one thing to pull a tiney teardrop with a small 6 cylender truck and get on here and attack any one that you don't like and its another thing to offer good sound adviese backed by many miles of experance and also commerical driving experance towing somthing for a liveing everyday.

My, my! First all of the questions and now this. Relax. Take a deep breath. Enjoy the campfire! This isn't even about you. It's about Jon's question. I'm sure your life experience will factor into Jon's build decisions, as applicable.

If you want to build that 3 foot wide camper Happy camper I will glady come up with a desinge and draw it out for you!!!!! Slow

Thank you for your kind offer. As I stated above, a three foot wide camper of any sort is currently redundant to my needs. But if you have the spare time to sketch a few designs, and use QCAD like me, please post them on the "forum". Maybe your design(s) can be added to the Forum's Design Library too!

Back to Jon P. Sorry the thread has gotten a little off track, and if I unintentionally led it that way.

You had asked for information about drag and reduced mileage under tow. It seems there is not a clear, unequivocal answer to your question. Others have remarked, and I agree, that even your "dirtiest" design might not be any worse than the commercially built trailers of approximately the same size. As to your current vehicle, I don't know what your actual mileage will end up being when used as a tug. I would doubt your mileage would be halved however. Given that you tow at a reasonable speed, your trailer isn't grossly overweight in relation to its size and at some point you change to a more effecient tow vehicle, I say GO FOR IT!

[Nobody]

Harvey (nobody), a couple of thoughts for you about the Scamp:

- The one place where the Scamp is well out of your Tacoma's wind shadow is underneath - it looks like the space under the Scamp is maybe less than half the space under the Tacoma. So half the air that went under the Tacoma now has to go over or around the sides of the Scamp - it's an air dam effect.

- On a related matter, the photo of the Scamp makes it look like the torsion axle may be on its last legs. They rode low from the factory back in the 1970s/80s but that looks even lower. Time to do a jump test - get someone inside to jump up and down and see how much the suspension moves - if there's little movement, the axle is a goner. I'm sure you've been over on FiberglassRV.com where there's lots more information about this, as 20-30 years appears to be the lifespan of most heavily-loaded torsion axles.

You're correct on both counts Andrew. The 'air dam' effect may be negated somewhat by the more rounded shape vis a vis the TD, hence the similar fuel economy.

And yes, the Scamp does ride lowwww... I've been trying to find out for sure what the original start angle was on the torflex type axle. Weather has been really 'crappy' here lately so I've not been able to pull the wheels to check. I think on these torsion arm suspensions that the square arm shaft that goes into the axle with the rubber, would have been 90* off from the square axle tube. I should be able to draw an extended line between the center of the square shaft & the spindle, then an extended line between the 'horizonal' corners of the square shaft, & thus get an approximate start angle. That's assuming of course that the rubber hasn't deteriorated so much that the shaft has actually 'turned' inside the axle.

Got an e-mail from the Scamp factory that said the original start angle should have been 22.5* down, on a leading arm, 4 lugs on 4.5" hub. That seems a little extreme to me; mine definitely has a leading arm but it's right now at about 20* or so up angle. If there'd been that much change in the angle orientation I'd probably have lost near 8" of height, again assuming about 5.5" torsion arm 'swing', & while it does tow low, it isn't that low. I have approx 2.5-3" clearance between the top of tires & underside of the wheel wells. I'm expecting to find the original start angle was near zero, or possibly 5-10* down. Oh yeah, I'll get my 290lb grandson-in-law over here to do the 'jump' test

In any case I expect I'll hafta change the axle. If I do that I'd want to add elect brakes. I've heard conflicting opinions re the effectiveness of brakes on a leading vs trailing arm. Switching to a trailing arm would involve considerable work to relocate the axle in that the Scamp has a 'drop' floor in front of the dinnette & the current axle is located just behind that 'drop'.

[Wolffarmer]

I don't see any reason brakes on a leading axle would be worse. In fact they could be better. With brakes applied the axle will just "rise" up a bit. My old BMW motorcycles with leading front axless stop real nice and level. but of course that is on the very front of the machine where "weight shift" is very noticeable.

Do they use a special built axle or can most any torque flex type axle be turned around? I have long wondered about building one like that. Could get more drop down storage in the cabin.

Randy

[Nobody]

I don't see any reason brakes on a leading axle would be worse. In fact they could be better. - - - - -

Do they use a special built axle or can most any torque flex type axle be turned around? I have long wondered about building one like that. Could get more drop down storage in the cabin.

Randy

Randy, the way I understand is that Dexter will build an axle to your specs (within their parameters of course). I'd think that unless each axle has a built-in 'toe-in/toe-out' you might be able to turn 'em around?? I'm pretty sure you can't just 'flip' 'em over as they have a built-in 'arc' to set proper camber.

It's pretty obvious to me that I'm gonna hafta replace the axle so after I get the wheels off, check the original start angle, & make full measurements, I'll call Dexter & see if they mfg what I need & for a price I can afford If I can get brakes that'll work with a leading arm at the correct start angle, I figure that'll be lots better (& easier) than trying to re-locate the axle for a trailing arm.

I've been on the Dexter website reviewing the 'torflex' info. Have been able to 'decipher' most of it but I'm still a little confused about the differences between top mount, side mount, & high profile brackets?? Seems each requires a little different measurement & I ain't real sure where to measure. Perhaps I'll just take a pic & measure what's on there now & go from there, especially if I can get what I want with a leading arm.

[aggie79]

Hi Harvey,

I think I may have deciphered Dexter's "code" for mounting options when I was figuring out my torsion axle specs some time ago.

They offer two mounting options. The top mount is for use if you have a "C" or "I" section from. For top mount installation, you bolt up through the axle bracket into the bottom flange of the frame rail. The side mount option is for use with tubular steel frame rails. For these, you have to buy a side mount hanger that is welded to the bottom of the frame rail. For side mount installation, you bolt sideways through the axle bracket into the side mount hangar.

The bracket on the axle is offered in two profiles - high and low. Either profile can be used with either mounting option. For a #9 axle, which is what I used, the difference in height of the bracket is about 1". In other words, the top of the high profile bracket is about 1" further away from the top of the axle tube than the low profile bracket (which is almost flush with the top of the axel tube.)

I hope this didn't confuse things. BTW, I really like your latest acquisition.

Take care,
Tom

[Nobody]

Ah-haaa, thanks Tom! Wasn't sure 'bout that. My axle mounting bracket is welded to the bottom of the tubular frame so it'll hafta be cut/ground off. The more I look at mine & read, it appears it may actually have had 22.5* down start angle. Considering the size of the original tires & pics I've seen of 'road height', a 10 or 22.5 degree down start angle would have shown pretty much the complete wheel & most of the tire. I'd hoped to get the wheels off today but we woke up with light show this AM, & I had to go to the AFB for medicine so that plan was 'shot'. Tomorrow's forecast is possible wintry precip with heavy snow starting tomorrow night thru Wednesday (wellll, heavy for us anyway, 4-6" ) so don't look like I'll be able to get any outside work done 'fore the end of the week at the earliest. I need a good sized shop but that ain't in the cards either.

Yeah, I think we're gonna like the Scamp if I can ever get started on it.

[angib]

I don't believe there is a problem with using a standard torsion axle in leading arm ("wrong way round") configuration. Dexter don't seem to like to advise doing this but one forum member has been told by them that it's OK. Note we are talking about turning the axle around from left to right, not flipping it upside down - so the camber remains correct.

That Flexiride diagram I posted in another thread did suggest they put toe-in on their axles (which would become toe-out when reversed), but maybe Dexter don't.

The one thing Dexter don't appear to do is to build axles this way round - so you have to buy a standard axle and swap the brakes from left to right yourself.

What is not good is leading arm axles with a big 'down' start angle and I think nowadays Scamp may want to do that to give "off road" ride heights, so that may be why they stopped building them this way - back when your trailer was built, they wanted them to ride low to fit in people's garages!

And I think the jump test is more important than the ride height - if it doesn't bounce, it hasn't got suspension and if it does bounce, it doesn't matter if the ride height is wrong.