Which way Canned Ham?

[jimqpublic]

No, I'm not about to build a trailer this year... but I keep thinkin' and thinkin...

If (IF) I were to build a trailer, I'm torn between a mega-teardrop (meaning rear outside kitchen), or a Canned-Ham type (meaning inside kitchen). The tiny Shasta parked up the street from me got the thought processes going...

I want the axle to be fairly far back with weight concentrated as close as possible to it for towing stability. I'll probably keep towing with a low vehicle- both of ours have rooflines right at 5' from the earth.

Most of the "canned ham" or "egg" styles I've seen have the taller end at the front. I'm thinking that it might be better to put the tall end at the back. A couple reasons:

-Better aerodynamics. Try to get the pointier end down so the wind from above my car doesn't hit a vertical surface.
-Better reaction to side winds. With the axle set back, the side surface area ahead of and behind the axle would be well balanced so a gust won't tend to make the trailer try to steer the car. (let's say the body is 13' long with the axle 7.5 feet from the front, 5.5 from the rear)

Of course, to make standing headroom at the door, the door has to be behind the axle. My floorplan with this design would be as follows:
-Front dinette/double bed (crosswise) 4.5' of length
-Rear double bunks (crosswise) 2.5' of length
-Off-doorside wet bath (just in front of bunk- minor problem with wheelwell!) 3' of length
-Off-doorside drawers, closet &/or fridge- 3' of length

Doorside-
-Door just in front of bunks- 2' of trailer length
-Kitchen counter/stove/sink- 4' of trailer length.

It could trim down a bit, or if there wasn't a need to sleep four people it could be shorter

[angib]

-Better aerodynamics. Try to get the pointier end down so the wind from above my car doesn't hit a vertical surface.

This isn't correct. If you have any sort of curvature on the front edge, drag will be less if the roof tapers gently downward towards the back, so all those canned hams are the right way round.

Gut feel may tell you a knife shape is aerodynamic, but it ain't. As I've said here before a good few times, you must not confuse what advertisers call 'aero styling' with streamlined design - they are opposites as often as they are alike!

If you put a square edge on the front, it probably doesn't matter which way round it is - it will be awful either way. A radius, preferably a big one,
on the front/top edge is vital to reduce drag.

-Better reaction to side winds. With the axle set back, the side surface area ahead of and behind the axle would be well balanced so a gust won't tend to make the trailer try to steer the car.

More awkward truth: what matters for stability is whether the side force is acting in front or behind the centre of gravity of the trailer, not its axle line. Again, this may seem to disagree with gut feel, but that's how the science goes.

In general a reversed canned ham shape will tend to be more stable as it has more of its side area further back, as long as its centre of gravity is in the same position. But if you move its centre of gravity back as well, its stability will get worse.

Andrew

[jimqpublic]

I knew there was a reason we pay you the big bucks!

[angib]

I knew there was a reason we pay you the big bucks!

What?..... Where?...... Am I missing something?......

Must go - the Rolls Royce catalogue needs looking at again.

Andrew

[cracker39]

JQP,

I went though this with Andrew early in my design phase. I have to agree with his scientific approach. As I see it, there are two problems that affect mileage other than weight…wind resistance and drag. Having a lower front may reduce the first, but the higher rear will induce more drag. Am I correct in this statement Andrew? So, I kept my design as is. The front of mine above the tow vehicle roof line slants back and up, merging into an upward curve on the roof to help reduce wind resistance, then it curves down to the back, which is angled, not flat, to help lower the drag. I liked the idea or a rear door and looked at Andrew’s compact for a while, but decided to stay with my design. Have you looked at the compact? It has a stand up area, without raising the roof to full standing height while driving, and can easily be modified for 2+2 sleeper. I did a drawing of it with a fold away front bunk over the main bed. I didn’t widen it to sleep across, but that is an easy mod.



The higher part of the roof is a pop up section. Check it out in Andrew’s Designs in the Hall of Fame.

And, for all the help Andrew provides to the members here, he deserves the big bucks.

[Arne]

One thing on my tear is my side windows open up as I drive. So, there has to be a vacuum going on there.... and the cheap roof vent used to do the same thing... now the top shape is changed and I've installed a fantastic vent, so not sure about that one.......

If you want to see something strange, the bows of all new ships have a bulbous shape to them, right below the water line.... not a seemingly better knife edge bow. The bulbous bow works so well, they are even retrofitted to older vessels..

http://www.brayyachtdesign.bc.ca/article_bbows.html

Maybe the best shap for a tear is one made from an old propane tank.

[angib]

As I see it, there are two problems that affect mileage other than weight…wind resistance and drag.

Er, sorry to spoil the party - wind resistance = drag. Drag is just the technical term.

Maybe you were talking about pressure on the front versus suction on the back - I think most people can see that pressure on the front slows them down, but can't see that suction on the back has the same effect. That's why most people seem to think only the shape of the front matters - which it does, but not much.

If you want to see something strange, the bows of all new ships have a bulbous shape to them, right below the water line.... not a seemingly better knife edge bow.

What the bulbous bow does is set up a wave that's the opposite of the wave that the bow of the ship would otherwise make - the two waves cancel each other out and so that bit of wave-making resistance is avoided. This only works because a ship is operating at the water-air boundary.

The propane tank bulbous bow might indeed by a cunning adaptation, but it would only be of benefit for trailers that were floating in water. Now you can call me old-fashioned, but I think a better alternative is not to put the trailer in the water......

Andrew, the former naval architect (=ship designer)

[cracker39]

As I see it, there are two problems that affect mileage other than weight…wind resistance and drag.

Er, sorry to spoil the party - wind resistance = drag. Drag is just the technical term.

Maybe you were talking about pressure on the front versus suction on the back - I think most people can see that pressure on the front slows them down, but can't see that suction on the back has the same effect. That's why most people seem to think only the shape of the front matters - which it does, but not much.
Andrew, the former naval architect (=ship designer)

Andrew, that's why I asked you to check my statement. That's exactly what I meant, but, misstated it. I think of pressure on the front as resistance and suction from the back as drag. Thanks for the corrected explanation. See, you just earned the big bucks again.

But, where are they???

[Wright]

Drew,
So with the new info you've just given us.... I have furiously rubbed my last two brain cells together... and am still a little confused... what I think you
said is... that Rik Keller's RoadToad would produce a bit less drag than a standard tear shape? I'm lost....
Wright

btw: the RoadToad is in the hall of fame, sorry I haven't figured out how to work the pic's yet...(hey it took me a couple months to get an avatar up)

[asianflava]

If you want to see something strange, the bows of all new ships have a bulbous shape to them, right below the water line.... not a seemingly better knife edge bow.

What the bulbous bow does is set up a wave that's the opposite of the wave that the bow of the ship would otherwise make - the two waves cancel each other out and so that bit of wave-making resistance is avoided. This only works because a ship is operating at the water-air boundary.

The propane tank bulbous bow might indeed by a cunning adaptation, but it would only be of benefit for trailers that were floating in water. Now you can call me old-fashioned, but I think a better alternative is not to put the trailer in the water......

Andrew, the former naval architect (=ship designer)

The newer Bullet trains in Japan are adopting this bulbous nose.

[angib]

what I think you said is... that Rik Keller's RoadToad would produce a bit less drag than a standard tear shape? I'm lost....

Wright,

I've got to put lots of disclaimers on it, particularly on what tow vehicle is used, but yeah, I would expect the Road Toad to have slightly lower drag as there's some chance of steady flow on its relatively long, steady top shape. But its sides will be just as bad as any teardrop, with that sharp front corner.

By comparison, you could say that the region on any teardrop behind a protruding galley hinge is irrelevant as the flow will detach at the hinge - everything behind that is just sitting in a dirty wake.

I wouldn't say that the Road Toad is as low drag a shape as possible - it would be better if the roof dipped down a bit but behind a tow vehicle it really doesn't matter a great deal.

asianflava,

That's an interesting train shape! I wonder if its purpose is to stop the airflow off the front of the loco scouring the ballast (is that the US word for the stone chips on the track too?)? I went to a lecture on the development of a fast British train and they said that it's actually the joints between the carriages that causes most drag on a train - they're so long and thin that the front shape isn't too important!

If you want nice streamlined train shapes, how about the predecessor to that bullet train:



or maybe the grandaddy of them all (from some tinpot little Yurpeen country whose name escapes me just at the moment):



Andrew

[bledsoe3]

ballast (is that the US word for the stone chips on the track too?)?
Andrew

Yes.

[mikeschn]

GM typically displays all kinds of wierd stuff in the hallway right near where I sit. This week, in addition to some exotic corvettes, they had the cylclone...
Try to design a teardrop around that baby!!!

Mike...

[TomS]

Geez Mike.

How many MiGs did that thing shoot down?

[Wright]

WoW

How the heck could you get the hatch to work with matching fins????

Wright