Teardrops n Tiny Travel Trailers

Micro-tear definition: what a man sheds during Toy Story 3 i.e. wife - "what that you crying" man - " you didn't see nothing!"

I going to go through a design exercise for a microcar teardrop. I currently have 2 microcars:

A 1963 FIAT 500 D

A 2004 smart fortwo convertible

There is a bit of a tradition with these microcars, as well as others like the Isetta (which I hope to one some day!), where they are able to tow extremely small cute things behind them like teardrop trailers, or small boats.



Obviously, these cars weren't really made for towing, and they also have a very limited amount of horsepower, and braking power! The FIAT 500 had just 17.5 Hp and the Isetta only about 10 Hp (US models were 300 cc single cylinder and European models were just 250 cc). Even the more modern fortwo only has just over 60 Hp, so getting the teardrop as light as practically possible is critical to making it at all useful. Additionally, the FIAT 500 is only 52 in. wide at it's widest (that's the fender flares!), so a 48 in wide trailer will be in consideration of the FIAT, since it's the smaller of the two microcars

I've seen that there are several light designs out there, but with very few numbers or actual analysis behind them. I'm a degreed mechanical engineer by education and have spent the better part of a full career doing structural analysis of flight vehicles; so, maybe I can bring a fresh prospective in how to design a very lightweight teardrop trailer, that is not exotic, not particularly expensive, nor should it be difficult to build. The key will be starting from scratch, at the bottom with the chassis, and working our way up with an understanding of how the structure works together. There's obviously been a lot of water under the teardrop bridge, and I'll certainly miss Andrew Gibb's practical design input, but much of it might be considered the coalescence of so many people's historical input into a coherent design. The key to an overall low-weight design is integrating the teardrop cabin, and frame, together as a package. This will hopefully reduce overlapping or redundant structure.

I thought it might be interesting for people to see the kind of typical process that an engineer goes through and the decision-making process of going through so many options to ultimately end up with a final design that supports or satisfies the initial goals. With absolutely everything there is always considerations of cost, schedule, manufacturing form, fit, function, and compromises in all of those at every branch of the decision-making tree. In the end, you should be able to get something that satisfies the goals while keeping the showstoppers at bay. It's not much use if you can save a 1/2 pound if that 1/2 pound costs you $10K! So, let us begin...

In this case the goals would be (kind of, but not necessarily in order):
1) light weight
2) easy enough to build that it could be done in a garage with 'usual' tools by semi-skilled individual
3) reasonable cost
4) no exotic materials (see 3 above)
5) outsourcing for welding is OK, or not so complicated that a hobbyist couldn't do
6) sleeps two full-sized adults
7) robust build that would be expected to last a lifetime
?

Overall configuration:
Wants:
width: 48" wide (minimize weight and drag)
Length: L<7 ft (84 in)
mattress: long enough for 6 ft 4 in person (76 in long)
Chassis: A-frame
height: tall enough to sit up in
2-doors

Don't need:
Galley (too much weight and complexity)
Heat
AC

For now lets leave virtually everything else open (materials, construction techniques)

I'll start with the chassis -- that thread can be found here: https://www.tnttt.com/viewtopic.php?f=35&t=75602&p=1299706#p1299706

This is the initial notional layout - evolving and subject to change

I initially laid it out with 8 in. tires, but I'm leaning toward the 12 inch tires as that is the tire size on the FIAT 500 (125/12).
Note at the front of the trailer the front wall goes straight down. Aesthetically, it's a slight compromise (IMO), but the distance to the ball is reduced, which lowers the stress on the tongue.

After reading your chassis thread, I'm surprised your sketch suggests so much framing. Thin ply well adhered to foam or honeycomb - stressed skin panels - should just about eliminate framing and a lot of weight.

Looking forward to this one.

I agree. I think beyond framing in the base you really only need extra support for the door, hinges and latch, and the vent fan. I used more structure than I needed. It was my first time working with foam. What wood I used was mostly western red cedar (extremely light).

The curve of the roof creates a stress that provides a good deal of structure. As always testing and experimentation
is recommended.

pchast wrote:I agree. I think beyond framing in the base you really only need extra support for the door, hinges and latch, and the vent fan. I used more structure than I needed. It was my first time working with foam. What wood I used was mostly western red cedar (extremely light).

The curve of the roof creates a stress that provides a good deal of structure. As always testing and experimentation
is recommended.

QueticoBill wrote:After reading your chassis thread, I'm surprised your sketch suggests so much framing. Thin ply well adhered to foam or honeycomb - stressed skin panels - should just about eliminate framing and a lot of weight.

Looking forward to this one.

Both your points are well-taken and the feed back is appreciated. The sketch is just that at this point -- a sketch. As long as there is a good connection between the walls and the floor, and the floor is attached well to the frame, there isn't much cabin framing that should be needed. I think at the door (for hinges and latches), it would be nice to have something for the screws/bolts to attach to. Depending on material selection most of the framing should disappear, or at least be greatly reduced.