ultra lightweight trailer chassis concept

[QueticoBill]

I agree the A works fine, especially with plumb cabin front. The floor if built right won't flex and cabin face will doubly assure that. It does need to be anchored.

Think unibody versus body on frame. Or airplane versus house.

Very exciting.

[TimC]

I like this design. I'll be watching closely.

I want to create an ultralight teardrop that my EV can pull without seriously limiting its range. I have an old boat trailer (4' wide between wheels/fenders) and I was going to convert that but maybe I'll use its axle and go this route instead. Anxious to hear from the engineer types to see the precautions of a chassis like this.

Tim

This trailer design is not new - there's some OLD posts around here with the simple A style chassis.

I've been admiring the Pico chassis since I started my first teardrop in 2016. I even researched doing the A-frame out of lumber which doesn't seem like a great idea.

[TimC]

I agree the A works fine, especially with plumb cabin front. The floor if built right won't flex and cabin face will doubly assure that. It does need to be anchored.

Think unibody versus body on frame. Or airplane versus house.

Very exciting.

Qbill, are you referring to a floor built with a proper torsion box design? For instance with ply and laminated ply sticks of ripped ply like Glulam? That seems like it would be an easy, DIY structure. A bit more labor involved in the construction but very strong and durable.

I'm thinking DIY glulam of appropriate dimensions, maybe 1 1/2" square, sandwiched between 3/8 plywood and properly sealed from road splash. Does that make sense?

Tim

[kennyrayandersen]

I agree the A works fine, especially with plumb cabin front. The floor if built right won't flex and cabin face will doubly assure that. It does need to be anchored.

Think unibody versus body on frame. Or airplane versus house.

Very exciting.

Qbill, are you referring to a floor built with a proper torsion box design? For instance with ply and laminated ply sticks of ripped ply like Glulam? That seems like it would be an easy, DIY structure. A bit more labor involved in the construction but very strong and durable.

I'm thinking DIY glulam of appropriate dimensions, maybe 1 1/2" square, sandwiched between 3/8 plywood and properly sealed from road splash. Does that make sense?

Tim

The floor can handle some torsion, although the torsion loads aren't that high (I don't think), but the floor isn't very tall, so the torsion really wants to go into the cabin (think convertible -- even though there ain't much to a hard top, the overall structure is WAY stiffer than the convertible version of the same car). Torsion is reacted by the area reacting the torsion. In a wing it's the area you get when you take a section through the wing xz plane where z is aft z is up and you are using the right hand rule (x cross y = z). That area pretty much looks like a wing rib, and that's the closed area that reacts the torsion. If you take a section cut trough the floor you don't get much area because the floor is only a couple inches deep, whereas if you take a cut through the cabin its' something like 4 ft x 4 ft or more! So, cabin want to react the torsion. At the door cutouts, if they are large, the torsion load would be split between the floor and roof, and differential bending between the floor and roof. In the end -- it's stiffer than it looks.

I've been admiring the Pico chassis since I started my first teardrop in 2016. I even researched doing the A-frame out of lumber which doesn't seem like a great idea.

The A-frame is probably the lightest way to build a chassis as there isn't much that's redundant. The Harbor freight and utility trailers have a LOT of structure that isn't really loaded up much, so in the end, you carry all that excess around. Wood is doable with a bit of engineering, but you have to ask yourself why? White oak is good for 10-12 Ksi in it's grain direction if I remember right, but wood isn't great in bearing, it's going to probably weigh a bit more, and the chassis is exposed to the worst of it weather-wise. If you want the woody look it would probably be better to keep it to the cabin and keep the chassis frame metal of some kind, though it certainly is possible!

[QueticoBill]

I agree the A works fine, especially with plumb cabin front. The floor if built right won't flex and cabin face will doubly assure that. It does need to be anchored.

Think unibody versus body on frame. Or airplane versus house.

Very exciting.

Qbill, are you referring to a floor built with a proper torsion box design? For instance with ply and laminated ply sticks of ripped ply like Glulam? That seems like it would be an easy, DIY structure. A bit more labor involved in the construction but very strong and durable.

I'm thinking DIY glulam of appropriate dimensions, maybe 1 1/2" square, sandwiched between 3/8 plywood and properly sealed from road splash. Does that make sense?

Tim

They were called stressed skin panels in my classes but torsion box may be same. The APA (American Plywood Association) has tech sheets on it. I agree whole cabin is important but each "side" has to be fairly stiff.

Whether the skins - and I presume plywood for teardrops is most practical - are separated by rigid members, honeycomb, foam, or other - is secondary. SIPs are just another variation.

[kennyrayandersen]

They were called stressed skin panels in my classes but torsion box may be same. The APA (American Plywood Association) has tech sheets on it. I agree whole cabin is important but each "side" has to be fairly stiff.

Whether the skins - and I presume plywood for teardrops is most practical - are separated by rigid members, honeycomb, foam, or other - is secondary. SIPs are just another variation.

Stressed skin panels are just panels. The core prevents the facesheets of the panel from buckling so that they can react more load. If you the use the stressed skin panels to make a box, the dimensions of the big box are now what reacts any torsion that is being applied. The stressed skin panel is generally used to react in-plane shear and axial loading as well as they can be good at reaction bending loads due to things like localized pressure, or some other distributed load. Once the big box is built they act together and each panel then would react the shear and axial loads and the overall torsion is reacted by the box, not the panels.

It's easy to do a kind of demonstration by using a shoebox. Without the lid on, the applied torsion is being reacted by the area you get when you make a section cut through the lid-less box (a thin-walled 'U'). We refer to this as an open section. Open sections are very weak in Torsion. Put the lid on and the torsional area becomes the whole area circumscribed by the bottom, sides and lid and the box is now very rigid in torsion (like a wing on an airplane), Each side of the box is then only reacting shear, but the box overall is reacting the torsion. By comparison each side's contribution to the torsional stiffness is miniscule (there is some, but it's tiny).

Now, if I built the same shoebox out of 1/4" corrugated cardboard, the box without the lid would be stiffer in torsion compared to the lid-less thin-walled box because the area I get when I make a section cut is thicker and it has more area. However, if I then put a lid on the corrugated box the stiffness would be very similar to the standard shoebox (assuming the total thickness of the panel skins are the same). The one difference would be that because the skins are stabilized in the corrugated box it would ultimately be able to carry more torsional load before failure (the box cold also carry more bending and axial loads as well).

So, we see that the floor of the tear reacts a few kinds of loading. It transfers load from the chassis to the cabin, it distributes the local loads to the chassis (people laying on the floor), and it acts as one of the walls of the box. As such, it carries very little torsion load because that is carried by the overall box. This assumes that the floor is strong enough to get the loads into the overall box (or cabin). Now, if there is a square door and it spans from the top all the way to the bottom so that there is no wall, then the torsion would be carried by the area of the floor, the roof, and chassis at that point. By retaining area above the door, and making the top of the door round, the side would still function as part of the torque box (although the overall stiffness has been reduced a bit since the side with the door is less stiff than a continuous panel).

This is why, IMO the design of the frame must extend aft past the cutout for the door, so that the frame can help with this torsion. It's also why a box-section (square or rectangle) is a better choice for the frame as 'L's' or 'T's' are open sections and less capable of reacting torsion.

Currently, in my cabin sketch I have crossmembers at floor locations with higher local loading like at the front and back of the cabin, the door entrance (where you sit), between the door and the front cabin wall (where your torso would be located when you lie down), and at the axle location. These locations would also be used as attach location to join the cabin to the chassis with the exception of the aft most crossmember as there is no frame there.

[kennyrayandersen]

Time for some more details

I looked at the chassis near the coupler and I've simplified that and eliminated the 2x2member and combined the (2) 1X2 into a 2X2 to be compatible with the coupler. THe frame would be cut, bent, and new pieces welded in combined with a 1/8 6061 plate welded top and bottom to reinforce the welded area. Since the weld area is very close to the coupler (load introduction point), the post-weld and aged properties will be sufficient to support the applied loads.

ult_lightweight_frame_fwd_detail - sm.JPG (42.91 KiB) Viewed 766 times

For the frame side rail to axle cross member detail, it allows the installation (bolting) of the Flexride 1/2 axles rated at 425 lb (FF-425-1-S). There is a matching plate (FS30-2). The idea would be to mount the plate on top, and the axle on the bottom of the 2x2 cross member

ult_lightweight_frame_aft_detail - sm.JPG (88.91 KiB) Viewed 766 times

flexride FF-425-1-S.jpg (56.79 KiB) Viewed 766 times

The welded are in an area with low bending stresses so the reduced allowable won't be an issue. Note the clips that will joint the frame to the cabin. The fasteners that attach the clips to the frame will be in-line and go through the center of the beam. In this area the bending stresses are very low so the fatigue stresses will be lower than the endurance limits for 6061.

In summary we don't have any welds in high stess areas, not are any fasteners located in high stress areas; furthermore, we don't have any welds located in high stress areas, so the fatigue life should be the life of the cabin! Next we'll look at where we are on weight. I'm waiting to hear back on a weight for the FF-425-1-S (I think the 4-4" version is 25 lb and the 5-4.5" lug pattern version is 27 lb). Can anynoe confirm that?

[twisted lines]

I will try and weigh mine. 1000 K lb

23 lb 11 oz No Grease With nuts.

[kennyrayandersen]

So, I threw together a BOM (bill of materials), with weights. I'll add costs later, although It wasn't as expensive as I thought it might be - especially at the lighter gages.

Trailer BOM with weights
(2) 8’ Al 1”x2” .062” rect. tube – 7 lb
(1) 4’ Al 2”x2” .062” square tube - 2.25 lb
(1) 3”X6” .125” Al plate (2) 3”x3” frame reinforcements - .25 lb
(1) 18” 2”X2” AL angle ((8)x2” frame-to-cabin clips) - .79 lb
(1) 1 7/8” ball coupler – 2.6 lb
(2) coupler bolts 3” 3/8”-24 with nuts and washers - .2 lbs
(16) frame-to-cabin clip bolts (.9 lb total)
(8) countersunk internal wrenching cap screw 3” ¼-20 with nuts and washers
(8) hex bolt 3” ¼-20 with nuts and washers
(1) FR-425-1-S 4-4” ½ axles with hubs – 25 lb
or
(1) FR-425-2-S 5-4.5” ½ axles with hubs – 27 lb
(2) FR30-2 steel plate for mounting ½ axle – 1.21 lb
(12) ½ axle bolts with nuts and washers 3” 5/16”-24 – 1.1 lb
(2) 4.80-8 wheels and tires – 20 lb for 2 (manufacturers weight)
Weight total for trailer (including cabin mounting hardware:
61.3 lb w/ FR-425-1-S, 63.3 lb w/ FR-425-2-S

Trailer jack comments – I couldn’t find one that fit quite right, and none of them were particularly light weight since they were mostly designed for 1500-2000 lb rather than a sub 300 lb teardrop. Keep in mind the wheels and tires were nearly a 1/3 of the total trailer weight! In rel life we used the total weight in all the calculations whereas the weight of the tires and most of the torsion axle weight goes directly into the ground and is not borne by the trailer (or tongue); so, in the end, most of the calculations are pretty conservative) - especially the over-the-road calculations. The sketch of the lightweight aluminum jack follows:

Lightweight trailer jack sketch.jpg (72.18 KiB) Viewed 713 times

Trailer jack BOM with weights
(2) 3.5”x3.25” .19 Al plate - .43 lb
(1) 4.5” 1.75” sqr. Tube .125” wall - .36 lb
(4) 2” ¼-20 bolts with nuts and washers - .1 lb
(1) 12” 1.5” sqr. Tube .125” wall - .81 lb
(1) ¼”x2” L-handle ball lock pin - .2 lb
(1) 3” locking swivel wheel caster
(1) 1" 1.25"x1.25" solid Al sqr. stock - .16 lb

Weight total for jack
3.16 lb

Trailer total with jack:
64.46 lb w/ FR-425-1-S, 66.46 lb w/ FR-425-2-S

With a mostly foamy (and a smaller, simpler configuration at that) -- it doesn't seem too hard to bust the 300 lb goal... 250lb would be nice!

[kennyrayandersen]

I will try and weigh mine. 1000 K lb

23 lb 11 oz No Grease With nuts.

That's actually a bit over a pound less than Flexiride listed on their site so that's good to know -- is that for the 4-4" wheel bolt pattern?

[twisted lines]

5 on 4.5 X 2

[kennyrayandersen]

5 on 4.5 X 2

Interesting -- that would be the 2-S which they list as a couple of lb heavier than the 4-4" version (1-S) they make another non-adjustable version that is lighter -- is your arm adjustable? How long ago did you buy it?

[twisted lines]

Non adjustable,
Had them about a year.

[kennyrayandersen]

Non adjustable,
Had them about a year.

Well, that explains that! I've only seen the adjustable ones for sale so far, and those are a couple pounds heavier! Having said that I think the weight is already pretty low so far!