Caravan Aerodynamics on a Budget

One of the things I want to achieve with Wanacat is obtaining the best practical Aerodynamic performance within "normal" van shapes. We are going to achieve this by:

• Reducing frontal area as much as possible (reduce width and height)
• Streamline without impacting building ease and sapce
• Boat hulls are very streamlined, but hard to build with
• Too streamlined, and you loose usable space

On the first point, we might be able to reduce area by 15% without too much (over a 2.4x2.1 standard height), but the main thrust (drag?) will be streamlining:

• No bluff front
• A nice gentle deviation of airflow from the car to the van sides
• make that fron area curved (no flat surface, or a very small one)
• Use the fact the bed is longways at the front to swap headroom for sloping front (unless you need to stand up on your bed...). You see this style of bed in catamarans and 5th wheelers
• Fairings around protrusions
• Little things add up (eg - mirrors on cars get a LOT of attention)
• Fair into awnings, windows, ...
• Don't forget air goes around and under the van, not just over the top
• Apparently up to 1/3 of the drag on a car can be caused by the underbody flows
• Covers/fairings over the entire underbody. My motorcycle would not be as smooth at speed without a thin layer of plastic streamlining over the engine.
• tuck the van plan shape into the airstream behind the car, not just in profile
• Spoilers and stuff
• Tricky this one - need a wind tunnel to measure effectiveness
• Simple things like little deflectors in front of the wheels might assist (ever noticed that 2 inch plastic mudflap in front of your tyres? That's an aero aid)

So, it's all good to have that, but how do you know if it works: Answer is windtunnel testing and CFD (Computational Fluid Dynamics). Oh, you don't have them? Neither do I. CFD may be a longer term goal (along with FEM structural models) but a wind tunnel is just too costly.

With the power of a smart phone, OBD2 interface on you car, and the appropriate roads, it might actually be feasible to do some relative measurements:

ODB2 is the engine diagnostics that comes out of that D plug near your steering wheel on modern cars, and can give fuel consumption, rpm etc in real time, and for about $40, you can Bluetooth it to your smartphone. In the smartphone is GPS (speed), accelerometers etc, so you can do run down tests, steady speed monitoring of fuel usage, and down hill coasting. Do all these without the van first as a reference, then with the van. It would at least allow scientific comparison of vans (if not absolute figures), and perhaps allow effectiveness of spoilers, vortex generators and other "add on" aids to be measured.

A run down, or coasting test is where you coast to a stop (or near) from speed on a flat road. At lower end of the speed, rolling resistance is the main contributor, and at higher speed, it's air drag. A variation is to coast down a hill, and measure your terminal velocity, or rate of acceleration. Your smartphone can help work out the slope, and determine how much drag is present. You do need accurate mass measurement though, as terminal velocity depends on mass and drag. You need the right slope too. You also need a very still day, with identical atmospheric conditions each time.

A great way to combine a love of technology with the caravan business. Looking forward to this part of the process greatly.

Here's Some links to useful info:
Instructables: Measure the Drag Coefficient of your Car

Some thoughts on stability

This is a repost of some stuff on my nerd blog:


Youtube is full of videos of caravans getting out of control, much to the delight of Top Gear fans (and hosts). One of my pet projects is the design (and hopefully construction) of a large (6m) lightweight caravan, which will be the subject of another post (or three). Anyway, one of my concerns has been the stability of the end result. What makes it a good tow, will it be safe to tow.

One thing that I've wondered about is the origin of the 10% rule that is part of caravan culture. Nobody has directly explained where it might come from, but many rightly state that it's not a rule, just a recommendation. The rule says something to the effect that "You must have at least 10% of the van's weight on the towball to be stable". This rule just didn't seem altogether right as an absolute, both from the perspective of the  load factor and the idea of absolute stability. In particular, my tow vehicle does not like heavy ball weights (poor load carrying springs), so I'm aiming to keep that as low as practical in my design.
What I have found is that the boffins say that essentially  all conventionally coupled and proportioned caravans can become unstable. The system is like a double pendulum, and at some point of operation, it can just go nuts with the right input and oscillation. The live variable that changes a stable towing experience to unstable is speed. Something that settles quickly at 90km/h might become uncontrollable at 100km/h with the same disturbance. What makes one van stable at 100, and another unstable is very complicated, and the change in mode is very rapid. Most mathematical models need to represent the system with 4 degrees of freedom to even explain basic dynamics, and many more if you want to include the vehicle.

The critical things that can result in poor stability seem to be (in no particular order - numbers for reference only):

1. centre of mass not far enough,or not in front of the axle 
2. a relationship between radius of gyration and product of distances between ball to Cm and Cm - axle (Rgy must be less than product)
3. large moment of inertia (either massive van, or high mass around periphery - Collyn Rivers talks about mass vs weight)
4. large mass (particularly wrt the tow vehicle)
5. large overhang on tow vehicle
6. aerodynamics matter

These all make sense, but I think that is largely in the context of a normal caravan. The first two are strongly related, and probably the same thing. The first can be directly used to say that there must be some weight on the ball, as the ball is a lever holding the front of the van up. If the centre of mass is over the axle, then we could effectively have zero ball weight. A normal van like that is typically massively unstable. However, if we look at the interpretation of that from the perspective of #2 (distance between CM and axle is zero in this case), we need to make sure that the distance from the axle to the ball is large. Imagine  a simple van represented by a massive blob of steel sitting on the axle. With a short drawbar, you can imagine it might be able to bounce around a bit (up and down as well as side-side), but with a very long drawbar, it will be reluctant to swing around (take it to infinity if you like). I can't see any reason why a zero ball weight van could not be stable with a long draw arm, but you may bang into practicalities like tow length and structural stuff.

The rest are mostly common sense (big weights keep moving once started, and big levers exert more torque).

So what would my van look like to achieve stability? I think it might have the following attributes:

• overall weight as low as possible (minimalist approach - it's a caravan, not a house on wheels, just make it warm, dry, pleasant and avoid taking the whole box and dice)
• front half (or more) of the structure would be very light, and avoid extra mass (small/no storage areas)
• most of the weight would be centred around (and slightly forward) of the axle
• most storage compartments would be clustered around the cenrtre of mass (low intertia)
• axle would appear to be set well back. The front of it is really part of the drawbar, and it's weight relative to the whole van should be small, as if the front was a tent sitting on a long draw-bar
• heavily sloping, streamlined front to move centre of effort from side winds back towards axle (but will make the front accomodation a bit like a boat, but that's no bad thing). This also give aerodynamic damping. As airflow becomes more oblique, drag increases, forcing the van in a straight line. It also reduces the force from trucks and the like

Here's a few references that are pretty interesting:
Vehicle Stability, Dean Karnopp, Marcel Dekker Inc 2004
Collyn Rivers paper on Vehicle Dynamics

Update 24/6/2011: Well bugger me - just stumbled across the Evergreen Element caravan in the US - Almost exactly how the idea brewing in my head over the last 4 years was panning out. 24ft van with a tare of about 1400kg .... and they thought I was dreaming.

Can an off-road van be a lightweight?

Yes, Wanacat thinks it can.


Firstly, I'll perhaps qualify off-road as anywhere you might comfortably take an unmodified 4WD, and further qualify it as something that would not void the vehicles warranty and only require basic recovery gear.

So why do I think lightweight is good? With heavily built construction, you get a spiralling, self-defeating escalation of mass:

• Heavy Body means heavy chassis
• Heavy chassis means heavy suspension
• Heavy overall weight means heavy recovery gear
• Heavy means massive loading, particularly when parts of the van that don't normally contact the ground make contact with the ground

Do you remember the trusty old Suzuki Sierra, or perhaps the Mini Moke. Those vehicles have gone places that have left Land Cruisers (which are awesome vehicles btw) buried, just because their light weight meant they didn't sink in the bog, it meant that they didn't snap suspension dragging many tonnes over a rock. It also meant that if they got stuck, light gear could recover them.

It also meant they couldn't carry all the worldly possessions of the owner, which may be a problem, but I have my own views on that, but for hard-core off roaders, doing extended trips away from services, they do need to carry a lot of stuff (and carry stuff they don't need). So in extreme cases, your payload is massive, which will probably put you back in the spiral of weight growth.

But what if you feed that spiral with the same construction methods we use in light vans? I think it can be done, particularly if you confine the heavy lifting to the suspension and draw-bar components.

What about repairs in the field? Easy. You won't get cracking or other failures in the body due to flexing (the body does NOT flex at all, ever, unless you overload it) but if you do, a little epoxy and cloth will probably fix it (permanently). If a rock penetrates the composite in an unprotected area, the energy is absorbed in that area. It does not bend the whole van or cause widespread damage. The cores used are not water absorbent, so provided there are no holes right thru, you can just motor on, or apply a temporary cover to be tidied up when you get home. There are no places in the body that fail in a way that causes catastrophic structural failure of the whole unit - it's a distributed structure, with every load spread over a large area.

There's still plenty of work to be done, and some proving to do, but I'm confident that the concept will progress  from tarmac/soft-roader that we are designing now to something that can go nearly anywhere your showroom 4WD can, and perhaps beyond that.

When is a 6m caravan not a 6m caravan

When it has the same mass and rotational inertia as a 3.5m caravan


That's what we're aiming to acheive at Wanacat. When you tow your family van with bunks, shower, permanent double, it should behave exactly like a little camper (without beds) at the end of an extended draw bar. Perhaps if you imagine a typical windup camper (with full headroom), you will get where I'm coming from. When you think about it, provided that camper can store all your stuff, the beds and shower are effectively voids, and should not add significantly to the overall mass and weight of the van.

Using this model works well, but to remain faithful to the idea, we need to limit storage, particularly storage that is outside that camper size envelope. What does this mean:

• No traditional boot at the front. Make a place elsewhere for that stuff, or leave it at home
• We do have a tunnel just under the foot of the front double that will replace this.
• Gas bottles and spare tyre get moved from the drawbar (or rear bumper) and are moved as close as practical to the centre of mass of the van
• Spare will be under the bed-head, in the middle, with 2 small lockers either side
• Gas (2x9kg) will be in a sealed cupboard right next to the appliances that use it
• Wardrobes around the bed will be a little different to what you're used too (as is the double bed). There will be plenty of space, just not right at the end of the van.
• Rear of the axle, there are no heavy items, just bunks and the shower, with some storage for light items (clothes etc).
• The loo is oriented so the holding tanks are as far forward as possible (ie least mass over the rear)
• The bunks are at the very rear (sideways)
• they're then only heavy when being occupied, which is fine. Tue weight distribution of a parked van is not that important, its a van in motion that is a problem.
• No bumper bar, no bike rack, no external lockers.
• Put the bikes on the roof of the car, or if you want, we can conceal a roof carrier style mount on the floor in the middle of the van, or a tie-down point.

Can you live with that? Perhaps not, but we've found that our family can. We have spent weeks at a time cruising the coast, and get by without packing ALL of our belongings. Next time you unpack after a trip, do a quick audit of what was used this trip and what was untouched. Also do this for stuff that is permanently in the van. When was the last time it was used, when is the next - do you really need it? It's quite refreshing to travel light.

Cheers

Is your van well hung?

Caravan suspension seems a forgotten aspect on most vans. They wobble about, and bounce around when things get a bit rough or windy. Why do we have suspension, and what features are important to look for.

Early vehicles had directly attached axles. Every bump, hole and cornering force was transmitted directly to the vehicle. The resultant forces applied to the body made for a bad ride, increased stresses on all the components (wheels and body) and poor dynamics.

The next step was to spring the axles, allowing the wheels to move independently of the body. This helped enormously, but without some damping, the ride was "softer" on the body, but dynamics were poor as oscillations could occur, leading to wild instability. If the springs were too hard, the system was no better than a fixed axle, and too soft mean the suspended part repeatedly smashed into the body.

The other big change in dynamics was to spring the wheels independently. This allows the wheels to follow their own obstacles without transferring motion to the other wheel. It also allows the path of the wheel to remain in-plane, so no gyroscopic effects are involved.

So what do we see on caravans today? All of the above. There are some well suspended systems, but an awful lot of over-speced or under loaded slipper springs that do not control the wheel properly, or transfer every motion into the van body. The three key types are slipper spings, air suspension and some sort of spring (lumping rubber springs and coil springs together herre).

Slipper springs work well right? Well, yes, but with limitations. They often rely purely on the spring leafs for damping, and only work smoothly for a limited range. They are cheap, easy to fix and replace though.

Rubber springs (like AL-KO) appear to be excellent. Reliable, smooth and controlled. They are tricky to fix though, but work well. Similarly, coil springs (with shocks) provide excellent performance inside the working range. And that's the key problem I have - working range. Light european vans typically use rubber torsion springs. It is a contributing factor in their carrying capacity. the working range is about 20% (30 300kg payload in a 1200kg tare van). Similarly, coil springs are like slipper springs - limited load range (for smooth operation).

Thats why I had to choose air suspension. It's adjustable. The Wanacat design nearly doubles its weight between dry and full (850-1400kg), and something like Criusemaster Air allows you to tweak for your current load, which means you always have maximum wheel travel available (fixed springs will nearly always have you at some sub optimal position). You also have minimum force imposed into the chassis and body too.

So in short, adjustable suspension is a must for a light van that needs to carry reasonable loads. It's important that all the impacts and vibration remain in the suspended components. This allows lighter bodied vehicles to carry more payload, carry it safely and carry it smoothly and stably. A stable, smooth van means less fatigue and less stress, which makes for a better holiday.

Why Go Light?

When you are lightweight from head to toe and properly suspended, it makes things easier:

• compliant suspension can absorb the bumps better
• When the van is forced to change directions rapidly, you have less weight to move, so forces are lower (and strength required is lower)
• You are more stable. Heavy vans feel stable, until that fateful day when they suddenly become unstable due to the wrong combination of loading, speed and disturbance. Once they get swaying, you're hosed. Keep it light, and it's kept in check, but load distribution is no less is important.

Towing is cheaper too. I expect to obtain figures similar to what cna be acheived with a 1200kg "Low Tow" van, that has a height of about 2m: Estimated Towing Costs (based on Hyundai Santa Fe 2.2R)

• Flat Highway cruising (100kph) - 11l/100km
• This is the big unknown for us right now, and is hard to estimate without a prototype, but this target is feasible.
• Stop/Start traffic  about 12-14l/100km
• this figure can be estimated by adding the percentage weight increase of the combined mass of the car and van due to the van and applying it to van free figures, so if you get 8/9 l/100km around town (at 1900kg, and add 1100kg - you'll use about 60% more fuel
• Hilly Touring (lower average speed, up and down) - We expect to be in the 12's with the Santa Fe here, which is what we achieve with our 1200kg low profile van at the minute

How to make a strong lightweight caravan

There is a feeling out there that light vans are not strong, and perhaps that's true. If you use traditional van building techniques in a lightweight manner, you get low strength. Staples and screws need support from a rigid chassis and frame, as they have no ability to handle loads on their own and light chassis (overloaded) tend to fatigue and crack, or just straight out bend.

However, if you throw that away, and make an integrated structure, where everything is strong, light and bonded together (like a catamaran), you get a very light but very strong structure. All your cupboards, benches, beds and seats are miniature structural beams that produce a very stiff, self supporting box. Your chassis is reduced to something that you bolt your (high quality) suspension onto, and connects you to the tow vehicle.

Staying light extends to your payload too - when you load your van, you need to consider if you really need it, and if a lighter alternative is available. It's all about keeping it simple, but complete. Another advantage of a light base weight, is that you get more payload.

So what sort of materials do we use:

A "classic" van consists of a drawbar attached to a large steel chassis, with suspension attached to that. The body is attached to that, and furniture screwed inside, which works well. It's flexible to build and adaptable to a variety of layouts. The chassis tends to be heavy, as it has to protect the body from twisting, or the chassis is light, and you have to avoid too much rough and tumble.

Our design is similar, in that you need a body, wheels and a drawbar. Our drawbar runs from towball to suspension, and is light, as the van body attaches much closer to the towball, so the bending loads are reduced allowing lighter sections to be used (although we still use a section similar to what you see on a 1400kg van drawbar for peace of mind).

The body (in our prototype) uses foam sandwich construction, with epoxy/glass sheeting, just like large motor yachts and catamarans. It's tough (epoxy is much more durable than polyester resin - does not crack and craze easily). We use medium tech foams to keep the price down, but for "off road" vans, we'd go higher tech to deal with the bigger impact loads, and perhaps increase payload, but at minimal eight gain.

The front bed forms a giant composite "I" beam, as to the cupboards and seats, is fully bonded and loads are distributed appropriately.

This makes the body stiff enough and strong enough that we could potentially attach the suspension to the body directly and similarly, almost do away with a steel drawbar. The caravan market is a bit too conservative for that just yet though, and we are talking about 80kg of steel, which is not much (but could be another water tank).

The closest analogy is that of a boat on a trailer. The trailer supports the boat, but the boat is strong enough that the trailer only has to provide a distribution point for the boat's weight. We have the additional advantage in that we are bonded to the drawbar, so can manage all sorts of other loading (like cantilevering he rear section)