Underfloor and Body design of Greenpower Car (similar to Electrathon)

[Bicycle Bob]

Don't write off underbody flow as a mess of large-scale turbulence. While there may be a lot of disturbed air near the ground in a breeze, you can also think of the ground plane as a layer of enforced laminar flow at free-stream speed. Your own laminar surface, if present, will be tripped by the front wheel openings, spreading across at 15 deg. You have to use a slight rake to allow for the progressive increase in thickness of the boundary layer.
Laminar flow is what got the Mustangs to Berlin, but it isn't easy. You need almost-shiny or better surfaces of near-optical precision to specific curves, protected from vibration. This is not a realistic goal for an inexperienced builder, but keep it in mind. It isn't hard to get laminar flow on the 1st 10%; it just keeps getting harder to maintain on the way to 70%.

[stealth]

Thanks Bicycle Bob and Joe for your messages.

This is the side profile of my body so far. The top of the bodywork increases in height until about halfway which is where the cockpit opening starts and I don't have much control after that due to all of the turbulence in the cockpit.

The channel underneath as seen in the picture, starts quite late, just in front of the front wheels and has a smooth transition from the nose to the fairing. The ground clearance at this point is the maximum I can have it at and it does increase in height at the rear, producing a slightly low pressure area underneath, which is at a lower pressure than the airflow on outside. I think this helps to reduce the amount of airflow bleeding from the channel to outside the body and keeps the airflow at the rear relatively neat. Hopes this makes sense.


The cockpit opening and the roll bar is fixed, can't do much about them.

One of the disadvantages of using tiny 305ERT wheels is that they have almost no deflection in the wheel and the smaller circumference makes it have higher rolling resistance. I have thought about using minimal suspension, but the amount of space I have inside the car means that chassis flex done correctly would be better solution.

Joe of Loath - I always have wondered what's it like to drive one of those, I raced closely against one with same design but in carbon fibre at the final of 2013 and it looked good, is it slightly more unstable in the corners or does it not matter too much? Also did you ever use or experiment with derailleur gearing?

[Bicycle Bob]

I see a huge variety of roll bars - surely you can minimize that drag. I don't see windshields and side windows, but I don't see a rule against them. Even with the increase in frontal area, completing the cockpit with an attached vortex looks like a win to me, if done well.

One nice thing about small wheels is the reduced surface area friction, but I'd choose wheels based on available rubber. I'd also think about adding a belt to any tires I could find, and doubling the pressure. That makes suspension more desirable. You want to keep the roll center very close to ground level to avoid undue scrub. With narrow tires, camber control is less of an issue for traction, but any suspension can help or hinder cornering speeds, depending on how it is done. Solid axles are a good way to get started with suspension, but you MUST have a bend in your tie rod, giving it a bit of flex, or the Axle Tramp can wallop you.

[freebeard]

https://en.wikipedia.org/wiki/Smith_Flyer

The traditional way to do a flex frame is as a 2-dimensional plate. Bamboo is a good suggestion.



Bike frames are made from bamboo with joints wrapped in hemp tape. You could also use basalt tape. Bamboo plywood in 1/4-1/2-3/4". You could CNC your single A-arm suspension pieces from this.

One of the disadvantages of using tiny 305ERT wheels is that they have almost no deflection in the wheel and the smaller circumference makes it have higher rolling resistance.

Deflection radially or laterally? Your cross-section shows a flat transom instead of a horn.

[stealth]

Only way that I can minimise the impact of the roll bar is to reduce the size of it. I can't have any fairing or aerodynamic aids on the top 150mm of the roll bar and this would include the helmet fairing. Therefore I only have 150mm of roll bar exposed above the fairing as I did on my last car. There has been trend in the last few years to make the roll bar smaller and hide them behind the helmet.


The racing is done normally on airports and racing circuits and therefore for a small car, the track is large, smooth and doesn't have many tight corners. Almost no teams run suspension, probably because it's not really necessary and can add weight and complexity to the car. Here is a video of me driving a previous car in 2013 final to show what a normal track and race would be. Also shows that only minimal steering range is needed.

https://www.youtube.com/watch?v=yKg6vsMjsU4

Nothing is allowed above the driver's eye level, even windscreens aren't allowed above that level due to this rule.
T7.4. Bodywork, including windscreens, to the front or sides of the driver’s helmet may not be higher than the eye level of every driver. This does not include front wheels or their coverings.

The 305ERT wheels would have less deflection up and down than 16" or 20" wheels and especially with my wheel which has a 28 or 32 spoke count, which is high for a such a small wheel.
Yeah, although the horn design did give a quite low drag value, my modified catamaran design had even lower drag, mostly due to its gradual increase in the ground clearance and is mostly likely my body of choice.

[Bicycle Bob]

You could still do a lot with clear panels up to the Driver's eye level, and accept that the cockpit opening ends with a rounded obstruction. A gentle lip around the opening might establish better flow. Some F1 cars in the mid 60s met that spec, and used a clear duct- a double windshield - to help throw air over the opening.
A streamlined structure can be added just in front of the roll bar so it can draft at 1/4 the usual drag, and minimal dimensions used. If people want a handle on their roll bars as often seen in pictures, make it longitudinal, or add a finger gap to one spot on the wind splitter ahead of it. Or, maybe it can legally almost fill a gap in a continuous headrest fairing.
I am well aware of the usual wisdom that suspension adds weight and complexity. That's why I demonstrated that it does not have to, except for the considerations in the frame design, always looking for useful flex instead of making everything rigid. If you only need short travel, it gets much easier. It saves the rest of the structure from vibration and shock, even if the driver is comfy enough, and is good practice for the real world. A pan chassis lacks torsional rigidity, but that only becomes important for tuning the oversteer/understeer on a poorly balanced design.
Sharp corners on the body are OK on aircraft, but likely to cause problems in crosswind driving, causing early separation. It is easy to get a smoother curve on Coroplast in 2-D by slitting the inside skin between a few ribs. Blunt the blade tip to make it easy to control and angle the knife to keep the cut edges from jamming.
Bamboo is lovely stuff. It is six times stronger than aircraft aluminum, although it bends six times farther before failing, so it works best in a space frame. Resin and fiber lugs are a good solution to the joining difficulties.

[freebeard]

What would you think of a 'buckboard' made from a sheet of bamboo plywood with L-shaped slots so the four corners are articulated A-arms? The width of the arm would depend on the Modulus of Elasticity of the plywood.

[Bicycle Bob]

Originally Posted by freebeard

What would you think of a 'buckboard' made from a sheet of bamboo plywood with L-shaped slots so the four corners are articulated A-arms? The width of the arm would depend on the Modulus of Elasticity of the plywood.

Those sound like very short, weak swing arms to me. If you had another set, about one wheel radius above the first, you might have reasonable A-arms. Bamboo or wood, plywood would not have an efficient arrangement of fibers to take the stress. Designs for unreinforced wood spring suspension do use much of the wheelbase for the large springs needed, so I'd be more enthusiastic about a plywood pan chassis that flexes overall, and has reinforced corners hung from solid axles.

[Frank Lee]

I'd also think about adding a belt to any tires I could find, and doubling the pressure.

How do you add a belt?

[Bicycle Bob]

Originally Posted by Frank Lee

How do you add a belt?

You sand off the tread, wind on a belt from cords producing a flat tread area, and then mold on a new tread. This isn't as good as a proper radial carcass, but it is a big step in the right direction.

Maybe somebody could convince Michelin to sell their Solar Racer tires as a regular catalog item.