Talk:Car/Design Criteria

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Re 4.5, 6.1 and 6.2, the Power Cube is about a million dollars shy of meeting EPA automobile engine certification, which is the emission standard. An existing EPA certified automobile engine in a custom Power Cube gives us a far easier and less expensive option. [JackMcCornack 2/21/12]

4 Wheels vs. 3 Wheels for the first prototype OS Car (please feel free to insert comments into the text --JMcC)

The main disadvantage of three wheeled road vehicles is their reduced rollover resistance, compared to four wheeled vehicles. All else being equal (which it isn’t; we’ll get to how to improve three wheel rollover resistance), a three wheeled vehicle has half the roll resistance of a four wheeled vehicle, since the single wheel end of the vehicle has no roll resistance.

The primary ways to improve three wheel vehicle roll resistance are:

  • Lower the center of gravity
  • Move the center of gravity toward the paired wheels
  • Increase the track of the paired wheels
  • Tilt the vehicle chassis to move the center of gravity toward the inside of turns (a la motorcycles)

In comparison with an equivalent four wheeled vehicle (built for the same purpose, with the same powerplant and same construction techniques) the trike has no advantage in center of gravity height. The center of gravity can (and should) be moved fore or aft as needed to put roughly equal weight on each tire, and if that is done, a trike can have equal rollover resistance to a car in steady cornering, if the trike has a 50% wider track than the car. This is why in practice, trikes do not have substantial aero drag advantages over cars—a tadpole trike is easier to streamline with a “boat tail” than a car, but at the cost of greater frontal area due to wider track.

The rollover disadvantage is worsened by weight transfer during acceleration or braking, which shifts the weight toward the single wheel (during braking for a conventional trike, during acceleration for a tadpole trike)…which is why I personally prefer tadpole trikes over trike-trikes for road use; since my experience is that braking is more useful than accelerating when attempting to avoid (or minimize damage from) an accident and that rapid turning may be required as well, I prefer a vehicle that gets more stable while braking instead of less stable. Unfortunately, though a tadpole trike gets more roll stable while braking, it gets less yaw stable because of reduced load on the single rear wheel, so while it is less likely to roll over, it is more likely to skid (or even spin) under combined braking and cornering loads.

The advantage of a tilting trike over a motorcycle is the trike can be stopped without requiring the rider to hold it up (via a light spring to keep the chassis upright when there are no significant cornering forces), which allows the cabin to be enclosed and the rider to keep his feet inside the vehicle. Otherwise the dynamics are very similar to a two wheeled vehicle and a tilting trike (barring computer control of hydraulic or electric motors to hold it in a partial tilt, making the vehicle only partially roll stable) needs to lean at 45 degrees in a 1G turn to avoid flipping over.

And here’s the problem with a Power Cube in a tilting three wheeled vehicle: the Power Cube is 30” x 27.5” x 24” (L x W x H) and though I don’t have any weight or CG figures, it’s clear in the photos that the bulk of the weight is in the upper half. In a corner, a tilting three wheeler (or two wheeler) adds load to it’s suspension; at 1G it’s about 1.4 times its load when driven level. If we give the vehicle 3-1/4” of suspension and don’t want the chassis to hit the road in a 1G turn, the bottom of the Power Cube will have to be mounted at least 17” above the roadway (width/2 + suspension travel) which puts its CG at least 30” above the roadway, and the top of the Power Cube 41” above the roadway. The Power Cube will be the most massive component of the car, and mounting it at that height will increase the forward weight transfer during braking, increasing risk of control loss through skidding.

Electric three wheelers have the advantage of great weight in a small space (the batteries) and by building the battery boxes into the floorboard, CG can be lowered (and thus track of the paired wheels narrowed) spectacularly. The same holds true for cars (google “tango car” which is a four wheeled tandem two seater that’s 39” wide) but it won’t work with a Power Cube.

Hydraulic energy storage and regenerative braking has great potential for improving vehicle efficiency (in both fuel economy and performance) and though the state of the art doesn’t make it practical in the first prototype (the EPA estimates “…projected high volume manufacturing costs of $7000” for a delivery van sized system) the concept is brilliant and an open source version would be good for the car and great for the GVCS Truck. The problem with the system is that it’s bulky-- the hydraulic fluid needs a high pressure accumulator when it’s charged, and a low pressure reservoir when it’s depleted. By my calculations (which are rough, and extrapolated from EPA data on larger systems) and OS Car version which could recapture, store, and reuse 30 horsepower for 30 seconds would require 15 cubic feet of empty space for the combined accumulator and reservoir, and a body streamlined to take advantage of the tadpole trike layout doesn’t have the necessary interior space.

In conclusion, I think a four wheeled vehicle is the superior platform for the OS Car, because four wheels are better suited to both the Power Cube and to hydraulic regenerative braking. Either one is reason enough, in my opinion, and I believe the first prototype OS Car should not be unnecessarily different from the eventual Final Release version. JackMcCornack 2/25/12

______________________________ Awesome analysis! I would now like to invalidate half of it. :)

:-) Fair enough, Matt, and if you can invalidate eventual PowerCube and hydraulic drive options with Marcin, you may be right. That's why I left the number of wheels open in my proposal to build the first OS Car prototype; see

I'm putting my comments re your comments in italics. JMcC 2/26/12

From my point of view, it doesn't make any sense for the OS Car to be hydraulic. The most obvious reason is that there is no need to drive every wheel. It will work just fine with 2-wheel drive, which means the engine can be directly connected to the closest pair of wheels. This minimizes complexity and weight while maximizing efficiency.

Right you are, and the majority of production cars built in my lifetime have followed that format, usually front engine front wheel drive (starting with Saab, 1947) but many mid engine rear wheel drive (starting with Lamborghini, 1965) and some production sports cars that took a front engine front wheel drive package and moved it back to the rear wheels (Pontiac Fiero, 1984, using Chevy Citation FWD engine/trans unit, and Toyota MR2, 1985, using Toyota Corrola FWD engine/trans unit). And there are rear engine rear wheel drive packages as well (VW Bug, Porsche, since the '30s) but they aren't worth considering here, as that platform has serious efficiency and enclosure disadvantages compared to transverse engine forward of transaxle systems.

Wikispeed's car works this way and even has an interchangeable engine unit, so we could simply copy their technique.

95% of modern automobiles work this way, and 99+% of modern automobiles have interchangeable engine units. Is there a technique peculiar to the Wikispeed car that we should simply copy, or will simply copying ChevyFordChryslerHondaSuzukiFiatEtc (including post '74 VW) techniques suffice?

I think we should avoid a 4-wheel vehicle because we want the car to be accessible to as many people as possible. A 3-wheel vehicle is often not regulated as strictly as a 4-wheel vehicle. It would also be cheaper and easier to build than a 4-wheel vehicle.

Good points and worth considering if Marcin decides the OS Car need never be adapted to the PowerCube -- if not, my CG commentary re the PowerCube in a trike stands.

Re "accessible to as many people as possible," emotional access is something to address. We're asking people to accept the new paradigm of open source economic development, and that's a fair leap for most folks; how much will we limit the OS Car market if they also have to accept three wheels? I'm just asking, that's not rhetorical.

3 wheel vehicles are indeed less regulated, but that's not a great advantage in my opinion. For example, trikes (and other vehicles defined as motorcycles) can have plastic (e.g. Plexiglas, Lexan) windshields and no windshield wipers, but these plastics scratch and haze more easily than glass, and if the cabin is enclosed then wipers (and washers) may be necessary to see the road in some conditions. Motorcycles don't require side marker lights, but the driver will want to be seen by other drivers so side marker lights are in order for safety, even if no regulations require them. And "cheaper and easier to build" remains to be seen; three wheeled vehicle design is not without its challenges, such as strict control of center of gravity (including passenger placement).

I think we can all agree that the power cube isn't going to be used in the car. The biggest reason is that it doesn't have the horsepower necessary to get the car up to highway speeds in a reasonable period of time. Also, its transmission is hydraulic, which is unnecessarily inefficient and complex in this application. It would be better to just use a small engine and automatic transmission.

There's nothing sacred about the current power cube. It was just the easiest thing to build to run the tractor. We've always known there would be a family of power units. What better time to work on a new one then when building a highway-capable passenger vehicle? - Matt_Maier 25FEB2012

I agree with you in principle, Matt. I don't think having a quick remove powerplant in the car, in order that the powerplant can be dropped into a tractor or CEB press, is valuable in practice. I think having a car around, ready to go, is a good idea for a family or a community. Unless there's a doctor in the house, there's always a chance that emergency transport will be needed, and if it takes 15 minutes to pull the PowerCube out of the Soil Pulverizer (or whatever) and 15 minutes to install it in the OS Car, that's half an hour you'll never get back. But while I think we all agree the PowerCube isn't going to be used in the first prototype, we're not all agreed it will not be used in future prototypes or the Final Release of the OS Car. I'm fine with a dedicated car powerplant, but you have to convince Marcin, you don't have to convince me. - JackMcCornack 2/26/12


I think your "99+% interchangeable engine units" metric is a bit skewed. Traditionally, the engine/transmission/running gear in a vehicle is not designed to be removed, and definitely not designed to be removed easily. There are a few exceptions like some military vehicles. What we need to maximize is modularity and interchangeability. We're not designing a car, we're designing the part of the GVCS that moves people along a highway. It just happens to look and act a lot like the things people call cars. So, the performance of the vehicle itself should take a back seat to the performance of the GVCS as a whole. If we need a power plant with better performance than the current power plant, it still needs to be modular. Just because it's FOR the car doesn't mean it's a CAR POWERPLANT. It's a stand-alone powerplant that happens to be in a car at the moment. I would argue that 99% of vehicles are designed in the exact opposite way.

The power cube just isn't powerful enough to move a vehicle at highway speeds. This one isn't up to Marcin. It's up to thermodynamics.

I agree that three wheels are pretty weird. However, I think bringing Wikispeed on to the team sort of solves that problem. They are already registered automobile manufacturers and they are already well along the way to a production-ready car. We can help them bring a 4-wheel automobile to market and they can help us bring a 3-wheel vehicle. The difference is that they would be putting out an open source "traditional" car and we would be putting out an open source "highly specific" car. Ours would be part of the GVCS, theirs would stand alone. If we try to make our car 4-wheel it greatly complicates actually getting the thing replicated. Dropping down to the bare minimum lets us, and our replicators, skirt around a lot of regulations.

We need proof of concepts. If we can just get the whole integrated system built we can demonstrate and improve it. Basically: publish early and often.

The point of a modular system is that we can change the paradigm. Instead of the designers (us) dictating how a person will use their machine, we can free them to do whatever THEY want to do with it. You're thinking old school. If the engine unit is removeable, then the people who want a car standing by for instant use in emergencies can have it. But they aren't REQUIRED to have it. If they have different priorities they can leave it sitting with an empty engine bay and that engine can be doing work rather than taking up space in the garage. That's an option that people simply haven't had before.

If there are people who still think the current power cube can be used in a highway vehicle we should be able to put that to rest pretty easily. FeF already has power cubes and wheel units. They can use some of those square tubes to quickly fab up a car-sized frame, toss the power cube and the wheel units onto it, and see what sort of top speed they can reach. You know, if the math of a 20hp 15gpm pump isn't enough to convince anyone. - Matt_Maier 27FEB2012