Talk:Car/Design Criteria
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. :)
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. Wikispeed's car works this way and even has an interchangeable engine unit, so we could simply copy their technique.
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.
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