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Hey everyone. I'm not sure if this is the right place to ask this, but I'll give it a shot.

Well, first off.. has anyone heard of the Shell Eco-marathon? If not, here's a brief description. Every year, Shell hosts a fuel economy event in California. I'm not sure what the requirements are, but I think you need enter as a school. Anyway, basically teams from all over the country compete their vehicles (designed and built by the school/students) for the best mileage. Last year, a high school took 1st place with 2843 mpg. This may seem like a lot of mpg, but to be fair, these vehicles are made for pure mileage, and are not meant for street use (super lightweight, not very rigid, poor handling/breaking, etc).

Well this year (2009) there's a new subgroup in the competition where the vehicles raced are designed to resemble consumer cars. Ie, they must have headlights, side mirrors, 4 wheels, etc... I joined my school's supermileage team and we're designing a car to compete in this new subgroup for the 2010 competition. I think this will be a great learning experience, but I am worried because the scope of the project seems huge and I really don't know a lot about building a car. I was wondering if you guys can help me out a bit, give me some tips, or point me to a place where I may find more help. Right now, our team has some experience in the competition since we've raced in the other subgroup for the past few years. So we are basically taking our ideas from that old vehicle and implementing them into this new one.

Our main focus is to design a lightweight car (~150-200 pounds excluding driver) with a low drag coefficient. We will try to use low rolling resistance tires and inflate them heavily. We are still deciding on an engine, but we will most likely be using a small gas engine (50-100cc). Those are our basic goals, but how we will achieve them is what we are still trying to figure out. One of the first things we need to design is the chassis and we have narrowed the options to three types of chassis: monocoque, spaceframe, and backbone. Material choices so far are carbon fiber, aluminum, steel, wood (maybe??), and some plastics. Does anyone have any experience with these chassis types and materials. Is there any you would recommend over the other? Is there something else we should consider before designing the chassis? These are the questions we're all wondering now and since we're all relatively inexperienced with this, we would like to get as much info as we can on this.

It would be great if you can give us any information at all and we will really appreciate the help. This is the first place I thought of to ask because it's a mileage forum, but I think it's more geared towards consumer cars so I'm not sure if it's the right place to ask. Anyway, if you think there is a better place for me to find some tips like at another forum, please point me that way. Thanks.

Hi Z:
This sounds like a really cool idea! I went to the www.shell.com/eco-marathon website and took a look at some of the info there. Apparently, you have until April 15 to finish you vehicle. That is a pretty agressive schedule.

You are right that this forum is aimed mainly at consumer automobiles, but there is a lot of information here that would translate well into your endeavor. We have a member here, basjoos, who has done aero mods to a ordinary Honda Civic, and who has a website describing his mods and the engineering behind them. You should study that info. Since you have to be "street legal" wrt wheels, rearview mirrors, etc, you will want to think closely about how you implement them. Side mirrors are a significant drag factor on a hightly tuned aero body. You might want to see if a rear facing camera is a viable option. You probably know already that you will want lightweight body panels on every surface. The rough underside of most cars is also a significant drag. You will also want to minimize the frontal area striking the wind, and you will want a boattail in back to minimize eddy currents.

While design of the car is a big factor in fuel economy, driving style is also a huge factor. You will want to investigate what driving styles are legal, and if possible consider using FAS P&G during the competition. It also means your choice of transmission is critical. Will you be using direct drive, belt, or gears? This stuff is pretty tricky, and above my head. Maybe some of the others can chime in here.

You request should generate a lively and informative discussion. Welcome aboard!

Larry

Sounds like a great project - one you'll learn a lot from. I think I'd go with fiberglass, or at least investigate it, over an aluminum frame, and motorcycle wheels. Find out what drivetrains previous winners used. Your whole class is going to learn from your experience.

Wow! You are pushing the time limit! Mater Dei High School in Evansville, Indiana (http://blogs.edmunds.com/greencaradvisor/2008/04/indiana-high-school-team-scores-2843-mpg-win-in-shell-eco-marathon-marked-by-drama-sportsmanship.html) is only about 60 miles from me and took home the top honor last year. It was some pretty slick looking vehicles. They had 2 cheese wedge shaped cars that recorded an unofficial 6000 mpg due to some harsh wind assist.

Thanks for the replies so far.

Larry, dont worry.. we aren't competing in the 2009 race so we should have until april of 2010 to finish building this car :) . Thanks for the tips so far. Love the rear-view camera idea, but the rules says we must have rear view mirrors on each side with a minimum surface area of 25cm^2.

I agree that the driving method also plays a huge role in fuel economy. We've been using the coast and burn method (which I'm guessing is similar to FAS and P&G) where we basically floor the car until it reachs ~25mph, shut off the engine, let it coast to a low speed, and repeat. Last year, we used chains and gears (I think) to drive the rear wheel (it was a 3-wheeled car) and I'm not sure what we want to do with our new car, but I'm assuming we're going with the same setup or something similar.

About fiberglass.. I didn't look into it personally, but I remember my teammates considering it but throwing it out because carbon fiber beat it in almost every category except cost. But if there are some good benefits of fiberglass, I would love to hear more about it.

Depending on air speeds (ground speed + wind) you are running at, you may want to consider how much weight to put into aero aspects of your car. There's a lot less to be saved by reducing aero drag at 15mph than there is at 60mph.

Carbon fiber is used in most rowing shells made today. Typically the hull consists of layers of resin-impregnated carbon fabric that sandwich a core of honeycomb mesh. This is done by making a mold and then putting in layers of the cloth, then the honeycomb, then more layers of cloth. The whole thing is put into a plastic bag and air is sucked out of the bag. This "vacuum bagging" applies pressure so the parts bond together as the resin cures. Sometimes the bagged item is put into an oven and heated, and the oven may be pressurized in some cases. The result is a stiff, strong, light structure. But it gets complicated because the layers on both sides of the honeycomb core must be the same: if one side is weaker than the other the layers can shear apart from the core.

A simpler method that might work well for you is to skip the core in most areas, using it only in places that carry heavy loads. In the cored areas, you can substitute foam core for the honeycomb to save money. The coreless areas will feel "squishy" to your hands but will probably have adequate "panel stiffness" to resist deformation due to wind pressures. I have a rowing shell built in this way and it works fine, the water pressure does not deform the shape.

Depending on where you are, you may be able to get help from aviation or marine users of carbon fiber. It's commonly used in racing kayaks, canoes, and rowing shells, in spars and masts for sailboats, and in wind turbine blades.

Wow! You are pushing the time limit! Mater Dei High School in Evansville, Indiana (http://blogs.edmunds.com/greencaradvisor/2008/04/indiana-high-school-team-scores-2843-mpg-win-in-shell-eco-marathon-marked-by-drama-sportsmanship.html) is only about 60 miles from me and took home the top honor last year. It was some pretty slick looking vehicles. They had 2 cheese wedge shaped cars that recorded an unofficial 6000 mpg due to some harsh wind assist.

Aside from a blown tire -- they pumped Michelin tires rated for 50 psi up to 100 psi to minimize drag -- Mater Dei encountered few problems Friday and Saturday.

I wonder what tires they used?

Thanks for the replies so far. Love the rear-view camera idea, but the rules says we must have rear view mirrors on each side with a minimum surface area of 25cm^2.
That is a square mirror 5 cm on a side or a circular mirror with a 5.6 cm diameter. I still worry about the eddy currents forming behind the flat surface of the mirror increasing drag... Lightfoot says
There's a lot less to be saved by reducing aero drag at 15mph than there is at 60mph.

and while true, that is because flow at 15 mph is a lot more likely to be laminar. But the mirror is an invitation to turbulent flow. So...

Does it say anything in the rules that you can't mount the mirror on the inside of the wind shell, or than you can't put something like a lexan shell on the trailing edge of the mirror to make a smooth surface along the body of the car.

We've been using the coast and burn method (which I'm guessing is similar to FAS and P&G) where we basically floor the car until it reachs ~25mph, shut off the engine, let it coast to a low speed, and repeat.
This is very good, it IS P&G. However there are a few optimizations that can be made.
First, the engine is not as efficient at 100% load as it is at about 75% load. Since you probably can't hook up a device like a Scanguage to measure load, you are probably going to have to figure out the optimum throttle opening by trial and error. Instead of flooring it, open the throttle until you get about 3/4 max power, and accelerate at that rate. You will also have to experiment to determine your best top speed. Now that leads to the second optimization. What do you do while the car is gliding? Is the engine running? If so, you will need an easy way to shut off the engine, and restart it with a "bump" start. This is so you don't burn gas during the glide. This means you will need
a clutch, and you simply restart the car by letting out the clutch. Maybe you do this part already. (BTW: P&G means pulse and glide, acceleration and coasting. FAS means full auto stop, that the glide portion is done with the engine off).

Good luck,
Larry

Actually, FAS stands for Forced Auto Stop -- it was coined because hybrids already had AS (Auto Stop) and users wanted to dictate when it occurred outside of what was allowed by the programming.

The description of the process was bang on, though. :thumbs_up:

Does it say anything in the rules that you can't mount the mirror on the inside of the wind shell, or than you can't put something like a lexan shell on the trailing edge of the mirror to make a smooth surface along the body of the car.

Hmm, not sure what you mean by the lexan shell idea, but i like the idea of having the mirrors inside the car. Here's what the rule says: "The vehicle must be equipped with a rear-view mirror on each side of the vehicle, each with a minimum surface area of 25cm^2. The visibility provided by these mirrors, and their proper attachment, will be subject to inspection. An electronic device must not replace rear-view mirror."

It sounds iffy. Supposedly Shell made the rules somewhat vague to allow for innovative ideas. But I'll talk to my teammates about these.

First, the engine is not as efficient at 100% load as it is at about 75% load.

Good point about the engine efficiency. I'm not sure what the best operating speed for our engine was, but I remember one of the guys saying that it's most efficient at full throttle. But I will talk to some of my teammates about that also. And during the glide part, I'm pretty sure we shut the engine off, but how we get it to start and stop, I'm not sure.


Alright, so right now we are trying to do some analysis on the vehicle's rigidity.. but we're kinda stuck. Actually I am really confused here.. Does anyone know how we should go about it for analyzing the chassis' strength/stiffness? I think we are going with a kind of carbon fiber monocoque chassis/frame, but instead of it being 1 piece, we're going to split it up into smaller pieces for the top, bottom, sides, etc.. Anyway, we're having trouble finding the right size of materials needed. The restriction is that the car must have at least a 10 cm clearance from the floor when the driver is seated. In our old car, the car bottom bended quite a bit once the driver sat in it, but we can't have that for this new car.

Would it be ok to model the car as a beam and then assume a distributed load across the entire car, with maybe some point loads where the driver is and where the engine is? And about carbon fiber.. what is a good estimate on its strength and stiffness? I really don't know a lot about carbon fiber, but the way I'm seeing it is that the actual carbon fiber material is very strong and light.. but we are using it as a composite, so how does that affect its properties and where can i find that info? What would be a good material to reinforce with carbon fiber? And is there some other simple analysis that can be done on the car in terms of its rigidity?

Oh, and another thing: Tires/Wheels.
So the rims must be either 16 or 17 inches in diameter and the tires have a minimum width of 90mm sidewall to sidewall. Apparently tires of this size are hard to find, because we're having trouble looking for tires that fit the requirements. We've looked at a bunch of stuff; golf cart tires, spare tires, honda insight tires, etc.. but nothing yet seems to serve our purpose and meet the requirements.. low rolling resistance and light. One of the professors here said he was going to talk to someone he knows who owns a tire shop, but we haven't heard back from him in a while. So yeah.. any ideas on where we can get tires for this??

For reinforcing/strengthening carbon fiber use kevlar. Before you ask "Where do I get it?" This link is what you need: http://www.fibreglast.com/

They will be able to answer all your c.f. questions, be aware that supplies will sometimes be limited so if your considering buying some, talk to them about availability now. Are you allowed to have sponsers? They might be a source for free or reduced prices for material if their name was visible on your vehicle. Good luck. - Dale

Thanks Dale. Yes we are allowed to have sponsors. We pretty much are asking everywhere we buy from for either some free stuff or at least a discount lol. We have gotten donations of carbon fiber before from sponsors but that may be a different story now in these economic times.