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I know, I know... this conversations has been beaten quite a few times already, but...

Would a 14-ga aluminum frame be feasable if using a low-hp and low-torque engine? I realize it would be recieving about the same road roughness, however, it would (hopefully) be both lighter, and slower, as not to be taking high-G corners. Objectively, everything would be aluminum. Frame, suspension (suspension arms would probably be solid stock), etc. Steel would only be needed when utterly required.

Hypothetically, the engine would be from a Honda Insight. Alas, being a FWD, the nice 5-speed they come with would have to be canned for a RWD transmission, and the engine would have to be doctered before installing as RWD--the only main thing I can think of that would have to be changed would be the oil pan, if that.

However, I am not totally sure how the Insight is set up--the electric motor may be a pancake motor lodged between the engine and the transmission, so going from FWD to RWD may not be a practical idea, and a mid-engine setup may be a better idea.

Either way, the ending result would be more like Jack's MAX concept; a streamlined Seven, with full aerodynamic bodywork, closed fenders, and a removable hardtop.

Is it feasable? I mean scientifically... What alloy would be suggested, what would the tensile strength be compared to mild steel, 16-ga steel or lighter vs 14-ga aluminum, etc

I would worry more about how to attach the suspension pickups and motor mounts etc. to the frame in such a fashion that they wouldn't work harden, crack, and rip off in time.

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Probably some form of extrusion. The Insight chassis is an aluminum-copper alloy extrusion that has good tensile strength, and will cushion an impact, but if you try to bend it back out, it'll snap. I wonder what grade it is hardened to... Really, when it comes to the metallurgy area, I'm woefully underqualified.

Yes, you need to worry about the alloy. Once upon a time a long, long time ago I grew up working with aluminum. We fabbed everything you can imagine from alum. and many of these pieces were used in ways like the car in that they got constant or frequent bending forces without failing. A lot went to the military.

I know it can be done, but alas I don't have the knowledge to know the alloys that won't be so brittle.

I'm sure there are people out there that could spec this out for you, but even if they did would it be a common enough alloy that you could get the tubing extruded at a cost a mere mortal could afford?

I'll have to shoot a letter to the instructor from my welding class. He'd know.

These posts pretty much sum it up: yes, it can be done, but we readily admit we don't have the engineering skills to design an alloy chassis that we can guarantee for xxxxx miles.

Be wary of people who say it's easy because they don't know what they don't know. Building a beautifully-crafted car has nothing to do with engineering it correctly.

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Kimini book: Designing mid-engine cars using FWD drivetrains
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I think most of the other posts (after about 15 minutes of wading, I still couldn't find the ones I was thinking of), most people were aiming for super-lite race cars, using larger engines.

I'm looking more towards Jack's idea. The Insight uses a 1.0 liter 3-cylinder engine and a 10kw motor, weighing 124lbs. It gives 67hp and 66ft/lb of torque That is going to give much less weight (and stress) upon a frame than a conventional 120+hp 300lb engine. I think the main concern would be not of the chassis alloy, but what the suspension and moving parts are made of. I would definately be using the factory rear suspension, so the only concern would be in the front. I'd have to study the factory mounts and duplicate.

How about you build up the suspension parts as subframes that bolted into the aluminum chassis? Probably take a small weight penalty but might be much easier to get to the end product.

Also it sounds like this is not a performance minded build so use soft squishy rubber to isolate things. Heck use the rubber to your advantage and build something like the Weissach axle.

It's all hypothetical... I have to finish the Seven GTR first... It is more like an idea for some other people in mind.

Hrmm... Perhaps steel suspension would be an idea, if using rubber bushings rather than rod ends to avoid galvanic corrosion.

The only real issue I have with aluminum is the benefit/cost ratio. The chassis is already so light that I wonder is it really worth the experimentation factor and extra costs to drop 40# or so?

Now if we had a heavy frame I'd say it would be worth it. We already have people on the list running heavy steel floors just to get the weight up so they can qualify in certain racing classes.

But that's not the goal. This isn't for racing if you read above. They are going for the biggest efficiency they can get. But on any build that is going for efficiency; why use a 4 wheel build? Especially if its only seating 2.

Honestly I don't think anyone here has actually built an aluminum frame and then had it fail. Sure that's how aluminum works, but I'd give it a shot, just be prepared to strip it down every once and a while and check the frame for cracks.

People build boats out of aluminum and beat the [PooPoo] out of them so it's not an unusable material, it just has different engineering properties.

Milo

Moly might be better though. or Ti if you are really interested, just make sure to back purge with argon.

Right, of which we don't know enough about to create a chassis that's any lighter than steel and as reliable.

Don't forget that aluminum and steel have the same stiffness per weight. If a chassis is built using the same plans as one made of steel, it'll be about 1/3 lighter.... and 1/3 less stiff.

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

Exactly. However, the stiffness of the .068 steel tubing on my seven is to be able to handle a 250+hp engine, along with serious driving and cornering, whereas the stiffness of .064 aluminum should be enough to withhold a 67hp engine, or 66ft/lbs of torque. Remember, the driver will weigh more than the engine.

They make airplanes out of Aluminum because it turns out stiffer. What's happening is that equal weight panels are thicker in Aluminum then steel. The increased stiffness from the thickness far outweighs the much smaller difference of a simple test sample loaded in compression or tension.

Making something thicker by %30 percent would increase it's stiffness by about 2.2 times. Subtract the %30 percent less stiffness for aluminum vs. steel and you still come out nearly twice as stiff.

I think a welded steel space frame is a sensible thing as is an aluminum monocoque which is riveted or bonded. For consideration of efficiency, I think many factors come into play, especially cost and familiarity. Bling or gee wiz factor should be very low on the list.

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