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PostPosted: November 28, 2007, 3:01 pm 
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Location: Bolton, Ontario, Canada
This project is in its very early stages, but I thought I'd post what I have been up to.

Very quick background. I've built 2 Cobra replicas in years gone by and have always thought about building a car from scratch. I was almost serious about a "standard" type of Locost 7. For me, the hardest part of a project like this is fiberglass bodywork (I built a hardtop for Cobra #1 out of foam/fiberglass), so I was playing around with various designs that have no doors and simple bodywork. Then I read Kurts book and shortly after, saw an At-om. Perfect. Mid-engine and virtually no bodywork.

I managed to spend some time measuring the frame and suspension details of an At-om (bit of a trick considering there is only 1 road-legal one in Canada right now). I came away with pages of notes and figures. I thought Miata brakes, steering, seats, spindles, etc, would be perfect for this project. So I offered to store my friends Miata in my garage for the winter so I could measure up the various bits I would need (he did say something about carefully checking that all the parts were there when he gets his car back in the spring).

I've started modelling it using a 3D solid modelling package. This will be my winter project. If I am still enthused about it in the spring, I'll start construction. I figure that the more time and detail I put into the model, the easier it will be to construct. Also, I can keep track of the weight since I'm modelling the proper size and thickness of all components, and applying the correct material weight.

Since I don't have a tube bender, and the At-om uses 2.25 dia tubing for the main frame rails, this would be a problem. I know others have made benders, or got the pieces CNC bent. After starting the frame model, I decided to see what it would look like with all straight sections. I was a bit surprised that, to me, it looks pretty good. So I'm going ahead with it using all straight sections.

I've attached a couple pictures. The first is the original frame that I started, with the curved main rails. The second is of the frame with straight sections. This one also has some more parts added to it (Miata wheels, tires, spindles). I'm currently working on the A-arms. So far it has been a lot of fun.

Rick
Bolton, Ontario, Canada


Attachments:
Atom frame - curved.jpg
Atom frame - curved.jpg [ 215.08 KiB | Viewed 9862 times ]
Atom frame - straight.jpg
Atom frame - straight.jpg [ 213.13 KiB | Viewed 9861 times ]


Last edited by 1 4 the road on December 1, 2007, 12:03 am, edited 2 times in total.
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PostPosted: November 28, 2007, 3:22 pm 
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That looks much better than I thought it would! Have you decided on a drive train?

Rod

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PostPosted: November 28, 2007, 3:51 pm 
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Quote:
Have you decided on a drive train?


I would like to use the Honda K20 that the Atoms use, but they are very pricey, so I'm reconsidering. The K20 is nice since the exhaust is on the back side of the engine, which works well in a mid-ending setup. After giving it a little more thought, I'm not sure how important that is. With the other (cheaper) Honda engines the exhaust is on the front side; maybe that means an off-the-shelf header could be used; saving a lot of finicky fabrication.

What I'm really after is something that is light with decent power, with a potential for an increase later (ie, maybe supercharging like the Atoms do). The North American Atoms have switched to the GM engine which is around 300lbs heavier than the Honda unit; don't really like that at all.

So I guess the answer is no, I haven't decided on a drivetrain yet, despite one of my racing buddies at work trying to talk me into a V-8. I told him I have no objection to a V-8 if he can find me one that will fit in the same space, with a transmission, as a front wheel drive 4 cyl package; he hasn't come up with one yet.

Rick


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PostPosted: November 28, 2007, 4:34 pm 
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Based on that, I'd recommend the Honda B18C3 or -5. Small, light, tons (and TONS) of aftermarket support. And they're getting much cheaper as kids move on to "the next big thing".

About the exhaust on the front of the head, it's really only an issue if it's all enclosed, like in a coupe. For your project, it doesn't matter, and in fact, you could even make it a feature, coming out above the engine like speedboat :wink:

Tell your friend that unless his car is going to beat your 5-6lb/hp, there's no reason to consider anything bigger.

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PostPosted: November 28, 2007, 5:31 pm 
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You could use the Fiero manual trans adapted to a Chevy V8. That conversion has been done countless times in a Fiero with great success. It fits in the engine compartment of the Fiero (minus the timing belt). The timing belt wouldn't be an issue with a scratch built car though as you can be sure to put enough space in for it. Just a thought...

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PostPosted: November 28, 2007, 6:02 pm 
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You could use the Fiero manual trans adapted to a Chevy V8


I have actually considered that. Adapters and kits are available (I used to frequent V8 Archies site). The big negative is the weight. I would estimate that the Chevy V8 with the Fiero transmission would be 300-350 lbs heavier than a Honda engine/tranny. Of course the Chevy almost stock would easily put out 350-400HP.

Quote:
It fits in the engine compartment of the Fiero (minus the timing belt)


Not sure what this means; Chevys have internal timing chains, not belts, as far as I know.

Somehow a heavy V8, even with a lot of power, just doesn't seem right for a car like this. But the price certainly is right. A Honda B18C engine/tranny seems to be around $2500; Chevys are probably the cheapest V-8's in existance.

I still have time to think about it while I model the suspension, steering, pedal box, seats...

Rick


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PostPosted: November 28, 2007, 6:44 pm 
Cool stuff.

One thing to keep in mind with transmissions is working out a shift mechanism. A cable operated box should be easier to cope with than a rod shift, assuming the shift mechanism is on the rear side of the transmission. I can tell you after driving a Honda-powered At-om that its shifter was so vague that selecting a gear was like playing russian roulette with half the chambers loaded.


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PostPosted: November 28, 2007, 6:45 pm 
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1 4 the road wrote:
I managed to spend some time measuring the frame and suspension details of an At-om (bit of a trick considering there is only 1 road-legal one in Canada right now). I came away with pages of notes and figures.


would you mind posting the dimensions you have on the At-om? i have been trying to come across them for awhile with no luck.


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PostPosted: November 28, 2007, 9:46 pm 
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would you mind posting the dimensions you have on the At-om?


I don't have any objections to doing this, I just don't know how to right now. My sheets with dimensions and notes wouldn't really make sense to anyone else (a lot of my shorthand stuff). Some of it is frame pictures, found on the net, with my dimensions added here and there. I haven't played around with the drafting part of the modelling package I'm using. I'll play around with that and see what kind of a dimensioned drawing I get. If anyone needs any specific dimension info, let me know. I can't promise what I have is dead accurate, but I think most of it is pretty good. It is tough to measure the curved frame rails in 3 dimensions.

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I can tell you after driving a Honda-powered At-om that its shifter was so vague that selecting a gear was like playing russian roulette


I'm really surprised to hear that. Reminds me of driving a friends Porsche 911 with the old 915 gearbox, nicely worn out; anyone that has driven one of those will know what I mean. Like you Keith, I've owned a couple of Miatas; what a perfect way to get spoiled for life on what a great shifter is like! As an aside, I think I remember you from the original Miata mailing list (back before these "interactive" forums existed).

I have to say that this forum is a fantastic source of information. Looking at the various build logs has already answered so many questions on how to design various bits and pieces. For anyone even considering buying either Keiths and/or Kurts books, do it. Great sources of information and will answer so many questions that you didn't even think of yet.

Rick


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PostPosted: November 28, 2007, 10:32 pm 
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You are correct about the Chevy timing chain. I think the extrernal part was for the water pump. It's been so long since I looked into the conversion. I know there was some part on the front of the engine that had to be run thru the wheel well because of space. Might have been the belt for the alternator.

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 Post subject: suspension dimensions
PostPosted: November 28, 2007, 10:58 pm 
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I'd love to see any measurements on the suspension pieces. Mounting points, arm lengths, upright dimensions.


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PostPosted: November 28, 2007, 11:39 pm 
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Very cool!!! I cant tell you how many people would be interested in this information.

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PostPosted: November 29, 2007, 12:46 am 
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Welcome Rick. Great drawings and thanks for sharing.

I'm no engineer, metallurgist, nor master fabricator, but I would think the curved main rails would be *much* stronger than the straight sections butt welded together. Personally, I would look for a local fab shop with a 3 roller bender and just pay to have the tubes bent.

Peter


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PostPosted: November 29, 2007, 2:14 am 
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I'm no E, M, nor MF neither, but I'd think the opposite. I'd expect a frame made of straight sections would be stiffer than one made of curved sections. A heck of a lot more expensive to produce, but the added stiffness might make it worth the trouble. Perhaps tight (small radius) bends at the tube junction points would be a good compromise.

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PostPosted: November 29, 2007, 2:19 am 
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The only time the curved tubes *might* be better would be in a side impact. They would straighten rather than bend, kind of like semi-truck trailers. Otherwise, its just going to cause the frame to flex and distort easier.

Do a simple test with a 2 paperclips - bend both straight then bend one into an arch (ala At-om frame sides). Try to compress both axially with your fingers. This is basically what happens when a space frame loads a tube axially.

Pure track car my foot...


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