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PostPosted: March 12, 2018, 7:50 pm 
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Joined: January 26, 2018, 9:43 am
Posts: 9
The more I learn, the less I know...

I had been deep in the planning of a mid-engine classic Mini build however after weeks of reading and design work in the computer, I’m pressing pause on the concept and am seriously reconsidering a mid-engine locost, albeit without the classic Lotus 7 proportions/style.

Among the many cars I’ve studied is the At-om and while I’m not looking to build a clone, I am curious about the chassis design. In particular, I’d really like to know about it’s weak points and what features may contribute to it’s allegedly flexible form. In another post a user claimed to have performed an analysis and determined the car to have less than 1,000 ft lb per degree torsional rigidity. This seems to be supported by many people on the inter webs but then again, the internet is what it is.

I would greatly appreciate the thoughts of any engineers, builders or drivers on whether the reputation is deserved and if so, why? Thanks!


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PostPosted: March 12, 2018, 8:57 pm 
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Joined: February 8, 2007, 4:20 am
Posts: 250
Location: British Columbia, Canada
I can't answer your question directly regarding the weak points of an Ar-i-el At-om, but if you are considering building a mid-engine Locost, I would highly recommend taking a look at the Midlana build, and the corresponding book, if you haven't done so already:

http://midlana.com/

The Midlana design is similar to the Lotus 7 design, with the proportions tweaked due to the mid-engine configuration, so it might not be what you are looking for aesthetically, based on your comment.

If you are not interested in putting in the time it takes to do a scratch build, you might also want to consider this kit:

http://dfkitcar.com/

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PostPosted: March 12, 2018, 9:06 pm 
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Joined: January 26, 2018, 9:43 am
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Thanks for the advice but I’m a step ahead of you. I purchased Kurt’s Midlana and Kimini books several months ago and have dog eared more than a few pages.

I’m definitely interested in the scratch-built approach. In fact, the idea of a design that incorporates the best of multiple designs combined with unique aesthetics is particularly appealing. That’s one of the reasons behind my At-om question. There are a few aspects that I like but I would like to learn more about the good and the bad of the chassis.


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PostPosted: March 12, 2018, 10:54 pm 
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Joined: January 10, 2008, 4:47 pm
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Location: Massachusetts
I think the At-om is designed to look pretty with it's tubes exposed. I don't know how stiff they are but I don't think it was the priority.

If you want the car to be stiff, the frame needs to connect the points where the coilovers are mounted efficiently. That means with a minimum of deflection and weight. You also need sufficiently stiff and strong mounting points for the suspension pickups, the drivetrain, any rollover protection and the driver's seat and harness etc.

Doing this basically requires triangulated pathways between the coilover mounts. We have some threads on this in the frame section of the forum. Here' a big thread, http://www.locostusa.com/forums/viewtopic.php?f=39&t=2224. It discusses using software to measure your frame stiffness and includes some models for some members frames and also the Locost frame.For people that don't don't enjoy groveling with computers you can also learn a lot from balsa or other wood models and things like welding rod models etc.

It's a lot of work, but I enjoyed it. You can can get comments here if you post pictures of what you want to do.

Another thing to consider is that Locost style frame don't have to look like Super 7s when you're done. You can put a body on it like Jack McCornack did.

I think the midship format is open for some more designs, but I think the Midlana is a good example of a transverse middy. Look at Audi and Passat transaxles for longitudinal setups...

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PostPosted: March 13, 2018, 12:40 am 
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Joined: December 17, 2010, 1:24 pm
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Location: Gainesville, Mo.
Just an observation, but just looking at the Ar-i-el frame I would tend to believe the low estimates of torsional rigidity. In addition to the comments from horizenjob, the At-om chassis seems to be lacking a lot of the three dimensional triangulation of a real space frame. Given in a historical context, a good Locost frame versus the frame from an At-om is like comparing a Lotus 19 (a "real" space frame) with a Cooper Monaco (a good multi-tubular Frame). When you look at it, the Ar-i-el frame is really little more than a "Ladder" frame with very tall, skeletonized side rails.

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PostPosted: March 13, 2018, 3:18 am 
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Joined: February 20, 2015, 12:04 pm
Posts: 281
Location: Cotswolds - UK
There are two fundamental problems with the At-om chassis:
1) Poor connectivity across the frame, between the main two side trellises (very poor across the bottom, in particular, because both engine and occupant space hang below the bottom rail of the trellises, thus preventing direct connections across).
2) The curved main tubes are effectively 'pre-buckled': the straight force vectors between node points therefore fall outside the cross-sectional area of the tubes, meaning that what would be pure tension/compression loads (structurally very efficient) in a well designed spaceframe become bending loads (structurally very inefficient) in the curved tubes of the At-om.

Structurally, the design is an abortion.


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PostPosted: March 13, 2018, 8:29 am 
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Joined: January 26, 2018, 9:43 am
Posts: 9
Marcus: Thanks for the advice and the link to the lengthy thread. I've started in on that to get some insight. I may construct a very rough layout to post and solicit some feedback before committing to any CAD time. I'm not so interested in full body to mask a typically angular frame but a more stripped down approach might work. Back to the drawing board....

Mike: Yes, I think you summed it up succinctly - it is not much more than a ladder frame. I hadn't considered that..probably distracted by it's form.

Sam: Thank you for the analysis. My simplistic non-engineer takeway - a curved form is best handled by non-structural elements.

Greatly appreciate everyone's time on this subject. One of the main reasons I love this site.


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