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Learning how to build Lotus Seven replicas...together!
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 Post subject: Monocoque - why not?
PostPosted: March 13, 2017, 3:21 pm 
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Joined: February 18, 2013, 9:37 am
Posts: 45
I love the out of the box thinking on this site and the ingenuity in the builds. One of the thoughts I had is it would appear to be faster maybe to build a monocoque maybe, but I'm way out in front of my ski's on this one. I'm curious why not?

I'm a big fan of race car replicas and Fran's methodology for building cars. really breaks it down. Problem is, his builds start at 45k for parts.

I was thinking why couldn't a box be built like this and make a locost, would appear to be easier to get the bigger pieces cut and panels cut vs. all the individual pieces.

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or one of these on google image search from pistonheads...

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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 4:05 pm 
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Joined: October 19, 2012, 9:25 pm
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Location: Summerville, SC
Search Pirate 7 on google.
you'll see an interesting take on a sheet metal 7 called a Centaur. I built one a few years ago.

Only thing I didn't like was how small the engine compartment was. it would need a redesign for a modern era 4 banger

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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 6:00 pm 
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Joined: February 12, 2017, 7:57 pm
Posts: 42
Someone on this forum did it, Its yellow, he is from Oregon or Washingnton . He did a good job.


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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 6:09 pm 
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Joined: April 23, 2006, 8:26 pm
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Location: SoCal
You're right that the cut and punched panels could likely go together in a day. The problem for most garage builders is engineering such a design properly due to aluminum's differing qualities from steel; aluminum chassis fabrication has generated huge and numerous threads. Not saying it can't be done, it has, but almost exclusively by companies with the deep pockets to do proper engineering and fabrication. I suggest doing a Lot of research before buying material!

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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 7:29 pm 
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Joined: October 24, 2008, 2:13 pm
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Location: Carlsbad, California, USA
Kurt (KB58) is right on two fronts: the engineering challenge; and the aluminum design part. But, here's some very encouraging news. The original GT40 from the Broadley/Lunn/Wyer team was actually done in 20- and 22-gauge sheet steel. It had a torsional stiffness of about 10K lb-ft per degree. That's stiff! All three of them were experienced engineers and/or fabricators however.

I think a redo of the Locost 7 design in sheet steel, would be a great project. However, you should either have some practical engineering and design experience, or access to a suitable FEA program that can handle thin materials (not all can), and the skills to produce viable results with the software.

It is do-able, I believe.

Cheers,

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Build Log: viewtopic.php?f=35&t=5886


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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 8:50 pm 
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Joined: November 15, 2009, 9:58 pm
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Location: Port Angeles. Wa
Here is my build log -> viewtopic.php?f=36&t=10658
My car was a semi-monocoque. Aluminum tub between steel tubing sub-frames.

Thick alum. tub that is welded (better be a professional) as shown in your first picture or .065" 6061-T4, a lot of expensive rivets and structural bonding adhesive. The later took 3 days of gluing/riveting AFTER all the cutting, bending, fitting and hole drilling! I also had access to a 12' shear and a numerically controlled press brake. A steel tubing frame connecting the sub-frames would have been much simpler but probably not as torsionally stiff.

It was a rewarding exercise that I designed and built but I would not do a pure monocoque without an engineer. The loads into the tub were spread over a large area of contact with the sub-frames so no point loads from shocks, suspension and engine. Note that the front sub-frame had tubing braces to the front roll hoop that was braced to the main roll hoop.

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My build log viewtopic.php?f=36&t=10658&start=0 NOW NAMED =The Wycked 7

My other build log viewtopic.php?f=18&t=15162 The Skayt'R6


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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 9:47 pm 
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Joined: November 11, 2013, 4:47 am
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Location: No. Nevada
I'm still looking at Plywood or CF for my Tatum BEC tub.
Making molds for CF would be fairly easy and then I could make several.
Also a bit expensive to do using the correct epoxies.

Using plywood is about the same work as making the CF mold, but only provides one tub.
Much less expensive without buying CF.

There is a lengthy thread on plywood chassis you might look at.

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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 9:56 pm 
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If I were to do this I would look very closely at how the Lotus 49 was built (I want to build a replica-ish 49 some day). Basically the monocoque is a aluminum cylinder, at each end there is a flat bulkead/ firewall riveted around the edges, inside the monocouque on each side there is a large sheet with a ~45* break in at about halfway up that run down the length of the tub. They used a bolt on front subframe/ suspension with the radius rods meeting up with the tub where it is internally strengthened by the dash panel. The rear suspension fed its loads through the engine and gearbox but also had radius rods that ran forward to the seam where the tub's outer skin and firewall met. All the brackets at the pick up points were also fabbed from sheet. It's all brilliantly simple and elegant, very lightweight and remarkably stiff.

You can build a car like this if you keep in mind a few things. The biggest thing is that all your loads have to be fed into the tub properly. They need to be spread out over as large of an area as possible. A 7 could be built with the same principles, albeit in a more box like fashion.

Rumor has it the Lotus mechanics accidentally built a tub with soft condition aluminum and it didn't work very well. Obviously alloy choice is important for a monocoque. I make no recommendation.


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 Post subject: Re: Monocoque - why not?
PostPosted: March 13, 2017, 10:51 pm 
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Mid-Engined Maniac

Joined: April 23, 2006, 8:26 pm
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That's another thing people overlook, the consequences of welding an aluminum chassis. It all goes dead soft so it must be tempered to regain its original stiffness. That involves controlled temperatures that we likely can't copy in a garage (safely).

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 Post subject: Re: Monocoque - why not?
PostPosted: March 14, 2017, 9:37 am 
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Joined: July 17, 2008, 9:11 am
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A monocoque Seven was attempted by a company called "Robin Hood" Do some research. In a few minutes I found that they changed the design to include a steel frame inside the aluminum shell. You didn't mention Ali specifically, but I assume that is what you are talking about. If you decide to go with an aluminum semi-monocoque like Kartracer47 did, I suspect that you might be saving maybe 20-30 lbs at best, and adding a lot of engineering to what would otherwise be a tried and true, forgiving method of steel tube.

I've seen and driven Kartracer47's car and it is a well engineered package. Keep in mind, it was not his 1st car build, or 2nd or maybe even 3rd. He is a seasoned builder.

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 Post subject: Re: Monocoque - why not?
PostPosted: March 14, 2017, 9:41 am 
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Joined: September 22, 2005, 8:12 am
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Location: 4AGE in S.E. Michigan
Aluminum welding, Yup " It all goes dead soft " plus weld distortion :ack: .
Even a slowly built prototype aluminum vette frame needed massive fixturing to keep it straight.
The production frames used a 5 axis CNC milling machine to correct for distortion and locate all critical points.
Dave W


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 Post subject: Re: Monocoque - why not?
PostPosted: March 14, 2017, 10:49 am 
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Joined: April 1, 2010, 10:26 am
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I have an interesting book on alternative building techniques that I have not been able to find for years:) I am hoping the house move will find it again. One very cool technique is using SIP's, Structural Insulated Panels. You use a router to cut a V grove in the panel, fold the panel and epoxy a plate over the inner side of the bend. So a tub is a origami exercise. But you can very quickly build very light, very strong tubs.

Graham


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 Post subject: Re: Monocoque - why not?
PostPosted: March 14, 2017, 12:06 pm 
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Joined: January 2, 2009, 1:45 pm
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Location: Halifax, Nova Scotia
The University of South Australia built its TREV electric vehicle using notched and folded honeycomb panels for the chassis. The fancy bodywork was added by gluing blocks of foam on the outside and then fairing and sheathing. There is an online article on their chassis/monocoque construction process. I can't find it right now, so further exploration is left to the reader.

TREV has been around the world, by the way.


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Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601
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 Post subject: Re: Monocoque - why not?
PostPosted: March 14, 2017, 6:05 pm 
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Joined: January 2, 2009, 1:45 pm
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TREV chassis ('tub') construction is described in http://www.trevipedia.net/tub

Note the materials costs in the article - $450AUS per sheet for five sheets of glassfibre-faced aluminum honeycomb. Plus epoxy, plus carbon, Kevlar and glass. Not 'cheap.' But elegant.


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Build Log: viewtopic.php?f=35&t=11601
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 Post subject: Re: Monocoque - why not?
PostPosted: March 17, 2017, 7:46 am 
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Good points gents! Like I said, I am way in front of the skis on this one. I wouldn't try it if I was the first one to do it. Something about a monocoque is intriguing and feels like it would be a superior design. I know I simply don't have the time now to master.

I think these guys did a fantastic job of updating what a cobra chassis could be. But sadly not much appears is left from the original guys that developed this chassis. http://www.jblmotor.com/chassis.htm

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