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Learning how to build Lotus Seven replicas...together!
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PostPosted: Thu Mar 27, 2008 10:01 pm 
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Always Moore!
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I guess the time has come to start one of these things...

I've been doing design work since May of last year in Solidworks. There are some pics of the final model below. I guess really the only unique part about the frame is the integrated leg hoop that forms the scuttle.

According to ANSYS, it has a theoretical torsional rigidity of 3400 ft-lbs/deg. The colors of the plot show that this is acting as essentially a back-bone frame rather than a true space frame. I wonder how much this varies between cars?

The design work is pretty much done at this point aside from the rear suspension. The geometry is set but I need to determine the fore/aft position of the inboard points and make sure the arms will clear the shocks and the shocks will clear the driveshaft and nothing else hits.

As has been discussed in this thread, I'm using relatively small KPI and caster on the front. I've attached text files of my geometry from Racing by the Numbers for the front and back incase anyone is interested. Ignore stuff like halfshafts; I didn't set them. I'm using 2.5" of bump and 3" of rebound as its set now for both ends.

From the start, I've been set on an I4 engine - originally I was going to use a Miata engine but at this point its a toss up between a Duratec, Zetec, and Miata engine. I'll figure something out once I get a frame together. I used an engine-like blob in the model thats slightly larger than most I4's so I should be alright. Once I get an engine, I'll get the rest of inner engine bay tubes sorted out.


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geometry.zip [2.45 KiB]
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PostPosted: Thu Mar 27, 2008 10:10 pm 
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I picked up a MGB steering rack for $10 from Ebay a few months back. Its points ended up matching my suspension geometry perfectly. After I got it, I found a few dune buggy sites have dirt cheap steering components that match up to the spline on the MGB's column.

I've found from past FSAE experiences that I am terrible at making people fit into CAD models so I built a wooden mock-up this time. The build table made a perfect platform for it. As a side note, I made the exact same mistake it seems like everyone else with a HF shop crane made with their build table.

A few months back, I was reading through this board and saw Chet's post about Miata halfshafts fitting RX7 diffs. About 2 days later, an RX7 diff popped up on Ebay for $160 BIN; I bought it. A month later, I found a junkyard a few hours away with 99 Miata halfshafts for $45 a piece; it was a sign. I'm also going to use front Protege uprights in the back since it will provide a nice mounting option for the rear toe-link.


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PostPosted: Thu Mar 27, 2008 10:17 pm 
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Always Moore!
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The actual build started in the middle of February.

I started out with some camber adjusters for the upper arms. I ended up doing them on the drillpress; unfortunately I don't own a lathe (yet). I'll post more once I get started on arms.

Next were the front uprights. I opted for a hub from the rear of a 94-97 Metro and Corolla. Its National Hub part number 512018. The only downside is the hub didn't fit inside of the Miata front rotor. I ended up borrowing a lathe to knock about 5/8" off of the OD.

The plate for the uprights is 1/8" 1018.


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 Post subject: Re: A.Moore's Build Log
PostPosted: Fri Mar 28, 2008 5:35 am 
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a.moore wrote:
... this is acting as essentially a back-bone frame rather than a true space frame...

You'll never come close to a true space frame, of course, but I think you could get the side trusses to help that backbone a lot more if you fix the broken triangulation at the roll bar base. As it is, all the rear load is more directly braced back to the tunnel, and only indirectly back to the side trusses. Might also be a safety issue of more leverage folding the roll bar forward due to its tall height, low mount, and only the tunnel tubes resisting.

You'd probably break even on weight (roll bar tubing is heavy) whether you address it with the traditional diagonal tube or a fabricated box member.

Image

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PostPosted: Fri Mar 28, 2008 8:43 am 
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Wow, that modeling is simply awesome. Good work!

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 Post subject: Upright fabrication
PostPosted: Fri Mar 28, 2008 2:46 pm 
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Hi Andrew - good luck with your build, and fascinating to see your uprights in the flesh as opposed to the CAD renderings. As you'll know form our other conversations I'm contemplating something similar so could you tell me what your end plugs are made from, and how are you connecting them to the ball joints?

Cheers,
James

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PostPosted: Fri Mar 28, 2008 8:35 pm 
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Thanks guys.

Good suggestion SCG. If I get a chance, I may try plugging it back into ANSYS and seeing what it says just for kicks. I wish I could lose the driver and engine and some other stuff so I could get a real space frame. :)

I'd be curious to see how much that tube will really do since the sides of the frame don't appears to be as stiff as the transmission tunnel under torsional loading. Even if that brace is stiffer, if the stuff its attached to isn't it probably won't carry the torsional load like we'd hope.

The safety point is valid. The one thing that makes that tube missing less scary is the support from the rear structure. The 'box' that the rear supsension mounts to should be a pretty good structure for the hoop braces to mount to. The pic shows how this all ties in.

Right now elbow room is the driving factor against adding more stuff there but I'll find out for sure once I get a chassis together with the steering and everything else. If it fits, I'll probably toss it in.

I've seen a few cars with the side bars like I've pictured below in the pic with the full cage; if time and space permit, I may go with these since they'll also double as side impact protection.

Puk, that block is just 6061 aluminum. The stud on the balljoint is tapered (1.5" taper per foot) so all I had to do was drill a hole and ream it until the 'joint' on the balljoint was in the correct position for the suspension geometry. The castle nut can sit inside of the upright then.

The balljoint is made by Afco and its part number 20034. The threaded housing is also from Afco and its part number 20043.


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File comment: I still need to sand the ends of the uprights to get all of the faces even
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PostPosted: Tue Apr 01, 2008 8:41 pm 
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I have the uprights just about finished - all thats left are the steering arms. I'm putting them off for now since I'd like to do bolt on ones to try to avoid any attention welded ones would bring when it comes time to get plates. Plus it will allow for ackerman adjustment.

Now on to arms - I got the front jigs drilled last night and tonight I trimmed the tubes for the front lowers. 2 arms down, 6 to go.


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PostPosted: Wed Apr 02, 2008 9:52 pm 
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I got the front arms tacked together tonight. Hopefully I can get them welded up tomorrow.

I realized about the time I was pulling out the camera that I only have two 1/2" rod-ends.


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PostPosted: Sun Apr 20, 2008 9:41 pm 
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I've gotten the front arms welded up since the last update; I'm tackling the rears while I'm waiting for the rest of my tubing to show up (thanks UPS for deliver the two packages with the tubing that I don't need right now...)

Its a small accomplishment, but the rear uprights have lower arms now.

I've also discovered that HF c-clamps are not the best tool for pressing in balljoints; I ended up "having" to get the 12 ton shop press - its much better. :D


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PostPosted: Sat Apr 26, 2008 11:51 pm 
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I've started on the frame; got the rear sub-frame semi-finished today and I'm on to the front now.

Hopefully I'll be connecting the two this week sometime.


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PostPosted: Sun Apr 27, 2008 12:29 am 
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Andrew,

Beautiful work - very clean and thought out. Excellent fab work too!

I have a question on the mounting of the rod ends though. It looks to me as though your travel is limited by the binding of the rod end since it is mounted longitudinally instead of laterally. Will you have enough room in the rod end to allow for adequate travel?

Hope that helps and makes sense...

Randy


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PostPosted: Sun Apr 27, 2008 12:33 am 
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Thanks.

Its a little close. I was planning on getting everything assembled and shimmed and if necessary, bevel the inside edges of the mounts some to allow the rod-end to clear the tube.

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PostPosted: Sun Apr 27, 2008 12:21 pm 
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I spy an RS. I want to say it's an 06-07?


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PostPosted: Sun Apr 27, 2008 12:34 pm 
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Nice work and methods.

Andrew


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