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Learning how to build Lotus Seven replicas...together!
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 Post subject: Re: Exo Hot Rod
PostPosted: August 20, 2016, 11:08 am 
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Here’s a sketch of the Generic Locost with the dimensions I will need. This sketch doesn’t show the diagonals yet since that was still in development. Also attached is the computer output with the diagonals. This sketch doesn’t show the deflected shape.

If interested, send me your dimensions and I will model it up for you. Are you using any computer files now? I can try to make my files compatible.


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 Post subject: Re: Exo Hot Rod
PostPosted: August 21, 2016, 12:33 am 
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It's a bit challenging with my chassis, because there are lots of bends (though I'm treating each bend as a node, so it's almost like a joint), and basically nothing is square. The planes shift from one triangulation to the next. There are a few places where it's simpler, like inside of the main roll hoop. So, maybe the way to do it would be to get you some numbers for some of the main tubes going front to back? Then maybe fill in the rest as I get the numbers? I'm just not sure the simplest way.

I have done none of this on the computer. To find a shape and proportions that I liked, I drafted a version on graph paper. But I only really designed from the side view. I think you can tell when you look at the car, as the shape looks 'designed' from the side, but is not as convincing from the other angles which I'm trying to sort out as I go. I just cut out the tube that was the top of the windshield tonight, because it looked wrong. I'm going to try a straight tube next, which will hit exactly in the middle of the bend. Seems better when I mock it up with tape anyway.

To test my idea of the structure, I made a wooden model that was neither in a particular scale nor proportioned exactly right (it's also missing some bits). I used this as a learning exercise by starting with the basic structure and then twisting it with my hands, grabbing at the 'subframes.' As I added each level of bracing I could see where things were still moving, which direction, and I could generally feel when it got a lot stiffer.

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The pictures show where I stopped this exercise, but the wooden model does not include all of the braces that will be on the finished chassis. I stopped at this point because I could not twist the wooden structure enough anymore with my hands to get it to move in any meaningful way, and when I really cranked on it I just broke the glue at one of the joints. The weak point in the model, and I think also in the real version, is the joint where the floor bars are welded to the spools at the front of the rear subframe, and probably in the analogous spools at the rear of the front subframe. I plan to install extra bracing in these spots and also a large gusset that wraps the joint.

Once that joint broke free on the model, I could, with quite a lot of force, twist the model meaningfully again. The movement that I observed at this point was the right side of the floor moving forward in relationship to the left side, or vice versa. That particular flexibility is probably the greatest weak point in this chassis, and I'm working on ways to address it without employing a traditional driveshaft tunnel. The problem is that I don't have a one-piece floor, but two parallel trusses that must be meaningfully linked over the top of the torque tube. I want to try to make that structural link at the front and/or rear firewalls.

I do know that the inaccuracies of the wooden model would tend to invalidate the conclusions. However, I did learn quite a bit and get a little better and seeing the load paths, especially understanding that movement in one plane, once braced, very easily changes to movement in another plane lying at a 90 degree angle to the first. So if I'm trying to keep something from bending one direction I need to also make sure it's fixed int he other two. Nodes really are a good idea! I understood that before, but I'm better at seeing it now and imaging better solutions.

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File comment: Front bulkhead.
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-Graveyard

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 Post subject: Re: Exo Hot Rod
PostPosted: August 21, 2016, 11:15 am 
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Superb! You're doing with wood sticks the same thing we do with electrons on the computer; build a model and break it! I can't beat what you are doing but I can give you numbers to help you decide what you should try to break next.

Just the basic frame member dimensions is a good start and I can send you some graphics to play with along the way. Nodes are good, curves can be filled in later.

But don't stop what you are doing and never stop building!


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 Post subject: Re: Exo Hot Rod
PostPosted: August 23, 2016, 12:03 am 
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Bobber,

Here is a link to a pdf which has the exact locations of the mounting points for the subframes:

http://corvetteforum.shelor.net/Docs/C5 ... Panels.pdf

The measurements you'll be wanting are in the last few pages.

I used these measurements to set up my subframes so that I get exactly the same suspension geometry as the C5 corvette. The spools that are bolted down to the subframes are centered on the 15mm holes. The rear hole for the front subframe is the Zero Line.

I think this might be an ideal place to start, because it will give us a set of common reference points from which to measure. Let me know what you think.

----

Yesterday I fitted the 'Duvall' bar for the center of the windshield, and decided I didn't like how it looked.

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I looked at it from every angle, took pictures, thought about it. Again, and again. But I just couldn't convince myself it looked right. Somehow, the angle from the cowl to the top of the windshield wasn't right, and the top bar looked like it was in the wrong spot because it was not centered on the bend. I had reasons for putting it where I did, but it didn't look right. Also the bend seemed to be going the wrong direction.

Anyway, I decided that a Duvall style windshield with the divider up the middle might not look good on this car, which meant I needed to rethink having a bent bar on the top. I cut out the top bar and the support bars, which I was going to upgrade to a larger size anyway, and fitted a straight tube on top right at the middle of the bend.

Here's the new top bar fitted:

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I'm not completely in love with it as is, but I think the little gusset tubes in the corners (marked by tape now) will make it look right. I'm still thinking about it.

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The support tubes are fitted, but not yet tacked in. Good fit though. This is the kind of fit I like for this wall thickness. It's close enough to not deal with any gaps, but there's lots of edge for good penetration. I like to weld this stuff hot (welder set for 10 gauge), but not slowly, and not more than a few inches at a time. I get good fusion that way, but don't overheat the steel or have much trouble with burn through. I also try to run each bead from thinner (steal) to thicker, ending at the beginning of another bead if possible. This helps me to get penetration in the thicker places, avoid burn through in the thinner spots, and ending at another bead helps blend them together and avoid cratering.

-Graveyard

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 Post subject: Re: Exo Hot Rod
PostPosted: August 23, 2016, 8:50 am 
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Yeah, "windshield 2.0" frame looks better than the one with the center bar. IMHO, my .02, etc.

The gussets do add something to the "look" don't they? It's funny to me that it makes such a difference, but I do agree with your observation.

:cheers:
JDK

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 Post subject: Re: Exo Hot Rod
PostPosted: August 23, 2016, 6:44 pm 
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Here’s the beginning layout. I wasn’t sure if their lateral dimensions were from the centerline or if they were overall width so I showed overall width. I threw in the biggest tires I could find for that year. What track will you be using? Feel free to mark this up as you have time. I can send a larger size if you want.
Your build is looking good!

Moderators, should I be sending this outside this forum?


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 Post subject: Re: Exo Hot Rod
PostPosted: August 23, 2016, 9:22 pm 
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JD, thanks for the input. Sometimes I've looked at something for so long that it's hard to really see it anymore.

Bobber,

The lateral dimensions are measured from the centerline.

My parts are from a 2000 Corvette, and the track width I've seen for that year is 62 inches, though I've seen other numbers too. It's variable depending on alignment. The camber is adjusted using eccentric bolts in the subframes, which would push the contact patch in and out. I don't know what specific track width I'll want to run; it will be adjustable as my top suspension mounts are going to be bolt-ons so that I have extreme adjustability.

I plan to run stock 2000 Corvette wheels, in the beginning at least, with the stock size of tire. The wheels came with the chassis, and they're very light (under 20 lbs. each for 17x8.5 and 18x9.5 inch wheels), forged, and nice-looking, so it would be hard to find anything better for cheap. Any C5, C6 or C7 wheels will fit. I have a second set that I picked up locally for cheap, so I can run a square setup in 17s or 18s if I ever care to.

At ride height, the rear mount points for the front subframe are at about 9 inches from the ground. Most C5 vette folks say you can run the car 1 inch lowered without any geometry problems, so it could be as low as 8 inches (at least in race trim).

I do not know the exact distance from the Zero line to the center of the wheels from any official GM source, but I measured from the front hole of the rear subframe to the center of the half-shaft (assuming it's aligned straight out of the diff.) as 5.75 inches. So, the center of the rear wheel would be 5.75 inches behind the front mount point of the rear subframe.

One of the things I like about this setup for my purposes is that it provides tremendous adjustability to be able to adjust both the top and bottom control arms. The front bottom A-arm can be adjusted in and out, both the front bolt and rear, 1.5 inches on each side. So, since I can adjust camber and caster with the top arms, I have 3 inches of track width adjustability in the front. Only the front bolt of the bottom A-arm is adjustable in the rear, intended as a rear caster adjustment I presume, so I can probably get an inch or so of track width adjustment out of that.

-Graveyard

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 Post subject: Re: Exo Hot Rod
PostPosted: August 24, 2016, 12:54 am 
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Here’s an update with the rest of the subframe mounts. Let me know if I got them right. I set their heights so the front mount was 9 inches above the ground.


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 Post subject: Re: Exo Hot Rod
PostPosted: August 24, 2016, 9:06 am 
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I just love the look of this. One thing that "bothers" me are the tubes over the engine bay. Of course I see why they're there structurally but can't get over the reduced access to the engine bay. Any consideration to making that a bolt-in piece? Sort of like a strut tower brace but running longitudinally. Just a thought. Keep up the great work!

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 Post subject: Re: Exo Hot Rod
PostPosted: August 24, 2016, 11:25 am 
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Bobber,

It looks pretty good. The front of the rear subframe and rear of front subframe mounting points are now correct. However, the front of the front and the rear of the rear are not correct.

For the rear, that hole is at: L:2665, W:359, H:574. You can see it on the chart, it's a 15mm hole.

For the front, the correct 15mm hole is 1.5 inches behind the 20mm hole. I don't know why the front bolt hole is not in the collision repair manual, but fortunately it's at exactly the same height and width as the dowels.

Here are some pictures of a section of frame rail where it bolts to the subframe, and the subframe mounting point with the locating pin. The mounting pin is what goes in the 20mm hole. On the passenger side, the 20mm hole is oblong, on the driver's side it's round. I presume this is for adjustability to make sure you can get the subframes square even if the mounting holes are slightly off.

Well, I was going to put the pictures in, but suddenly I'm getting a note that the picture of the subframe is bigger than 2MB so it won't go. Here's a picture of the subframe anyway.
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 Post subject: Re: Exo Hot Rod
PostPosted: August 24, 2016, 11:55 am 
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Keith,

Thanks for the compliment. It's important to me that I like the way the car looks, so I've prioritized that in this build.

As for the bars over the engine compartment, they really don't compromise access very much, though it's maybe hard to tell in the photos. There's a lot of clearance (I'm planning a tall intake), and it's just two tubes. It should be simple to change intake manifolds, swap heads, and even change headers. I'm also setting up the front so that I can do a cam change with the engine in the car. The only thing I won't be able to do is take the engine out through the top, but the C5 Corvette was designed to have the engine taken out through the bottom. That's what it specifies in the factory service book, and it's supposedly much faster and easier. I think this is mostly because of the torque tube setup. To take the engine out the top you'd have to be able to slide it forward enough to disengage it from the torque tube, but you can't tip it very much. So, to be able to take the engine out the top, I'd not only have to get rid of the top bars, but also move the front forward and down at least a couple of inches. I really thought about doing that, but it would compromise the look and structure of the car.

I suppose I could make the 'hood tubes', as I call them, removable pieces, but it's simpler, cheaper, lighter, and more rigid to have them welded in, so I'm going to try it this way first. The solution just ticks all the right boxes for me. If I find myself wishing for better engine access through the top, I'll just cut them out and fab up some plates for bolt-in rods (If I do that I'll use aluminum to make up for some of the extra weight).

Another thing is that part of my approach with this build is to take every advantage I can of the Corvette setup. I see two of those advantages being the ability to eliminate the driveshaft tunnel and the ability to triangulate over the engine bay.

The more I work with the C5 drivetrain and suspension setup, the more I see the brilliance of the design. I understand why GM is still using this setup 20 years after they introduced it.

-Graveyard.

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 Post subject: Re: Exo Hot Rod
PostPosted: August 24, 2016, 2:19 pm 
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Here’s the latest. Let me know if O.K. I like the bracing over the engine bay from a structural view point. And pulling a chassis off an engine makes sense when the engine weighs more than the chassis.

So, I had to put a supercharged small block in the picture, just because. And Bono wants to try it on for size.


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 Post subject: Re: Exo Hot Rod
PostPosted: August 25, 2016, 10:55 am 
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Bobber,

The top drawing is now correct. The rearmost does appear to be wrong in Bono's car though.

I almost wish the engine were set back as far as it is in Bono's version.

I'll get you measurements for the spools that are bolted into those subframe holes, if you think that's a good next step.

I did leave room for a super charger. :D

-Graveyard

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 Post subject: Re: Exo Hot Rod
PostPosted: August 25, 2016, 3:11 pm 
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I'm ready for all the measurement you have. If it's easier, you can send them to me at rgarner2@san.rr.com

But keep posting photos here. I usually draw in AutoCAD and I can save files to dxf or? I think I can transfer structural files to dxf.


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 Post subject: Re: Exo Hot Rod
PostPosted: August 26, 2016, 12:44 am 
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Bobber,

I'll just reply here and keep it short, only because I'm going to be adding pictures and references with my measurements, and I think they add to the substance of the build log.

To interface with the subframes I built 8 'spools'. Each is made of 1.75" round tubing with .134 wall thickness (heavy stuff), plated with 1/8" on each end, and with a 1" (.083) compression tube welded to the plate on each end. The 1/8" plate was highly beveled before welding, and then both the compression tube and weld bead were ground flat. Here's a picture of a trial spool I made to see if I liked the idea:

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The four spools that interface with the rear subframe are 4.25" tall. Four inches of tube with an eighth of an inch plate on each end. You can see the basic setup here:

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As you can see, the spools are connected with 1.75" tubing (.095 wall, in this case, as I had some leftover still). The tubes on the sides are pushed up as high as I could get them on the spools, so that they give adequate clearance for the half-shafts coming out of the differential. This clearance is the reason the spools in the rear are a full inch taller than the ones in front. The tube in the rear connects the rear spools and is pushed up as high as possible with deference to the side tubes. There is no tube connecting the front spools on the rear subframe, since the transmission is in the way.

The four spools that interface with the front subframe are 3.25" tall, otherwise built exactly the same way as the rear spools. The tubing connecting the spools is also 1.75" with .095 wall, and similarly has the side tubes pushed up as high as possible and the front tube bumped up to the limits imposed by the side tubes. Of course, there is no rear connection, as the engine is in the way.

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The stiffness of the subframes and the nature of how I interface with them is, I think a critical aspect of what will be the functional stiffness of this chassis. After studying the subframes and how they are built (rather thick cast aluminum, by the way), I think they will be as rigid as anything I can build to mount to them, as long as each point of interface is not allowed to move in relation to the others on the same subframe. In other words, I think the subframes are extremely stiff when bolted to a stiff structure, but they were engineered assuming as much. GM had them bolted directly to stiffened points on the very-large, hydroformed, frame rails. There were no bushings, and the bolts went through the frame rails (and 1" compression tubes, which is where I got the idea), but the steel plate GM used to contact the subframe mounting points is not as thick as what I'm using, or as directly tied in to the next structural level. The spools I've built contact the entire interfacing surface, so there would be nothing to be gained by making the bases of them larger.

I believe that the weakest point of the way my tube structure interfaces with the subframes is the lack of bracing between the front two spools on the rear, and the rear pair in the front. To be kept from bending inward, toward one another, the pairs of spools will need to be triangulated to the nearest hard points; hard points being my name for nodes already fixed in three dimensional space. The hardness of the point is a function of its fixity.

In the rear, the hard point will be the middle of the harness bar; this will be a very hard point. In the front, I think the hard point will be the bottom of the firewall triangulation; this is less hard than the rear, and I may look to either make the point harder somehow or double up by bracing to a second hard point.

I don't expect anyone to actually read most of the above, by the way. I'm mostly putting it down for my own build record.

-Graveyard

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