Exo Hot Rod
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Author:  Omaha Vette Graveyard [ September 7, 2016, 1:54 am ]
Post subject:  Re: Exo Hot Rod


I would argue that my cage bars do hit the corners, if I were the arguing type. Here are some photos for reference; tape is standing in for tubes that still need to be added for the cage proper:

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Compare this to Kurt's "integrated SCCA-legal roll cage" on the Midlana.

I would argue that the top of my cage is of a similar design, but much stronger than that on the Midlana. Please, though, don't take that as a critique or criticism of Kurt's design, as I think it's excellent. My cage is also heavier.

Thanks for the question.


Author:  Omaha Vette Graveyard [ September 7, 2016, 1:31 pm ]
Post subject:  Re: Exo Hot Rod

Okay, Bobber, so here are a bunch of measurements.

First, the inside base tubes:

They are parallel with the outside base tubes and are welded to to the outside base tubes 1.5" from where the outside tubes hit the spools. The distance from the outside of the outside base tube to the inside of the inside base tube is 23" for the entire length where they run parallel. The front bend line for the inside base tubes is 24" from the zero line, the rear bend line is 60" from zero.

Next, the top outside tubes. These are the top tubes that connect the main roll hoop with the front hoop.

These tubes hit the middle of the bend at the main hoop, and the middle of the bend at the front hoop. The high point of these tubes, at the cowl, is 4.5" higher than the middle of the bend where they connect to the main hoop. At that same point, at the cowl, they are 50" total width, outside of driver's side tube to outside of the passenger's side.

Next, the two top bars that connect the top of the roll hoop to the cowl.

These tubes connect to the top of the main hoop 6.5" from the centerline. That is, the inside edge of the tube is 6.5" from center.

They bend down and the front top edge of the tubes hit the high points of the top outside tubes.

The tube that connects these two sides is right at the middle of the bend (the top of the windshield) and the top of this windshield tube is 2" lower than the top of the main hoop.

The distance from the front edge of the top of the main hoop to the rear edge of the top windshield tube is 27".

Last, the cowl tube, which connects the inside of the middle of the bend of the two top outside tubes.

This tube is bent right in the middle, with the bend being higher and forward of the ends. The center point is 4" forward of the ends, and 1.75" higher.

All of this tubing described above is 1.75" with .120 wall.


Author:  Bobber [ September 9, 2016, 2:03 pm ]
Post subject:  Re: Exo Hot Rod

I have the lower tubes drawn but I’m having some trouble seeing the upper framing. Could you mark up a sketch? Thanks.

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Author:  KB58 [ September 9, 2016, 4:27 pm ]
Post subject:  Re: Exo Hot Rod

Omaha Vette Graveyard wrote:

Compare this to Kurt's "integrated SCCA-legal roll cage" on the Midlana.

I would argue that the top of my cage is of a similar design, but much stronger than that on the Midlana. Please, though, don't take that as a critique or criticism of Kurt's design, as I think it's excellent. My cage is also heavier.

Thanks for the question.


No offense take, though the above comes across as if strength wasn't considered during Midlana's design - it was indeed. It's all about balancing access with outright strength; had a diagonal been run from the top corner of the windscreen frame back to the main hoop belt line it would be really strong - and an absolute PITA to get in and out of. You pick your battles and I chose the compromise of being able to live with it. Consequences of the design - and the options of alterations - are fully discussed in the Midlana book :)

Put a seat in your frame and try getting in and out - you'll see!

Author:  Omaha Vette Graveyard [ September 9, 2016, 5:41 pm ]
Post subject:  Re: Exo Hot Rod


I had no intention of implying that strength wasn't considered. I do think your cage is plenty strong.

My cage is not stronger because I think it needs to be stronger, but because I'm trying to get more rigidity out of it. The additional strength is a by-product.

The vertical support pillar in my design, which connects the bend from A pillar to roof line, is crucial to both the strength of the cage and the degree to which top provides additional rigidity. It has no effect on ingress or egress, though there is a small weight penalty and a small reduction in outward visibility.


Author:  Omaha Vette Graveyard [ September 11, 2016, 5:00 pm ]
Post subject:  Re: Exo Hot Rod

Here are some really terrible drawings. Hopefully they make the measurements clearer than I was able to communicate via text alone.

The inside base tubes look exactly correct.

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I just realized that I didn't give the location of the bend on the top outside tubes. The middle of the bend is 24" rearward of the zero line.

I'm wishing I still had my drafting papers. They had everything drawn to scale with annotations, but they were getting pretty dirty and I didn't think I needed them anymore. Oops.


Author:  Bobber [ September 18, 2016, 9:25 pm ]
Post subject:  Re: Exo Hot Rod

Here’s the latest. I still need some help figuring out the framing. Thanks.

My computer crashed so I had to take a couple of days to replace. Still avoiding Windows 10.

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Author:  Omaha Vette Graveyard [ September 23, 2016, 12:58 am ]
Post subject:  Re: Exo Hot Rod


That looks pretty close. Just a couple of adjustments; the top side tubes hit the front hoop only 2.5 inches from the top, measured from the very top of the front hoop to the middle of the side tube. In your drawing it looks like it's at 5 inches. Also, the top and A pillars are single bent tubes, not mitered. Maybe you have that, but it kind of looked like a mitered angle in the drawing.

I'll get you some specific info on other tubes. I've been kind of busy this week so I haven't had much garage time. Hopefully I'll get some stuff done this weekend.


Author:  Omaha Vette Graveyard [ October 1, 2016, 10:10 pm ]
Post subject:  Re: Exo Hot Rod

A few things done in the last week or so.

I've redone the front bulkhead between the passenger compartment and engine compartment. My new solution uses heavier tubing, but should absolutely prevent the engine from ever entering the passenger compartment, and the changed structure puts more cross section where I really need it. I'll post pictures when it's all tacked in.

Also, I picked up some 1.75"x 11 gauge square tubing for my shock mounting bits. I'm figuring on using this for a few small bits around the chassis. It's stronger in bending than the round tubing, and more to the point, an easier interface for bolting on brackets. The shock mounting tube will be drilled for compression tubes, 2 through the front, one from the side. This should make for a very simple and strong interface with any bracket based on 2" angle.

File comment: Front shock mount tube.
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I've been trying to buy a drill press, but it's hard to find a decent one locally that doesn't cost more than I have to spend. They're either expensive, or cheap, if you know what I mean.


Author:  robbovius [ October 5, 2016, 8:06 am ]
Post subject:  Re: Exo Hot Rod

Omaha Vette Graveyard wrote:
I've been trying to buy a drill press, but it's hard to find a decent one locally that doesn't cost more than I have to spend. They're either expensive, or cheap, if you know what I mean.


I built the B-3 using a Porter-Cable bench top drill press that I got at Lowe's...$130 or something? Did everything I needed, and its still going strong.

Author:  Bobber [ October 5, 2016, 2:22 pm ]
Post subject:  Re: Exo Hot Rod

I'm thinking of a Craftsman 10" 1/2 hp or the Skil 3320. But this will just be for drilling aluminum.

ps: other computer crashed. no drawings.

Author:  Omaha Vette Graveyard [ October 5, 2016, 11:38 pm ]
Post subject:  Re: Exo Hot Rod

I'm trying to finalize the exact position of my upper control arms.

From the factory, the lower front control arms are parallel to the ground while the upper ones angle back about 6.5 degrees from front to back. This gives a little more anti-dive than I think I want, but having not driven the car I'm not sure. I know of a few professional, high-end companies that use the C5 running gear for other applications, and they change the geometry for less anti-dive.

Look at the pictures here:

The AVI Racing version of the Corvette uses not only the running gear, but also the subframes and wheelbase. They advertise a revised suspension geometry, and if you look at the pictures it's clear the the upper front control arms are parallel to the ground, or at least much closer to parallel.

There are a few other examples of this change, though there are also some that appear to copy the C5 setup pretty exactly.

I will have adjustability in the mounts, so I can make some changes to this later, but If I build for 6.5 degrees laid back and decide I want to go to zero anti-dive I'll be pushing right up against the limits of my adjustment range, and maybe going beyond where I'd like.

One option I've been thinking about is designing for 3 degrees of angle, so that the stock configuration is at one end the adjustment range and zero anti-dive is at the other.

Otherwise, my original plan was to have all of the structure set up so that the default would be basically right at the stock configuration. That seems the safest.

I have lots of steel drilling to do in the next several months, and I really hate drilling holes in little pieces of eighth-inch steel with a hand drill, so I'm probably going to try out the Harbor Freight 13-inch floor standing drill press. I've got a coupon and can get it for $230 out the door. There is a new version in this size, and it looks quite a bit nicer than the old. Better casting, nicer fit and finish, bigger chuck, more powerful motor, etc.. I was able to check it out at my local HF, as they had one on the floor, but they didn't have any in stock, and didn't know if or when they'd be getting any more. So, I think I'll drive to Lincoln (about 40 minutes) tomorrow morning to the HF store there, as they've told me over the phone that they have a couple of these in stock. I'm just hoping the have the new ones. I'm also going to pick up a better chop saw while I'm there, if they have one that will work.

I hope it's not a wasted trip.


Author:  Omaha Vette Graveyard [ October 24, 2016, 12:50 am ]
Post subject:  Re: Exo Hot Rod

A few updates, but not a lot to show for all of the work I've put in over the last few weeks.

First, I bought the 13 inch drill press from Harbor Freight. It cost just over $200, and it's pretty good for that price. They are making new versions of the larger presses, and they are better than the old ones. Casting, chuck, fit and finish, motor; everything's just better. I also picked up a real chop saw while I was there. Finally. I wish I had bought both of these a year ago.

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First thing I did with the new drill press is start drilling 1" holes in 11 gauge steel. It does just fine if you go slowly. This part is the first piece for my front upper control arm mount setup. I changed directions on this, as I wanted something cleaner and simpler. I had do drill a bunch of holes though. The square tube is 1.75" x 11 gauge. Heavy stuff, but it's going to do some important jobs and I'm not using that much of it.

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I then beveled the holes pretty significantly and welded in 11 gauge compression tubes. Everything was ground flush and I hit it with a flap disk to clean up. Then I did the same thing with a smaller square tube that I welded on right in the middle, at 90 degrees. Both the longer and shorter tubes got end plates welded on. Finished and mounted, but without any of the connecting round tube bits, they look like this:

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The centers of the holes will give me the C5 Corvette factory geometry, only with an extra 3/8" room for additional negative camber. So I can either adjust the alignment with the eccentric bolts on the lower control arms, as normal, or use shims on the upper control arms. However, you can see that I also have one hole in the top of the UCA mount tube between each pair of holes in front. This allows for greater adjustability. I can simply bolt a 5" piece of 2" angle to the square tube and weld connection points for whatever UCA geometry I'd like to try. So I can change roll center height (has very little effect on roll center fixity with this setup), change anti-dive amounts, go a little crazy with caster, or whatever. I toyed with making the basic through-the-bolt-holes setup have a little less anti-dive, as I've seen that most race car custom setups with the C5 parts look like they run very little, but in the end I decided the factory location was a better default.

The square tube on the top is the shock mount tube. You can see the two holes in the front, but there's also a transverse hole that is exactly between. This is essentially the place where I'll bolt on the shock mount bracket. Again, I can make the bracket-to-tube interface a simple piece of 2" angle, or I can make it more complicated. I decided to go with a bolt-on bracket because I don't know what shocks I'm going to run. I already have the Corvette transverse springs and factory shocks, so it will be free to run those and I plan to do that in the beginning. However, I suspect I'll eventually want to go to coilovers, and a mount that works for the factory shocks will not support other kinds of units very simply. The obvious solution is a bolt-on bracket. I also like it that it allows me to change the angle of the shocks some, and to use different lengths of shocks. I can run whatever parts I can find for cheap, just have to build a new bracket.

The caps on the ends of the square tubes are the connection points for round tubing that will interface with other parts of the chassis. I decided on this as a reasonable way to get square tubing to work with round. As the square is 1.75" like the round, it interfaces nicely as long as they're on the same plane, but as soon as they're not it's a problem. I really wanted to use square tubing for the shock and control arm mounts because it's stronger/stiffer in the direction of the primary load and it makes a much simpler interface for brackets. But it's heavy and only plays nicely with round in one plane. So i'm using enough square to get the job done, then plating the end and switching to round.

The little pointers in the picture are my fixed points in space that I used to locate the UCA mount holes. First I took the tube chassis off the subframes (no, the chassis is not as heavy as the picture makes it look, this was just a simple solution), which only took about 10 minutes:

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Then, aligned, bolted down, and measured the mounting points. I measured the height of the bolt holes from the floor (my build table, it's pretty flat), the fore/aft from the front machined surface of the LCA on the subframe, and the width from the LCA bolt in its widest position. I measured both sides and found ways of measuring where I could get consistent numbers, mostly using a pair of squares a level and a few straight edges. I think the tolerances are within 1/16".

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Then I bolted the chassis back down square on the mounting pads (at least in the front), located the points in space, and fitted the UCA/shock mount assembly accordingly.

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Finally, I fitted a round tube on the bottom to hit one of the 'hard points'. After tacking on the round tube I removed the assembly, fully welded the round tube, and final fitted each side. I'll have one more look and measure before I tack them in, but I think they're pretty close to dead on.

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The UCAs won't bolt onto the holes directly, but to hit the factory locations they need to sit 1 3/8" out board, so for now I'll probably just drill holes in some 1.25" square tube and use little pieces of that for spacers. Eventually, if and when I keep the factory geometry, I'll use solid aluminum for the spacers. I'll just need a bar the right size that I can cut and drill.

Hopefully this big update makes up for the last few weeks.


Author:  mjalaly [ October 27, 2016, 9:39 am ]
Post subject:  Re: Exo Hot Rod

Not being critical here just wondering, why incorporate anti-dive at all?

Author:  Omaha Vette Graveyard [ October 27, 2016, 10:15 pm ]
Post subject:  Re: Exo Hot Rod

In reply to mjalaly,

I don't take it as a critique, but I am open to criticism. I find it very productive.

I'm really only incorporating anti-dive because it's part of the stock geometry. I know that the stock setup works extremely well in lots of different contexts, so in the end I think it will function as a good baseline. The idea of a 'tried and true' baseline was more appealing to me than a guess at an improvement (although I think it's a good guess).

In the end I suppose it doesn't really matter what the baseline is if I end up running anything else. A bracket is a bracket. I have lots of ideas about how to improve the geometry, and one of the first things I plan to test is running the car without anti-dive. But I had visions of testing version after version only to end up with a carefully crafted bracket that got me back to the stock geometry. Maybe I'm just covering my butt.


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