30 Sep 2008

A little surprising, of the comments received regarding round-versus-square tubing, all the replies were pro-round. So that answers that; the only square tubing will be where it makes sense, for attaching paneling or suspension brackets.

CAD work continues; the latest puzzle is determining how to most effectively get coolant from the front of the car to the back. My fabricator buddy, Alan, says to put the radiator in back; I’d love to but don’t trust that there’ll be decent airflow through the side radiators. There’s already some risk with the intercooler being back there but it’s a calculated risk. I’m much less willing to experiment with the main radiator, though.

For routing the coolant, the various ideas are to: run coolant through chassis tubes; run coolant pipes down the center then up and over the fuel tank; run them along the floor and under the tank; or something I haven’t thought of yet. I’m not too excited about running coolant through chassis tubes, not so much for corrosion but fear of spraying boiling coolant on someone in even a minor accident (never mind burning someone if they touch the tube.) A compromise is to run a tube-within-a-tube to both protect the occupants and isolate the heat somewhat. However, if the outer tube is on the floor, it’s only natural to want to use it structurally and to rivet the floor to it. Having an aluminum pipe inside it means the inner tube will be sitting on the top of the rivets – possibly wearing through – though I doubt that’s much of a problem. A good solution is to use stainless coolant pipe because it’s much tougher than aluminum, has much better heat-insulating properties, and it obviously doesn’t corrode.

Then there’s how to balance that requirement against mounting the emergency brake lever and shifter. The shifter is going to be extra challenging because someone (me) designed things so that there’s only about 3″ between the seats. Either the shifter has to go ahead of the seats, above them, or be made really narrow. I have an OEM Honda shifter assembly and it’s, well, huge, about 6″ x 10″. I might do a subproject and construct one from aluminum and rod-ends, which would look very appropriate in a car like this rather than a huge ugly plastic OEM assembly. We’ll see.

Like pushing toothpaste out of its tube, the problems are slowly being pushed out and dispensed with. My brother keeps nagging me about when it’s “going to metal” – when the CAD work is done. I’m going to use the plans myself to insure they make sense, dimensions aren’t missing, and that whatever I build first will be just like the one that you’re going to build 😉

21 Sep 2008

I’m debating the use of round versus square tubing. Many builders prefer square because they say it’s easier to cut. On the other hand, round tubing weighs 21.5% less, is equally strong in all directions, and looks nice. There’s something about a round-tube chassis that looks… what, smooth, more refined somehow. I’ve used both types and don’t feel strongly one way or the other. I do have a tubing notcher for doing round tubing so maybe I’m biased. Many builders say square is easy to cut with a hacksaw but I don’t think they’ve cut many two or 3D junctions. The worst is a square tube that meets up with two other tubes with none of them at 90 degrees. It forms a tricky three-dimensional cut with each side of the tube having its own angle. Yes, single plane cuts are easy though I read that even two-dimensional cuts drive some builders crazy.

However, I realize the car’s supposed to be easy to build and that means having easy-to-follow drawings. Showing drawings of round tubes with curvy ends isn’t going to be very helpful; about all I can do is show the overall length. I can “unwrap” the tubes so paper templates can be made, wrapped around the tube, a line scribed, the tube cut, and presto… Only – it doesn’t quite work out that way; square tubing isn’t really square, the corners are radiused by varying amounts. Also, tubes have finite wall thickness, something that “unwrap” drawings can’t take into account. The problem is that the inside of the tube might need a fairly different length than the outside at the same point on the edge. Some tube drawings could take a lot of fiddling to make them work right. (OTOH, many of the tubes have to be round due to being part of the rollcage. So, while a tubing notcher isn’t required, there’s not getting around dealing with round tubing.)

Also, as Gibbs notes in his book, having tube drawings with infinitely accurate dimensions is very misleading due to welding heat warping the chassis, cut variations, and very slight mistakes accumulating through the chassis such that by the time the builder gets to the other end, the lengths are way off. Regardless, drawings of some sort are needed. I’ll probably make drawings for square tubes where there’s a choice and let builders decide whether they follow the drawings or use lighter round tubing instead… I know which my car will use. (I should add that using square tubing off-axis, and unwrapping tubes in general, aren’t something that SketchUp’s particularly good at and takes a long time for each tube).

In other news, the engine tray won’t be removable; Turns out the drivetrain can come out by removing only one tube. It saves a few tubes and does away with half a dozen bolted connections, a dozen screw connections, and all the threaded inserts. The one removable tube will be the diagonal above the drivetrain and will have rod-ends to make sure it fits in spite of build tolerances and heat distortion.

While the drawings may look a lot like the last ones they’re more polished and… focused. The engine bay is fairly complete, missing only a couple tubes on the lower panel and engine mounts (the rear one is in place.) I’m working my way forward, making final tweaks and working on tube intersections so individual tube drawings can be made. The upright cylinders at the back are the shocks while the large horizontal cylinder is the muffler – with 3″ inlet and outlet 😉

The last picture shows the diffuser. Kimini is very stable as speed – I did something right – so Midlana is getting the same treatment with a smooth undertray in addition to side-exiting radiator ducting. As an open car there’ll be more turbulence but it’s worth a try and a future iteration might be a hard top!

I’ve got some good leads on engine builders and Kimini’s sale helps that happen; I just have to decide when. The bent roll cage tubes will be the first tubes fabricated, followed by the bottom rails. After that it pretty much depends which end of the chassis is built up first. Starting at the front gives plenty of time for the engine to be done by the time I get to the engine compartment.

And finally, I’m selling two of the shocks, QA-1 DDR7855. They’re too long for the rear suspension, except these exact units are used at the front, so consider it the first two components for your future car! Brand new, never used, still in the box, $470 shipped anywhere in the lower-48. Contact me if interested.

14 Sep 2008

Now that the rear suspension is settling down the real drawings are starting (instead of the endless try-this-no-how-about-this-no-try-this-instead thing.) The engine tray is first only because it’s the last thing figured out and makes a nice compartmentalized sub-project. I’ll probably build it up in steel after finishing the drawings to have a sense of tangible progress. It would be nice to support the engine on it but the design is setup the other way round – to have the main chassis suspend the engine at each end while the engine tray simply keeps it from rotating. I know everyone wants to see pictures but it’s pretty dull right now, lots of half-rendered lines on the computer screen is about it.

I may sell the shocks due to them being rather long for my needs; the design changed after buying them. Than again if there’s no interest I may just use them instead of taking the loss. I’ll have to double-check the fit-up at the front suspension, where the long shock body may actually lay-out better than a short body, one requiring the top shock mount to extend further out from the chassis than it does now.

I’m starting to look for engine builders. Initial contact was so-so; they need to understand that customers have nothing other than response time to judge them initially. When a week goes by before a reply they may as well not even bother in my opinion – they don’t seem to understand who writes the checks. If I get annoyed enough I’ll just rebuild it myself; the trick is finding a really good machine shop that knows what it’s doing and have them do all the precision stuff – I just bolt it together, or that’s the theory.

8 Sep 2008

The rear suspension is slowly taking shape. True A-arms will be behind the drivetrain because trailing links didn’t work for several reasons, the main issue being the use of Miata rear uprights. Between the wheel offset and upright design, they’d had to angle inboard 30 degrees, fouling chassis tubes and the drivetrain.

The shocks will be in the traditional outboard position, tucked nicely into otherwise-wasted space. The installation ratio was the driving factor, where the initial try resulted in a value of 0.6 – bad news. Compared to a shock which moves the same as the wheel, a 0.6 results in springs having to be 1/(0.6)^2, or 2.78 times stiffer. This would be a real problem because there’s roughly 350 lbs sprung weight at each rear corner. A typical soft suspension starting point is to have the springs equal the sprung corner weight which would be 350 * 2.78 = 972 lbs springs, rates that are impossible to source, never mind that springing the car for the track may double that value. I managed to get the installation ratio up to 0.75 but it’s still marginal.

The Miata rear upright has two pickup points at the bottom, inline with each other such that a long bolt passes through both. It’s natural to use rod-ends, the problem being that the spring force bends the rod-ends radially. A spherical bearing in a welded-in cup will be used, with the other being a rod-end to allow adjusting toe; the upper joint will adjust camber.

Oh, and just for fun I moved some tubes around in the rear area to create a diffuser, much like Kimini’s which made the car feel very stable. Of course Midlana being open top will be – to coin a currently-popular phrase – like putting lipstick on a pig – but why not.

The free drawing program Google SketchUp is looking better and better after discovering plugins that make it more than adequate. It’s still going to be a lot of work, but that’s true of any CAD system.

5 Sep 2008

Our company began a 9/80 work schedule; 9-hr days with every other Friday off and today is our first Friday off – cool!

After some research I’ve backed away from using an air-to-water heat-exchanger in the engine compartment (for the turbo) with its associated pump and lines to the front where another radiator rejects the heat. These setups work great for drag-racing but for road racing they heatsoak and the overall efficiency takes a dump. Between that and the weight, expense and complexity, a straight air-to-air intercooler will be used, getting air from one of the side ducts. What’s unknown is how much airflow there’ll be.

It seems like it should be decent since there’ll be a belly pan under the engine and the windscreen and curved engine cover should create low pressure behind it. Also, with the side vents being immediately ahead of the rear fenders, there should be high pressure piling up ahead of them. “Should be” means I really don’t know for sure; if airflow is insufficient, there’s always the McLaren F1 solution. It has a horizontally-positioned engine compartment fan that sucks air from below the car into the engine compartment, blowing it out the back. Doing it that way, the fan could push/pull air through the intercooler at the same time. This means a constantly running heavy fan, something I rather avoid, so we’ll have to wait and see. One perk of the McLaren setup is that the fan generates significant downforce!

Yes, an F1/Atom type center-mounted engine air duct has been considered but doesn’t seem appropriate. Since the car will have a windscreen it may prevent the duct from working at all, then there’s my real reason: the bane of composite construction and having the rear view mirror completely blocked by the duct. I would find it very annoying. On the other hand, my brother says he uses only his side mirrors in his Stalker. Eh.

What the intercooler really does is break the mental block I’ve been wrestling with, how to handle the rear suspension. I had all but decided to use trailing + lateral links but it means there’s zero room for the intercooler (and air intake, and maybe an oil cooler.) This is pushing the design back towards having true A-arms located entirely behind the engine. Now if I could just get rid of the darn rocker-arms!

I’ve received a few notes that hint at, “What’s taking so long, when are you going to start cutting metal?!” As I’ve said, it’s much creating blueprints for a big building. Rushing at this points sets the stage for disaster when I get to the third floor and discover some dire mistake made in the foundation. It has to be right, carefully thought through now so I don’t screw myself later on. Doing this paid off big time with the Mini so I’m not rushing this.

2 Sep 2008

The rear suspension is a tough cookie and making it easy to build makes creating the first one difficult, not a one-off where anything goes, lol. It’s easy to make it complicated… I’m spending hours staring at the mockup trying to come up with a simple and elegant solution and it’s hard. I really want to avoid push-rods and rocker-arms.