Several people have written questioning whether the upper chassis is following SCCA and NASA rules; specifically, how the roof tubes cross instead of running parallel to the centerline of the chassis. I don’t think I’ve broken the rules but would appreciate input if you feel otherwise, especially if pertanent rules can be quoted. The reason the roof tubes cross is to make it easy to get in and out. I really don’t want a true boxed-in cage, where getting in and out resembles an old episode of “Dukes of Hazzard”, never mind having a girl get in wearing a dress! I noticed with disappointed amusement that NASA, like the SCCA, allows using 1.375″ OD “unobtainium” tubing.
So let me know; it’s trivial to correct now; much tougher later on, never mind the liability of having a bunch of books in circulation saying to do it wrong…
One of the consequences of designing a car is often not knowing dimensions of components without first having them in-hand. Other times it’s guessing about what will fit where and placing orders before all the details are known – that doesn’t work sometimes. Such was the case with the floor-mount Tilton pedal assembly. With perfect hindsight, there is indeed room for an overhung top-pivot design, which fits better, is more ergonomical, and costs much less. The good thing is that it makes the overall project that much less expensive to build and fits together a bit better. Hard to argue with that, just wish I hadn’t been so quick to order the wrong part but so it goes.
I’ve been exchanging ideas with reader Dean, who’s comments have helped make the chassis look a bit less dunebuggy-like and a bit more sports-car-like. Never mind the ugly nose, I’m too busy working on the rest of the chassis to create a proper render. Note that the side windscreen tubes now bend directly into the roof instead of around the windscreen as before. The foot of the windscreen frame now extends forward all the way to the floor. I also triangulated the bulkhead around the knees, though it remains to be seen if it’ll interfere with the drivers legs or feet. I hope not because it hugely stiffens a bulkhead that’s normally left open on most cars.
What’s still half-baked is where to run the coolant lines. At the moment I’m back to running them down the center and if that doesn’t work out it’s back to outside the chassis. My buddy Ron suggested using large 3″ angle for the lower outside frame rails and tucking the coolant lines inside, covering them with a radiused cover. That would look really sweet but there isn’t 3″ to give away outboard of the seats – things are that tight. I can’t justify widening the car 6″ just for coolant lines so if down the center doesn’t work it’s back to along the outside of the chassis, covered, ala Cobra side exhaust-style.
Kimini is officially sold; the trailer has already left for the new owner’s home in Utah and next week Kimini will make her journey inside a proper transporter.
Blunting the impending emotional loss is the arrival of a Garrett twin-scroll GT3071 0.78 A/R turbocharger, also an emotional item, and admittedly not a necessary component, never mind this early in the build. Its purpose is to serve as an emotional goal, to look forward to hearing it whistling someday 🙂
The emergency brake system is done. Next is determining shifter, dash, and steering wheel (and steering shaft) placement. This naturally flows into dictating the cowl dimensions. It’s all but been decided to use aluminum for the cowl rather than messing around with adapting an existing – but wrongly sized – composite cowl. (The term “cowl” refers to the assembly that houses the dash and extends under the windscreen base, meeting up with the hood.)
I knowingly swapped the Miata rear uprights side-for-side to bias the upper pickup point toward the rear, making it more of a straight shot to the upper pivot points. What I didn’t realize was what I was doing to the emergency brake actuators… now pointing toward the rear of the car… doh! Fixed, it’s an awkward reach for the upper A-arm, but oh well.
The gas tank is done, complete with hanging brackets, baffles and hose connections. Since it’s triangular-shaped, not too much capacity was lost by shortening it a couple inches, improving cable routing for the shift cables and emergency brake. It does lower capacity to about 13 gallons but that’s still a decent size and, hey, it lowers the CG a bit, too. Now I know how much work I avoided by not having an emergency brake on Kimini, it’s a pain. The good news is that a stock Miata emergency brake lever and cables assemblies are used, no modifications necessary.
Had the day off so good progress was made on the gas tank, either aluminum or stainless, seen here in its protective frame. This fits up behind the seats from below, fastening with only two bolts – pretty sweet I think. Sketchup says it’s 14.23 gallons which is a good size. Still to be added are internal baffles, doors, filler, vent, etc, etc.
Reader Dean suggested running the coolant lines along the ouside of the chassis instead of through the chassis tubes. I’m good with that as long as it looks okay. Maybe something like those muffler covers on Cobras but smaller. The trick is finding something that requires little to no work; maybe perforated stainless sheet rolled over a form. I’m open to ideas
Because space is so limited between the seats, a couple different arrangements are being considered. One rather massive idea is side pods, running the coolant lines through them instead of through the passenger area. Side pods have interesting possibilities: improved torsional stiffness, impact protection, and depending on their size, possibly blocking the line-of-sight path that rocks kicked up by the front wheels take to hit the passengers. They could even possibly provide some storage space. And, from a wishful-thinking standpoint, inverted wings could be incorporated into their undersides, though I’d expect the net effect to be about zero due to the 4″ ride height, never mind getting all the dirty air off the front wheels.
They’d really change the look depending how big they are, making the car look much wider and perhaps less dunebuggy-like. It makes sense to make them about a foot wide, the same width as the rear fenders but geez, I don’t know. Maybe I’ll draw it up in CAD but I think they’ll look, um, bad. Side pods look okay as long as they’re more-or-less parallel, like on an F1 car, but a Locost tapers towards its narrow nosecone. Having big 12″-wide side pods angling towards each other conjures up odd images, but who knows. Besides the oh-my factor, they also weigh more, cost more, and extend construction time. Frankly, right now I can’t see enough benefit to warrant the extra work and expense.
Another way is to run coolant through the lower-outside chassis tubes, now being reconsidered after several people I respect said that it’s fine, running hot water down one side and cool water up the other… hmmm. It certainly is the most efficient and lightest way to go, with corrosion inhibitors in the coolant preventing rust. It means that rivets can’t be installed into the tubes, though welding on the floor or riveting it to a small angle bracket solves that. There’s time to roll this idea around until it settles; let me know what you think.
In other news, the Walbro GSL392 fuel pump, plus a factory Honda RSX-S ECU arrived and was shipped off to be converted into a Hondata KPro tunable ECU. The cool thing is that it’ll have a preloading basemap that should work well with my engine components; it “should just start right up”… we’ll see.
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 😉
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.
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.
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.
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.
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.
Changed the rear suspension layout yet again. The shocks have moved to the back corners where they’re more accessible and the suspension arms now consist of lateral and trailing links – basically, really big A-arms – all to give the drivetrain plenty of room. Once the decision was made to have the engine tray removable, it’s natural to attach all the suspension links to it, and maybe even the rocker-arm pivots. Though things are still in flux the design is settling down, really.
Yesterday was materially non-productive but a fundamental decision was made; the bottom engine frame/tray will be removable. It allows running tubes as needed to produce a strong structure without compromising them to get the engine in and out. The tray will bolt across the base of the main roll hoop and to the rear down tubes – just like Kimini. This stuff is the hardest part of the project, where all the decisions will come back to bite me if they aren’t thought through very carefully. Everything that comes after this is icing on the cake!
Being back at my day job gives time to reflect on various design decisions – during break time of course. One consideration is whether to push on with a traditional A-arm rear suspension, or change over to what was used on Kimini, long lateral links and even longer trailing links. Doing so would eliminate one of the two bulkheads behind the engine, freeing up room and simplifying the layout. My only concern is the trailing links, long tubes heading back from the main roll hoop plane to the suspension uprights. In the case of an accident, they might be driven forward into the gas tank(!), or even into the passenger compartment. One solution is to use tall U-shaped channels to attach them to, which also act as “catch fences” for broken or bent trailing arms. The large bracket catches the failed tube and prevent it from passing forward of the main roll hoop plane.
Even if the above is done, it doesn’t avoid the need for one transverse bulkhead behind the engine to attach the lateral suspension links to. The bulkhead is a prime issue when it comes time to install or remove the drivetrain. The trick is to have few – or no – tubes that have to be removable; I’m not sure yet if it’s achievable.
In other news, philanthropist/actor/race car driver/race team owner Paul Newman is expected to pass away within a few weeks – cancer. I had the honor to see him in person once in 1984 at the Los Angeles Times Grand Prix. I was walking through the pits and there he was, sitting on the end of a bench all by himself, race suit around his waist, kicking his feet back and forth like a little kid. I was to shy to walk talk to him but in hindsight I know he’d have liked to talk about the Datsuns we both once owned. Anyhow, that’s how I’ll remember him, someone who obviously really enjoyed what he was doing. When he was out on track he was just one of the guys, and a very competent driver in his own right. With the millions donated by his “Newman’s Own” food brand, he’s done a lot to help people. I read one time that he said that he acted in order to fund his racing and I don’t doubt it. I enjoyed him most in “Butch Cassidy and the Sundance Kid” and “The Sting” – a simpler time in Hollywood. Yes, he’s had a pretty good ride. I think that’s the best we can do here, where when it’s over, people say, “you done good.”
To end on a higher note, Kimini is virtually sold; the deposit has been received and the buyer has through September to pick her up. Her new home will be Salt Lake City, Utah, where the new owner said that I can come visit her. I might just take him up on that.
Heard from Mitchell Software… my mistake – big surprise there. I somehow changed the rotation point, an obscure parameter I had a hard time finding in order to set back to zero. How I managed to accidentally get into that menu, set it wrong, and forget doing so beats me.
Between catching the flu and awaiting an answer on strange results I’m getting from the Mitchell suspension software, I’m starting on the CAD drawings – may as well do something productive. Doing the drawings is already proving educational, forcing the need to address tubes that aren’t even on the mockup yet. One thing to decide is how to pass the coolant lines, shifter and throttle cables, brake and clutch lines, and wires from the front of the car to the back. Some people have suggested running the coolant lines outboard, but that doesn’t really simplify anything. They still need to be covered to avoid burns or having boiling coolant spraying about in case of a cracked line or accident. Even if they were run there, the throttle and clutch lines lend themselves best to running down the center of the car anyway, so may as well run everything that way.
Today the decision was made on where and how the rear shocks will mount.
There’s going to be rocker-arms at the rear. Not because it’s ubersexy, just that it’s the most appropriate solution. Why? The Miata rear upright has its upper suspension pickup cushioned in a big rubber bushing. Putting a screwdriver through the bushing and pulling it from side to side shows that it’s fairly soft. I was going to mount the bottom of the shock to it, but applying an offset load of ~400 lbs to it isn’t a good idea. A solid bushing could be substituted for the rubber, but one design goal is to avoid lathe-work. On top of that, the steeply-inclined shocks gave too low an installation ratio, requiring spring rates higher than what’s commonly available in 10-12″ springs. They were also too close to the engine and exhaust for comfort, and that’s on my drivetrain. Who knows how close they’d be to whatever drivetrains builders are going to install. Tilting the shocks to be more upright wasn’t an option, either, because they’d stick through the engine cover with nothing to attach to. I would have attached the shocks directly to the lower suspension pickup point on the upright, but they’d would have to hang off a bolt in single-sheer about an inch out… won’t go there.
The best solution appears to be to keep the shocks away from the drivetrain and keep them low. While putting them behind the drivetrain makes them accessible, the exhaust is back there, too, cooking anything close by. For these reasons they’re being placed vertically, about a foot forward of axle centerline.
Speaking of the rubber bushing, if a single rod-end is bolted to one end, it’ll twist the bushing. If a U-shaped bracket is used so a bolt passes all the way through the assembly (like a Miata) then it’ll both bind slightly when toe is adjusted, and move “some amount” during cornering. I’ll probably leave it as-is until I figure out how much of an issue it is.